Power

BOOK No 11 Version 0

Global Organization

Innovative Solutions Product & Substation System Business Business

Power Systems for Industry

BA THS / BU Transmission Systems and Substations LEC Support Programme

Suggestions for improvement of this book as well as questions shall be addressed to: BU TS / Global LEC Support Programme C/o ABB Switchgear AB SE-721 58 Västerås Sweden Telephone Telefax Telex

+ 46 21 32 80 00 + 46 21 32 80 13 40490 abbsub s

Copyright  BU Transmission Systems and Substations

2

BA THS / BU Transmission Systems and Substations LEC Support Programme

Power Systems for Industry Welcome to the handbook "Power Systems for Industry". It is an extract from the ABB Industrial Manual, published by the Automation Segment in Sweden. This extract includes the Industrial Power Systems part. The reprint has been made by courtesy of the A - segment in Sweden in order to increase the mutual understanding between the two segments. We hope you will find the booklet useful in your work. The authors welcome any idea you may have to improve the quality of this booklet as well as the other ones.

Best of luck!

BA THS / BU Transmission Systems and Substations LEC Support Programme

BA THS / BU Transmission Systems and Substations LEC Support Programme

INDUSTRIAL POWER SYSTEM Industrial Power system (IPS) Design - an Overall Approach

INDUSTRIAL POWER SYSTEMS Contents page 1 Industrial Power System (IPS) Design - an Overall Approach

1

2 Low Voltage Distribution

12

3 Medium Voltage Distribution

57

4 Transformers

68

5 High Voltage Switchgear

77

6 Fault Control

90

7 Industrial Power System Control

104

8 Industrial Cogeneration of Power and Heat

114

9 Standby and Uninterrubted Power Supply (UPS)

126

10 Prefabricated and Mobile Substations

139

11 Bibliography

147

INDUSTRIAL POWER SYSTEM (IPS) DESIGN - AN OVERALL APPROACH Contents page 1 General

1

2 Safety

3

General supply being ”only” a support function - becomes a “necessary evil”. However, it is the responsibility of the IPS designer to communicate the fact that the manufacturing function or process will not be more reliable than the electrical system supplying power to it. The conceptual design phase is often the most crucial in the chain of events leading to a new, expanded, or modernised electrical power system, as well as of a co-generation project. Conceptual design studies produce the criteria

The electrical power system of an industrial plant represents only 5-10 % of the total capital investment. This is perhaps the main reason why it is sometimes difficult for the IPS electrical design engineer to convince project management that an economical and reliable power system requires a great deal of careful analysis and planning, perhaps more than for some of the more expensive parts of the project. The manufacturing process is always of primary concern to project management, and electrical 1

5.1

1

97-03-05, 13.14

INDUSTRIAL POWER SYSTEM Industrial Power system (IPS) Design - an Overall Approach IPS conceptual design studies. There are - with few exceptions - no college courses or engineering handbooks to help guide interested engineers in such assignments. Fig. 1 suggests a procedure for IPS design execution. In the absence of applicable literature, course curriculum or training courses, this procedure is offered as a basic framework for conducting and evaluating or assisting in audits of IPS design.

for the best single line architecture, the best voltage levels, the most efficient system earthing and optimal fault protection consistent with long-range safety, reliability, flexibility, maintainability and lowest lifetime costs. The achievement of a sound IPS design helps assure a “first-time quality” system, while avoiding the agony of failure that costly corrections of unforeseen flaws might otherwise require. There are no international industry standards, guidelines, or customer specifications for

Initial conditions

Plant Layout Power company supply Production Capacity LOAD AND SUPPLY PLANNING

SELECTING VOLTAGES

SELECTING SYSTEM NETWORK

Safety Simplicity Maintainabily Flexibility Reliability Economy

SHORT CIRCUIT AND DYNAMIC ANALYSIS

CALCULATE VOLTAGE VARIATIONS SELECT SYSTEM EARTHING SELECT SURGE PROTECTION RELAY PROTECTION AND SELECTIVITY POWER FACTOR CORRECTION

Specification and procurement

Fig 1 Conceptual Design of an Industrial Power System

2

5.1

2

97-03-05, 13.14

INDUSTRIAL POWER SYSTEM Industrial Power system (IPS) Design - an Overall Approach 2) Risk analysis, ergonomics and subjective feeling of risk. So far, risk analysis has had its application mostly in larger systems of more complex nature (e.g. nuclear power stations or large power systems). Risk analysis has much in common with reliability analysis and is based on statistical data concerning various types of equipment failure and accidents. Such data is still not easily available, especially because health risks in electrical plant are low compared to other industrial hazards, and since there has been little urgency expressed regarding the need to collect such data on an international basis. It may, therefore, still take some time before risk analysis of industrial electrical systems and equipment will be commonplace, and before ergonomics (man-machine theory) will be generally applied to improve electrical operator safety. One area that is often overlooked in personnel safety is that there may be little correlation between the statistical probability of a certain type of injury and the subjective or psychological risk experienced by the operators (for example the fear of health hazards from electromagnetic non-ionising fields). This may admittedly lead manufacturers to improve aspects of their equipment, but they would be measures that only marginally improve ”real world” safety.

This guideline demonstrates how the various parts of the design process are intimately interconnected. This procedure does not deal with the selection of hardware itself, but rather with those qualities a system should have, and those planning activities that lead to the successful specification of hardware and subsequent procurement. It should be noted that the approach described and the categories chosen in the following represent just one of many possible ways of representing a complex undertaking. IPS design mastery cannot be gained simply from a lifetime of working in an engineering office where project engineering production is the primary criterion of performance. Only long-term, hands-on experience under proper guidance, in addition to selected graduate courses can bring about the development of a seasoned conceptual system designer.

Safety From the designer’s point of view safety consists of two main areas: 1) Safety codes and regulations, (such as ISO 9000 Quality Standards) In most countries power systems are required by law to follow applicable safety codes or wiring regulations. This guarantees some degree of safety through the quality of the equipment and workmanship thus stipulated. Also, the International Electrotechnical Commission and national bodies provide recommendations that further ensure that the power system will conform with a certain class of safety. (It should be remembered that Quality Assurance can be no better than the specifications used in the project.) However, the final test of safety to personnel, plant and environment will come after the system is commissioned and when electricians and operating staff take over the daily running of the system. Simple, readable operating instructions are needed, together with a thorough knowledge of how all the equipment will behave, both under normal and abnormal conditions. There is no place for guesswork and indecision in emergency situations where the health and lives of personnel can be endangered. This is when the designer’s ability to envisage the operational phases will truly show up.

Simplicity A simple single-line diagram and a plot of the physical and mechanical layout of equipment will assist operational safety because the risk of making mistakes during commissioning, operation and maintenance is reduced when things are easy to find and overview both in documents and in the form of hardware. It is therefore important to make electrical systems and plant as simple as possible. This requirement is somewhat in contrast to needs for reserve capacity and redundant feeders, which tend to make networks more complex. A balance has to be reached which requires the IPS designer to have a full understanding of what actually goes on in an electrical operation and maintenance department. Expandability - Flexibility For future growth of the power system to be 3

5.1

3

97-03-05, 13.14

INDUSTRIAL POWER SYSTEM Industrial Power system (IPS) Design - an Overall Approach still much to be done. Some large international industrial users have collected their own statistical failure data, but this is generally not available to outsiders.

included in planning, it is necessary to know something about possible future expansion and increased production capacity. Where such plans do not exist, it will be helpful to clarify how much expandability the system has and where the bottlenecks to growth may lie, and to inform the project team about it.

Total economy It is well known that good economics for an IPS is not only a question of the lowest purchase price of equipment. Total life-cycle cost of a power system depends on equipment purchase price and quality, construction and installation costs, operating costs including losses, outage costs, repair costs, useful lifetime of the equipment and administrative costs. A particular manufacturer’s likelihood of surviving into the future and being able to supply spare parts is also an important consideration in the procurement evaluation. This holistic economic view has been termed “terotechnology”, but is not always applied where IPS planning and design takes place.

Maintainability In order to make a power system and its equipment easy to maintain, it is important that the maintenance programme be kept in mind already at the planning stage. If continuous round-the clock operation of all or parts of the process is required, the network must have reserve feeders or separate supply to these components. Otherwise, provisions for maintenance of live equipment must be made. In order to answer these questions, it will be helpful to clarify when and for how long the process equipment itself will require maintenance downtime. The importance of a rigorous maintenance programme and a meticulous spare parts policy in order to maintain high continuity of operation is well known. Various reliability surveys conducted for industrial plants provide plenty of evidence for this. Inadequate or improper electrical maintenance is a significant factor in all in-plant outages. And yet the owner will be prepared to have work going on round the clock to remedy the failure once it occurs. The paradox is obvious and well known, and demonstrates the need to draw attention to preventive maintenance as a major factor in power supply reliability.

Planning activities Even if engineering judgement is also required in what follows, precise calculation and analysis are more of a priority than in the previous ”qualities” discussed. Fig. 1 describes how the conceptual design work may proceed. At an early stage all four of the first activities are normally carried out simultaneously. This is a highly iterative process for a new project, where equipment layout, production capacity and the power company supply points represent initial conditions. Fig. 1 is by no means a strict guideline, but rather a reminder of how the activities are dependent on each other. Therefore, the feedback loops and the number of adjustments that have to be made to the IPS design will vary from one project to another. The work will also depend on how much and how often the initial conditions are changed (which in some projects constitute continual ”harassment” of the electrical discipline).

Reliability - Availability The technique of reliability analysis is well known and widely used for large power systems and might be considered a well established analytical tool. However, until recently, reliability analysis has not been applied widely to IPS planning for the simple reason that statistical failure data for medium and low voltage components have not been readily available. With the advent of the IEEE “Report on Reliability for Industrial Plants,” such statistics have become available on American equipment used under American conditions, but for international applications elsewhere there is

The diagram in Fig. 1 illustrates the necessity for a central IPS engineering body to be in charge of the overall view and make sure that nothing is forgotten. The diagram represents an ideal situation where all considerations and analyses have been attended to before the electrical equipment is finally specified and pur4

5.1

4

97-03-05, 13.15

INDUSTRIAL POWER SYSTEM Industrial Power system (IPS) Design - an Overall Approach relay and overcurrent protection should • All function for both maximum and minimum

chased. In reality, the specifications and equipment data sheets, and the purchase of hardware, have sometimes been completed while some system analysis is still going on. This may not always work out well, and there is a great risk that costly changes may come, or that some important requirements will have to be neglected as a consequence.

available short-circuit current. During the relay co-ordination study it usually becomes apparent that available short-circuit current from small in-plant generators is very limited. This should be kept in mind when selecting both generator and relay protection, which preferably should operate in a selective manner even in emergency conditions.

Load and supply planning Some of the fundamental questions which determine network architecture and layout concerning the planned production capacity are: How many kW / kVA are required? Where are the major load points located on the plot plan? What types of load (motors, furnaces, etc.)? How much power is required at stage 1, 2, 3, etc., of the process?

Choice of system network - single-line diagram One of the main criteria, as mentioned under “Simplicity”, is: Do not make the network more complex than necessary! The simpler the single-line diagram is, the easier it is for the operator to avoid mistakes during switching operations. The more complex the system is, the larger will be the requirement for interlocking and automatic control with corresponding investment cost. Also, from a service point of view, a simple radial system will provide less risk of ”back” voltage and therefore is safer to maintain. ”Simple” usually also means lower initial cost. A well planned maintenance programme and a scrupulous spare parts policy will have so much influence on reliability that it may outweigh a very simple network diagram with restricted redundancy. See also “Maintainability”. The simpler the network, the easier it will be to visualise and carry out quick and accurate system estimates for load flow, short circuit and voltage dips on motor starting by hand rather than by computer. The priority of certain loads will, of necessity, require standby supply. The more alternative ways of supplying each load or load centre, the more complex the network will become.

• • • •

•

The answers to these questions cannot be absolute at an early stage, but assumptions for different alternatives must be made to prepare for the power company negotiations. Whether to consider in-plant or co-generation will depend on: What tariff will the power company use? What outage frequency and power quality can the power company provide at the point of supply? What will be the cost of process shutdown? Must part of the process keep running during power company outage? Will primary energy be available at low cost (e.g. process steam or gas)? What plans does the power company have for future expansion, with additional lines into the area that could improve availability?

• •

•

• • • •

•

Discussions with the power company will also concern the following: Present and future supply voltage and shortcircuit levels. These will determine what voltage disturbances and harmonics there will be due to the starting of motors, arc furnaces, variable speed drives, etc., both within the plant itself and from feedback into the supply power system. This will also determine if and how much of the plant will have to be shut down to allow for motor restart/re-acceleration after return of supply voltage following short outages.

•

Some questions which should be asked about the system network are these: What might future expansion look like and how will it affect the network? What kind of system network would the plant operation engineers prefer? How much overload can be expected at each load centre? (Transformer fan cooling is a cheap short-time reserve, but there are high losses).

• • • 5

5.1

5

97-03-05, 13.15

INDUSTRIAL POWER SYSTEM Industrial Power system (IPS) Design - an Overall Approach main transformer and main distribution switchgear.

With these questions in mind, the selection of the network may proceed along the following lines: In order to simplify the discussion it is necessary to use a block diagram representation for a collection of hardware which can naturally be lumped together, such as shown in Fig. 2. Each small section of single-line diagram is represented by a box as shown. Figs. 3 and 4 then illustrate the two types of systems discussed, the radial and the loop system.

Receiving or incoming switchgear In the 66-145 kV range this switchgear is usually in the form of an outdoor switchyard. If space is limited or pollution is a problem, GIS SF6 metal-enclosed gear is used. Normally one or two incoming lines from the power company will supply the industry. The example in Fig. 2 with a single busbar for this function is only one of many possible arrangements, but more than one busbar is seldom chosen since the overall reliability does not improve much with more complex arrangements. (This is normally different for larger power company substations) Main transformer The size may vary according to load levels, but maximum sizes of 40-50 MVA seem usual for this type of substation. The size will primarily be determined by the available short-circuit level on the low voltage side, as well as the limitations and cost of the distribution switchgear and associated protection. Transformer capacity is usually duplicated to give back-up reserve in case of failure. Through extra fan cooling one transformer may be able to handle all or part of the substation load during a period of limited supply or maintenance work. Main distribution switchgear The voltage of this kind of switchgear may be 6-36 kV. The hardware is usually of indoor type in a separate building or in an operating room which is part of the plant building. Large industries have one or two switchgear assemblies per substation, and they may or may not be in the same physical location. Double bus switchgear of the duplex type may also be used for this important function, where one of the buses carry all high priority loads.

Fig. 2 Functional blocks and description of IPS components

Secondary or sub-distribution switchgear These switchgear assemblies are usually similar to the main distribution switchgear and carry the same voltage. Single bus is mostly used, with the addition of sectionalising in order to segregate high priority loads. The secondary distribution switchgear may wholly or partially be a high voltage motor control centre.

The terms used are as follows: Main or primary substation Depending on the size of the loads, there may be one or several main substations. As in Fig. 2, this usually consists of three separate functions in the single-line diagram: Incoming switchgear, 6

5.1

6

97-03-05, 13.15

INDUSTRIAL POWER SYSTEM Industrial Power system (IPS) Design - an Overall Approach Radially fed secondary substation This type of substation consists of 0.5-2.5 MVA, dry or oil-insulated transformers with low voltage switchgear. Using dry transformers, the substation may be located inside the manufacturing process building on the shop floor. Due to fire regulations, oil-filled transformers are placed in concrete “boxes”, normally outdoors. As the substation is radially fed, it may not have any high voltage equipment at all. In some cases two transformers provide back-up for each other with sectionalised low voltage switchgear where high priority loads are connected to one of the sections. Automatic changeover may provide continuous operation with a short break if one of the incoming feeders or one transformer experiences a failure. Loop-fed secondary substations Differing from radial substations, these have high-voltage switchgear consisting of load break switches with current-limiting fuses or circuit breakers for transformer protection. The substation may have 1-4 transformers, usually with segregated low voltage switchgear for each transformer. The high voltage switchgear is sometimes called a “ring main unit” in countries with British electrical influence.

Fig. 3 Radial distribution

Fig. 4 Loop distribution 7

5.1

7

97-03-05, 13.15

INDUSTRIAL POWER SYSTEM Industrial Power system (IPS) Design - an Overall Approach Fig. 3 illustrates the radial system. The diagram does not show the electrical paths, only the functional blocks and their position. The system is the simplest type possible and allows for great flexibility and expandability. The radially fed secondary substations may be located close to the load centres, thereby saving losses and keeping the expensive low voltage network to a minimum. The primary substation may be the same for the loop system in Fig. 4 as for the radial system. The loop system concerns only the feeding of the secondary substations from the secondary distribution switchgear. The rings or loops are normally open in the middle so that in case of failure only one half of the loop will experience outage until the faulted section is isolated and supply can continue. The radial and loop systems are often mixed to utilise the combined advantages of both. The network layout will be influenced strongly by requirements to maintain limited power supply in case of power company supply failure. The operation of the plant may be divided into three categories.

Reduced or vital operation may be maintained with standby generation or from a separate power company supply. When part of the power company supply fails, a preset load-shedding procedure may automatically leave only high priority loads connected. For emergency operation only such equipment as is necessary for personnel safety and preventing catastrophic failure of the process equipment is maintained.

1. Normal operation 2. Reduced or vital operation 3. Emergency operation

Fig. 6 System with three levels of local generation Single-line diagrams for “typical” industries are represented by the examples in Figs. 5 and 6. Fig. 5 shows a typical simple system without any stringent requirement for redundancy or reduced operation. In case of power company failure on both incoming lines, the whole plant is down. This may be acceptable if the process does not require gradual shutdown. The system in Fig. 5 is purely radial, except from the reserve feeders between adjacent secondary distribution switchgear assemblies. A somewhat more advanced system in Fig. 6 has two stages of local generation, first a larger cogenerator for reduced operation, and secondly two small emergency generators. Choice of voltage(s) The size of the industrial process, i.e., the power requirement, will determine whether one or several voltage levels are necessary. A small industry may well manage with only low volt-

Fig. 5 System without internal generation 8

5.1

8

97-03-05, 13.16

INDUSTRIAL POWER SYSTEM Industrial Power system (IPS) Design - an Overall Approach voltage 6-36 kV • Distribution The distribution voltage will depend on the

age, while a larger one could require 3 - 4 voltage levels. The choice of network and voltages are so closely linked that for all practical purposes they can be regarded as a single integrated procedure. Depending on the country and the standards it is influenced by, there are a great variety of voltages available which, when selected, may determine and limit the number of manufacturers that can submit competitive offers for the equipment. Selection of voltages is, therefore, the most vital consideration for the designer in preselecting the supplier(s) of hardware. Therefore, in selecting voltage levels, it should be recognised that limitations to international trade may be imposed on a project. The following main voltage areas may be identified:

total power demand by the plant, as well as on the short-circuit level required to handle disturbances and harmonics. Short-circuit study - stability analysis The calculation of available short-circuit currents in the network is the basis for selection of switchgear, cables, relays and protection coordination study. Further, the short-circuit level at each point in the network will determine voltage variations during starting of motors, welding machines, furnaces, or other intermittent loads. The stronger the system is, the less disturbances there will be. What the optimum short-circuit level will be is thus an economictechnical judgement limited by available apparatus and acceptable voltage variations. Some reminders:

voltage 400-1000 V • Low There may be one or two voltage levels

and simple hand calculations may be • Quick carried out using only power company sys-

below 1000 V. Usually, lighting and general purpose loads such as heating and air conditioning will have a separate voltage, usually a 400/230 V system where both single-phase and three-phase loads are connected. If there is a large number of motors above 50 kW, they may require a somewhat higher voltage level of 500, 600 or 690 V. Almost without exception, low voltage systems are radially fed from the secondary substations discussed before. It is important to recognise that no manufacturing distinction is made in IEC between low voltage switchgear and motor controlgear as is done in ANSI standards.

tem, large generator and transformer reactance, ignoring all other impedances. This will give a good first estimate of the shortcircuit level, and motor and small generator contribution may be added as the system layout takes shape. A number of good PCoperated computer programs are available to-day for modelling the system, and for gradually adding new information as design progesses. reactance (size of the trans• Transformer former) and current-limiting reactors, to-

motor voltage 3-11 kV • Special The decision as to which voltage is right for

gether with sectionalising, are the main tools in manipulating the short-circuit level.

large motors and generators depends both on the cost of the motor itself, cable and bus duct, as well as the cost of the starting method and switchgear involved. For certain smaller motors the starter may cost more than the motor itself. For very large motors, it is worth while looking at the motor-transformer unit principle, i.e., a separate transformer for each motor that will give a smooth start with minimum disturbance along with lower cost for the motor-starter-transformer combination. If the plant distribution voltage is different from the motor voltage, this may well be a preferred solution.

aware that IEC 909 does not represent the • Be full international story regarding short-circuit calculations. Unfortunately, IEC 363 also stipulates rules for calculation in marine systems. The IEC technical committees have not been able to agree on one common set of rules for both land and sea. Also be aware that ANSI standards are different from those of IEC since the method of rating circuit breakers is different. The smallest available short-circuit level may be critical for protective relaying and selectivity to function during periods of reduced operation, e.g., during emergency or standby operation with only local generation source connected. 9

5.1

9

97-03-05, 13.16

INDUSTRIAL POWER SYSTEM Industrial Power system (IPS) Design - an Overall Approach economy of each alternative will decide which one to select. At the same time, the available starting torque of the motor should be checked against the load torque to allow the motor to accelerate to full speed before becoming overheated. The relay protection requirements of the motor are thus considered at this time to allow for a co-ordinated and integrated drive package. Loads with fast-varying power demand, such as welding equipment and arc furnaces are often placed separately in the network (on separate transformers) to allow for minimum disturbances. Static converters create more complex problems as they require both varying reactive current at the power frequency and, in addition, create harmonic frequencies. Combinations of filters and switched capacitors are used to improve the voltage quality.

When a short circuit occurs in the power system, voltage drops will occur throughout the system. Similarly, during short power company outages (auto-reclosing) the power system can experience a period of severe disturbance. When the short circuit is removed and voltage is restored, synchronous machines may have fallen out of step, or induction motors may have lost speed to such an extent that there is not enough strength to re-accelerate all of them simultaneously. A transient stability study will give the answers to what happens during such disturbances. This is achieved through the use of a computer program, and the outcome shows what measures will have to be adopted in terms of protective relaying, load shedding, restart sequence, etc. A type of automatic load shedding that is often forgotten is inherent in the large number of voltage-held contactors present in nearly every industrial plant. Contactors drop out at approximately 65% of rated voltage if no steps are taken to delay the drop-out.

Choice of system earthing The main purposes of connecting the system´s neutral point directly to earth or through some impedance is to: a) produce a current or voltage during an earth fault with a magnitude sufficient for selective tripping or alarm signal b) limit the earth fault current to keep personnel hazard and material damage to a minimum c) limit overvoltages that may occur during earth faults on a completely unearthed system The selection of system earthing is, therefore, closely linked to the choice of protective relaying and voltage surge protection. Without going into the merits of the various methods of system earthing, the following points should be kept in mind:

Calculations of Voltage Variations - Load Flow Study Where a power system is relatively complex, with a number of alternate feeding paths for each load centre, a load flow study by computer will be necessary. Sometimes this study is a prerequisite for the short-circuit study. It provides data for the sizing of cables and bus, and determines the resulting voltage drop due to steady-state loads with high power factor. When a simple radial system is used, load-flow calculations may easily be carried out by hand, although a PC may be helpful in cutting down the work. Apart from the above steady-state conditions, there are some loads which at times will require high reactive currents. The most common type is the starting of motors. With the aid of the reactance diagram arrived at during the short-circuit study, very fast and simple offhand estimates of the voltage drops due to starting may be carried out assuming a completely reactive circuit at the instant of start. For more comprehensive answers giving the system performance during the entire run-up period, a dynamic study is required. If the network requires reinforcement, or if reduction of starting current is achieved by some reduced voltage starting method, the

neutral points are not avail• Ifable,transformer there will be an extra cost for special earthing transformers to establish artificial neutral points. often have special earthing re• Generators quirements which may deviate from the rest of the system. Low voltage systems up to 400 V are mostly solidly earthed, while 690 V systems more frequently are unearthed, or, high-resistance-earthed to allow for delayed disconnection of the faulty circuit until such time in the plant process as is convenient to repair it. Internationally, high impedance or Petersen coil (resonant) earth10

5.1

10

97-03-05, 13.16

INDUSTRIAL POWER SYSTEM Industrial Power system (IPS) Design - an Overall Approach relays, earth fault relays, etc., is an elaborate procedure closely related to the choice of system network, system earthing and short-circuit and stability analysis. This is covered in rather more detail in the section ”Fault control”. Instrument transformers and their ratios and burdens must also be considered, as well as the setting ranges of the relays. It will be necessary to carry out a protective co-ordination study for all series-connected overcurrent devices. With the inverse characteristics of fuses, overcurrent and overload relays, direct acting trips, etc., it will only be possible to demonstrate on timecurrent log-log paper the degree of selectivity obtained. On the same graph, such data as maximum and minimum short-circuit level, starting currents of motors (including starting time) and l2t curves for cable and transformer withstand ability may be plotted for a total view of the protective function. Co-ordination with the power company´s protective relaying will be directed by agreement as to how the two power systems should interact with each other during heavy disturbances.

ing is common in medium and high voltage (3-100 kV) systems, although each country has developed some degree of individual practice. power company will be responsible for • The system earthing on the supply voltage side. Voltage surge protection The most common harmful overvoltages in an industrial power system stem from: surges on power company lines or • Lightning overhead lines within the plant itself. • Switching overvoltages. or arcing earth faults in un• Intermittent earthed systems. Those components specifically vulnerable to overvoltages are the low BIL and high-investment parts; primarily motors. For the first two types of overvoltages the use of surge capacitors at the terminals of each large motor should be considered to protect the turn-turn insulation against surges with steep front, and lightning arresters to protect the total winding insulation against the amplitude of the surge voltage. Which of these methods or what combination of them should be used will depend on the size of the motor, its voltage rating and the degree to which the network is exposed to lightning and switching voltages. Dry type transformers also have a low BIL and may need lightning arresters. Arcing or sputtering earth faults in unearthed systems may build up harmful overvoltages in the system´s equivalent capacitance. This may be avoided by earthing the system through a relatively high resistance.

Power factor correction What power factor is considered acceptable will depend on two cost assessments: First, to what extent does the power company tariff penalise the use of reactive power? Secondly, how much can be saved in cable sizes, transformer capacity and loss reduction through the installation of capacitor banks? The optimum location of the capacitors in the system network will have to be calculated too. The closer to the load the capacitor is located, the more effective the compensation will be, but there is also a higher cost per compensated kvar. Do not forget to include the price for switching equipment, if applicable. In choosing between large synchronous and induction motors, it should be remembered that synchronous motors may be over-compensated to improve a poor plant power factor. Induction motors have no such facility. Fast varying var requirements may require more sophisticated compensation, such as thyristor-switched capacitors or synchronous condensers. The problem here is the voltage disturbances caused by the varying load, rather than the price of kvar used. This is covered in rather more detail in Chapter 6 ”Reactive power compensation”.

Protective relaying and co-ordination study The sophistication and complexity of relay and fault protection equipment necessary for each part of the power system will depend on the investment in the object protected, and the degree of availability and personnel safety offered by the protection. The aim is to disconnect selectively the faulty part of the system as fast as possible to minimise damage. The choice of overcurrent relays, fuses, direct-acting trip devices, overload relays, differential relays, overvoltage and undervoltage relays, frequency 11

5.1

11

97-03-05, 13.16

INDUSTRIAL POWER SYSTEM Low voltage distribution

LOW VOLTAGE DISTRIBUTION LOW-VOLTAGE SWITCHGEAR AND CONTROLGEAR ASSEMBLIES LV ASSEMBLIES Contents page 1 General

12

2 Requirements

17

3 Sizing faktor

25

4 Plant design

27

5 Apparatus and combination of apparatus

32

6 Selection of short-circuit protective device (SCPD)

43

7 Selection of overload protection

48

8 Microprocessor-baserad control technique for low-voltage distribution systems

50

9 Sizing of devices and cables

51

10 Project planning

51

11 Information to be provided on an LV ASSAMBLY

53

General Introduction In an industrial low-voltage distribution system the loads consist of motors and electrical devices in some producing processes, see single-line diagram Fig. 2, p. ? below and the section Sizing factors, p. ?. Larger industries have their own receiving substations, from where medium-voltage (6-36 kV) power is distributed. Each sector has an incoming medium-voltage switchgear and controlgear assembly with a circuit-breaker, see Medium-voltage distribution in Chapter 5.4 on p. ?, which feeds a transformer of up to 3.15 MVA, see Distribution transformers, p. ?. The most common secondary voltages (low voltages) are 400, 500 or 690 V. See the section Voltage levels, p. ?.

The transformer is mostly placed near the lowvoltage switchgear and controlgear assembly located in a locked operating room to which only skilled persons have access. The abbreviation LV ASSEMBLY/IES is hereafter used for a ”Low-voltage switchgear and controlgear assemblies”. The LV ASSEMBLY can be divided up into main distribution and sub-distribution. The main distribution LV ASSEMBLY is then placed in a locked operating room and the sub-distribution LV ASSEMBLY can be located close to process objects. In many countries it is common that not only the main distribution LV ASSEMBLIES but also the sub-distribution LV ASSEMBLIES are placed in operating rooms. See the section Plant design, p. ?. The main incoming unit in an LV ASSEM12

5.2

12

97-03-05, 13.17

INDUSTRIAL POWER SYSTEM Low voltage distribution

Fig 20, Typical LV-ASSEMBLY stated by the manufacturer the time is 1 s.)

BLY is normally a circuit breaker cubicle. The circuit breaker should be sized to trip at the maximum short-circuit currents, and to continously carry the total current load. The circuit breaker is supplied with a short-circuit protective device (SCPD) and overload protection, which break the circuit in the event of a short circuit or an abnormally high load current. Circuit breakers are described under the section Circuit breakers, p. ?. To achieve a very short breaking time if an arc arises, it is recommended that the LV ASSEMBLY be supplied with arc monitors. The co-ordinat system is described in the section with the same name on p. ?. The rated short-time withstand current of an LV ASSEMBLY is normally in the range between 20 kA and 100 kA. (Unless otherwise

The rated current (load current) of an LV ASSEMBLY is normally between 800 A and 6000 A. The rated voltage of an LV ASSEMBLY does not exceed 1000 V AC. Main distribution - Load Center, LC The breaker that is directly connected to the secondary side of the transformer, feeds the main busbar in the LV ASSEMBLY. Outgoing units are connected to this busbar: for sub-distribution for resistive loads for motors and motor drives for other types of loads See the section Sizing factors, p. ?.

• • • • 13

5.2

13

97-03-05, 13.18

INDUSTRIAL POWER SYSTEM Low voltage distribution starters for heavy motor start • Soft switches for short circuit and earth• Earthing ing of the busbars related to maintenance on

Sub-distribution: LV ASSEMBLIES with outgoing units for motor drives, Motor Control Center, MCC. LV ASSEMBLIES with outgoing units for distribution. Multibox-type LV ASSEMBLIES with outgoing units for lighting panels, fan motors, ventilation equipment, heating equipment, pump motors, etc.

• • •

disconnected part of the plant. In the section Apparatus and combination of apparatus, p. ?, more information can be found on the above devices. Power factor control The loads in a process industry mainly consist of motors, which work with magnetic fields needing current. This current is reactive and is not transformed into energy, or useful work.To compensate this, it is appropriate to connect an automatically power factor control device to the LV ASSEMBLY. The equipment consists of capacitors which are switched on and off by contactors, guided by an electronic reactive power control unit that measures the power factor, cos j. Fig. 3 below shows the principle applied. Power factor control is described in more detail in Chapter 6, Reactive power compensation (power factor control) on pp.?-?.

Examples of outgoing units:

(mechanical) + fuses • Switch breakers (normally as incoming unit, • Circuit see above). case circuit breakers, MCCBs • Moulded • Disconnectors Switch-fuses • Fuse-switches, boards with D-type fuses or • Distribution with miniature circuit breakers, MCBs. starters (the most common outgoing • Motor units in an MCC). converters for speed adjusted DC • Current motors converters for rev/min adjusted • Frequency AC motors.

Fig 21. Single-line diagram 14

5.2

14

97-03-05, 13.18

INDUSTRIAL POWER SYSTEM Low voltage distribution The main busbars (primary system) are assembled horizontally, and are connected in each cubicle to vertically assembled busbars (secondary system). Functional units (in most cases outgoing units) are connected to the vertical busbars. The primary busbar system has a rated current in the range 800 A to 6000 A and the rated current of the secondary busbar system is between 300 and 2000 A. Besides the horizontal and vertical busbars the phase conductors (L1,L2,L3) - there is also a neutral conductor (N), a protective conductor (PE) or a combined neutral and protective conductor (PEN). The neutral conductor (N) is insulated, while the protective conductor (PE) or the combined neutral and protective conductor (PEN) is mounted with good contact to the cubicle. See the section Types of distribution system, p. ?.

Fig. 3 Principle of power factor control

Degree of protection, IP CODE (IEC 529) The enclosure protects against: Unintentional contact with hazardous live parts Ingress of water or moisture Ingress of solid foreign objects and dust

Design and construction The LV ASSEMBLY is designed in the form of cubicles (floor-standing) containing modularised incoming and outgoing units. See Fig. 1 above. There is a separate compartment for wiring (cables), for busbars and for functional units (incoming and outgoing). See more about separation in the section Protection against electric shock, p. ?. The cubicles are mostly manufactured from steel sheet (1.5-2.5 mm gauge), treated or painted. The most common dimensions (including cable compartment) are: Width 600-1200 mm Depth 300-800 mm Height 1800-2300 mm.

• • •

Cable compartment The cable compartment is generally separated from the apparatus compartment (functional units) and from the busbar compartment and has its own door.

The classifying of the degrees of protection is described in International Standard Publication IEC 529. The degree of protection provided by an enclosure is indicated by the IP Code (IP is an abbreviation for International Protection). In the code IP 21C, for instance, the first numeral indicates the degree of protection against solid foreign objects, the second numeral the degree of protection against water and the additional letter the degree of protection against access to hazardous parts. Fig. 4 below gives a brief description of the IP Code elements. The sections Enclosure and degree of protection, p. ?, and Environmental aspects, p. ?, give more information on requirements for protection.

Busbars The busbars are made of copper or coppercoated aluminium and separated from functional units and from the cable compartment by barriers or partitions. See the section Protection against electric shock, p. ?.

Incoming and outgoing units The incoming and outgoing units in an LV ASSEMBLY are built as complete functional units. They contain all the necessary apparatus, components and connections. All this is assembled in a modularised mechanical unit. 15

5.2

15

97-03-05, 13.19

INDUSTRIAL POWER SYSTEM Low voltage distribution

Fig. 4. Degree of protection (IP Code)

Removable and withdrawable parts Removable and withdrawable parts can be safely removed or installed in the LV ASSEMBLY without the busbar system needing to be disconnected. This allows removal or installation of an unit while other units in the LV ASSEMBLY are live. To make this possible, the removable and withdrawable parts are connected by plugging in to the secondary busbar system. The difference between removable and withdrawable parts is that the withdrawable parts have plug-in contacts on the cable connection side (outgoing side) as well, and have fixed ”Connected”, ”Test” and ”Disconnected” positions. In the ”Test” position the main circuit is disconnected while the auxiliary circuit is live. In the ”Disconnected” position both the main circuit and the auxiliary circuit are disconnected.

To provide a high degree of protection against unintentional contact with hazardous live parts in adjacent functional units, and protection against the passage of solid foreign bodies from one unit to an adjacent unit, each unit is built as a separate, screened compartment with its own door. See the section Protection against electric shock, p. ?. The units can be designed as: Fixed parts Removable parts Withdrawable parts

• • •

Fixed parts The fixed parts are fastened by bolts to the secondary busbar system (the vertical busbars). This means that the unit cannot be removed, drawn out or installed in the LV ASSEMBLY without the busbar system being disconnected. 16

5.2

16

97-03-05, 13.19

INDUSTRIAL POWER SYSTEM Low voltage distribution

Requirements

Multibox-type LV ASSEMBLIES Multibox-type LV ASSEMBLIES are built with separate boxes for incoming and outgoing units. The boxes are designed for wall mounting. The boxes are modularised and designed for standard apparatus up to a rated current of 800 A. The degree of protection is usually IP 43 or IP 65. See the section Degree of protection, p. ?. The boxes are generally made of treated or painted sheet steel. The busbars are made of copper or aluminium and are available for rated current up to 800 A. The busbars can be placed in a separate row of boxes or built in to the same box as the devices. The apparatus is internally connected with cables or with copper bars. Plug-in connection of the apparatus is also available, a safe and quick system that eliminates subsequent adjustment of bolts and simplifies assembly. Boxes can also have covers of plastics. The material should be polycarbonate, which has good impact resistance, resistance to ageing and is recyclable. The most usual degree of protection for this type of box is IP 54 or IP 65.

Introduction Many electrical accidents take place when working with live LV ASSEMBLIES. Most of these accidents occur in plants with an operating voltage below 1000 V. A large proportion are arc accidents (short-circuit phase to phase or phase to earth), with operators suffering serious burn wounds, and/or major material destruction causing expensive shutdown. The main reasons for arc accidents are: uninsulated tools changing of fuses voltage testing falling metal objects uninsulated wire ends overheating

• • • • • •

The accidents thus often happen when an electrician is working with a live LV ASSEMBLY. The requirements of an LV ASSEMBLY is that it should be properly designed and well maintained, so that risks of overheating, short circuits and arcing inside an LV ASSEMBLY are eliminated as far as possible. The requirements are documented in several international standards. See Bibliography: Applicable IEC standards, p. ?. See also the section Standardisation in Chapter 14.1 , p. ?.

Apparatus commonly found in boxes:

+ fuses • Switch Blade fuses • Switches • Moulded case circuit breakers, MCCBs • Distribution boards with D-type fuses or • with miniature circuit breakers, MCBs

The main standard for low-voltage switchgear and control gear assemblies, LV ASSEMBLIES, is the International Standard Publication IEC 439-1, ”Requirements for TypeTested and Partially Type-Tested Assemblies” Service conditions LV ASSEMBLIES are normally designed specifically for indoor installation, except for multibox-type LV ASSEMBLIES, which can also be installed outdoors. Normal service conditions The ambient air temperature does not exceed +40oC and its average over a period of 24 h does not exceed + 35oC. The lower limit of the ambient air temperature is -5oC for indoor installations. For outdoor installations the lower limit of the ambient air temperature is -25oC in a temperate climate and -50oC in an arctic climate.

Fig. 5 Multi-box type LV ASSEMBLY 17

5.2

17

97-03-05, 13.20

INDUSTRIAL POWER SYSTEM Low voltage distribution form and polarity which the circuit of an LV ASSEMBLY is capable of withstanding without failure under specified conditions of test and to which the values of clearances are referred. Before sizing of minimum clearances and creepage distances the following must be determined: a) Material group (7.1.2.3.5 in IEC 439-1) b) Polution degree (6.1.2.3 in IEC 439-1) (the standard polution degree is degree 3) c) Rated impulse withstand voltage (table G.1 or G.2 in IEC 439-1) d) Rated insulation voltage (4.1.2 in IEC 439-1) e) Altitude (6.1.3 and table 13 in IEC 439-1) f) Test voltages (table 13 in IEC 439-1) g) Homogeneous (uniform) or non-homogeneous (non-uniform) field (2.9.15 and 16 in IEC 439-1) h) Rated operational voltage (4.1.1 in IEC 439-1) j) Overvoltage category (table G.1 or G.2 in IEC 439-1 k) AC r.m.s. star or delta (table G.1 or.2 in IEC 439-1, distribution system with or without a neutral point)

The air is clean and its relative humidity does not exceed 50% at a maximum temperature of +40oC for indoor installations. For outdoor installations, the relative humidity may temporarily be as high as 100% at a maximum temperature of +25oC. The standard degree of pollution for industrial applications is degree 3, which means: Conductive pollution occurs, or dry, non-conductive pollution occurs which becomes conductive due to condensation. The altitude of the site of installation does not exceed 2000 m. (More information is given in the section Clearances, creepage distances and insulating distances, p. ?.) Special service conditions The user is to inform the manufacturer if exceptional service conditions exist. See more about special service conditions in the section Environmental aspects, p, ?. Conditions during transport, storage and erection. Unless otherwise specified, the following temperature range applies during transport and storage: between -25oC and +55oC and, for short periods not exceeding 24 h, up to +70oC. Equipment subjected to these extreme temperatures without being operated, provided that it does not suffer irreversible damage, is then to operate normally in the specified conditions.

After this determination, minimum clearances and creepage distances can be sized according to tables 14, 15 and 16 in IEC 439-1. Example: a) = II; b) = 3; c) = 8 kV; d) = max. 690 V; e) = sea level; f) = 9.8 kVAC peak and DC f) = 7 kV AC r.m.s.; g) = non-homogeneous; h) = max. 690 V; j) = III; k) = AC r.m.s. star or delta, all live parts insulated from earth (no neutral point). Result: Minimum clearance = 8 mm and Minimum creepage distance = 9 mm

Design and construction Mechanical design General The LV ASSEMBLIES are to be constructed only of materials capable of withstanding the mechanical, electrical and thermal stresses, as well as the effects of humidity, which are likely to be encountered in normal service.

Apparatuses forming part of the LV ASSEMBLY are to have distances complying with the requirements of their relevant specifications (IEC standard), and this is to be maintained during normal sevice conditions. When arranging apparatuses within the LV ASSEMBLY, the specified creepage distances and clearances or impulse withstand voltages are to be complied with, taking into account the relevant service conditions. For bare live conductors and terminations

Clearances, creepage distances and insulating distances Determination of clearances distances, creeping distances and insulating distances in LV ASSEMBLIES is normally based on the Impulse voltage rating. The rated impulse withstand voltage is the peak value of an impulse voltage of prescribed 18

5.2

18

97-03-05, 13.20

INDUSTRIAL POWER SYSTEM Low voltage distribution If connecting facilities for incoming and outgoing neutral (N), protective (PE) and combined neutral and protective (PEN) conductors are provided, they are to be arranged in the vicinity of the associated phase conductor terminals.

(e.g. busbars, connections between apparatus, cable lugs), the creepage distances and the clearances or impulse withstand voltages are, at the least, to comply with those specified for the apparatus with which they are directly associated. The insulation of withdrawable parts is, at the least, to comply with the requirements for disconnectors (IEC 947-3).

Enclosure and degree of protection (IEC 529) The classification system for the degree of protection, the IP system, is described in the section Degree of protection, p. ?. The degree of protection of an enclosed LV ASSEMBLY is to be at least IP 2X after installation. In locations with high humidity and temperatures varying within wide limits, and in locations with heavy pollution of the air by dust, smoke, corrosive or radioactive particles, suitable arrangements (ventilation and/or internal heating, drain holes, etc,) must be made to prevent harmful corrosion and condensation within the ASSEMBLY. See more about special service conditions in the section Environmental aspects, p. ?.

Terminals for external conductors The terminals are to be such that the external conductors may be connected by a means (screws, connectors, etc.) which ensures that the necessary contact pressure corresponding to the current rating and the short-circuit strength of the apparatus and the circuit is maintained. The available wiring space is to permit proper connection of the external conductors of the indicated material and, in the case of multicore cables, spreading of the cores. The conductors must not be subjected to stresses which reduce their normal life. Terminals for neutral conductors are to allow connection of copper conductors having a current-carrying capacity: – equal to half the current-carrying capacity of the phase conductor, with a minimum of 10 mm2 if the size of the phase conductor exceeds 10 mm2. – equal to the full current-carrying capacity of the phase conductor if the size of the latter is less than or equal to 10 mm2. Note: For certain applications in which the current in the neutral conductor may reach high values, for example large fluorescent installations, a neutral conductor having the same current-carrying capacity as the phase conductors may be necessary.

Temperature rise The temperature rise in an LV ASSEMBLY must not be allowed to damage apparatus, components, devices, wires, connections, terminals, etc. In service, normal live parts are not permitted to have a temperature that causes overheating with the risk of internal arcing accidents. External surfaces are not permitted to be so hot that skin contact is made impossible. The table below shows the highest temperature rise allowed for different parts of the LV ASSEMBLY.

Temperature rise limits (IEC 439-1) Part of LV ASSEMBLIES

Temperature rise (oK over the ambient temp.)

Built-in components

In accordance with the relevant requirements for individual components, if any, or , in accordance with the manufacturer´s instructions, taking into consideration the temperature inside the LV ASSEMBLY 19

5.2

19

97-03-05, 13.20

INDUSTRIAL POWER SYSTEM Low voltage distribution

Terminals for external insulated conductors

70oK

Busbars and conductors, plug-in contacts of removable or withdrawable parts connected to busbars

Limited by: 1). mechanical strength of their conducting material; 2.) possible effect on adjacent equipment; 3.) permissible temperature limit of the insulating materials in contact with the conductor; 4.) the effect of the temperature of the conductor on the apparatus connected to it 5.) for plug-in contacts, the nature and surface treatment of the contact material

Manual operating means: – of metal – of insulating material

15oK 25oK

Accessible external enclosures and covers: – metal surfaces – insulating surfaces

30oK 40oK

Discrete arrangements of plug and socket-type connection

Determined by the limit for those components of the related equipment of which they form part

Protection against electric shock The LV ASSEMBLY is to be so designed and manufactured that the risk of unintentional contact -direct contact- with live parts - is eliminated as far as possible and any exposed conductive part is not, through a fault, to become a dangerous live part -indirect contact.

disconnected before the door can be opened (for example by interlocking the door with a disconnector). The LV ASSEMBLY is to have internal separation by barriers or partitions. By dividing LV ASSEMBLIES by means of partitions or barriers (metallic or non-metallic) into separate compartments or barriered subsections the following conditions can be attained: – protection against contact with live parts belonging to adjacent functional units. The degree of protection is to be at least IP 2X or IPXXB. – limitation of the probability of initiating arc faults

Protecting against Direct contact created by enclosures and barriers. All external surfaces are to conform to a degree of protection against direct contact of at least IP 2X or IP XXB. All barriers and enclosures are to be firmly secured in place. They are to have sufficient stability and durability to resist the strains and stresses likely to occur in normal service without reducing clearances. Removal, opening or withdrawal must necessitate the use of key or tool. All live parts which can unintentionally be touched after the door has been opened must be

Note: The effects of an arc can be reduced by use of means limiting the magnitude and duration of the shortcircuit current. See the section Arc guard system, p. ?

20

5.2

20

97-03-05, 13.21

INDUSTRIAL POWER SYSTEM Low voltage distribution 2. The protective circuit may not include switches. 3. Terminals for incoming protective conductors are to be bare and normally be designed for copper conductors. 4. A separate terminal of adequate size is to be provided for the outgoing protective conductor(s) of each circuit. 5. If the protective conductor is an insulated single-core cable it is to be greenyellow (twin-coloured). See the section Identification, p. ?.

– protection against the passage of solid foreign bodies from one unit of an LV ASSEMBLY to an adjacent unit. The degree of protection is to be at least IP 2X. The following are typical forms of separation (IEC 439-1): Form 1 No separation Form 2 Separation of busbars from the functional units. Form 3a Separation of busbars from the functional units and separation of all functional units, but not of their terminals, from one another. The terminals for external conductors need not be separated from the busbars. Form 3b Separation of busbars from the functional units and separation of all functional units from one another. Separation of the terminals for external conductors from the functional units, but not from each other. Form 4 Separation of busbars from the functional units and separation of all functional units from one another, including the terminals for external conductors which are an integral part of the functional unit.

The cross-section of protective conductors (PE) in an LV ASSEMBLY is to be determined in one of the following ways: a) The cross-sectional area of the protective conductor is to be not less than the appropriate value shown below: Cross-sectional area of the protective conductor (IEC 439-1) Cross-sectional area of Minimum crossphase conductors sectional area of the corresponding S protective conductor Sp

Protection against Indirect contact by using protective circuits A protective circuit in an LV ASSEMBLY consists of either a separate protective conductor or conductive structural parts or both. It provides the following: – protection against the consequences of faults within the LV ASSEMBLY – protection against the consequences of faults in external circuits supplied through the LV ASSEMBLY. Constructional precautions are to be taken to ensure electrical continuity between the exposed conductive parts of the LV ASSEMBLY and between these parts and the protective circuits of the installation. Types of system earthing are described in the section Types of distribution system, p. ?. The main requirements are: 1. It should not be possible to break the protective circuit. For example, when a part in the LV ASSEMBLY is removed.

mm2

mm2

S 800

S 16 S/2 200 S/4

b) The cross-sectional area of the protective conductor is to be calculated with the aid of the formula: Sp= √I2t / k where: -Sp is the cross-sectional area, in square millimetres -I is the value (r.m.s.) of AC fault current for a fault of negligible impedance which can flow through the protective device, in amperes -t is the operating time of the disconnecting device -k is the factor dependent on the material of the protective conductor, the insulation and other parts, and the initial and final temperature 21

5.2

21

97-03-05, 13.21

INDUSTRIAL POWER SYSTEM Low voltage distribution rent including DC component) for determining the electrodynamic stresses is to be obtained by multiplying the r.m.s. value of the short-circuit current by the factor n. Standard values for the factor n and the corresponding power factor are given in the table below.

Requirements related to accessibility in service by authorised personnel The LV ASSEMBLY must be designed in such a way that its accessibility when the LV ASSEMBLY is in service and under voltage is as high as possible, with very low likelihood of accidents. Accessibility is of three types: 1. Accessibility for inspection and similar operations, e.g. replacement of fuse-links 2. Accessibility for maintenance, e.g. changing of contacts in a contactor 3. Accessibility for extension under voltage, e.g. extension of the LV ASSEMBLY with additional functional units

Peak withstand current factor n (IEC 439-1) R.M.S. value of short-circuit current P.F. n 5 kA 10 kA 20 kA 50 kA

Short-circuit protection and short-circuit withstand strength LV ASSEMBLIES are to be so constructed as to be capable of withstanding the thermal and dynamic stresses resulting from short-circuit currents up to rated values. LV ASSEMBLIES are to be protected against short-circuit currents by means of, for example, circuit-breakers, fuses or combinations of both, which may either be incorporated in the LV ASSEMBLY or arranged outside it.

< < < <

I >/= 5 kA I
0.7 0.5 0.3 0.25 0.2

Arcing inside an LV ASSEMBLY, Internal arcing tests It is desirable that the highest possible degree of protection for personnel should be provided in case of a fault leading to arcing inside an LV ASSEMBLY, although the prime objective should be to avoid such arcs by suitable design or to limit their duration. The tripping time should be less than 0.1 s (100 ms). An incoming circuit breaker which has a fast-working short-circuit protective device as well as arc guards with detectors in each cubicle is recommended. See the section Arc guard system, p. ?. It is recommended that internal arcing tests be carried out in accordance with a relevant test method.The purpose of internal arcing tests is to verify that the enclosure of the LV ASSEMBLY provides the necessary protection for persons in front of or in the vicinity of the LV ASSEMBLY in the event of internal arcing. The testing is intended to verify that the enclosure withstands the pressure and the temperature arising from internal arcing with shortcircuit current and thus provides the necessary protection against injury by radiation from the arc, glowing particles, ejected hot gases or flying parts. The requirements differ, depending on whether accessibility to the vicinity of the LV ASSEMBLY is restricted or not. The recommended test method can also be used to verify the performance of partitions.

Short-circuit rating The manufacturer is to state the short-circuit strength in one or more of the following ways: a) The rated short-time withstand current together with the associated time, if different from 1 s and the rated peak withstand current. Note: For times up to a maximum of 3 s the relationship between the short-time withstand current and the associated time is given by the for-mula: I2t=constant, provided that the peak value does not exceed the rated peak withstand current.

b) The rated conditional short-circuit current c) The rated fused short-circuit current For items b) and c) the manufacturer is to indicate the characteristics (current rating, breaking capacity, cut-off current, I2t, etc.) of the short-circuit protective devices necessary for the protection of the LV ASSEMBLY. Relationship between peak and r.m.s. values of short-circuit current The value of peak short-circuit current (peak value of the first loop of the short-circuit cur22

5.2

22

1.5 1.7 2 2.1 2.2

97-03-05, 13.21

INDUSTRIAL POWER SYSTEM Low voltage distribution nents, for example co-ordination of motor starters with short-circuit protective devices, are to comply with the relevant IEC standards.

It should further be observed that the intended protection against internal arcing faults is only obtained if the doors of the LV ASSEMBLY are properly closed and if the LV ASSEMBLY has been installed according to the manufacturer´s instructions. The recommended arcing tests are not intended to give any guidance on service availability after an arcing fault. Because of the extensive material damage caused by free arcs of short-circuit currents during normal tripping times, it is not possible to put an LV ASSEMBLY back into service immediately after an arcing fault.

Installation Switching devices and components are to be installed in accordance with the instructions of their manufacturer (position of use, clearances to be observed for electric arcs or for the removal of the arc chute, etc.) The apparatus, functional units mounted on the same support (mounting plate, mounting frame) and the terminals for external conductors are to be so arranged as to be accessible for mounting, wiring, maintenance and replacement. In particular, it is recommended that the terminals be situated at least 0.2 m above the base of floor-mounted LV ASSEMBLIES and, moreover, be so placed that the cables can be easily connected to them. In general, for floor-mounted LV ASSEMBLIES, indicating instruments which need to be read by the operator should not be located higher than 2 m above the base of the LV ASSEMBLY. Operating devices such as handles, pushbuttons, etc., should be located at such a height that they can easily be operated; this means in general that their centreline should not be higher than 2 m above the base of the LV ASSEMBLY. Actuators for emergency switching devices should be accessible within a zone between 0.8 m and 1.6 m above servicing level. The switching devices and components are to be installed and wired in the LV ASSEMBLY in such a manner that its proper functioning is not impaired by interaction. More specified requirements are stated in the standard mentioned.

Co-ordination It is very important that the short-circuit protective devices in a plant (distribution system) be co-ordinated. See the section Choice of short-circuit protective device, SCPD, p. ?. Where there are considerable demands of continuous service, the short-circuit protective devices and their settings are co-ordinated so that a short circuit is disconnected by a breaker or a fuse without disturbing any other circuits, i.e., selectivity. See more about selectivity in the section Selectivity plan, p. ?. Unprotected conductor Within a section in an LV ASSEMBLY, the conductors (including distribution busbars) between the main busbars and the supply side of functional units, as well as components included in these units, may be rated on the basis of the reduced short-circuit stresses occurring on the load side of the respective short-circuit protective device within each unit, provided that these conductors are arranged so that under normal operating conditions, an internal short circuit between phases and/or between phases and earth is only a remote possibility. Such conductors are preferably of solid, rigid manufacture. Insulated flexible conductors may be used provided that they are securely fastened.

Fixed, removable and withdrawable parts The three types of functional units are described above in the section Incoming and outgoing units, p. ?.

Switching devices and components installed in LV ASSEMBLIES Selection of switching devices and components Switching devices and components incorporated in LV ASSEMBLIES, as well as coordination of switching devices and compo-

Markings Inside the LV ASSEMBLY, it must be possible to identify individual circuits and their protective devices (switching devices and components). Items of equipment in the LV ASSEMBLY 23

5.2

23

97-03-05, 13.22

INDUSTRIAL POWER SYSTEM Low voltage distribution are to be designated, and the designations must be identical with those in the wiring diagrams which may be supplied together with the LV ASSEMBLY, and are to be in accordance with the International standard publication IEC 750.

rials to the temperatures reached, are all to be taken into consideration. Connections between current-carrying parts are to be established by means that ensure a sufficient and durable contact pressure.

Identification Where appropriate, the identification of the conductors of main and auxiliary circuits is to be in accordance with international standard publications IEC 445 and IEC 446. The protective conductor (PE) is to be readily distinguishable by shape, location, marking or colour. If identification by colour is used, it must be green and yellow (twin-coloured). When the protective conductor is an insulated single-core cable, this colour identification is to be used, preferably throughout its whole length. Any neutral (N) conductor in the main circuit should be readily distinguishable by shape, location, marking or colour. If identification by colour is used, it is recommended that a light blue colour be selected. Any combined neutral and protective conductor (PEN) is, like the PE conductor and the N conductor, to be readily distinguishable by shape, location, marking or colour. When the PEN conductor is an insulated single-core cable it is recommended that a green and yellow (twin-coloured) scheme be selected, preferably throughout the whole length, with a light blue marking at the connection points (the end points). The terminals for external protective conductors (PE) are to be marked with the earth sign as per international standard publication IEC 445 This symbol is not required where the external protective conductor is intended to be connected to an internal protective conductor which is clearly identified with the colours green and yellow.

Electronic equipment supply circuits and electronic equipment incorporated in LV ASSEMBLIES General When electronic equipment is incorporated in LV ASSEMBLIES it is important to take into consideration the interference factor, or electromagnetic compatibility (EMC). There are several sources of interference that must be taken into account to ensure interference-free operation. They come under the following headings: interference •ThisConducted group has a very wide frequency range and can be subdivided into the following groups: a) Low-frequency interference b) Medium-frequency interference c) High-frequency interference In addition to conducted interference, electronics can be affected by: discharges (ESD) • Electrostatic interference • Radio magnetic fields • Low-frequency voltage variations • Input • Overvoltages • Waveform variations in voltage and • Temporary frequency The requirements are stated in the standard mentioned. It is recommended that Chapter 13.9 Electronic and magnetic disturbances be read. Test specifications (8 in IEC 439-1) General The tests to verify the characteristics of an LV ASSEMBLY include : Type tests Routine tests

Electrical connections inside an LV ASSEMBLY: bars and insulated conductors General The connections of current-carrying parts are not to suffer deterioration as a result of normal temperature rise, ageing of the insulating materials or vibrations occurring in normal operation. In particular, the effects of thermal expansion and of electrolytic action in the case of dissimilar metals, and the reaction of the mate-

• •

Type tests (8.2 in IEC 439-1) Type tests are intended to verify compliance with the requirements laid down for a given type of LV ASSEMBLY. 24

5.2

24

97-03-05, 13.22

INDUSTRIAL POWER SYSTEM Low voltage distribution Note: The performance of the routine tests at the manufacturer´s works does not relieve the firm installing the LV ASSEMBLY of the obligation of checking it after transport and installation.

Type tests will be carried out on a sample of such an LV ASSEMBLY or on such parts of LV ASSEMBLIES manufactured to the same or similar design.

Summary It is very important that there be a dialogue between the manufacturer and the user so that the LV ASSEMBLY meets the customer´s (the user´s) expectations.

Type tests include: a) verification of temperature-rise limits b) verification of dielectric properties c) verification of short-circuit withstand strength d) verification of the effectiveness of the protective circuit e) verification of clearances and creepage distances f) verification of mechanical operation g) verification of the degree of protection (IP Code)

Sizing factors Different types of loads It is very important to clearly specify the type of load when planning. Motor data alone is not sufficient information to determine what motor starter is needed. For correct sizing, more information about the type of machine that the motor will drive is necessary. The following points are to be considered.

These tests may be carried out in any order and/ or on different samples of the same type. If modifications are made to the components of the LV ASSEMBLY, new type tests have to be carried out only insofar as such modifications are likely to adversely affect the results of these tests. The type tests are described in detail in the above standard.

Motor load The driven machine generates requirements for starting method, starting time, starting current, utilisation category (AC category), required lifetime, etc. The starting current, for example, can cause problems when a circuit breaker is chosen as short-circuit protective device. Even with theoretically correct sizing, problems can occur, for example, when the motor is oversized, since the connection current is proportional to the size of the motor. See Chapter 4.4 Motor starting systems.

Routine tests (8.3 in IEC 439-1) Routine tests are intended to detect faults in materials and workmanship. They are carried out on every new (delivered) LV ASSEMBLY after its assembly or on each transport unit. A further routine test at the place of installation is not required. LV ASSEMBLIES which are assembled from standardised components outside the works of the manufacturer of these components, by the exclusive use of parts and accessories specified by the manufacturer for this purpose, are to be routine-tested by the firm which has assembled the LV ASSEMBLY.

Distribution transformers For optimum setting of the protection the starting current must also be considered here. In unfavourable conditions it can be over 30 x In, which requires a time-lag on the short-circuit protective device. Another important point is the load of the transformer. The transformer can temporarily cope with an overload. The protective breaker does not need to be oversized to manage this, but the protection must cover the transformer´s whole field of application. See Chapter 5.4 Transformers, Distribution transformers, p. ?.

Routine tests include: a) inspection of the LV ASSEMBLY including wiring and, if necessary, an electricaloperation test b) dielectric test c) checking of protective measures and of the electrical continuity of the protective circuit These tests may be carried out in any order.

Furnaces Large current peaks can occur, depending on 25

5.2

25

97-03-05, 13.22

INDUSTRIAL POWER SYSTEM Low voltage distribution (gas-tight joints). Contacts in breakers, contactors, switches, etc., plug-in contacts for main circuits and auxiliary circuits and plug-in contacts for electronics are examples of contact surfaces that are not protected against corrosive gases. In hostile environments special measures must be adopted to prevent corrosion and the subsequent rise in resistance. The consequences for main current paths (busbars, busbar connections, breaking contacts, fuses, plug-in contacts, etc.) can be high temperature rise, flashover and arcing, with damage to material, long periods of downtime (loss of production) and worst of all, serious injuries to operators, etc. Where electronics is concerned, the attack on contact surfaces could lead to temporary or permanent interruption in control circuits; interruptions that lead to operational disturbances and loss of production.

the type of furnace. An arc furnace generates complicated overtones. Power electronics for rev/min-adjusted drives (Current converter, frequency converter, soft starter) Generating of overtones is common in all applications with power electronics that can disturb sensitive equipment. This imposes special requirements on the earth fault equipment in a non-direct-earthed circuit. More problems can occur when a thyristor and a frequency converter are in the same system. This type of equipment is more frequently seen in modern LV ASSEMBLIES and the temperature rise becomes an important sizing factor. See Chapter 4 Electric drive systems, p. ?. Lighting Fluorescent tube fittings also generate overtones that can be disturbing in a system. These also consume reactive power and might need power factor correction. See the section Terminals for external conductors (sizing of the neutral (N) conductor) under Mechanical design on p. ?.

Which gases are the ”villains”? In industrial environments the most common aggressive gases are sulphur hydrogen (H2S), chlorine (Cl2), nitrogen (NO2) and sulphur dioxides (SO2). All of these gases are separately corrosive, but combined, they lead to even faster degradation.

Capacitor banks Power factor correction and the need for filters in distribution systems are described in Chapter 6 Reactive power compensation, p. ?. Capacitor inputs and outputs require special connection devices. In general, they must be over-sized by 30 % and the possibility of reignition during the breaking sequence must be minimised. Re-ignition generates overvoltages which can damage the capacitors and other devices.

occurs in the petrochemical, pulp and • SO paper, and metal-producing industries. 2

• •

Environmental aspects As mentioned above, LV ASSEMBLIES are usually designed for indoor location in normal service conditions. See the section Service conditions, p. ?. In some industrial environments, however, there is a high level of corrosive gas, which can have an adverse effect on the functions of the components and devices. The most sensitive parts are the contact surfaces, where it is impossible to maintain a very high contact pressure

•

•

SO 2 attacks all metals except precious metals such as gold, silver and platinum. NO2 occurs when burning oil, petrol and coal (fossil fuels). NO2 is aggressive towards copper and brass and, together with SO2, increases the degradation of many materials. (Together with reactive chlorine even gold can be affected.) H2S occurs in the pulp and paper and chemical industries. H2S attacks all copper based metals. In dry environments even silver can suffer damage. Cl2 occurs in bleaching plants, in pulp and paper industries and in the production of PVC. Reactive chlorine, Cl2, affects most metals, even in low concentrations. In combination with other pollutants its effects are aggravated. Cl, chlorides, mostly arise from seawater. They attack most metals.

26

5.2

26

97-03-05, 13.22

INDUSTRIAL POWER SYSTEM Low voltage distribution smoke and dust attract other kinds • Grime, of pollution and moisture, increasing the

that apply are to be complied with, or else special agreements are to be reached between user and manufacturer. The user should inform the manufacturer if such exceptional service conditions exist. Such special service conditions are, for example: 1. Values for temperature, relative humidity and/or altitude that differ from normal service conditions. 2. Applications where variations in temperature and/or air pressure take place at such speed that exceptional condensation is liable to collect inside the LV ASSEMBLY. 3. Heavy pollution of the air by dust, smoke, corrosive or radioactive particles, fumes or salt. 4. Exposure to strong electric or magnetic fields. 5. Exposure to extreme temperatures, for example radiation from sun or furnaces. 6. Attack from fungi, vermin, rodents, etc. 7. Installation in locations where fire or explosion hazards exist. 8. Exposure to severe vibrations and impacts. 9. Installation in such a manner that the current-carrying capacity or breaking capacity is affected, for example equipment built into machines or recessed into walls. 10. Appropriate measures to counteract electrical or radiated interference need to be taken into account.

rate of corrosion.

(together with pollution) acceler• Moisture ates corrosion. (Note. To avoid static electricity discharges that interfere with electronics, the relative humidity should not be less than 30 %.) If the proportion of pollution in the air is too high, 1-100 m/m3, steps must be taken to decrease the risk of corrosion in and on the LV ASSEMBLY. Location of the LV ASSEMBLY in the same operating rooms as air treatment equipment (ventilation systems) is common in the heavy process industries (pulp, paper, petrochemical and steel industries). In such harsh environments surface coating of copper and/or aluminium bars with tin, or in some cases silver, is recommended. The trend towards dividing the LV ASSEMBLY into main distribution (Load Centre, LC) and sub-distribution (Motor Control Centres, MCC), has the consequence that the MCCs are located as close as possible to the process equipment and are exposed to aggressive gases, which can be removed in the operating room through filtering in the air treatment equipment (see above). See the sections Introduction, p. ?, and Plant design, p. ?.The surfaces of the enclosures for these MCCs should be treated to resist corrosion. Built-in devices and components must be chosen with the best contact material possible. For more information on corrosion it is recommended that”Corrosion of electronics” a handbook based on experiences from a Nordic Research Project (Bulletin No 102, Swedish Corrosion Institute, August 1991) be read. See also Chapter 3.5Industrial Environments, p. ?. Apart from air pollution, the location and surroundings of the LV ASSEMBLY can involve problems, e.g. vibrations and humidity and large differences in temperature between night and day (leading to condensation). It is, as mentioned earlier, vital that the manufacturer obtain information from the user regarding all factors that affect the design of the LV ASSEMBLY.

Plant design Introduction The planning of an industrial system involves a great deal of money, both in direct investments and future running costs. However, a systematic approach and an overall view of the problems are often lacking. Designing an industrial low-voltage distribution network requires experience, extensive knowledge, intuition and analytical ability. How should one start to design and size an industrial system, what considerations need to be examined and what are the most important issues to be addressed? These problems are not new and most of the design technique has been available for decades. However, over the last few years new technology has been developed that offers fresh

Special service conditions Where any of the following special service conditions exist, the particular requirements 27

5.2

27

97-03-05, 13.23

INDUSTRIAL POWER SYSTEM Low voltage distribution MCCs) out closer to the process equipment has always existed. However, this arrangement has not been viewed with favour in the past due to the adverse effects of the surroundings (see the section Environmental aspects, p. ?) on the bars, fuses, circuit breakers, movable contacts (e.g. plug-in contacts) and electronics. In recent years, though, equipment has been developed to protect the electrical parts when they are located close to the process. This distribution system (with the distribution LV ASSEMBLY or the MCC located close to the process equipment) results in lower shortcircuit currents in the LV ASSEMBLY concerned. This design results in lower investment costs for the LV ASSEMBLY and lower costs for installation (e.g. cable costs) and for ventilation equipment.

opportunities in plant design. Further, maintenance costs can now often be reduced. The power system represents some 10-15 % of the capital costs for a major industrial project. There are several ways to plan an LV ASSEMBLY design, but two main principles apply: 1) Place the transformer next to the LV ASSEMBLY and collect all units and devices in a single LV ASSEMBLY located in a locked operating room. 2) Split the LV ASSEMBLY equipment up into main distribution Load Centre (LC) and sub-distribution Motor Control Centres (MCCs). See the section General, Introduction, p, ? and the single-line diagram in Fig. 2, p, ?. The load centre (LC) is direct fed from the transformer and is normally designed for high currents. It distributes the current to sub-distribution MCCs (which can be placed near the process). Sub-distribution units (distribution switchgear and MCCs) are most often fed via cable from units in the load centre. The cable area and length (impedance) determine how much of the short-circuit current is limited at the sub-distribution point. This limitation makes it possible to design sub-distribution for the reduced shortcircuit current. It also provides the possibility to use circuit breakers with a lower tripping value in the outgoing feeder units. See the section Short-circuit levels, p, ?. A common arrangement is to place the transformer near the LV ASSEMBLY and collect all units and devices in this LV ASSEMBLY. This design gives a good overview of the distribution system, which is located in a locked operating room. See above in the section Introduction, p. ?. The operating room is cleaned and ventilated to protect the electrical equipment against corrosion. See the section Environmental aspects, p. ?. This kind of design normally implies long cables and high short-circuit currents in the LV ASSEMBLY. High short-circuit currents make it necessary to select more advanced apparatus. The possibility of moving the LV ASSEMBLIES (distribution LV ASSEMBLIES or

Voltage levels The size of the process, i.e., the power required, determines to a certain extent if one or several voltage levels are needed. Selection of voltage level cannot be made until after the circuit system has been settled on. For major industries with a considerable number of large motors it may be more advantageous economically to select two main voltage levels, 400 V and 690 V. If a main voltage of 690 V is used, a voltage of 10 kV can normally be used as motor voltage for high voltage drives. For an AC system with a nominal voltage of 120 to 1000 V and with the appropriate equipment, the values shown in the table below apply. Three-phase 4-wire or 3-wire system (IEC 38) Three-phase, four-wire Single-phase, or three-wire systems three-wire systems Nominal voltage (V) voltage (V)

Nominal

– 230/4001) 277/4802) 400/6901) 1000

120/240 – – – –

Under normal system conditions it is recommended that the voltage at the supply terminals should not differ from the nominal voltage by more than +/- 10 % 28

5.2

28

97-03-05, 13.23

INDUSTRIAL POWER SYSTEM Low voltage distribution The lower value states the voltages between phase and the neutral conductor (N conductor) and the higher value states the voltage between phases. Three-phase systems with a voltage of 400/ 690 V are intended only for heavy industrial applications or similar. Some industries have an internal distribution system using a main voltage of 500 V.

1) The nominal voltage of existing 220/380 V and 240/415 V systems is to evolve towards the recommended value of 230/400 V. The transition period should be as short as possible and should not exceed 20 years aftter the issue of the IEC standard publication (= year 2003). During this period, as a first step, the electricity supply authorities of countries having 220/380 V systems should bring the voltage within the range 230/400 V +6 %, -10 % and those of countries having 240/415 V systems should bring the voltage within the range 230/400 V +10 %, - 6 %. At the end of this transition period the tolerance of 230/400 V +/- 10 % should have been achieved; after this a reduction of this range will be considered. All the above considerations also apply to the present 380/660 V value with respect to the recommended value 400/690 V. 2) Not to be utilised together with 230/400 V or 400/ 690 V.

Short-circuit levels - Rated current and Short-circuit current The transformer data and wiring give the necessary information on the principal data needed for the LV ASSEMBLY. The most important data are: voltage and frequency • Main current • Rated level • Short-circuit Connection •

As can be seen, the table includes singlephase circuits connected to three-phase, fourwire systems.

Example of calculation of rated current and short-circuit current Transformer data: 2 MVA, 10,000/400 V (10/0.4 kV), uk= 6.25 % Rated current, In= 2,000,000/400 x 3= 2890 A Short-circuit current, Ik= 2,000,000 x 100/400 x 3 x 6.25= 46 kA Ik depending on transformer and cable, U= 400 V Pn

kVA 1000 1250 1600 2000 2500

uk

% 5.00 5.00 6.25 6.25 6.25

Ik

kA 29 36 37 46 58

Ik limited by the cable impedance Cable Cable 25 mm2 95 mm2 Cu Cu Al Length m Length m 25 100 25 100 8 3 14 5 8 3 15 5 8 3 16 5 9 3 16 5 9 3 18 6

Cable 240 mm2 Length m 25 100 19 10 20 10 22 11 23 11 27 11

Ik= Short-circuit current Pn= Rated power uk= Short-circuit voltage in % Diversity factor The rated diversity factor of an LV ASSEMBLY or part of an LV ASSEMBLY having several main circuits (e.g. a section or a subsection) is the ratio obtained from the relation-

ship of the maximum sum, at any one time, of the assumed currents of all the main circuits involved, to the sum of the rated currents of all the main circuits of the LV ASSEMBLY or the selected part of the LV ASSEMBLY. 29

5.2

29

97-03-05, 13.23

INDUSTRIAL POWER SYSTEM Low voltage distribution Directly earthed system, TN system, (TN-C and TN-S) Figs. 6 and 7 (figur 24 and 25, p. 332)

Standard rated (recommended) diversity factors (IEC 439-1): Number of main circuits Diversity factor 2 and 3 4 and 5 6 to 9 inclusive 10 (and above)

0.9 0.8 0.7 0.6

Types of distribution system; TN, IT, and TT systems (IEC 364-3) The following characteristics of the distribution system are to be assessed: of live conductor • types • types of system earthing

Not-directly-earthed systems, IT systems These have all live parts isolated from earth or one point connected to earth through a high impedance, the exposed conductive parts of the electrical installation being earthed independently or collectively or connected to the system earthing. Not-directly-earthed systems are only utilised in industrial establishments and are to have an earth fault protection.device that indicates earth faults. An earth fault protective device of this kind can be arranged through zero point equipment.

Types of live conductor systems: AC systems DC systems Single-phase 2-wire 2-wire Single phase 3-wire 3-wire Two-phase 3-wire Two-phase 5-wire Three-phase 3-wire (3 phase conductors + PEN conductor) Three-phase 4-wire (3 phase conductors + PE conductor + N conductor) Types of system earthing Directly earthed systems, TN systems These have one point directly earthed, the exposed conductive parts of the installation being connected to that point by protective conductors (PE). Three types of TN system are considered according to the arrangement of neutral and protective conductors, as follows: TN-S system:

TN-C-S system:

TN-C system:

Fig. 8 Not-directly-earthed system, IT system

in which, throughout the system, a separate protective conductor (PE) is used;

TT system s have one point directly earthed, the exposed conductive parts of the installation being connected to earth electrodes electrically independent of the earth electrodes of the power system.

in which neutral and protective functions are combined in a single conductor (PEN) in a part of the system;

Note: TT systems are to be used very restrictively.

Demands on availability Maintenance work should be kept in mind during the projecting/planning period. If continuous operation is required, the system must be designed so that bypass links and standby supply possibilities are ensured so that neces-

in which neutral and protective functions are combined in a single conductor (PEN) throughout the system.

30

5.2

30

97-03-05, 13.24

INDUSTRIAL POWER SYSTEM Low voltage distribution 2. By incoming disconnector With a normal disconnector as incoming device, making or breaking cannot/should not be carried out using the disconnector. See the section Disconnectors, p. ? 29. A disconnector on its own normally has no breaking/making capacity and should therefore be operated only when not under voltage. With a switch-disconnector, voltage can be switched on and off, but making and breaking of a high-current load is not recommended. A switch-disconnector can be supplied with auxiliary contacts and an interlocking magnet, which prevents incorrect manoeuvring. The disconnector allows switching off so as to carry out work on the LV ASSEMBLY, (see the section Disconnection for work purposes, p. ? 27) but no protection functions can be accomplished through automatic disconnection. A disconnector is often used as an inexpensive solution, or when there already is an effective protective device in the supply system. Sometimes disconnectos are used by selectivity reasons, i.e., when a chain of selectivity cannot be built up without an incoming circuit breaker tripping. This is a bad argument since selectivity can often be achieved by means of time characteristics. The breaking and protection functions must be located in the supply system.

sary inspections of those components that require the circuit being broken can be carried out. See also the section Requirements related to accessibility in service by authorised personnel, p. 13 ?. Selectivity (p. 332 in the Swedish edition) Selectivity refers to disconnecting only part of the equipment that is affected by the fault. Selectivity is brought about by co-ordinating different current and time curves, and/or with time functions. More detailed information on selectivity is provided in the section Selectivity plan, p, ?. (p. 315). Connection to the LV ASSEMBLY Connection of incoming supply conductors from the transformer to the LV ASSEMBLY can be effected in one of the following ways: 1. By incoming circuit breaker 2. By incoming disconnector 3. By switch and fuses 4. By direct connection from the transformer to the main busbars in the LV ASSEMBLY 1. By incoming circuit breaker An incoming circuit breaker is the best and most dependable (in many countries a demand) solution for connection to LV ASSEMBLIES. Having a circuit breaker allows quick disconnection in the event of faults. See the section Circuit breakers, p.?. If work is needed on the LV ASSEMBLY it is then easy to disconnect and selectivity can be accomplished through time functions in the breaker´s protective device. Circuit breakers normally have overload protection, delayed and instant shortcircuit protection, earth-fault protection and undervoltage protection. The circuit breaker can be remotely controlled by a motor-operated apparatus. Measuring, automatic making/breaking of contacts and reconnection are possible. If the LV ASSEMBLY is supplied with arc guards connected to the circuit breaker on the low-voltage side of the transformer, a fast tripping time is obtained even when arc faults occur giving limited fault currents in the LV ASSEMBLY. See the section Arc guards, p. ? 39.

3. By switch and fuses Breaking and protection against overloads and short circuits can be achieved by using switches and fuses in the incoming unit. The units can hardly be rated for more than 800 A (largest fuse) and only occur in small LV ASSEMBLIES and multibox-type LV ASSEMBLIES. If only switches are used, the protection function is lost, but breaking is naturally still possible. 4. By direct connection from the transformer to the main busbars in the LV ASSEMBLY The breaking and protection functions have to be located in the supply line (on the primary side of the transformer, a medium-voltage circuit breaker with protective devices is necessary).

31

5.2

31

97-03-05, 13.24

INDUSTRIAL POWER SYSTEM Low voltage distribution The first two are not to be recommended. The safest, most up-to-date and most used method is to fit earthing switches in the incoming circuit breaker cubicle in an LV ASSEMBLY (often both before and after the circuit breaker). The earthing switch can be operated from the front of the cubicle with door closed. The primary side of the earthing switch is connected to the phases and the secondary side is short-circuited and connected to the protective earth bar (PE). The earthing switch is to be interlocked with the circuit breaker via auxiliary contacts and can further be interlocked with a blocking magnet to prevent incorrect operation. See the single-line diagram below.

Disconnection for work purposes Disconnection of a plant part for work purposes should be executed such that power supply is prevented from all directions. Fully adequate safety devices should be used. Where the power supply is from several directions, or where there are no electrical switches, special care needs to be taken to prevent errors. To ensure that the necessary level of security is achieved, disconnection is required using one of the following methods: 1. Disconnection of a voltage not exceeding 1000 V AC : 1.1 Use of disconnector or switch-disconnector with visible breaking points 1.2 Use of switch-disconnector without visible breaking points, provided with reliable position indications. 1.3 Use of ”safety switch” 1.4 By removing fuse-links 1.5 Use of other electrical switches than under items 1.1 and 1.2 in combination with earthing for work purposes behind closed cubicle front or similar. It is presupposed that: – interlocking between electrical switch and earthing device is so arranged that it is not possible to perform earthing before the electrical switch has broken the circuit in all phases. – earthing can be carried out without risk of injury to operators, etc., even if it is performed on live parts. See the following section Earthing for work purposes. 1.6 Use of withdrawable units in disconnected position (see the section Incoming and outgoing units, p. ? 6) 1.7 Other ways than those under items 1.1 to 1.6; methods that can be easily checked visually.

Earthing device with earthing switches Fig. 9 ?

Apparatus and combination of apparatus Disconnectors (IEC 947-3) A disconnector is a mechanical switching device which, in the open position, complies with the requirements specified for the isolating function (isolating distance). A disconnector is capable of opening and closing a circuit when either negligible current is broken or made, or when no significant change in the voltage across the terminals of each of the poles of the disconnector occurs.

Earthing for work purposes Earthing of an LV ASSEMBLY when working in it, or close to it, is a precautionary measure to make sure that no accident will occur. There are three methods: 1. Earthing device with earthing cable 2. Earthing device with earthing lance 3. Earthing device with earthing switch 32

5.2

32

97-03-05, 13.24

INDUSTRIAL POWER SYSTEM Low voltage distribution contact separation of main contacts will be prevented. It should be possible in all cases to perform operations with satisfactory safety. If the switch is to be used as a switchdisconnector when working on a part of a plant the switch must be chosen to conform with Utilisation Category AC 21 (see Utilisation categories above and the section Disconnection for work purposes, p. ? 27). Switches can normally be supplied with auxiliary contacts for indicating of on/off position.

It is also capable of carrying currents under normal circuit conditions and carrying for a specified time currents under abnormal conditions such those of short-circuit. A disconnector normally has no breaking/ making capacity and should only be operated when under no voltage. Switches (IEC 947-3) A mechanical switch is a switching device capable of making, carrying and breaking currents under normal circuit conditions, which may include specified operating overload conditions, and also carrying for a specified time currents under specified abnormal circuit conditions such as those of short-circuit.

Fuses (IEC 269) A fuse is a device that, by the fusing of one or more of its specifically designed and proportioned components, opens the circuit in which it is inserted by breaking the current when this exceeds a given value for a sufficient time. The fuse comprises all the parts that form the complete device. The fuse represents a weakness in the wire system, and contains a meltable wire called a fuse-element. The fuse-element is designed and sized to melt when the current exceeds a certain value, and consequently the circuit is broken. In most fuses the wire is enclosed in a fuselink made of porcelain and filled with quartz sand. The quartz sand makes it easier to switch off the arc inside the fuse-link at the moment of tripping. The most common material in the fuse-element is silver for low currents and a thin silver-plated copper strip for higher currents. The temperature rise in the fuseelement is proportional to the square of the rated current. The higher the current, the quicker the temperature rise. When the fuse-element melts and the metallic connection breaks, an arc ignites over the breaking gap. The heat of the arc melts and turns the fuse-element into gas. When the impedance becomes too high through the arc, the energy is no longer enough to maintain the arc and this is extinguished. The breaking sequence when the fuse blows can be divided into two phases, the melting time and the arcing time. The total breaking time for the fuse is thus the sum of these two times. Fuses can be divided up according to two different principles, those of mechanical design: D-type fuses (diazed fuses) and bladefuses, and those with a disconnection function: gG and aM fuses.

Note: A switch may be capable of making but not breaking short-circuit currents.

Utilisation categories (The utilisation categories define the intended applications.) AC 21 Switching of resistive loads including moderate overloads AC 22 Switching of mixed resistive and inductive loads, including moderate overloads AC 23 Switching of motor loads or highly inductive loads The requirements of switch tripping and breaking capacity are related to which of the three utilisation categories the switch comes under. A switch should always be protected by an overcurrent/overload protective device (fuse). In the data for the switch, the maximum rated current for the pre-connected fuse, related to the prospective short-circuit current in the actual circuit, should be mentioned. The fuse is normally connected after the switch because it is considered that the design is such that a short circuit will not occur in the fuse-combination unit (switch+fuses). Any fault is expected to take place after the combination. The requirement of short-circuit safety is related to the prospective short-circuit current of the actual circuit and the maximum stated fuse size for the switch. If a fault occurs, this combination will provide protection so that switching can be performed without, for example, contact welding occurring, or if the fault occurs during running, 33

5.2

33

97-03-05, 13.25

INDUSTRIAL POWER SYSTEM Low voltage distribution gG fuses have a disconnection characteristic which ranges from 1.6 times the rated current as stamped on the fuse up to current-limiting characteristics at very high fault currents. aM fuses have no thermal protective function. Their breaking characteristic does not start until at 6.3 times the rated current as stamped on the fuse. At higher currents, on the other hand, the aM fuses have a steeper breaking characteristic, which makes it easier to size the apparatus behind them. The rated current for blade-fuses is from 6 A up to 800 A with rated voltage = 690 V. The rated current for D-type fuses is from 2 A up to 100 A with rated voltage = 500 V. A longer type of fuse-link exists for 690 V rated voltage.

Current limiting diagram, Fig. 11 ? I2t diagram The I2t value is a measure of how much energy the fuse lets through for different fault currents. This diagram is thus of great significance when sizing, for example, apparatus, cables and selectivity calculations involved in the circuit.

Melting-time diagram For melting times above 100 ms the arcing time has a very little significance, since this time is relatively short. For a melting time below 100 ms the arcing time assumes greater significance.

Melting-time diagram Fig. 10 ?

I2t diagram, Fig. 12 ?

Current-limiting diagram In the case of very high currents through the fuse, any fault current present will be limited. This means that the circuit will be broken before the fault current reaches its highest value.

Contactors (mechanical) (IEC 947-4-1) A contactor is a mechanical switching device having only one position of rest, operated otherwise than by hand, capable of making, carrying and breaking currents under normal circuit conditions including operating overload conditions. 34

5.2

34

97-03-05, 13.25

INDUSTRIAL POWER SYSTEM Low voltage distribution Note: Contactors combined with suitable relays and which are intended to provide short-circuit protection must also satisfy the relevent conditions specified for circuit-breakers (IEC 947-2).

Other advantages of some contactor relays are fast mounting on profile bars and high-speed plug-in connection. There are two types of contactors:

Brief description and requirements The contactor must be able to withstand a short circuit if it is combined with a suitable shortcircuit protective device. The contactor does not need to be able to break short-circuit currents. The contactor is normally operated by an electro-magnet but some pneumatic controlling devices also exist. A contactor cannot be operated manually. Contactor features include a high operating frequency, a long life, and the possibility of remote control and of automatic operation via a pilot switch or a sensor.

for main circuits • Contactors relays for auxiliary circuits. • Contactor This type of contactor is not described in this book. The normal rated current range for contactors is from 9 A up to a maximum of 800 A (for motors, the power range 4 kW to 450 kW). Recommended utilisation categories (The utilisation categories define the intended application.) The rated current for a contactor depends on how the contactor is to be used.

Standard utilisation categories for contactors and motor starters Type of current Utilisation Typical categories applications AC

AC-1 AC-2 AC-3

Non-inductive or slightly inductive loads, resistance furnaces Slip-ring motors: starting, switching off Squirrel-cage motors: starting, switching off motors during running Squirrel-cage motors: starting, rheostatic braking, inching, reversing

AC-4 DC

DC-1

Non-inductive or slightly inductive loads, resistance furnaces Shunt motors: starting, rheostatic braking, inching, reversing Dynamic braking of DC motors Series motors: starting, rheostatic braking, inching, reversing Dynamic braking of DC motors

DC-3 DC-5

General For a contactor to work well, there are requirements of correct auxiliary circuit design and that the tolerance level of the operating voltage is not exceeded. The contactor is the most hard-working power switch in an auxiliary circuit, with its whole chain of conditions and demands. An incorrectly designed auxiliary circuit can result in a total breakdown. A contactor can normally be operated much faster than the time de-ionising of the extinguishing segments takes. When faults occur in the auxiliary circuit (rebounds, unreliable contact-making or any other type of disturbance)

the operating frequency can be so high that it results in flashovers due to ionised gas or abnormal contact wear. In most cases where a breakdown occurs, such faults are the cause. It is very rare that the fault is in the switchgear itself. The contactor must always be combined with a short-circuit protective device, normally fuses, and for motor drives, overload protection as well. MCCBs may also be used as short-circuit and overload protective devices. See below the section Selection of short-circuit protective device (SCPD), p. ? 41.

35

5.2

35

97-03-05, 13.26

INDUSTRIAL POWER SYSTEM Low voltage distribution Service and maintenance Normal service includes checking the wear on the main contacts, and replacement when the wear level is such that the spring force for a switched-on contactor has reached the limit value which is fixed for each contactor type. The expected contact life is related to the type of drive/lifetime curve and this should be borne in mind when sizing the equipment. The contactor is usually fitted with a number of auxiliary contacts.

with a cold relay, and here too there is a reduction to about a third for a hot relay.

Overload relays (IEC 255-8) An overload relay is an overcurrent relay or release intended for protection against overload. An overcurrent relay or release is a relay or release which causes a mechanical switching device to open with or without time-lag when the current in the relay or release exceeds a predetermined value. An overload relay is mostly used in combination with a contactor in starters. See the sections Motor starters, p, ? 36 and Selection of overload protective device, p. ? 47.

protection • Overload Phase failure protection • Re-start protection • of starts per hour) (counting the number • Symmetry protection

Electronic overload relays The use of electronic overload relays is becoming increasingly common. They are expensive, but normally have a wide range of applications with several functions that are not available with mechanical overload relays. An electronic relay requires a current transformer. Among their functions the following can be mentioned:

The relays are microprocessor-based (see also the section Microprocessor-based control technique for low-voltage systems, p. ? 49) and sample values, ”draw curves” and compare actual values with reference values. When these diverge, the relay will give a signal, or trip. The big difference between the electronic overload relay and the thermal overload relay is the possibility the former has of varying the tripping time and setting the alarm before tripping.

Thermal overload relays (bi-metallic relays) A thermal overload relay is an inverse time-lag overload relay or release depending for its operation (including its time delay) on the thermal action of the current flowing in the relay or release. The most common overload relays are thermal overload relays, which are simple, robust and cheap. They are either direct-fed or fed via a current transformer. The relays should be adjusted to the motor´s rated current (for stardelta starters = rated current /÷3), and are normally temperature-compensated from - 20oC to + 65oC. This means that the relays can work in the most usual ambient temperatures without any special calibration. Modern overload relays are often fitted with protection against phase failure. Should there be a phase failure, the relay then trips the contactor before the motor´s two ”healthy” phases become overheated. Overload relays normally have two tripping characteristics. Normal relays have a possible starting time of 8-10 s when the relay is cold. For a hot relay the possible starting time is reduced to about a third. Relays for heavy-duty starting have a possible starting time of approx. 15-20 s

Circuit breakers A circuit breaker is a mechanical switching device, capable of making and breaking currents under normal circuit conditions and also making, carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those of short-circuit. Circuit breakers normally have both shortcircuit protective devices and overload protective devices. Undervoltage protective devices are usual. (See the section Fault control, undervoltage protection, p. ?) The following types of circuit breaker are standard: circuit breakers, ACBs (IEC 947-2) • Air An ACB is a circuit breaker in which the

•

contacts open and close in air at atmospheric pressure Moulded case circuit breakers, MCCBs (IEC 947-2)

36

5.2

36

97-03-05, 13.26

INDUSTRIAL POWER SYSTEM Low voltage distribution MCB MCBs are available for rated currents up to 125 A, but 63 A is usually the highest value. There are various designs, depending on breaking capacity and characteristics. See Fig. 14, p. ? 35 (below). MCBs are mainly used in auxiliary circuits and in distribution boards (instead of fuses).

An MCCB is a circuit breaker having a supporting housing of moulded insulating material forming an integral part of the circuit breaker circuit breakers, MCBs (IEC 898) • Miniature An MCB is a circuit breaker intended for protection against overcurrents in wiring installations in buildings and in similar applications; they are designed for use by inexpert people and do not need being maintained

Differences between ACBs and MCCBs Being subject to the same design standard it is not surprising that the differences between ACBs and MCCBs are becoming finer. Where they exist it can be said that the main difference lies in the possibility to add accessories and opportunities for service and maintenance. In this respect MCCBs have limitations compared to ACBs. Further, there are better possibilities of delaying the short-circuit protection in an ACB, as an ACB can withstand a higher short-circuit current for a longer time than an MCCB. This is shown by the rated short-time withstand current, the 1-second value, (see the section Short-circuit protection, p. ? 41), which is much higher for ACBs than for MCCBs.

ACB ACBs are available for rated currents from 630 A to 6300 A, but the range most frequently used is 1250 A - 3200 A. Until recently all ACBs were of non-current-limiting type with breaking capacity up to 85 kA. Now there are also current-limiting ACBs with tripping times
Current-limiting circuit breakers One often refers to current-limiting and noncurrent-limiting circuit breakers, see Fig. 13 ?, p. ? 35 (below). All MCCBs and MCBs are more or less current-limiting, since the current forces that arise upon a short circuit press the contacts apart. This usually results in an arc, and thus an impedance which limits the current allowed through. A current-limiting circuit breaker is a breaker which is able to extinguish an arc before the current passes through neutral for the first time after a short circuit has occurred. This gives extremely short break times. Non-current-limiting circuit breakers have to wait for the current to pass through neutral before they extinguish the arc generated. This means that the break time is around 20 milliseconds. It should be observed that current-limiting breakers need a theoretical short-circuit current of 20-30 kA to work correctly.

MCCB MCCBs are available for rated currents from 1 A to 3200 A, but the range most frequently used is 10-630 A, and for incoming breakers up to 1600 A. Current-limiting and non-currentlimiting breakers are available from most suppliers. The non-current-limiting breaker is often available in more designs with different levels of breaking capacity. Development is moving towards a better current-limiting capacity. In the MCCB field too there are protective devices based on electronics that are becoming more and more sophisticated. The overload protective devices in MCCBs are mainly used for protection of cables and conductors, but there are breakers in the lower current area where the overload protective device is designed for motor protection and with the extra functions that this application needs.

Breaking capacity Breaking capacities for circuit breakers are 37

5.2

37

97-03-05, 13.26

INDUSTRIAL POWER SYSTEM Low voltage distribution described in the section Selection of shortcircuit protective device, p. ? 41.

Motor starters (IEC 947-4-1) See also Chapter 4.4 Motor starting systems (p. 140-164). A motor starter is a combination of all the switching means necessary to start and stop a motor in combination with suitable overload protection, a manual externally-operated switching device and a short-circuit protective device, mounted and wired in a dedicated enclosure. The switching and short-circuit protective devices may be a fuse combination, a switch with fuses or a circuit breaker with or without an isolating function.

Utilisation categories Category A: Circuit breakers not specifically intended for selectivity Category B: Circuit breakers specifically intended for selectivity

Strömbegränsande

Ej strömbegränsande

I kA

I kA

50

50

20

20

Note: 1. A dedicated enclosure is an enclosure specifically designed and sized for its application, in which all tests are conducted. 2. The manually-operated switching device and the short-circuit protective device may be a single device and may incorporate the overload protection as well.

5 ms 13 ms

I1

I2

}

T (s)

K

General: The most common outgoing units in a Motor Control Center, MCC, are motor starters. The main parts in a motor starter are: Contactor for on/off switching of the motor current (see the section Contactors, p. ? 31) Short-circuit protective device, SCPD, switch+fuses or circuit breaker, often an MCCB (see the sections Switches, p. ? 29, Circuit breakers, p. ? 34 and Selection of SCPD, p. ? 41) Thermal overload relay, in some cases an electronic overload relay (see the section Overload relays, p. ? 33)

Termisk utlösning

K B C K D

103

The rated current for motor starters is limited to a maximum of 700 A (400 kW output for motors).

102

101 K U

L

100

B

3

D

C

10

5

×In

}

1

Momentan utlösning I3 I4

Figs. 13 and 14, Current-limiting diagram and non-current-limiting diagram, Thermal and instantaneous tripping of different MCB types.

Fig. 15 Motor starter 38

5.2

38

97-03-05, 13.27

INDUSTRIAL POWER SYSTEM Low voltage distribution

1

5

9

I>

4

I>

I>

4

2

2

4

2

3

3

6 8

7

4

4

2

2

3

3

Fig. 16 Typical variants of motor starters, circuit diagrams. Type 2 co-ordination requires that, under shortcircuit conditions, the contactor or starter should cause no danger to persons or installation and must be suitable for further use. The risk of contact welding is recognized, in which case the manufacturer is to indicate the measures to be taken with regard to the maintenance of the equipment.

Utilisation categories Recommended utilisation categories for motor starters are the same as for contactors, see p. ? 32. Co-ordination with short-circuit protective devices Types of co-ordination: (Conditions after a short-circuit test carried out with bolted short-circuited outgoing terminals.)

Important:After an actual short circuit the location of the fault must always be pin-pointed and the motor starter examined and repaired before re-starting.

Type 1 co-ordination requires that, under shortcircuit conditions, the contactor or starter should cause no danger to persons or installation and may not be suitable for further service without repair and replacement of parts.

See also the section Inspection of condition, p. ? 45. 39

5.2

39

97-03-05, 13.27

INDUSTRIAL POWER SYSTEM Low voltage distribution Main types of motor starters: 1. Direct-on-line starter This is a starter which connects the line voltage across the motor terminals in one step. Direct-on-line starters are designed to start and accelerate a motor to normal speed, to provide means for the protection of the motor and its associated circuits against operating overloads, and to switch off the supply from the motor. This also applies to reversing starters.

Note: In star connection, the current in the line and the torque of the motor are about one-third of the corresponding values for delta connection. Therefore, star-delta starters are used when the inrush current due to starting is to be limited, or when the driven machine requires a limited torque for starting.

4. Pole changing starter This is a starter for motors with multiple speeds. These motors could have two windings, with a different number of poles for different speeds, or a switchable winding so that that the number of poles is divided into halves upon switch-over, a so-called Dahlander connection. Pole-changing starters for Dahlander-connected motors have an extra contactor for the switch-over from high speed to low speed or vice-versa.

2. Reversing starter This is a starter designed to cause the motor to reverse the direction of rotation by reversing the motor´s primary connections while the motor is running. See also above under direct-on-line starters, second paragraph.

5. Two-step auto-transformer starters This is a starter designed to start and accelerate an AC induction motor from rest with reduced torque to normal speed, to provide means for the protection of the motor and its associated circuits against operating overloads, and to switch off the supply from the motor. Auto-transformer starters are normally not intended for inching duty or reversing motors rapidly and therefore Utilisation Category 4 does not apply.

Current /rated operational current I e

Current /rated operational current I e

3. Star-delta starter This is a starter for a three-phase induction motor such that in the starting position the stator windings are connected in star and in the final running position they are connected in delta. Star-delta starters are designed to start a three-phase motor in star connection, to ensure continuous operation in delta connection, to provide means for the protection of the motor and its associated circuits against operating overloads, and to switch off the supply from the motor. Star-delta starters are not normally designed for reversing motors rapidly and therefore Utilisation Category 4 does not apply. 6

ID 5 4 3

IY

2

I L 0.8 tap

6

IR

5

IT

0.8 tap

4

IT

0.65 tap

IL

0.5 tap

I T 0.5 tap

I L 0.65 tap

3 2

Synchronous speed 1

Synchronous speed 1 0.8 1

CM D 1 Synchronous speed

CM R

CM T

CR

CR 1

Synchronous speed

0.65 tap

0.8

Speed/rated speed

Fig. 17 Typical curves of currents and torques during a star-delta start.

40

1

Speed/rated speed

Fig. 18 Typical curves of currents and torques during an auto-transformer start. 40

5.2

Speed/rated speed

1

CM Y

0.8

1

Speed/rated speed Torque/rated torque

Torque/rated torque

0.8

97-03-05, 13.27

INDUSTRIAL POWER SYSTEM Low voltage distribution Note: In the starting position, the current in the line and the torque of the motor related to motor starting with rated voltage are reduced approximately as the square of the ratio: starting voltage/rated voltage. Therefore, auto-transformer starters are used when the inrush current due to the starting operation is to be limited, or when the driven machine requires a limited torque for starting.

Voltage DoL

58%

Fig. 20. Motor voltage when starting with different starting methods. Current limiters Current limiters are used as complementary short-circuit protective devices for smaller motors (max. approx. 22 kW, max. rated current approx. 50 A). A current limiter, based on polymeric material, acts as a conductor at rated currents but becomes high resistive if a short circuit occurs. Consequently, the fault current is limited. A current limiter is connected in series with a breaker (mostly an MCB) and will limit the fault current to a value under the breaking capacity of the breaker. The breaking capacity of the circuit breaker is thus enhanced. Using conventional technology, it is not only the short-circuit protective devices, SCPDs, that have to be upgraded, but also any auxiliary equipment, such as contactors, due to higher let-through energies (I2t). The busbar system and cables often need to be upgraded too. Using a current limiter, the fault levels are reduced to a much lower level, e.g. 6 kA instead of 50 kA, and subsequently the let-through energies involved will be linked to the limited fault level. There are current limiters for use in MCC (Motor Control Centre) applications with motor starters up to 690 V, 63 A, and others for distribution applications with standard MCBs (miniature circuit breakers). In order to ensure good interaction between breakers and the current limiter, only breakers specified in official co-ordination tables should be used.

Soft starters A soft starter is a starter which is supplied with six anti-parallel-connected thyristors. The thyristors´ ignition angle is controlled by a microprocessor that increases the voltage to the motor during starting. There is a scale on the soft starter where the starting time can be adjusted so as to achieve a soft start. The soft starter makes it possible to control the voltage to match the actual need, so that power losses are minimised and smoother, more sparing operation is obtained, with longer life as a result. Soft starters are becoming increasingly usual in process industries and for motor sizes of 160 kW and above.

DoL

Star/Delta Softstart

Arc guard system General Very short disconnection times are the best way of limiting the risk of injury to personnel and damage to equipment due to arcing faults.

Speed

Fig. 19. Starting torque with different starting methods. 41

41

60%

10%

• • • • • •

5.2

Softstart

Adjustable starting ramp

Accessories Motor starters can be supplied with various accessories such as: indicating lamps on and off pushbuttons auxiliary relay for remote control switch for choice of remote control and local control ammeter remote reset by thermal overload relay

Torque

Star/Delta

100%

97-03-05, 13.27

INDUSTRIAL POWER SYSTEM Low voltage distribution A combination of an arc guard system and highspeed circuit breakers provides the best means of ensuring a minimum of damage and injury. Cleaning and repair times become shorter, operating reliability and equipment availability increases, and downtime is dramatically decreased. Potential breakdowns are very largely avoided, production losses are drastically cut and there are clear savings in costs.

2

kA S

Q2 F11

D

Fig. 22 Arc monitor with detectors, circuit diagram

➂

➁

Operational features The arc monitor delivers the trip signal in approximately 1 to 2 milliseconds. The actual disconnection time depends on the type of circuit breaker used, but the entire process is over in less than 50 milliseconds if an LV ASSEMBLY has been provided with an arc guard system.

Steel Fire

Cable Fire

➀ 0

35 ms

100 ms

200 ms

400 ms

Q3

500 ms

Fig. 21 Arc duration and resulting damage

Current sensing unit The detectors can also be sensitive to other forms of intense light, such as camera flashes, lightning, direct sunlight, switching arcs in circuit breakers and other large apparatuses. By combining the arc monitor with a current sensing unit set at just over the normal operating level, a current-dependent condition is introduced which prevents triggering from irrelevant light sources. This prevents spurious tripping of the LV ASSEMBLY, causing nuisance power outages.

Description The watchful eye (the detector) of the arc guard system detects any large increase in light intensity. The detector transfers light from the arc through a state-of-the-art solid-state electronics package (an arc monitor). Within an interval of one to two milliseconds, the system sends a trip signal to the disconnecting circuit breaker (upstream) located in the LV ASSEMBLY, bypassing delays caused by selective features of relaying schemes. This protects both equipment and personnel.

T3

The system includes: Arc monitor with detectors (normally one detector in each cubicle in an LV ASSEMBLY) Current sensing unit Breaker fault unit

•

F12

• •

Q2 Q3

F11

All communication between the detectors, arc monitor, current sensing unit and the breaker fault unit is through fibre optics. Fibre optics eliminate the risk of interference from extreme high electromagnetic fields, especially during an arcing fault.

D

Fig. 23 Arc monitor with detectors and current sensing unit, circuit diagram. 42

5.2

42

97-03-05, 13.27

INDUSTRIAL POWER SYSTEM Low voltage distribution arranged to give maximum coverage to the compartment in which they are located. Additional detectors are to be added as required to obtain complete coverage. Tripping should be arranged so that all internal feeders connected to a faulty section of main busbar are disconnected. To cover the situation when feeder circuits act as incoming or outgoing circuits, the current measuring input for each section of main busbar is to be operated from current transformers.

Breaker fault unit The breaker fault unit provides a safeguard against failures in downstream circuit breakers (on the low-voltage side of the transformer where the circuit breaker is mounted in the LV ASSEMBLY). In installations with access to the main upstream circuit breakers (on the medium-voltage side of the transformer) the addition of a breaker fault unit can be utilised. This ensures protection if the downstream branch circuit breaker does not function. If the downstream breaker fails to disconnect the supply within the set time, the breaker fault unit will override and trip the main upstream breaker (on the medium-voltage side).

Selection of short-circuit protective device (SCPD) Introduction Short-circuit protection is a question of selecting between fuses and circuit breakers. Both types of protection are still highly applicable in plants and described below is what application they work best in. Sizing of circuit breakers or fuses requires somewhat different input data in order to make a fair comparison. Circuit breakers are more dependent on voltage and short-circuit power than fuses are. This means, in turn, that there are a great many circuit breaker variants. See Chapter 4.4, Motor starting systems, p. ?........... An analysis of short-circuit currents that occur in main distribution and sub-distribution systems has always been important as an instrument to check selectivity and trip conditions, for example. With circuit breakers in the plant, the analysis will also have a considerable effect on the cost level. Unfortunately, this analysis mostly concerns the highest short-circuit current occurring, even though the lowest short-circuit current is at least as important to know for various fault cases, so that tripping of the circuit occurs within a few seconds. A simple definition of the best-working shortcircuit protective device is that it should not trip at overcurrents (starting currents) occurring in service, but that it should for the lowest shortcircuit current in the plant. The relationship between operating current and switching currents occurring in the application determines the selection of tripping level:

Q1 F13 T3 F12 Q2 F11

Q3 D

Fig. 24 Arc monitor with detectors, current sensing unit and circuit breaker, circuit diagram. LV ASSEMBLY protection An LV ASSEMBLY protection system is based on the ABB arc guard system, or equivalent, operating in conjunction with an arc monitor. At least one detector is to be included for each compartment of the LV ASSEMBLY. In the LV AssemblY, optical detectors are to be located in each vertical busbar section, including detectors in the main horizontal busbar compartments, in each circuit breaker compartment and in the compartments where the connections for outgoing cables are to be monitored. Detectors are also to be located to monitor the main busbar, breakers, and connections to outgoing cables. All detectors are to be

For Utilisation Category AC21 (resistive load) the relationship is Imax to In = 1 43

5.2

43

97-03-05, 13.28

INDUSTRIAL POWER SYSTEM Low voltage distribution ment in the event of a short circuit, even if the fuse is sized to trip the circuit within a few seconds. In order to always have the best short-circuit protective device, selection must be based on the need of short-circuit current, i.e., the switching current occurring in the circuit, and the tripping level should be chosen accordingly. A rule of thumb for this, representing a ”safe level”, can be given for Utilisation Category:

For Utilisation Category AC22 (distribution) the relationship is Imax to In = 2.5 -3 For Utilisation Category AC23 (threephase induction motor) the relationship is Imax to In = 7 Transformator 10/0,4 kV 800 kVA I = 23 kA k

AC21: Tripping level 2 x In AC22: Tripping level 5 x In AC23: Tripping level 10 x In

AC22

Stum kortslutning i centralen ger 5,7 kA. Ett fas–jordfel ger 3,3 kA. Med hänsyn taget till dämpning av felljusbåge 30 % är lägsta kortslutningsströmmen 2,3 kA.

The calculated values should be checked in relation to these levels and any relationship between the requirement level and the results from the calculation should be assessed. When studying this and starting to evaluate different short-circuit protection alternatives, differences between fuses and circuit breakers are soon observed. With these differences established and set in their right context in the plant, good progress has been made in pursuit of the best possible solution; a solution that takes into account costs due to unplanned production downtime in relation to the investment in electrical equipment.

Säkring 250 A 4 × 240 mm2 Al-kabel 200 meter Undercentral 250 A

Driftström inkl. startström

2,3 kA

Fas–jord 3,3 kA Fas–fas 5,7 kA

➀ Figs. 25 and 26 Example of short-circuit category AC 22.

Special terminology Before making any direct comparisons between different short-circuit protective devices, a number of terms need be clarified.

According to the fuse´s tripping curve, the tripping time is 0.5 seconds at a fault current of 2.3 kA. The plant´s rated current is 250 A and starting currents in service for Utilisation Category AC22 are stated at 2.5-3 times the rated current. The area (1) between the lowest calculated value and the operating current value is also to be considered as a fault area. It is impossible to predict what current will result from a fault. In the case of a fault with higher absorption of a fault arc, for example a 1000 A fault current, the same fuse would have a tripping time of about 20 seconds, with the obvious risk of fire breaking out in cabling and equipment. In other words, there is nothing to ensure that there will be no damage to cables and equip-

Cascade connection Cascade connection is the term used when a breaker with a high tripping capacity backs up a breaker with a lower tripping capacity. This is a very useful economic alternative if limited selectivity can be accepted, since both breakers in the chain trip for currents exceeding the breaking capacity of the lower breaker. The most common application is when an MCCB backs up a number of MCBs in a distribution board. (MCCBs and MCBs are described under the section Circuit breakers, p. ? 34.) Note that accurate sizing necessitates joint testing of apparatus. It is very difficult to achieve the required function by combining different devices merely on the basis of catalogue data. 44

5.2

44

97-03-05, 13.28

INDUSTRIAL POWER SYSTEM Low voltage distribution The difference in the new standard is that it is now required that the two values (ICU and ICS) be presented on the breaker´s rating plate. This was not the case previously, which resulted in problems since not all suppliers clearly stated which value it was that was stamped on the plate. It is worth noting that the standard does not differentiate between ACB and MCCB, which means that an ACB too is only able to withstand a few short circuits at its maximum breaking capacity. The difference is that the ACB´s design makes it quite easy to replace arc contacts and quenching chambers, which extends the lifetime of the ACB. In practice, however, it is extremely unlikely that the same breaker will be repeatedly exposed to fully developed short circuits. A fuse has, in principle, unlimited breaking capacity.

a

b c

F

M

M

M

Fig. 27 Cascade connection

Breaking capacity Breaking capacity describes the breaker´s capacity to trip short-circuit currents. It is important to know how the breaking capacity is defined in order to impose the right requirements on the apparatus. An IEC standard clearly specifies how the test must be conducted when the breaking capacity is to be verified. There is some difference here between the old standard, IEC 157-1, and the new one, IEC 947-2 (see the section Circuit breakers, p. ? 34), but the test procedures are virtually identical. In both cases two different values for breaking capacity are verified by two different test series. After the first of the test series, a limited function is accepted for the breaker and this value is termed the P1 value, as stated in IEC 157-1, and the ICU value, as in IEC 947-2. After the second test series the breaker is still to retain its full function and this value is termed P2 in IEC 157-1 and ICS in IEC 947-2. These values are of course at a lower level than the values P1 and ICU.

Thermal trip conditions There is another description of a breaker´s performance where it is important to know the background. According to IEC 947-2 the breaker´s thermal overload protective device should be calibrated at +30o C unless otherwise stated. However, the calibrating temperature is more usually +40oC or +45oC. The standard for circuit breakers, IEC 947-2, has wider tolerances than the standard for motor starters, IEC 947-4-1. The MCCB is quite simply primarily designed for the protection of cables and normally does not have the kind of overload protection required to safeguard motors. An example of how the calibration temperature can play an important part is an MCB

Test of short-circuit breaking capacity, test sequence Standard

Test sequence

Requirements after test

IEC 947-2: ICU ICS

O-CO O-CO-CO

Limited function Full function

IEC 157-1: Cat. P1 Cat. P2

O-CO O-CO-CO

Limited function Full function

O = one break CO = one make, automatically followed by one break 45

5.2

45

97-03-05, 13.28

INDUSTRIAL POWER SYSTEM Low voltage distribution current-limiting circuit breakers. See the section Circuit breakers, current-limiting circuit breakers, p. ? 35. The goal is to have the same low level of letthrough energy (I2t) for circuit breakers as for fuses at high short-circuit currents.

installed in a car engine heating distribution board located outdoors. This breaker may be calibrated at +20oC, whereas the ambient temperature might be -20oC. This naturally means that the breaker will not perform as intended. × Ie 1,40 IEC 947-2

Ip (kA)

1,30 1,20 IEC 947-4

100 1,05 1,00 –10 –5

0

10

Effektbrytare 160 A gG 160 A aM 100 A aM

20 30 40 °C Omgivningstemperatur

10

Fig. 28 Thermal trip conditions Ambient temperature Co-ordination type Types of co-ordination for motor starters are described in the section Motor starters, p. ? 36.

104

Let-through energy (I2t) and let-through current peak Let-through energy and current peak are two important magnitudes that give us reason to examine the differences between fuses and circuit breakers.

105 I (A)

I2t-värde, A2s 107 8 6 I2t 660 V I2t 500 V 4 I2t 380 V

Fuses The breaking procedure for fuses depends entirely on the size of the short-circuit current. The higher the current, the shorter the breaking time is.This means that the let-through energy (I2t) is relatively independent of the shortcircuit current at high levels of fault current (see the graphs below). Note that the shortcircuit current must be at a relatively high level for the fuse to be able to limit the current peak (see the section Fuses,current-limiting diagram, p. ? 31).

2 106 8 6 4 2 105 8 6 4 2 104 8 6 4

Circuit breakers The circuit breaker´s short-circuit protective device, on the other hand, trips at approximately the same time, whatever the size of the short-circuit current. This means that the letthrough energy (I2t) will increase with higher short-circuit currents. This is counteracted by

2 103

63

125

250

400

46

Märkström, A

Figs. 29 and 30 I2t curves and current limiting 46

5.2

630

97-03-05, 13.28

INDUSTRIAL POWER SYSTEM Low voltage distribution Let-through energy (I2t) and current peak are important data for fuses and circuit breakers, since they are of importance to the sizing of other components in the circuit, not forgetting cable sizing (see the section Cable sizing, p. ?...........).

Comparison of fuses and circuit breakers as short-circuit protection The comparison is based upon three basic conditions: 1. The short-circuit protective device is not to trip if the rotor is locked in the motor. Overload currents up to that level are to be taken care of by the thermal overload protective device.

Condition monitoring The problem is to find out what the breaker has been exposed to and what condition it is in. Such checking is very much a question of subjective judgement and requires some experience. A visual inspection of this kind should be included in a periodic inspection control plan, as well as after a breaker has tripped for a short circuit. In the regular inspection the breaker should also be operated a few times. Furthermore, the trip test button that is to be found on most MCCBs today should be tested. A more objective assessment of condition can be made, after a trip, by measuring the contact resistance over the main contacts, as well as the insulation resistance to earth. Tables of acceptable values are available. It is also important to remember that the breaker that can carry its rated current can also cope with tripping for a short circuit. It is the characteristics of the thermal overload protective device that may be adversely affected.

2. The motor starter is not to trip during normal starting of the motor. 3. The contactor in a motor starter is to have a higher making and breaking capacity, than the tripping value of the short-circuit protective device, so that the contactor is able to break the fault current when the therrmal overload protective device gives an overload signal. Fuses It has been described above how fuses should be sized so as to fulfil the first two requirements. It is the third requirement that causes problems, since the fuse performance in the overlapping area is rather difficult to control. With short-circuit currents that are too low, the breaking times for the fuse are too long. Often the contactor has to withstand and interrupt fault currents which the fuse should really have dealt with. The current-time curves given for fuses do not provide much help as they only describe the melting time for the fuses. The total breaking time is often far too long to protect the contactor effectively. Often the fuse will not blow fast enough, or in the worst case not blow at all if, for example, there is an attempt to switch on a contactor when there is an insulation fault in a motor. This is more than the contactor can handle and causes damage to the contacts and often results in contact welding. The contactor is often given the blame for defective operation but in reality it is the short-circuit protective device that did not work correctly.

Dependable disconnection Not all experts approve of using the same device for breaking short-circuit current and employing it as a disconnector when carrying out work on equipment (see the section Disconnection for work purposes, p. ? 27). Technical solutions that ensure dependable disconnection have been developed, for example taking out and turning round the thermal overload relay in a breaker and using it as an disconnector. In heavy industry, withdrawable units (see the section Incoming and outgoing units, p. ? 6) are used in LV ASSEMBLIES. The units are located in a disconnected position - a reliable solution. For those who do not wish to choose withdrawable units there are today several switchgear alternatives. Plug-in MCCB types can, for example, be used as disconnectors when isolated or withdrawn.

Circuit breakers With a circuit breaker as the short-circuit protective device, the first requirement above is, in theory, easy to fulfil. The breaker has a very steep characteristic in the short-circuit range, and it is therefore easy to adjust to match the 47

5.2

47

97-03-05, 13.29

INDUSTRIAL POWER SYSTEM Low voltage distribution

10-1

for the difficulties is that the breaker lets through more energy the higher the short-circuit currents become. Apparatuses and devices after the breaker have to be selected so that they can cope with the amount of energy involved. The next general rule is that sizing is easy with fuses when there are fully developed, direct short circuits. The problem of the fuse´s long breaking time in the case of limited short-circuit currents is most critical for larger motors (> 70 kW. However, it may be solved to some extent with aM fuses. See the section Fuses, p. ? 30 and Chapter 4.4 Motor starting systems., p. ? The graph below shows the problems and also the possibilities in selecting a short-circuit protective device in a motor starter. When it comes to pure distribution, the advantageous properties of the circuit breaker can be fully utilised. The weaknesses of circuit breakers at high short-circuit currents are far less marked in pure distribution than in the protection of motor starters, since no co-ordination is required with other components. In the United Kingdom and Italy, for example, fuses are very seldom used in pure distribution. On the other hand, there are still fuses in around 50 % of all motor starters. The reason is not lack of technology; it is often due to oversizing of other components in trying to achieve sizing equivalent to that of fuses at higher short-circuit currents.This has in its turn resulted in higher initial costs. Nowadays, there are more, improved current-limiting breakers available, allowing contactors to be better utilised. If the voltage is limited to 500 V and the short-circuit current to 50 kA, the differences today are small compared with fuses as short-circuit protection. Recent price development, too, has made the MCCB more attractive for motor starter applications.

10-2

See also Chapter 4.4 Motor starting systems

motor´s starting current. On the other hand, only a very short current over the tripping value is needed for the breaker to react. Once it has started to move there is no return, and the breaker will trip. The very short current peak, which inevitably occurs when starting a motor, is quite sufficient to trip the breaker. This means that this peak determines sizing of the setting of the short-circuit protective device. Since it is much easier here to predict a breaker´s operation than that of a fuse, it is also easier to choose a contactor with sufficient performance to fulfil the three requirements mentioned earlier. As a general rule it can be said that it is much easier to size with circuit breakers as the short-circuit protection at these limited short-circuit currents. Difficulties with circuit breakers arise in combinations with equipment in a motor starter. These combinations have to be protected against a bolted short circuit (no arc) directly on the outgoing side of the combination. The reason T

(s) 102

101

Termiskt relä

MCCB

100 A aM 100

160 A gG

Motor

Selection of overload protection 102

103

Introduction It is primarily in the selection of short-circuit protection that a comparison between fuse and circuit breaker assumes most importance. Irrespective of the choice of short-circuit protec-

104 I (A)

Fig. 31 Current-time curves. ComparisonFuses/MCCBs 48

5.2

48

97-03-05, 13.29

INDUSTRIAL POWER SYSTEM Low voltage distribution Conventional thermal overload relays The simplest and still the most common overload protective device is the thermal overload relay. See the section Overload relays, p. ? 33. This is a good solution at a good price which gives enough protection for most motors.

tion, perfectly adequate overload protection is obtained for the cable. The question is whether the protection is enough to protect the object in question. To protect a motor from overload, the fuse´s thermal characteristic is obviously not enough. Further, the suitability of the integral thermal protective device present in most MCCBs as overload protection for a motor should also be questioned. Development is moving towards more stringent demands regarding plant availability. In connection with this, demands for adequate protective devices of different kinds are increasing. Therefore, selecting an overload protection with worse capacity than traditional thermal overload relays (see the section Overload relays, p. ? 33) cannot be correct. Consequently, both fuse and MCCB should be supplemented with the best possible overload protection feature for the motor. The object, i.e., the motor and the machine the motor is to drive, determines the nature of the required protective functions.

Electronic overload relays Vital continuous drives often require functions that are not available with thermal overload relays. A current-transformer-fed electronic overload relay is then a good alternative. See the sections Overload relays, p. ? 33 and Microprocessor-based control technique for low-voltage distribution systems, p. ? 49. Electronic overload relays with communication possibilities To ensure maximum availability of a plant, preventive maintenance is important. It is then required that various protection functions provide an alarm for impending and incipient faults. These could include measuring of temperature at different points, operating hours and the number of makes and breaks. All this can be measured and stored in a monitoring system. See the sections Overload relays, p. ? 33 and Microprocessor-based control technique for low-voltage distribution systems, p. ? 49.

Different types of overload protective devices Standard protective device in MCCBs The thermal relay built into all MCCBs is designed to protect cables. It does this job better than a fuse, which means that in most cases the cable can be better utilised if a circuit breaker has been selected. However, as mentioned previously, the let-through energy (I2t) upon short circuit for cables < 25 mm2 must be checked. Manually controlled direct-on-line starters and various types of combination starters (see the section Motor starters, p. ? 36) that have been developed in recent years contain thermal overload protective devices that fulfil the IEC standard for contactors and motor starters (IEC 947-4-1). One or more of the following functions are, however, often missing:

Temperature measurement It is not the overload itself that is damaging to the motor; it is the elevated temperature that the overload causes. So long as all that is required is to measure the current to avoid damage to the motor, there is optimum protection as the current characteristics of the protection are just above the current characteristics of the motor during inrush, start and continuous operation. However, in the case of abnormal temperature conditions in the LV ASSEMBLY or at the site of the motor, current measuring gives only a poor indication of how severely the motor is loaded. To obtain complete protection, temperature measurement of the motor´s windings is necessary. An increasing number of motor starters can accept signals from, for example, a thermistor (built into the windings of the motor) for tripping or warning.

contacts that differentiate between • Signal overload and short circuit. • Temperature compensation. • Phase fault protection. of automatic resetting (and • Possibility thereby remotely controlled resetting via electrical interlocking). 49

5.2

49

97-03-05, 13.29

INDUSTRIAL POWER SYSTEM Low voltage distribution

Microprocessor-based control technique for low-voltage distribution systems

All this helps lead to more efficient production through optimum use of the electrical plant. System principles There are three types of electronic microprocessor-based systems for Motor Control Centres, MCCs, on the market. 1. Systems with motor protection functions (electronic overload relay) 2. Communication systems that have bus communication between MCC and master process control system 3. Systems that include both motor protection and bus communication with master process control system, for example ABB´s INSUM.

General It is not only the supply to the motors but also the protection and control functions that are attended to by an LV ASSEMBLY. Normally, these protection and control functions are realised by combining apparatus with the right functions and data, where the choice of apparatus depends on both motor data and the application. The apparatus should usually be connected in a certain manner to obtain the required operation for the motor drive. With the help of microprocessor technology such electrical installations can today be built up in a much easier way. All monitoring and control, for example of motor drives, is concentrated to a common master operation centre. Information that needs to be known about a motor in the plant can be read on a display, for example current level, fault messages, operating hours, maintenance period and a great deal more. This information can also be found on control and indication panels on the LV ASSEMBLY. The large number of monitoring functions, and the possibility of getting warnings at programmed-in measuring levels, results in more efficient process monitoring. The warning signals make it possible to take steps in time and breakdowns can thus be avoided. At the same time as control and monitoring functions are being improved and extended, the actual design of the electrical plant is being simplified. Point-to-point conductors are being replaced with bus connections, which reduces the number of cables needed.

Describtion of type 3 above (ABB´s INSUM system) All conventional components are replaced by a microprocessor-based device. This consists of measuring and control units that have all functions for protection, controlling, monitoring, measuring and operating of, for example, a motor drive. Each unit collects, processes and stores measured values and transmits control signals to the motor starter. A current transformer ensures galvanic separation between main circuits and auxiliary circuits and provides a suitable measuring level. In an operating and indicating unit the bus connection is able to handle up to 32 measuring and control devices, communicating with a master process control system. The functions that are monitored in a motor drive are:

• Overload failure • Phase rotor in the motor • Locked • Underload fault • Earth Temperature • Undervoltagerise •

Some of the advantages of this technique are: monitoring and controlling • Improved Improved motor protection • More reliable system processing • Extended protection • Simpler plant design functions • Quicker and more efficient plant • construction effective maintenance • More Easier and faster troubleshooting •

There are also units for controlling and monitoring circuit breakers. Together with the master process control system, for example the ABB Master system, controlling and monitoring of the electrical plant can be integrated in the production process. The operator and the plant electrician are 50

5.2

50

97-03-05, 13.29

INDUSTRIAL POWER SYSTEM Low voltage distribution short circuit (short circuit without arc) must be determined. See the section Motor starters, p. ? 36. In the suppliers´ tables a certain type of motor is used as the point of departure. A 4-pole motor of a well-known make is usually chosen. An important feature where motor makes differ is the power factor, cos j, during start-up. The current peak generated during the start is entirely dependent on the P.F. or cos j. Modern, high-efficiency motors have a lower P.F. and this results in a higher switching current surge when starting. This current surge determines sizing in setting the short-circuit protective device in the circuit breaker. Corresponding problems can arise when changing to circuit breakers in an existing plant where the motors are often over-sized. Unfortunately, the short-circuit protective device cannot be set with too broad a margin, since this directly affects the maximum cable length, which is also dependent on the short-circuit protective device setting. The manufacturer can provide information on which power factor has been calculate with for different motor outputs. This information must be checked against all actual motors in the plant. In the sizing tables there are recommended settings for the short-circuit protective device. With help of these figues the cables can then be sized. See Chapter 10 Cables, as mentioned above. Note that when calculating the nominal setting of the circuit breaker, a possible voltage variation of +/- 10 % needs to be taken into account, as well as a tolerance of +/- 20 % in the short-circuit protective device. With regard to the maximum settings, based upon defined tripping with a certain cable, a 30 % voltage drop in any arc that occurs can be reckoned on, and a tolerance of +/- 20 % in the breaker´s set value.

thus equipped with an effective tool to control and run the process. All necessary functions for process control, drive equipment, motor control, machine control, instrumentation, drive optimisation, quality control, etc., can be included. ABB Master

BAG INSUM

Startkopplare

Luftbrytare Megamax

Max 32 stycken

MSG INSUM

M

Lokal manöverlåda

T.ex. Nödstopp

Fig. 32 The INSUM system

Sizing of devices and cables For direct outgoing units (motor starters, for example) this is largely a question of selectivity and cable sizing. This requires, as input data, the calculated short-circuit current at the different locations where the short-circuit protective device is to be installed, and the short-circuit current for various fault cases at the item to be protected, With circuit breakers, the requirements made regarding the condition of the breaker after a bolted short circuit (short circuit without arc) in the LV ASSEMBLY must also be determined. Further, co-ordination tables from the supplier of the circuit breaker are needed in developing selectivity plans. See more about selectivity under the section Selectivity, p. 26 ?.......... Cable sizing is described in detail in Chapter 10 Cables, Cable sizing, p. ?.......... For sizing of motor starters, the supplier should be able to supply complete co-ordination and sizing tables. One should avoid combining a motor starter on one´s own. Correct sizing requires careful joint testing of the components (i.e., the complete starter, see the section Motor starters, p. ? 36). For combination starters, the requirements made of the device´s function after a bolted

Project planning Planning steps Aspects affecting the design and the selection of components for the LV ASSEMBLY have been described earlier. This section constitutes a summary and a check list for planning steps in logical sequence. 51

5.2

51

97-03-05, 13.30

INDUSTRIAL POWER SYSTEM Low voltage distribution Step 5: Determination of incoming supply from the transformer See the section Connection to the LV ASSEMBLY, p. ? 26.

Step 1: Determination of plant design See the section Plant design, p. ? 22. Alternative 1

Alternative 2

Place the transformer next to the LV ASSEMBLY and gather all incoming and outgoing units in a common switchgear assembly located in an operating room. Divide the LV ASSEMBLY into a main distribution Load Centre (LC) located in an locked operating room, and sub-distribution Motor Control Centres (MCCs), placed close to the process equipment.

circuit breaker • Via disconnector • Via Via switch + fuses or MCCB • Direct connection • Step 6: Determination of short-circuit protective device for the outgoing units See the section Selection of short-circuit protective device (SCPD), p. ? 41. Alternative 1 Switch + fuses Alternative 2 MCCB The requirements of selectivity must be closely considered.

Step 2: Determination of rated data for the switchgear - rated current, rated voltage, rated short-time current See the section Plant design, p. ? 22.

Step 7: Determination of the design of outgoing units See the section Switching devices and components installed in LV ASSEMBLIES, p. ? 15.

Data for the feeding transformer and the sum of the load currents, taking into consideration the diversity factor (see the section Diversity factor, p. ? 24), form the basis for the rated data of the LV ASSEMBLY. Future extensions and any requirements regarding maximum temperature rise should also be considered.

Alternative 1 Fixed groups Alternative 2 Removable groups Alternative 3 Withdrawable groups Step 8: Determination of utilisation category See the sections Contactors, p. ? 31, Motor starters, p. ? 36 and Selection of short-circuit protection device (SCPD), p. ? 41.

Step 3: Determination of system earthing See the section Types of distribution system, p. ? 25.

Step 9: Determination of power factor control, where necessary See the section Power factor control, p. ? 4.

earthed • Directly directly earthed • Not 3-wire • Three-phase Three-phase 4-wire • Step 4: Determination of degree of protection, IP Code and environmental adaptation See the section Service conditions, p. ? 8.

Step 10: Control of power losses Resistances in conductors, connections, apparatus (coils, for example) and resistors give rise to power losses and temperature rise.

normal design Jag • IPIP 21, 41, protection • againstextra direct contact with live parts 54, waterproof design • IPAnother • adaptationIP Code and environmental

A number of guidelines If the diversity factor is in accordance with the standard values, see under the section Diversity factor, p. ? 24 (see also Step 2 above), there is usually no need for special consideration.

•

52

5.2

52

97-03-05, 13.30

INDUSTRIAL POWER SYSTEM Low voltage distribution the diversity factor is higher than the • Ifstandard values, the LV ASSEMBLY

• • • •

ence and submits a preliminary tender to the user - the client. The latter makes his own adjustments and thereafter a final tender can be arrived at. Clients and consultants having PCs can be given access to the program.

should be fitted with a fewer number of ”separated” groups (the space in the cubicle thus not fully utilised). The rated currents for groups and for the equipped cubicle could also be increased in relation to the operational currents. For enclosure class IP 21the power losses of the horizontal busbars need not be considered. For enhanced enclosure class IP 41 the rated current must be reduced by 10-30 %. For enclosure class IP 54 the rated current is reduced by 30 % for horizontal busbars. When sizing the ventilation system for the operating room, a power loss of 1 kW for a normally equipped cubicle can be used as a guideline.

Information to be provided on an LV ASSEMBLY Documentation (5 in IEC 439-1) The documentation constitutes an important feature of an LV ASSEMBLY delivery. It can be divided up into the following groups: 1. Catalogues, pamphlets, brochures and other printed sales matter 2. Test reports 3. Instructions on installation and maintenance 4. Customer documentation for the delivery in question

Planning tools (p. ? 350) Large manufacturers of LV ASSEMBLIES all have some type of computerised system for project planning and producing tenders and documentation. The following are usual features:

1. Catalogues, pamphlets, brochures and other printed sales matter A catalogue supplies technical information on data, dimensions, functions, standards, etc. Pamphlets, brochures and other sales matter are intended to stimulate interest. They may contain a certain amount of technical data but the information in the catalogue is what counts when planning.

a questionnaire • Preparing tender, designing documenta• Preliminary tion for buildings changes • Making tender • Final • Ordering • Documentation input data for the creation of manu• Providing facturing documents. The complete switchgear and controlgear standard is stored in the system, including all standardised units, enclosures, accessories, etc. Input particulars include primary data for motors, loads and feeding transformer. The system creates a ”proposal” for a complete LV ASSEMBLY, including price and the following documents:

2. Test reports A type test report is required. The tests can be performed in the manufacturer´s own laboratory, with or without witnesses, or in another independent test facility. See the section Test specifications, p. ? 17.

diagram • Single-line and floor layout, drilling plans • Front diagrams • Circuit of name-plates • List • List of apparatus

3. Instructions and manuals There are a number of product-related instructions that concern mounting, assembly,

The proposal is scrutinised by the planner, who then makes adjustments based on his experi53

5.2

53

97-03-05, 13.31

INDUSTRIAL POWER SYSTEM Low voltage distribution service and maintenance, etc. Instructions often form part of complete customer documentation, together with catalogues and order—related documentation, e.g. circuit diagrams, lists of apparatus, etc.

assembly drawing (with dimensions • General for the LV ASSEMBLY, cubicles, units, etc.) layout • Front • List of apparatus Possible additional inclusions: tables (cubicle level) • Wiring of name-plates • List plans • Erection Instructions and manuals for installation • and maintenance

4. Customer documentation Customer documentation normally contains: diagrams • Circuit diagrams (and/or overview • Single-line diagrams)

54

5.2

54

97-03-05, 13.31

INDUSTRIAL POWER SYSTEM Low voltage distribution

Bibliography Applicable IEC standards As mentioned in the introductory section, p. ? 2, the International Standard Publication IEC 4391, Low-voltage switchgear and controlgear assemblies, Part 1: Requirements for TypeTested (TTA) and Partially Type-Tested (PTTA) Assemblies is the main standard for LV ASSEMBLIES. The object of the standard is to establish requirements. The standard has eight main divisions: 1. 2. 3. 4. 5. 6. 7. 8.

General, 1.1: Scope and object, 1.2: Normative references (=applicable IEC standards, see list below) Definitions Classification of ASSEMBLIES Electrical characteristics of ASSEMBLIES (rated voltage, rated current,etc.) Information to be provided on the ASSEMBLY Service conditions Design and construction Test specifications

List of applicable IEC standards Name Coding of indicating devices and actuators by colours and supplementary means

Number IEC 73: 1991

Degree of protection provided by enclosures (IP Code) IEC 529: 1989 Electrical installations of buildings

IEC 364

Assessment of general characteristics.

IEC 3643:1977

Protection for safety; Protection against electric shock.

IEC 364-4-41

Choice of protective measures against electric shock in relation to external influences.

IEC 364-4-481

Selection and erection of electrical equipment; Earthing arrangements and protective conductors. Graphical symbols for use on equipment. Index, survey and compilation of the single sheets.

IEC 417: 1973

High-voltage test techniques

IEC 60

IEC standard voltages

IEC 38: 1983

IEC 364-5-54

Identification of equipment terminals and of terminations of certain designated conductors, including general rules for an alphanumeric system. IEC 445: 1988 Identification of conductors by colours or numerals.

IEC 446: 1989

Insulation co-ordination, Part 1: Terms, definitions, principles and rules.

IEC 71-1: 1976

Insulation co-ordination for equipment within low-voltage systems, Part 1: Basic principles and requirements.

IEC 664-1: 1992

55

5.2

55

97-03-05, 13.31

INDUSTRIAL POWER SYSTEM Low voltage distribution International Electrotechnical Vocabulary (IEV)

IEC 50

Switchgear, controlgear and fuses

IEC 50 (441): 1984

Insulators

IEC 50 (471): 1984

Generation, transmission and distribution of electricity, Operation

IEC 50 (604): 1984

Item designation in electrotechnology.

IEC 750: 1983

Low-voltage fuses

IEC 269

Low-voltage controlgear; Semiconductor contactors (solid state contactors)

IEC 158-2: 1982

Low-voltage switchgear and controlgear

IEC 947

General rules

IEC 947-1: 1988

Circuit breakers

IEC 947-2: 1989

Switches, disconnectors, switch-disconnectors and fuse-combination units.

IEC 947-3: 1990

Contactors and motor starters; Electromechanical contactors and motor starters.

IEC 947-4-1: 1990

Method for determining the comparative and the proof-tracking indices of solid insulating materials under moist conditions.

IEC 112: 1979

Method of temperature-rise assessment by extrapolation for partially type-tested assemblies (PTTA) of low-voltage switchgear and controlgear.

EC 890: 1987

Miniature Circuit Breakers, MCBs; Circuit breakers for overcurrent protection for household and similar installations. (Amendment No. 1-3): 1990

IEC 898

Semiconductor convertors; Semiconductor self-commutated convertors

IEC 146-2: 1974

Standard directions of movement for actuators which control the operation of electrical apparatus.

IEC 447: 1974

Surge arresters; Non-linear resistor-type gapped surge arresters for AC systems.

IEC 99-1: 1991

Thermal electrical relays, Overload relays

IEC 255-8: 1990

Literature: ”Corrosion of electronics”, A Handbook based on Experiences from a Nordic Research Project Authors: Jan Henriksen, Risto Hienonen, Torbjörn Imrell, Christofer Leygraf, Lena Sjögren

Bulletin No. 102 Swedish Corrosion Institute, August 1991

56

5.2

56

97-03-05, 13.31

INDUSTRIAL POWER SYSTEM Medium Voltage Distribution

MEDIUM VOLTAGE DISTRIBUTION Contents page 1 Design

57

2 Switchgear

58

3 Equipment items

61

4 Charactristics of various circuit breakers

63

5 Measuring transformers and relay protection

65

6 Condition monitoring

66

Design switchgear in such cases. Moisture is also a serious problem in switchgear. Condensation on insulation surfaces, particularly in combination with dust, nearly always leads to flashover and consequent damage. Cubicles in operation usually develop sufficient heat to prevent condensation, but there is great risk of condensation in cubicles not operating (standby cubicles) if air humidity is high and the temperature falls. In cases where there is risk of condensation, each cubicle should be provided with a heating element, the switching on and off of which needs to be controlled by a hydrometer if condensation is to be prevented. A simpler but completely reliable alternative control system for the heating element is to arrange for it to be disconnected when the connection equipment is in operation, so that when the latter switches off, the heat element comes on automatically.

Medium-voltage switchgear for industry is nearly always of in-house design. Under IEC 298, temperatures in switchgear rooms may range from -5 to +35°C (+40°C peak value) and the air in them may have a maximum 24hour average moisture content of 95% and must be free from pollutants such as dust, smoke, corrosive or explosive gases, etc. Environmental aspects Environmental problems are liable to arise in many industries, e.g. the dust often released into the atmosphere by the cement industry, smelters, etc. Switchgear rooms have to be ventilated in such a way as to prevent dust making its way in and affecting the operation of switchgear. It is, of course, important that the filters that have in many cases to be used in ventilation ducts be cleaned at regular intervals. Other types of environmental problems arise where there are air pollutants that have chemical effects on switchgear. Sulphur, for example, is capable of reacting with copper and silver to form sulphides. A coating of sulphur on contact surfaces may lead to increased resistances, higher temperatures and eventually to overheating and damage. Industries liable to create explosive atmospheres have to avoid using certain kinds of equipment capable of generating even tiny sparks (auxiliary contacts, motors, microswitches, etc.). Very severe requirements therefore have to be imposed for

Location Switchgear in industry may be housed in separate buildings but is often located in special operating rooms within industrial buildings. As there are so many different kinds of industry, the range of possible environments is far too great to be able to give any general indications about the design of industrial switchgear. Where it is in special operating rooms, however, there are a number of points that have to be catered for, such as pressure relief and personnel evacuation. 57

5.3

57

97-03-05, 13.31

INDUSTRIAL POWER SYSTEM Medium Voltage Distribution for calculating these pressure stresses are available from ABB. Low ceilings may sometimes make it impossible for gases due to arcing to be discharged upwards sufficiently quickly, in which case it may even be necessary to reduce the shortcircuit current in order to cope with potential pressure increases caused by arcing.

Operating rooms, construction requirements Pressure relief Switchgear should be arc-tested to IEC 298 standard. Arcing results in large amounts of gas that have to be discharged to prevent serious pressure damage to the switchgear. This gas has to be removed from the operating area and led out into the open air to prevent anyone being harmed by its toxic components. The pressure created by an arcing fault may amount to several tonnes per square metre and can be calculated approximately by means of the empirical formula P=

Escape routes In principle, no-one should ever be more than 5 m from an exit. This means that if the switchgear room is longer than 5 m there has to be an exit at each end. However, if the switchgear meets the arcing requirements and the tripping time does not exceed 0.15 s (e.g. by means of arcing monitors), it is possible to have an exit at only one end of switchgear rooms up to 10 m long. The width of the operating area should be such that there is always a clear escape route at least 0.5 m wide even when cubicle doors are open. Immediately outside the doors of switchgear rooms there must be a level area of at least 2 x 1.5 m in the same plane as the switchgear floor. If the exit is directly onto open ground, there may be a certain difference in height, but preferably not more than 250 mm. These requirements relate, of course, to the need for it to be possible to evacuate the room and look after injured persons immediately after an accident. Switchgear housed in industrial buildings may sometimes cause problems. The switchgear room may be well below ground level or high up in a building. In new buildings, this matter should always be resolved at the design stage, but where switchgear has to be installed in existing buildings, evacuation requirements often cause problems. The running of pipes, cables, etc., through switchgear rooms should be avoided.

2xNxUxIxt V

where P = positive pressure in atmg N = number of arcs U =arc voltage in kV I = arc current in kA t = time in seconds before cubicle pressure relief vents open V = volume in m3 The following criteria are used for evaluating an arcing test: doors, enclosure parts, etc., that are closed • Do and locked in the intended manner remain

• • • • •

closed after the test? Do parts of the metalclad switchgear capable of harming persons become loose? Have holes been caused in freely accessible external parts of the enclosure by burnthrough? Do cotton indicators positioned vertically 30 cm in front of the switchgear ignite (disregarding those ignited by burning paint or flakes)? Do cotton indicators positioned horizontally over the operating area ignite (same parenthesis)? Do the earth current paths remain operational after the test?

Switchgear General, standards All medium-voltage switchgear is governed by international standard IEC 298 and the voltage levels mostly used for industrial applications are 7.2 and 12 kV, which means test voltages of 60 and 75 kV peak voltage and 20 and 28 kV AC voltage, respectively. Short-circuit currents and operating currents

In industry it is not unusual for switchgear to be situated far from any outside wall, which means that the pressure relief ducts may be very long, requiring verification that pressure stresses in the switchgear do not exceed the values obtained in type tests. Special computer programs 58

5.3

58

97-03-05, 13.32

INDUSTRIAL POWER SYSTEM Medium Voltage Distribution are very high, partly because of the rather low voltages used, but mainly because most industries have large power requirements. Operating currents of up to about 4000 A are not unusual, which corresponds to the data of ABB’s Safesix switchgear.

Such arrangements provide the continuous possibility of supply via another path if a fault occurs anywhere. Double busbars may be placed side by side in the same cubicle, or in what is known as a duplex arrangement, i.e., two rows of cubicles (see Fig. 48), or in a single row of cubicles with sectioning switches and different incoming supplies (see Fig. 49).

Composition One of the most characteristic features of industrial switchgear is the requirement for high operational availability, particularly in the case of process industries. Industrial switchgear is therefore often designed so that there are alternative incoming supplies. This involves various kinds of double busbar arrangements (see Fig. 46).

Fig. 48 Safesix in duplex form Systems with double busbars in the same cubicle or variants of such systems are now in decline. Having two busbar systems in the same cubicle increases not only the risk of both systems being affected by faults but also the risks to personnel who have to work on one system while the other is energised. Duplex systems have therefore become more usual, despite their somewhat higher cost. The safest is the duplex front-to-front, which involves two rows of cubicles installed facing one another (see Fig. 48). This means that the two busbar systems are several metres apart and a fault on one side will not affect the other. Anyone working on one busbar is also a safe distance from the other. Front-to-front arrangements are also relatively economical on space, since they require only one operating area. Duplex back-to-back arrangements involve two operating areas and a consequently larger switchgear room. The fact that back-to-back arrangements make it impossible to see the A and B sides at the same time also increases somewhat the risk of incorrect action. In some situations it may be more economical to place all the cubicles in one row with sectioning switches between them, in which case each busbar has its own incoming supply and it is possible, for example, to use two incoming supplies to cater for a number of

Fig. 46 Double busbars

Strömbegränsande

Ej strömbegränsande

I kA

I kA

50

50

20

20

5 ms Fig. 47

13 ms 59

5.3

59

97-03-05, 13.32

INDUSTRIAL POWER SYSTEM Medium Voltage Distribution cial measures such as painting of busbars may also result in increased radiation, allowing higher currents without exceeding standard temperature values. If rectangular busbars are used, every possible endeavour should be made to position them edgewise, as this provides optimum cooling.

different supply possibilities (see Fig. 49). Certain industries also prefer to have each busbar system in a different room to further reduce the risk of loss of supply. Cooling High currents are usual in industrial applications, which means that particular attention usually needs to be paid to cooling. IEC 298 permits temperature rises to a maximum of 50°C for bolted connections between bare copper busbars in air. The temperature permitted in the case of silver-coated contact surfaces is 75°C. The permitted temperature rise for plugin contacts is 65°C for silvered copper contacts and 35°C for unsilvered contacts. The current that a given busbar system can carry depends partly on the materials and surface treatment of joints and contacts and partly on what cooling can be provided. Forced cooling by fans means that higher currents can be used, but this does, of course, depend on the fan system always functioning. It is therefore usual in most cases to try to manage with natural ventilation. Properly thought-out design can achieve automatic air circulation and consequently good cooling even without fans. Spe-

Cubicle types There is, in principle, an infinite variety of switchgear for industrial applications. A small engineering establishment may have switchgear consisting of a few individual cubicles of very simple design with one incoming supply breaker and a few outgoing load isolating cubicles (Fig. 50). At the other end of the scale, switchgear for oil platforms, nuclear power stations, paper mills or rolling mills sometimes involves very complicated equipment incorporating isolating cubicles, load isolating cubicles, metering cubicles, breaker cubicles, contactor cubicles, etc. IEC 298 divides metal-enclosed indoor switchgear into three main categories: switchgear • Metalclad switchgear • Compartmented Cubicle switchgear •

Fig. 49 Double incoming supply with sectioning breakers. 60

5.3

60

97-03-05, 13.32

INDUSTRIAL POWER SYSTEM Medium Voltage Distribution can be provided with capacitive voltage indicators built into the insulators of the earthing switch, in which case the cubicle front incorporates a warning device to show whether the phases are live or not.

Metalclad switchgear has its components in separate cells with intermediate walls made of metal (see Fig. 51).

Equipment items General, various operating situations All kinds of operating situations occur in industry. Power supply may be from subordinate transformers within the industrial area in cases where the switching on or off of power is only required extremely rarely. Another possibility is the connection and disconnection of a bank of capacitors for controlling reactive power offtake, with switching taking place several times a day. Different equipment is also used depending on whether operation is to be continuous with stringent requirements for uninterruptible power supply or whether interruptions can be tolerated without unduly expensive consequences. The appropriateness of each piece of equipment depends on the type of load. Achieving an optimum solution requires a clear picture of the types of currents and voltages that will occur, how often switching will take place, what interruptions of operation are acceptable, etc. Only on the basis of this kind of information is it possible to decide on the most suitable equipment.

Fig. 51 Safesix type metalclad switchgear. Compartmented switchgear also has all its components in separate cells but now the intermediate walls are made of non-metallic materials. Cubicle switchgear is any switchgear which is not classifiable as metalclad or compartmented, e.g. it may have the breaker and the cables in the same space. Personnel safety is, of course, at a considerably lower level, since cubicle switchgear has no automatic protection against energised cables harming anyone drawing out the breakers.

Disconnectors A disconnector may be a separate piece of equipment or simply a mechanism for connecting/disconnerting, for example, a cirquit breaker. A disconnector has neither breaking nor making capacity and may only be operated when there is no current in the conducting path, so it always has to be interlocked so as to be only operable in the de-energised state. It also has to withstand not only its rated current continuously without dangerous temperature rise but also a short-circuit current characterised by the impulse current (peak value at the beginning) and short-time current (r.m.s. value during short circuits, usually 1 second). A disconnector involves more stringent requirements for insulation across open separation points than the separation point of a breaker does. In principle, flashovers should be to earth

Earthing for work Some kind of integrated earthing for work is a common feature of all three types of cubicle. The safest kind is an earthing switch with making capacity for the full short-circuit current for the earthing of cables and, in most cases, the busbar as well. The earthing switch should always be interlocked so that it cannot be operated against voltage. Mechanical interlocking is the first choice but is not always possible for earthing switches in incoming supply cubicles. If energisation checks are desired before earthing, ABB’s Safesix type switchgear 61

5.3

61

97-03-05, 13.32

INDUSTRIAL POWER SYSTEM Medium Voltage Distribution The mechanical life of the motorised spring operating devices incorporated in all circuit breakers is limited to 10,000-20,000 switching operations. This means, for example, that the service life of a drive that has to be switched three times per hour on a two-shift basis will be only 1 to 2 years, which is not a particularly economical solution. The connecting equipment for this kind of drive should not be a circuit breaker but a highvoltage contactor, which is mainly available in vacuum or SF6 versions. Vacuum contactors usually have longer service life (several million switching operations) but have the disadvantage that in certain positions they are liable to generate overvoltages that may, for example, damage motors that lack suitable protection. Contactors are available for 3.6, 7.2 and 12 kV, operating currents are usually 400 or 630 A and breaking currents between 5 and 10 kA. As their breaking capacity is limited, they are often provided with built-in fuses as protection against short-circuit currents. It is also possible to use relay protection to block the contactor and a circuit breaker to take care of short-circuit currents. Contactors thus have the great advantage of very long mechanical life, partly because the movable contact is operated by means of a solenoid and not by a spring operating device. The result is an operating device with few moving parts that copes with a very large number of switching operations (over a million).

rather than across the separation distance. A separation distance should preferably also be visible, unlike a breaking distance. If the contact position indication is satisfactory, however, there is no need for the separation distance to be visible. Switch disconnectors Switch disconnectors are used for disconnection and connection of operating currents. They can be switched against a specified short-circuit current and can also carry the short-circuit current for a certain time in the closed position. They can break the load current, i.e., their rated current, usually 400 or 630 A, but cannot break short-circuit currents. The short-circuit protection used in some cases consists of builtin fuses that break short-circuit currents. To avoid that just one phase is broken by a singlephase fault, these fuses are provided with a striker pin that trips the opening mechanism of the switch disconnector. Switch disconnectors are usually operated by means of a spring operating device tensioned manually or by motor. Fuses are often used as short-circuit protection in conjunction with load disconnectors and contactors. Each fuse consists of two contacts associated with one or more fusible conductors made of silver or copper. These fusible conductors are surrounded by an extinguishing medium consisting of quartz sand of suitable grain size. The fusible conductors and the extinguishing medium are enclosed in an insulating cartridge body. To ensure controlled melting of the fusible conductor, it is usually provided with weak points in the form of punched holes. Each fuse may be provided with a warning device. The cartridges are fitted to load disconnectors or contactors. When a high fault current passes through the fuse, the fusible conductor becomes hot and melts. A fuse creates a fault current limit that may, for example, be useful in the dimensioning of cables.

Circuit breakers Circuit breakers are the most common type of switch in industry. They can make and break the full short-circuit current and are provided with a motorised spring operating device. They may have an operating sequence known as rapid auto reclosing which is particularly important in industries in which periods of interruption are very expensive. This means that a temporary fault will make the breaker switch off but then immediately switch back on. If the fault has by then disappeared, the process will continue as usual, but if the fault persists, the breaker switches off again. IEC 56 defines various test cycles for breakers, but the one most commonly used is 0-0,35CO-3 min-CO. A point to be particularly noted for industrial

Contactors Operating situations sometimes occur in industry in which operating currents at mediumvoltage level have to be connected and disconnected several times an hour, e.g. for the switching of motors, capacitor banks, furnaces etc. 62

5.3

62

97-03-05, 13.33

INDUSTRIAL POWER SYSTEM Medium Voltage Distribution applications is that switching operations may be very frequent, as in the case of breakers for capacitor banks, certain motor applications, arc furnaces, etc. Breaker selection therefore involves looking not only at optimum service life but also at servicing and maintenance costs. The various types of breaker available each have their advantages and disadvantages. When assessing possible breakers, it is therefore important to consider all parameters. The breakers most commonly used are of the vacuum and SF6 types. A great many minimum-oil breakers, magnetic air breakers and compressed air breakers are also still operating but are no longer used in new installations except in very special cases, e.g. for generators.

The opening pulse makes the cylinder move downwards, the upper normal current contacts open and the current is commutated to the arcing contacts. At the same time, the gas in the cylinder is compressed. As the opening operation continues, the arc is drawn out between the arcing contacts. The compression of the gas in the puffer is increased by the heat of the are itself. At current zero the compressed, cool, gas mixes with the hot gas in the former are region, preventing reignition and completing the interruption. SF6 breakers without mechanical compression, known as self-blasters, are also available. (ABB HA)

Characteristics of various circuit breakers

Vacuum breakers There are several differences between vacuum and other types of breaker. One is that vacuum breakers contain from the outset no gas that could be conducting. The parting of the metal contacts during breaking causes an arc that burns in metal vapour from the contacts. The hot base points of the arc give off sufficient vapour for the arc to persist even when the contacts are rather far apart. When the current passes through zero, vapour ceases momentarily to be given off and existing vapour condenses on available surfaces. If vapour development is not resumed, the breaking operation is complete.

Industry involves many different operating situations for which different kinds of breaker are more or less suitable, depending on the types of current and voltage that occur. Breaking short-circuit currents The breaking of short-circuit currents subjects breakers to great stresses, both mechanical (electrical forces, pressure development in poles) and thermal. As breakers are put through comprehensive type tests involving a variety of short-circuit currents, these stresses are normally no great problem. Where an installation incorporates a number of parallel turbogenerators with switchgear installed very close to them, as on oil platforms, it is possible for there to be a very high DC component at the beginning of a fault. As all breakers need a zero

SF6 breakers using the puffer principlec (ABB’s type HPA breaker, see Fig. 52) When the breaker is in the closed position, current flows via the normal current contacts.

Fig. 52 SF6 breaker using the puffer principle, ABB type HPA. 63

5.3

63

97-03-05, 13.33

INDUSTRIAL POWER SYSTEM Medium Voltage Distribution overvoltages may be generated where there are small leakage capacitances (e.g. very short cable between breaker and load), but such overvoltages are fairly low-frequency (a few Hz) and can be taken care of by surge diverters, if installed. Considerably more serious is another phenomenon, known as voltage escalation, which only occurs with vacuum breakers. The “problem” is that they are too good at breaking highfrequency currents, so if they restrike (which happens when the contacts do not part sufficiently the first time the current passes through zero), the high-frequency current is broken after restriking at the first zero crossing. The voltage rises between the contacts, a further restrike occurs, and so on. Hundreds of restrikes of constantly increasing amplitude over a few milliseconds have sometimes been observed. Depending on the characteristics of the circuit connected, this may result in very high overvoltages, but above all in very large and steep voltage jumps distributed unevenly over the load. If the latter is, for example, a motor, 90% of the sharp voltage rise takes place across the first winding turn and destroys its insulation. In such cases, vacuum breakers have to be fitted with special forms of protection, like RC-links.

crossing to enable them to break, the associated arcing times may be long and result in excessive stresses. Connecting capacitive currents Capacitive currents (no-load line, cable or capacitor bank) have to be broken without restriking and this is usually coped with by all modern breakers. Connecting individual capacitor banks is likewise no problem, although the connecting current peaks during the connection of parallel banks may sometimes cause problems. Minimum-oil breakers are more sensitive in this respect than SF6 and vacuum breakers, so damping reactors are often used in conjunction with them. Certain breakers, e.g. type HPA, are available with synchronised closing, which largely eliminates the connecting current and the distortions it might cause to the mains supply. Breaking small inductive currents Industries often use high-voltage motors of various sizes. This frequently creates a need to break small inductive currents, which may involve special problems. All breakers have a tendency to “chop” the current off before the natural zero crossing. If this happens during the disconnection of an inductive load, the energy that is in the inductor at the time is released and then oscillates between the parallel capacitance and the inductance of the load. This may result in high overvoltages. The high chopping level (2030 A) of compressed air breakers has caused many problems. The chopping level of minimum-oil and SF6 breakers depends on the parallel capacitance. This means that a higher capacitance results in higher chopping levels but also more “spare” for the energy released, which means that the overvoltage level becomes largely independent of the capacitance and depends only on the inductance. Minimum-oil and SF6 breakers are approximately similar in this respect, with chopping levels of 2-3 A. These “chopping overvoltages” are fairly low-frequency, so they represent no great danger to motors, etc. The chopping level of vacuum breakers, which is 2-5 A with the most commonly used contact materials, is rather independent of the parallel capacitance. This means that high

Electrical endurance Some industrial applications involve a high breaking frequency, i.e., many switching operations per day, which calls for breakers that provide the installation concerned with optimum economy. Generally speaking, vacuum breakers may be said to have the longest electrical life, with up to about 100 full short-circuit interruptions. SF6 breakers have the second longest life in this respect and minimum-oil breakers come third. The number of rated current breaks (load breaks) performed by breakers is considerably higher, often up to about their mechanical life, 10,000-20,000 connections in the case of vacuum and SF6 breakers. Operating situations where breaking has to take place several times an hour are not suitable for circuit breakers but require the use of contactors. Maintenance The various types of breakers differ radically in their maintenance requirements, as illustrated 64

5.3

64

97-03-05, 13.33

INDUSTRIAL POWER SYSTEM Medium Voltage Distribution moulded case insulation. They are usually installed in such a way as to act also as supporting insulators for busbars and contacts. These current transformers are designed according to IEC 185 and may be provided with various types of core. Measuring cores have great accuracy at normal operating current but low saturation limits. Relay cores have to register fault currents and therefore have a high saturation limit but relatively low accuracy. A current transformer may contain several cores, usually two or three. The following considerations apply when selecting current transformers:

by the following description of major overhauls after a large number of operations: Minimum-oil breakers: Overhauls involving the replacement of oil, contacts and possibly some breaking chamber parts have to take place more frequently than in the case of other types of breaker, but they are very easy to carry out, can be undertaken by the user’s personnel in a normal room and should take not more than 4-5 hours after simple training. An overhaul interval of 6-10 years may normally be adopted. Vaccum breakers and SF, breakers need a minimum of maintenance. The electrical endurance is very high. In case of mal function the poles have to be exchanged. The mediavisions are normally to be inspected every 5000 operations. SF6 gas may be refilled in use of a pressare alarm.

•

•

1. At least the same insulation level is required as for the rest of the switchgear. 2. The relevant rated current has to be adopted. For certain types of current transformer there is the possibility of changing the rated current by primary changeover, e.g. when the load increases.

From the operational availability point of view, continuous monitoring of the breaking medium is very important. No breaker can break particularly high currents if the breaking medium is replaced by air, although it should be noted that an SF6 breaker of the buffer type can break the rated current and small short-circuit currents even when the positive pressure has completely disappeared, provided that the poles mainly contain SF6 gas. The maintenance costs of SF6 and vacuum breakers are generally regarded as lower, which compensates for their usually higher purchase price as compared with minimum-oil breakers.

3. The short-circuit current, i.e., the maximum current the transformer can cope with for 1 second, also has to be tested at peak current, which causes mechanical stresses. The peak current is normally 2.5 times the short-circuit current. Caution is required if primary changeover of the current transformer is possible, as series connection results in lower short-circuit strength than parallel connection of the primary windings. 4. The rated burden is stated in VA (voltamperes) and represents the load which the secondary side of the transformer can cope with. It may be for driving an ammeter or a relay but it is also necessary to allow for line losses, particularly if the relays are situated in a separate control room.

Measuring transformers and relay protection An important aspect of medium-voltage switchgear is the various kinds of measuring transformers used for obtaining necessary information about currents and voltages, not only for charging purposes but also for detecting abnormal states. Measuring transformers convert currents and voltages from very high values (kA, kV) to values that can be used for driving relay protection, meters, etc. (V and A).

5. The overcurrent factor (ALF, accuracy limit factor) denotes the overcurrent as a multiple of the primary rated current up to which accuracy is maintained. Voltage transformers (IEC 186) Voltage transformers for medium-voltage switchgear are always of the magnetic type and insulated with cast resin. Single-phase versions are normally used, although two-phase are also available. As in the case of current transform-

Current transformers (IEC 185) The current transformers used in medium-voltage switchgear are mainly of the type with 65

5.3

65

97-03-05, 13.33

INDUSTRIAL POWER SYSTEM Medium Voltage Distribution

Condition monitoring

ers, there are different accuracy classes depending on whether the transformer is to be used for measurement or for relay protection. Here again the choice of transformer is based on insulation level, rated burden, accuracy, etc. See also the chapter on measuring transformers in ABB’s Electric Power Manual, pages 219-230.

One of the essentials for good operational availability is to always have good knowledge of the state of the installation. The possibility of continuously monitoring temperature changes on contacts and busbars provides a way to avoid ever being exposed to incidents due to overheating. Being able also to check continuously that the content of the breaking poles is correct means that there is no need to risk incidents due to leakage. Condition monitoring is thus an important factor in industrial applications, many of which involve very high standstill costs. It may be used not only to reduce the risk of unscheduled downtime, but also to achieve better planning of maintenance work and hence lower maintenance costs. ABB offers the Safe Guard condition monitoring system for the switchgear and the circuit breakers.

Relay protection There has been discussion over the years as to whether relay protection should be situated in the switchgear or in a separate relay cubicle. Its location used to be partly based on how safe to personnel the switchgear was considered, but most switchgear is now regarded as so safe that relay protection can normally be installed in switchgear cubicles. The fact that relay protection for busbar faults has been speeded up has also reduced the risk of harm arising from faults. Efficiency test It is always necessary to perform an efficiency test by using the measuring transformer to check that the earth fault protection actually works at the intended primary value. This is particularly important in cases involving what is known as summated current connection whereby three single-phase transformers are connected to one neutral and the relay protection is provided with power supply. Earth fault protection usually takes the form of cable current transformers. Optimum protection is achieved by placing them round the outgoing cable. Their cores are normally made of silicon alloy sheet metal, although in some cases this may be replaced by mumetal, which has very little phase angle error. Cable current transformers are normally openable, i.e., the core can be opened to make it easy to fit round a cable, although this procedure cannot be applied to transformers that have mumetal cores, which lose their characteristics when they are divided.

Breaking media of circuit breaker For vacuum breakers there is at present no method for continuous monitoring of negative pressure during normal operation, although SF6 breakers are easy to monitor by means of pressure monitors that give an alarm if the pressure in the poles falls below a certain level. Operating devices The Safe Guard system has an optical measuring arrangement for the circuit breaker to measure the contact velocity during each closing and opening operation and the closing and opening times. Any unduly high or low value trigger alarms. The mechanical system is thus monitored and any imminent fault may also be detected, since data from up to 32 events can be stored. Contact wear The breaker monitor also records the wear from each breaking operation. A large number of experiments have recorded electrical life as a function of the breaking current. The breaker monitor then measures the level of the breaking current and generates the value that corresponds to the inverse of the number of possible breaks. This value shows the amount of wear on arcing contacts and other wearing parts of the breaking poles. An alarm may then be set to operate

Switchgear relay boxes have to include space for terminal blocks, meters, switches, warning lamps, relays etc. In cases where there is considerable integrated relay protection, this sometimes has to be moved to a control room because of insufficient space. 66

5.3

66

97-03-05, 13.34

INDUSTRIAL POWER SYSTEM Medium Voltage Distribution at a certain level, e.g. at 80% of their electrical life. This makes it possible to achieve optimum utilisation of the electrical life of the breaker and to plan maintenance work in good time, resulting in more efficient use of the installation.

in between. As thermovision measurement necessarily involves visual contact with high voltage, personnel safety is not best served by this method. Temperature indicators are affixed at exposed points and can be read from the cubicle front. Their colour shows the temperature, which can therefore be checked on each visit to the switchgear room. None of these methods provides continuous temperature monitoring, nor do they trigger alarms to signal incipient overheating. ABB has therefore developed a new monitoring system whereby the temperature on highvoltage equipment can be continuously monitored and recorded (see Fig. 53). This makes it possible not only to prevent unscheduled stoppages due to overheating but also to use the installation more efficiently by controlling power offtake due to temporary overloads.

Temperature monitoring Switchgear overheating is a universally known possibility, perhaps particularly in industries that release corrosive gases into the atmosphere. Here again it is of course extremely important to be able to detect abnormal temperature rises in good time so as to be able to deal with faults before they become incidents. The two methods so far developed for indicating temperatures on high-voltage equipment are thermovision measurement and temperature indicators that change colour. Thermovision measures the temperature of a busbar at the exact time of measurement. The next measurement may perhaps be a year later and nothing may be known about what happens Fig. 53 Temperature monitoring system. = measuring points

•

67

5.3

67

97-03-05, 13.34

INDUSTRIAL POWER SYSTEM Transformers

TRANSFORMERS Contents page 1 Standard specification

68

2 Three-phase connections and nation of phase displacement

69

3 Short-cicuit impedance and voltage drop

71

4 Parallel connecition of transformers

73

5 Agering considerations and emergency service

74

6 Inrush current

75

7 Capitalisation of losses

75

8 Installation considerations

76

Standard specifications a desiccant breather or with a membrane for prevention of contact with the atmosphere. Another system, in American practice, has a cushion or ”blanket” of nitrogen gas above the oil in a hermetically sealed or controlled-pressure tank. There is also a type of distribution transformer of European design with corrugated tanks (cooling fins), completely filled with oil and hermetically sealed. The corrugations take up the volume expansion elastically (although one must be alert to possible overloading capability). Liquid-immersed transformers with nonflammable liquid (earlier: chlorinated biphenyls - many trade-names exist) were marketed aggressively in the past, but have been banned nowadays for environmental reasons. Alternative liquids have been proposed, but have had little market penetration as yet.

Some general terms and concepts A power transformer is a transformer intended for transmission of power - at least 5 kVA three-phase or 1 kVA single-phase. Such transformers are specified and tested according to Power Transformer Standards. There are other standards for smaller transformers, for certain special transformers for protection and safety, and for instrument transformers (measuring transformers). A distribution transformer is the last link in the chain of step-down transformers. It steps down from local distribution high voltage (6, 10, 20 kV) to consumer low voltage (not above 1 kV). According to European tradition, distribution transformers are three-phase units. In the American tradition, single-phase distribution transformers are common. The usual range of rated power is from 30100 kVA to 1 000-2 000 kVA. Standards have set a limit at 3150 kVA. Distribution transformers are either oil-insulated or of dry type.

Dry-type transformers are generally available in the distribution transformer range of voltage and power rating. Two different practices are followed: bare windings with air ducts for insulation, and encapsulated (resin-cast) windings. The types differ mainly in terms of immunity against moisture and pollution.

Oil-immersed and dry-type transformers. Large power transformers are oil-immersed and provided with some kind of oil preservation system. The more common system employs a ”conservator” (expansion vessel) with

Items to be specified. Power transformer standards contain lists of items - in addition to the 68

5.4

68

97-03-05, 13.35

INDUSTRIAL POWER SYSTEM Transformers low voltage winding is to carry the taps. In the European tradition, the taps and the on-load tap-changer are usually fitted on the high voltage side. In America it has traditionally been the other way round. It may be that the untapped winding faces the system having the widest variation of service voltage, and the tapped winding runs with nearly constant voltage. This means that the volts-perturn will vary when tappings are changed (variable flux regulation). The latest IEC Standard, Publ. 76-1 (199303) and national standards in conformity therewith prescribe that the notation of tapping range is to be related to the variation of turns in the tapped winding, but it is also recommended that a full table should be provided on the nameplate, indicating for every numbered tapping the maximum service voltages on both the tapped and the untapped winding. This table also deals with optional truncation of ultra high voltages or extremely high currents towards the ends of the tapping range. This is elaborated in Chapters 5 ??? and 7 ???, and Annex B of IEC Publ. 76-1.

rated electrical quantities - that have to be specified for a customised transformer - such as cooling system and oil preservation system (self-cooling, or forced cooling with fans and/ or oil pumps), insulation levels, tappings and tap-changer, desired value or range of shortcircuit impedance, etc. At medium system voltages the insulation levels are fixed in relation to the system voltages, but at higher voltages there are alternative options. Recommended lists of auxiliary equipment for protection and supervision are given in national standards for certain categories of transformers, and in manufacturers’ catalogues. The term regulating transformer has a different meaning in different countries. It is sometimes used for any transformer provided with a tap-changer for adjustment of its ratio - in contrast to a fixed ratio transformer. Otherwise the word is used specially for a separate booster unit (inserted for voltage correction in a weak system - having a variable ratio close to unity), or for a transformer which is to deliver secondary voltage over a wide range from nearly zero up to a maximum value (e.g. testing equipment). Special process transformers for furnaces or electrolysis, delivering very high secondary current, may consist of a set of two units - one wide-range, high voltage regulating transformer and another fixed-ratio, step-down transformer fed from the regulating unit.

Tap-changers. As the term indicates, an onload tap-changer is capable of changing the ratio of the transformer without interruption of service. The off-load tap-changer is a simpler device that must not be operated unless the transformer is disconnected. The extent to which medium-size step-down transformers for utilities are provided with onload tap-changing varies from country to country. The conventional width of the tapping range in per cent also varies. Distribution transformers are as a rule provided with a small range of off-load taps (plusminus 5 per cent in three or five positions). A suitable tapping is not selected until the transformer is installed, and will usually not be changed later.

The tapping range of a transformer is usually expressed as a rated voltage, plus-minus a number of equal tapping steps (e.g. 120 kV ± 9 x 1.5%). According to older practices, the numbers may either indicate the specified range of variation of service voltage on the winding in question (regardless whether the winding is tapped or not) or, instead, the range of variation of turns available on the winding. In international standardisation a comprehensive treatment of transformers with considerable tapping range has not existed until quite recently. It is important to establish what the name-plate information on older transformers really means! In actual service, the voltages will vary more or less on both sides of the transformer, but only one of the windings is tapped. There is not a free choice whether the high voltage winding or the

Three-phase connections and notation of phase displacement. (The following refers to IEC Standards. American practice differs somewhat.) Short notation of three-phase winding connection and phase displacement employs a code 69

5.4

69

97-03-05, 13.35

INDUSTRIAL POWER SYSTEM Transformers power minus power loss and reactive power consumption in the transformer. These conventions are independent of direction of power flow through the transformer. U.S. Standards interpret rated power as delivered secondary power. Primary input power and voltage will be rated power and voltage plus the consumption and voltage drop. Step-down transformation is presupposed, and the power factor is to be at least 0.8 (inductive). In most practical cases the difference between the two systems of standards is apparent only at the specification stage, and will be of little importance. Once the ratings are fixed, tests and other related data (losses, short-circuit impedance) come out the same.

of letters and numerals, e.g. YNyn0, Dy11, YNautod5. The system is as follows: windings are noted beginning with the • The high voltage winding, and with following

• • •

• •

windings in the order of falling rated voltages. The high voltage winding connection is noted with capital letters; the others with lowercase letters. The connections are Y(or y) for star, D (d) for delta, Z (z) for zigzag, and I I I (i i i ) for open windings (all ends brought out separately) A neutral terminal taken out from a y or z winding is indicated by the letter ”n”: YN, zn Phase displacement between ”corresponding terminals” of the windings is expressed by the ”clock method”. The high voltage winding phasor is understood to be at 12 o’clock (not indicated). The other winding or windings are given clock hour numbers showing the direction of their respective phasors in the phasor diagram. The clock hour number is written after the connection letter. All usual connections lead to full hours. The system is best understood by studying the examples in Fig. [ ? ]. Auto-connection between two y windings is indicated - for example as YNauto or YNa0 A separate equaliser delta winding (not available for loading) is indicated as ”+d” , e.g. YNy0 +d

Rated voltage is a quite critical parameter nowadays, since transformers with modern core steel are operated with flux density close to saturation. Overfluxing a transformer results in abnormal magnetising current, and may cause overheating of steel parts in the vicinity of the core. For a transformer with tappings, the ”tapping voltages” for specific tapping numbers (indicated on the name-plate table) have the same significance as the rated voltage has for the ”principal” (central) tapping. Difficulties with overfluxing may occur if a transformer is connected to a load with a very low power factor (capacitor bank or compensating reactor). The numerical voltage drop across the transformer is then at its maximum (positive or negative). Such cases should be indicated and analysed before the transformer is ordered. The important fact is whether the direction of reactive power flow is from an outer winding towards the winding closest to the core, or the opposite. (A priori it is often not known whether the low voltage or the high voltage winding is closest to the core - this is not apparent from the name-plate, for example.)

Note that there are different systems of terminal markings in use in different countries: ABC - RST - UVW - H1H2H3. National standards specify this. Specified ratings and loadability in service Rated quantities are reference values for guarantees and tests. The no-load loss is referred to rated voltage. The short-circuit impedance and the load loss are referred to rated current. During the temperature rise test, the total loss to be injected also corresponds to the rated quantities. IEC Standards interpret rated power as incoming power (rated voltage, rated current). On a loaded transformer, if the primary voltage is equal to rated voltage, the secondary voltage will be equal to secondary rated voltage minus voltage drop. Delivered power is incoming

Rated current is not critical in the same sense. Temporary loading of the transformer above rated current leads to higher winding temperature rise above surrounding oil, and (more slowly) increasing oil temperature in the whole tank. The ordinary winding loss and eddy current loss rise strictly by the square of the current. The stray flux between the windings rises in 70

5.4

70

97-03-05, 13.35

INDUSTRIAL POWER SYSTEM Transformers small separate ”earthing transformer” with a YN or ZN winding. Distribution transformers feeding four-wire or five-wire low voltage networks with singlephase loading between phase and neutral (”zero sequence”) are specified with mixed connection: Dyn, or Yzn. These connections provide neutralisation of the secondary neutral current, so that this is not carried over to the primary system. The primary system is usually resonant-earthed in some European countries.

proportion to the current, and its stray loss heating effects on adjacent steel parts rise by about the square. In a well designed transformer there is considerable margin between rated current loading and an overload which would create immediate risk of serious damage to the transformer. Cyclic or temporary loading up to high values with corresponding temperature swings, however, results in mechanical dilation of the windings and corresponding permanent set of the insulation structures, which may reduce the rigidity of the design in case of heavy through-fault current. See also the section on ageing and emergency loading below. General information on loading characteristics of representative power transformers is presented in the form of ”Loading Guides” by standardisation bodies (IEC and ANSI). These documents, however, cannot give guarantees for any specific transformer; only general guidance.

Type of cooling is indicated by a four-letter code (IEC). The common alternatives for oilimmersed and air-cooled transformers are: ONAN: self-cooled by air - without oil pump or fan ONAF: natural oil circulation - forced air cooling by fans OFAF: oil circulation through coolers forced by pumps - air flow by fans ODAF: oil flow by pumps, directed right into the windings - air flow by fans (mainly for large transformers, above 100 MVA)

Three-phase connections are selected based on several considerations. The simplest combination is Yy. It is used in large system transformers - often with autoconnection. In separate-winding transformers either the high or the low voltage winding, or both, may have their neutral terminals taken out for system earthing according to different principles (see Chapter ??? on this subject). The connections are then YNy, Yyn, or YNyn. If either of the connected systems carries zero-sequence current (current to the neutral either single-phase load between phase and neutral or earth-fault current) which cannot be transformed across the transformer to the opposite system, then it may be required to provide an equaliser delta winding, in which compensating zero-sequence current will be induced (circulating around the delta winding). This ”reduces the zero sequence impedance”. (See the section on system earthing under Chapter ??? , Relay protection, p. [?]

Alternative cooling systems may be specified for the transformer. Information is thereby given about, for example, the permissible load on an ONAF transformer if the fans are stopped (the transformer becomes ONAN). According to IEC, the rated power figure is unique, and relates to the highest cooling capacity. According to U.S. Standards, there are multiple power ratings assigned for the different cooling steps.

Short-circuit impedance and voltage drop Short-circuit impedance (or short-circuit voltage) is the series impedance that the transformer presents when current flows through it. The name derives from the measurement of this quantity in one of the routine tests on the transformer, the short-circuit test. A reduced voltage is applied to the terminals of one of the windings with the opposite winding short-circuited. The test voltage is raised, until rated current flows - this is the ”short-circuit voltage”. It is expressed as a percentage of the rated voltage of the energised

”Mixed connections” are, for example, the combinations YNd, Dyn, or Zyn. Again, the selection of these combinations is related to system earthing and earth fault considerations. As there is no neutral terminal on a D winding, it may be necessary to provide an artificial system neutral point at the transformer by connecting a 71

5.4

71

97-03-05, 13.35

INDUSTRIAL POWER SYSTEM Transformers limitation for cable systems, accuracy of reactive power compensation, etc.

winding. The percentage figure is the same, regardless of whether it is measured from the high voltage or the low voltage winding. The ”short-circuit impedance” is the same percentage of the ”reference impedance” of the transformer, i.e., the impedance that the loaded transformer represents when it passes rated current at rated primary voltage. (The percentage figure also indicates the apparent power in MVA that the transformer itself absorbs under the same conditions, as a fraction of the rated apparent power).

Three-phase conventions for symmetrical quantities: Current I is line current Voltage U is line-to-line voltage Three-phase apparent power S = ô3 ⫻ U ⫻ I U Impedance per phase Z = 3:I ô (”As in Y-connection”) Other expression for Z = U 2 S where S is the three-phase power (Z in ohms, U in volts, S in Volt-amperes, or Z in ohms, U in kV, S in MVA)

The short-circuit impedance is represented in the equivalent diagram of the transformer as a resistance, representing the load loss, in series with an inductive reactance, representing the reactive power required by the leakage flux betweeen the current-carrying windings (cf. equations below).

Rated quantities for the transformer are denoted with index ”r” Sr, Ur, Ir, and Zr All percentage figures are referred to the rated quantities. They are noted with common letters, not capitals.

The reactive part of the short-circuit impedance dominates, and the larger the transformer, the more this is the case. A 1 000 kVA distribution transformer has a loss figure of the order of 1 % of the rated power, while the short-circuit reactance is typically 6 %. At higher power ratings the percentage loss becomes lower, but the percentage reactance increases. A ”natural” reactance of a transformer emerges from economic optimisation of the design with regard to capitalisation of noload loss and load loss. A transformer with low average loading has a low load loss capitalisation rate; this results in a transformer with light magnetic core, low no-load loss, high load loss and high percentage short-circuit reactance.

Short-circuit voltage: uk = 100 ⫻ Uk Ur (dimensionless) Short-circuit impedance ZT = RT + j XT (impedance per phase) Percentage short-circuit impedance ZT zT = rT + j ⫻ T = 100 ⫻ Zr (This number is also dimensionless and is identical with uk) The voltage drop across the transformer is the product of the load current, multiplied by the transformer short-circuit impedance. However, this ”vectorial voltage drop” is not in phase with the current, leading it by nearly 90 degrees, since the short-circuit impedance is mainly inductive. The conventional, ”arithmetic” voltage drop which is observed on the voltmeter is the ”arithmetic” difference between the voltage at no load and the actual voltage with the load applied. Depending on the phase angle of the load, this quantity may assume any value between zero and the full vectorial voltage drop, and may even be negative (the secondary voltage of a transformer will rise, not fall, when

In some countries there is a praxis that the user specifies fixed values of series reactance. This may be in order to facilitate parallel operation between arbitrary transformers of the same range; cf. the section below on parallel operation of transformers. Typical reactance values for normal utility step-down transformers are astonishingly different in different countries; a 10 - 20 MVA transformer may have values ranging from below 10 % up to 20 - 25 %. These traditional practices depend on different philosophies regarding the layout of medium voltage systems: average distance between stations, installation of spare capacity, short-circuit power 72

5.4

72

97-03-05, 13.36

INDUSTRIAL POWER SYSTEM Transformers section below on ageing and emergency service). Under the light load season one of the units may be taken off (the no-load loss in the station is then reduced to half). Under normal conditions with both units available it is possible to sectionalise the secondary busbar (reduced shortcircuit fault current). ”Dual-secondary” transformers have two secondary windings with the same rated power, and with low coupling between them (”double-concentric” winding system or ”vertically split windings”). They work, as it were, like two parallel transformers on a common core.

capacitive load is added). The voltage drop is largest for low power factor, inductive load. This explains why there is such a heavy voltage dip when a large motor is started - the starting current is large, and the power factor is low. Assume stiff source voltage on the primary side of the transformer. The secondary voltage at not load is then U20 Apply an apparent power load S S=P+jQ= cos j + j sin j This load represents an impedance per phase ZL = U2 ⫻ (cos j + j sin j) S

Tap-changer mismatch by one step not a problem When two parallel transformers are equipped with tap-changers, there are different systems to coordinate their movement. During the short interval while the two tap-changers rest in dissimilar (staggered) positions, there will be a small superimposed current that circulates between the transformers. The e.m.f. which drives this current is one-step voltage, and it meets the series-connected short-circuit impedances of the two transformers. The circulating current will be of the order of 1/10 or 1/20 of rated current. It is essentially reactive, and combines vectorially with the load current. Obviously, the whole phenomenon is in fact negligible. The estimate is also applicable in cases of paralleling non-identical transformers which have slightly different rated or tapping voltages (see the following section).

The secondary voltage changes by the drop DU from U20 to U2 U2 = ZL (this equation is complex) ZL+ZT U20 After algebraic operations an approximate expression of the voltage drop is optained, expressed in percent of rated voltage: Du = S ⫻ (xT sin j + rT cos j), Sr I or ⫻ (xT sin j + rT cos j) Ir Example: Load is 50 % of rated current or rated apparent power. Power factor is 0.8, inductive Transformer load loss rT = 08 % of rated power. Transformer rectance xT= 12 %. (If cos j = 0.8, sin j = 0.6). Du = 0.50 (12 ⫻ 0.6+0.8 ⫻ 0.8) = 3.9% - say, 4%

Paralleling of transformers with non-identical data Parallel connection of transformers having nonidentical data is possible under the following conditions:

Parallel connection of transformers

must have the same clock-hour phase • They relationship. (There are stratagems with re-

Two units with the same data - spare capacity consideration A normal utility installation practice is having two identical units side by side. If the rated power of each unit is, say, 70 % of the full load in the station, then there is 40 % overcapacity under normal service. However, if one of the units should fail, continued emergency service may be provided by overloading the remaining unit, in the worst case up to the proportion 100/70. This may still be acceptable (cf. the

•

versed phase sequence, etc., which make a few additional combinations possible; consult experts or the IEC standard) They must have identical or closely similar rated voltages. Units with tap-changers need not have precisely the same tapping voltages, but there must be a sufficiently wide overlap of the tapping ranges. A small difference in tapping voltages between the units would be

73

5.4

73

97-03-05, 13.36

INDUSTRIAL POWER SYSTEM Transformers

•

of either turns ratio, or differing short-circuit impedances. This implies that requirements regarding matching transformer data should not be exaggerated. A more rational approach in the enquiry would be to request from the designer a calculation of the resulting load sharing. The load must be described with both apparent power and power factor, or by separate figures for active and reactive load power. A reasonable, allowable tolerance (e.g. 10 %) for the load mismatch should be specified.

limited to not more than one tapping step if the co-ordination of the tap-changer controls is optimised. The remaining mismatch of current sharing should not be given exaggerated attention (cf. preceding paragraph about staggered tap-changer positions). They must have approximately the same short-circuit impedance in per cent. If the short-circuit impedances in per cent are the same, the transformers will share the load current in proportion to their rated currents (or rated power values). It is easy to understand that if one of the units has a lower reactance it will absorb a relatively larger proportion of the load current (so that the voltage drops become equal for both transformers). The result will be that the sum of the rated power values for the two transformers cannot be utilised fully. Standards sometimes give the recommendation that the reactance values should not differ by more than plus/minus one-tenth of the average value. For two units having the same rated power this would result in a reduction of their common loadability of the order of 10 per cent.

Ageing considerations and emergency service. Rated current or rated power is not a precise limit of loadability for the transformer. However, loading beyond name-plate rating naturally means more rapid wear and tear on the unit. It is very difficult to express this in quantitative terms. An effort to provide a scientific background is based on spontaneous chemical deterioration of the insulation system as a function of the service temperature. The rate of reaction of the chemical oxydation processes grows exponentially with the temperature. An addition of a certain number of degrees of temperature corresponds to a multiplication of the rate of deterioration by a certain factor. Conventionally, an increment of 8 Kelvin is supposed to correspond to a doubling of the rate of deterioration. Or, about 25 K higher temperature raises the deterioration rate by a factor of 10. Emergency service implies that the unit takes over load which would normally flow through a different route. The pattern of variation over the day will remain unchanged, but the level will be higher - in the worst case perhaps close to double the load. Carrying the above example further, a month of moderate emergency service, leading to a representative temperature 25 Kelvin higher than before, would then affect and shorten the life of the transformer about as much as a full year of undisturbed service would. If such scenarios are identified and recognised during strategic planning, and the transformer ratings selected to cope with them, then the installed overcapacity becomes a kind of calculated insurance premium. There is no basis, though, for calculations of consumed or remaining lifetime of a trans-

One should not connect transformers with widely differing power ratings together. The risk is that the small transformer will be the one that sets the limit of common loadability since it is likely to have a lower short-circuit impedance in per cent. Standardised distribution transformers often have short-circuit impedance values fixed in large steps: 4, 5, 6%, increasing with the rated power. Specification of a new transformer intended for parallel operation with an existing unit. Suppose that it is not possible to obtain a true repeat of the old transformer. The manufacturer may have modified his design standards or completely changed his range of products. Or else, the enquiry may be directed to a different factory. It is then often difficult to obtain close agreement with the previous data, particularly the variation pattern of the impedance along the tapping range of a regulated transformer. This is dependent on the physical arrangement of the windings with respect to each other, and is also subject to several design considerations. In the previous paragraphs of this section it has been explained how to estimate the magnitude of a circulating current that results from mismatch 74

5.4

74

97-03-05, 13.36

INDUSTRIAL POWER SYSTEM Transformers as of a full short circuit through the transformer: far above rated current peak. This worstcase current amplitude is possible only if the transformer is energised on the inner winding, which is usually the low voltage winding. Energising from the low voltage side should therefore be avoided. (However, it should be noted that the oscillatory, transferred switching overvoltage on the non-connected high voltage side is not without risk either.) A particular phenomenon has been observed when a second transformer is energised in parallel with another, both at no load. The transient asymmetric saturation may then oscillate back and forth between the two for a considerable time, which may confuse the relay protection. The inrush current is sensed by the differential protection relay, as well as by the neutral current protection and overcurrent protection. The characteristic content of even-order harmonics has, however, made it possible to design a blocking feature against inrush current tripping. Another, more simple method is to install a time-lag device, so that the inrush transient gets time to decay before any tripping signal is released.

former in absolute terms: years, or percentages. Besides, there are many other factors, e.g. in the form of occasional overvoltages or overcurrents, or neglected maintenance and inner contamination, that may contribute to the breakdown of a transformer. Some conventional guidelines concerning emergency service have been agreed upon (IEC Loading Guide): During ”long-time emergency loading” - lasting for weeks or even months - it is recommended that the hottest part of the windings should never exceed 140°C in a medium-size transformer, or 150°C in a distribution transformer, while the current should never exceed 150 % or 180 %, respectively, of the rated figure. The top oil temperature should never exceed 115°C in any transformer. ”Short-time emergency loading” is loading which is so high that some remedial action has to take place; within half an hour or so. This may be by manual intervention or through tripping by thermostats for oil temperature or winding temperature, well before the temperature in the transformer has reached its critical value. The top oil rise limit is again limited to 115°C (tripping at 120°C). The maximum allowable current is tentatively limited to 180 % for a medium transformer (overcurrent tripping possibly set at 200 %). Note that it is also usual to have a blocking function in the tap-changer which prevents operation when the current exceeds 200 %.

Capitalisation of losses Utilities apply capitalisation of loss figures for the evaluation of transformer bids. They submit separate capitalisation rates per kW of no-load losses and of load losses. These capitalisation rates are based on the applied interest rate for investment, and - for the variable load loss - on the expected utilisation of the transformer, expressed as utilisation time of the maximum load in hours per year. In industries there are some transformers which see load variations over the day and over the year that are similar to utility loading characteristics, with a typical utlisation time of 4000 hours. For such transformers, the capitalisation of load loss should be given a rate 1/3 to 1/5 of the rate applied for no-load loss. The absolute figures depend on the interest rate for investment money. Process transformers (e.g. for electrolysis) may have utilisation times ranging up towards 7000 hours. The correct rate for load loss capitalisation would then rise to about 2/3 of the noload loss rate.

Inrush current When a transformer is energised, a high inrush current sometimes occurs at random. This is caused by transient saturation of the core steel. Its magnitude depends on the remanence condition of the transformer core since the previous disconnection, and on the point of wave of the AC voltage at which the phases are closed. The inrush current does not have symmetrical half-cycles, but has a pronounced DC component, and contains harmonics of even numbers of order. The whole phenomenon decays in the course of a few seconds. It is accompanied by a distinctive sound - a heavy thud. The maximum possible current peak is of the same order 75

5.4

75

97-03-05, 13.36

INDUSTRIAL POWER SYSTEM Transformers

Installation considerations Most transformers are air-cooled. When they are installed in closed cells for fire protection, there must be adequate provision of cooling air. For satisfactory cooling, the temperature rise of the air cannot be taken higher than about 10 Kelvin. This corresponds to a consumption of about 5 cubic metres of air per minute for each kW of dissipated losses. All ducts must be sized for this capacity. ONAF transformers with radiators and fans are usually arranged so that the fans blow horizontally into the radiator banks. The warm air flows upwards spontaneously. It is important to arrange the cell so that this warm air will disappear out, and not be recirculated down again in the cell by the fans. Either there have to be sufficiently wide ducts for incoming and outgoing air, or separate fans for supplying sufficient cooling air to the cell. In case of fire in the cell, the large flows of cooling air, sometimes forced, would be an aggravating factor. Therefore, automatic flaps or shutters have to be installed to stop the supply of air when fire in the cell is detected. In metallurgic plants large transformers are installed close to the furnaces, where the ambient air is full of dust and fumes. The transformers are then often specified as having water cooling. A different solution has been practised with advantage, namely that of designing for OFAF cooling and running oil pipes to compact oil-to-air coolers located outside the building. Distribution transformers in a factory typically feed widespread motor drive installations. If oil-insulated transformers are used, they have to be placed in cells outside the building, out of regard for the risk of fire and oil spillage. In order to limit the cost of heavy low voltage cables and also the losses in these, a popular solution is to distribute the load centres over the floor area. This then leads to the choice of drytype units which can be installed in simple metal-clad cubicles with self-cooling by ambient air through venting slots.

76

5.4

76

97-03-05, 13.36

INDUSTRIAL POWER SYSTEM High voltage switchgear

HIGH VOLTAGE SWITCHGEAR Contents page 1 Air insulated (AIS) and Gas Insulated Switchgear

77

2 Switchgear components

78

3 AIR Insulated Switchgear (AIS)

80

4 GAS Insulated Switchgear (GIS)

84

Air Insulated (AIS) and Gas Insulated Switchgear (GIS) General Until the 1970s, air insulated switchgear (AIS) was the type most in use. AIS requires large distances between earth and phase conductors and therefore a good deal of space. Hence, for the higher voltages, this type of installation was only feasible outdoors. In some cases it was possible to install indoor air insulated equipment at voltages up to 145 kV, but above 36 kV, outdoor installations had traditionally been the rule. Germany and Japan first introduced SF6 (sulphur hexafluoride) as an insulating medium in switchgear enclosures in order to reduce phaseto-earth distances. By giving the gas an overpressure of approx. 500 kPa, this further improved insulating properties and led to a reduction in space requirements. There are now approximately ten world manufacturers of GIS switchgear, most of them in Europe and Japan. The advantages of GIS as compared to AIS are as follows:

out easily. When equipment capacities need to be uprated, or the network arrangement has been altered, new equipment may be more easily added or changes made. With regard to investment costs for the switchgear alone, AIS is considerably less expensive than GIS. However, when costs for maintenance and land area are added, the difference becomes less.

space requirements - especially in con• Less gested city areas sensitivity to pollution, as well as salt, • Less sand or even large amounts of snow personnel safety - enclosed high volt• Higher age and insignificant EM fields Fig. 1. Comparison of dielectric strengths.

Conventional AIS is constructed outdoors with free-standing switchgear components joined together by lines or tubular buswork. The advantage of such an arrangement as compared to GIS is mostly that modifications may be carried 77

5.5

77

97-03-05, 13.37

INDUSTRIAL POWER SYSTEM High voltage switchgear

Switchgear components High voltage equipment has to match or exceed the available short-circuit level at the location in the network where it is installed. It is also commonly agreed that the equipment must withstand this short-circuit current for the time taken for the back-up protection to clear the fault. Circuit breaker This is the most important switchgear component. It must be able to break and make normal load currents, but above all be able to interrupt short-circuit currents due to faults in the system. Modern circuit breakers have interrupting times as short as 20 milliseconds. Such an operation is performed automatically in response to signals from fault sensing relays. The main components of a circuit breaker are

Fig B Centerbreak disconnect switch the interrupting chamber and the mechanical operating mechanism. Energy is stored in charged springs in the operating mechanism, and when called upon, this energy is released such that the breaker contacts are forced apart. The arc now establishing itself between the parting contacts is then extinguished by high pressure oil or SF6 gas blown towards the arc. The stored ener-gy must be sufficient in order for the circuit breaker to manage one complete open-close-open sequence. There is a tendency for SF6 gas to become a common interrupting medium for circuit breakers used in outdoor AIS as well. Disconnector Disconnectors are used in order to switch during no load conditions. In order to make a circuit breaker ”dead” (without voltage) and

Fig A Live tank circuit breaker for outdoor switchgear 78

5.5

78

97-03-05, 13.37

INDUSTRIAL POWER SYSTEM High voltage switchgear thereby available for service or maintenance, a disconnector may be used on either side. Disconnectors may also be used in order to change the switching arrangement. A disconnector has a very low breaking capacity and may only be operated under no load (no current) conditions. The disconnector’s main purpose is to provide an open circuit on both sides of an item of equipment prior to working on it. Opening or closing a disconnector takes several seconds to complete. Earth switch An earth switch is a special kind of disconnector, used to connect parts of the switchgear to earth, also prior to working on otherwise live equipment such as lines or cables. Earthing can also be done in AIS by means of manual earthing hooks or portable earthing and short-circuiting devices. However, in GIS there is no access to phase conductors in this manner. Therefore, permanent earthing switches must be installed. Some of these are designed to connect to earth under full voltage, which requires them to withstand, both electrically and mechanically, closing of a full short circuit. Surge arrester Surge arresters protect switchgear against high overvoltages caused by for example lightning or switching overvoltages. They work on the principle of leveling the incoming voltage wave and diverting the energy as a current. When the voltage returns to normal, the current is reduced to zero. A good surge arrester has a low ignition voltage and a high extinction voltage, thus providing a non-linear function between current and voltage. Earlier makes of surge arresters generally had silicon carbide resistors and a spark gap (valve type) to obtain a high non-linear function between voltage and current. Modern zinc-oxide arresters have no spark gap and provide many advantages such as low voltage during dissipation, elimination of reignition transients as well as a high capacity for diverting energy. Correct insulation co-ordination is achieved through properly selecting and locating surge arresters in the network. The most strategic locations for surge arresters are at the terminals of transformers, reactors and incoming overhead lines.

Fig C Zink-oxide surge arrester with polymeric insulator

Current and voltage transformer (instrument transformer) Instrument transformers are required in order to provide signals for metering and protective relaying. Depending on the purpose of the signal, the accuracy may vary greatly. Current transformers operate under almost short-circuit conditions, while voltage transformers operate at no load. Current transformers normally have a magnetic coupling between primary and secondary side. Magnetic flux follows cores of different magnetic material, depending on whether its use is for metering or protection purposes. Protection current transformers must withstand both the peak short-circuit currents as well as the time taken for the longest set back-up relays to operate. It is very important to match the relay burden to the current transformers in 79

5.5

79

97-03-05, 13.38

INDUSTRIAL POWER SYSTEM High voltage switchgear that it is important to specify data for the total relay and metering burden. Voltage transformers are usually connected phase to earth.

order that saturation of the cores at high currents does not trick the relays into thinking they are seeing a different current from the actual one. Voltage transformers can be divided into inductive and capacitive types. The inductive type is normally used up to approximately 145 kV, while the capacitive type is used above this voltage. If power line carrier frequencies are required for communication or for protective relaying, capacitive types may be used for this purpose at lower voltages too. The metering range for the secondary voltage transformer winding is 80-120% of the nominal voltage. The relay winding has a range from 5-150 or 190%. The different secondary windings of voltage transformers have a coupling dependency on each other (which is different from current transformers), and this means

Fig D 2 Top core type current transformers Line traps Line traps are installed in power systems where high frequencies are used for communication via the high voltage system itself. The line traps are in reality a barrier that effectivily prevents high frequency from entering the substation. In this way a frequency can be reused in other parts of the network.

AIR INSULATED SWITCHGEAR (AIS) Busbar arrangements Some common arrangements for industrial applications are:

Fig D 1 Hair pin type current transformers 80

5.5

80

97-03-05, 13.39

INDUSTRIAL POWER SYSTEM High voltage switchgear Single busbar Arrangements of this type are the most common. The majority of Scandinavian power companies use this arrangement up to 145 kV, and it is growing in popularity in other countries too for ordinary, uncomplicated industrial purposes. Single bus with transfer bus The transfer busbar is used only as an auxiliary busbar when maintenance is being carried out on a line circuit breaker. The breaker can be disconnected on both sides while the line remains in service. One limitation of this arrangement is that the whole station is shut down when a fault occurs on the main busbar. Double busbar This system has two identical buses, i.e., one can be a standby for the other. This arrangement guarantees uninterrupted service in the event of a busbar fault. The circuit breakers,

Fig D 3 Magnetic voltage transformer No busbar This arrangement represents the simplest design for small transformer substations. When a fault occurs, the whole station is shut down but, on the other hand, initial costs are the lowest.

Fig. 2. Some common switchyard arrangements.

Fig D 4 Capacitive voltage transformer 81

5.5

81

97-03-05, 13.39

INDUSTRIAL POWER SYSTEM High voltage switchgear and also involves different design problems.

however, are not available for maintenance without the associated bay being disconnected. Apart from that, operating flexibility is good.

For low current ratings requiring only lightweight conductors, a strain bus may result in a more economical design, i.e., with lower costs for foundations, structures, insulators, conductors, etc., as longer spans can be used than with tubular conductors. On the other hand, increased loads, consideration of the dynamic stresses imposed upon buses, together with a preference for the low-profile concept, have combined to bring about a move towards a supported tubular bus rather than the use of heavy stranded bundle conductors. The bus design usually includes aluminium conductors with bolted connectors. The fully welded design, however, is also used. There are some problems involved in welding on site as this must be done in a protective gas flow. Furthermore, special reinforcements have to be introduced into the design to compensate for the material strength reduction resulting from welding temperatures. On the other hand, these joints will be of the highest quality from the electrical point of view. Tubular sections are usually chosen for reasons of economy, strength and symmetry and the mechanical problems of tubular bus with spans up to around 20 metres are not difficult to solve . Long tubular spans are subject to aeolian vibrations, although there are various solutions to this problem. One is to lay aluminium cable inside the tubes to damp these vibrations. The bundle conductor arrangement of strung type busbars constitutes a special calculation and evaluation problem due to the ”snatch” forces caused by short-circuit currents (parallel conductors forcibly pulled together by magnetic attraction cause high axial forces in the conductors). On installations with very high voltage, corona rings and large radius bends are used to achieve a low value of electric stress on the conductor surface. The busbar insulators must be carefully selected to meet electrical requirements, i.e., insulation level, pollution corona, coefficients for temperature and altitude. Mechanical requirements are: withstanding short circuits, seismic and wind forces, normal and composite loads.

Design considerations for civil works A large part of the design work for an outdoor switchyard is civil and structural engineering. Levelling of ground, sizing of foundations, layout of cable ducts, etc., require civil works drawings for foundation plan and cable ducts. For manufacturing and ordering of equipment, for erection and commissioning of the plant, further drawings are required for steelwork and other hardware, as well as layout (plan and section) drawings, material lists, purchase orders, cable schedules, etc. Today this documentation is mostly produced by computers. When establishing the design parameters for steel structures and foundations, it is logical to evaluate the maximum force which any piece of equipment is expected to withstand in its lifetime. Mechanical forces acting on electrical equipment and support structures are as follows: dead weights from conductor loads and ice covering, wind load, short-circuit loading, mechanical operation and forces due to earthquake.

• • • • •

The worst case for the electrical equipment and support structures arises from a combination of these factors. Increasing fault levels in power systems require an accurate understanding of the mechanical forces resulting from shortcircuit currents. Bus design Designing the bus includes consideration of a number of factors, the following being the most important: max. load current max. withstand impulse voltage mechanical strength to withstand dynamic forces atmospheric contamination

• • • •

There are two basic kinds of bus arrangement; the flexible stranded conductor or strain busbar, and the supported tubular busbar. Each gives the outdoor switchyard a different appearance 82

5.5

82

97-03-05, 13.40

INDUSTRIAL POWER SYSTEM High voltage switchgear 1. 2. 3. 4. 5. 6. 7. 8.

Power transformer Circuit breaker Disconnector Current transformer Voltage transformer Surge arrester Earth switch Post type insulator

Distances between feeders (centre to centre C-C) and total length for various voltages. Voltage

(kV)C-C (m)

L (m)

6,5 7,5 11,0

21,0 21,8 25,3

52 72,5 145/123

Fig. 3. (earlier Fig. 58) Outdoor switchyard for single bus arrangement. 83

5.5

83

97-03-05, 13.40

INDUSTRIAL POWER SYSTEM High voltage switchgear Countermeasures against pollution are:

Safety clearances Selection of electrical clearances is fundamental in AIS design. To ensure reliability and the safety of operating personnel, it is essential that adequate clearance is provided for live parts. There are two main aspects here:

creepage distance • increasing washing the insulators • applying silicon to the insulators • (encapsulating thegrease pollution) Earth grid and safety earthing The purpose of the earth grid is to provide a reliable low resistance path for dissipating large currents to earth from faults on the system or from lightning surges. The lower the resistance the better, and the voltage build-up will then be smaller. The earth grid is usually composed of bare copper conductors laid in the ground at a depth of about half a metre. To make the voltage distribution as uniform as possible, the grid should consist of uniform rectangular meshes. Furthermore, earth rods penetrating a few metres into the ground are usually connected to the earth grid, giving dual action between the components. The purpose is to limit the step and touch voltages to safe levels during fault conditions. Both resistance to earth and step and touch voltages are dependent on earth resistivity. Measurements are recommended to check the actual soil resistance and potential gradients. Temporary maintenance earthing in switchyards during work is an important safety procedure. Higher demands on reliability have led to a growing use of permanently installed earth switches. However, the most frequently used method is still portable maintenance earthing. The conductor size for this equipment has increased considerably with increasing earth-fault currents, which has made earthing work strenuous and time-consuming. Some aids have been introduced, such as suitable lifting tackle and shortening of the conductors by placing the earth stud at the top of the equipment stand.

1) insulation clearances 2) personnel safety clearances A high degree of insulation level standardisation has been established, although IEC recommendations include alternatives to match established practice in different countries. Common practice for voltages up to 300 kV is to use standard minimum clearances in air, the same for phase-to-phase and phase-to-earth. Personnel safety clearances are established by adding to non-flashover insulation clearances the dimensions of a typified body. Space and electrical clearances must also be provided for equipment moving in or out of position. Air pollution In recent years air contamination in switchyards has become a growing problem. The contamination is mainly of two kinds, coastal and industrial. A dry, clean insulator has the highest voltage withstand value for outdoor insulation. A clean insulator in fog conditions has a slightly lower withstand value, and the dirty-wet combination has the lowest. This dirty-wet condition can, in extreme circumstances, be disastrous to the power system. The pollution problem is a very complex one, influenced by wind, gravity, electrical forces acting on particles suspended in the air, adhesion, as well as the insulator shape and its position in service. In addition, rain can counteract or partly rinse the pollution, or at least wash away the soluble ingredients. Industrial pollution flashover is a long-time phenomenon of a thermal nature, evolving slowly as the salts first dissolve, and since dry band formations develop only gradually. Flashovers from coastal salt pollution evolve more rapidly since the salts are already dissolved and the whole surface suddenly starts conducting.

GAS INSULATED SWITCHGEAR (GIS) Gas insulated switchgear take advantage of the excellent properties of SF6 gas, both as an electric insulation medium, and as arc extinguishing medium in the circuit-breakers. As a result, the designs are compact, with small requirements on floor or ground area. 84

5.5

84

97-03-05, 13.40

INDUSTRIAL POWER SYSTEM High voltage switchgear Pure SF6 gas, as used in GIS equipment, is nontoxic. Electric arcs in the gas will initiale some chemical reactions in the gas and with solid matrials, forming a number of decomposition products. This will occur in circuit-breakers, and in the extremely unlikely event of insulation failures in a GIS compartment. Well established procedures exist to safely handle these decomposition products during maintenance and repair work. SF6 leaking into the atmosphere does not influence the ozone layer of the earth, and gives minimal greenhouse effect in the concentrations which maf occur. Emission og SF6 to the atmosphere should, however, be restricted as much as possible. At the end of the lifetime, and during repair of the GIS, care should be taken to prevent gas from being released. Arrangements should be made with either the manufacturer or specialized contractors to reuse the gas, or to dispose of it in a conrolled manner.

A 72 kV GIS cubicle concept Using a low differential pressure of only 20 kPa makes possible an enclosure design of cubicle shape, similar to traditional metal-enclosed medium voltage switchgear. The box shape also makes it possible to use disconnector and earthing switch designs common in medium voltage switchgear. SF6 gas, even at this low pressure, provides the dielectric strength required for 72.5 kV with a cubicle size only slightly larger than the standard 24 kV air insulated cubicle. Figs. 4 and 5 show the exterior view and cross-section through a typical double busbar feeder. The cubicle is sub-divided into the following four separate compartments, which can vary depending on the arrangement: compartment • Circuit-breaker equipment compartment • Feeder I compartment • Busbar • Busbar II compartment

Fig. 4. Cubicle type 72.5 kV SF6 switchgear for indoor installation. 85

5.5

85

97-03-05, 13.41

INDUSTRIAL POWER SYSTEM High voltage switchgear

Fig. 5. Cross-section of a 72.5 kV double bus feeder. 1. 2. 3. 4. 5a 5b 6 7

8 9

Voltage transformer V.T. disconnecting device Cable end unit Disconnector with earthing switch Busbar system I Busbar system II Busbar insulator Current transformer

10 11 12 13

Terminal bushing for C.T. and V.T. secondaries Pressure relief device with deflector funnel Gas barrier insulator Circuit-breaker Operating mechanisms for disconnectors C.B. operating mechanism

feeder disconnector with its integrated • the fast-acting maintenance earth switches • instrument transformers.

Two circuit breaker quenching systems are used: 1. The interrupter functions on the compression piston principle, i.e., puffer-type breaker

Each of the separate busbar compartments contains a busbar and associated disconnector. In addition to the maintenance earth switch on the circuit breaker side, the disconnector also accommodates a fast-acting earth switch for the busbar. The busbars are furthermore fitted with gas barriers between feeder sections, making each feeder autonomous.

2. The energy required for interruption of the short-circuit current is mainly taken from the arc itself, i.e., self-blast breaker The feeder compartment contains the following equipment: end unit (or alternatively a terminal • cable bushing), 86

5.5

86

97-03-05, 13.41

INDUSTRIAL POWER SYSTEM High voltage switchgear The individual compartments are gas-tight and segregated from each other. The conductors pass through gas-tight cast resin barriers. The walls of the separate compartments are of gastight welded sheet steel. Ribs of U-profile steel are welded to the enclosure walls to give additional mechanical strength. Each compartment is fitted with a pressure relief valve to relieve excessive overpressure in the unlikely event of an internal arc fault. The switchgear equipment - disconnectors, circuit breaker, instrument transformers - is installed through openings in each individual compartment, closed by special sheet steel covers. These access openings make possible equipment inspection and repair or replacement without removal of the enclosure. The access opening to the circuit breaker compartment is designed such that the puffer-type circuit breaker, pivoted at the base plate, can be swung out through an arc of 30 degrees, facilitating inspection and replacement of interrupter contacts without removing the circuit breaker itself. The operating mechanisms of disconnectors and earth switches are located in a compartment at the front of each feeder. The SF6 gas filling points, together with the gas density relays and supervisory equipment, are also located in the same compartment. Furthermore, control, interlocking, supervisory and other secondary equipment is accommodated in the same atmospheric control compartment, fitted with doors with mimic diagram, control switches, semaphores and instruments. The complete feeder together with its control equipment forms a rugged and free-standing shipping unit for mounting on an aligned concrete foundation. The weight of a fully equipped double-busbar feeder is approx. 3000 kg.

The SF6 in the various compartments is maintained at an absolute pressure of 120 kPa. Because of the cubicle shape, the compartments are not evacuated prior to filling gas, but filled as if it was a liquid, exploiting the fact that SF6 has a specific density of approx. five times that of air. The gas is introduced through a pipe leading to the bottom of the compartment and the air escapes through a pipe at the top. This method of filling will permit a small amount of air to mix with the SF6 gas due to a certain amount of turbulence. On the other hand, an SF6/air mixture with up to 20 % concentration of air exhibits a higher dielectric strength than pure SF6. Each individual gas compartment, including those of the circuit breaker interrupters, is monitored separately by a density relay. Referred to 20oC, an alarm is initiated when the insulating gas pressure drops to 110 kPa. In the circuit breaker an alarm is initiated when the pressure drops from 700 to 620 kPa, and a second alarm also blocks the circuit breaker from operating when the pressure drops to 600 kPa. This is the lowest pressure at which the name-plate rating of the circuit breaker is still maintained. A 145 kV GIS concept This type of design uses corrosion-proof aluminium as equipment casing. All three phases are enclosed within the same enclosure, as compared to one phase design often used for higher voltages. Due to low specific weight, floor loading is at a minimum, and expensive foundations are avoided. A double busbar feeder bay weighs approx. 3700 kg. The individual equipment modules are connected such that expansion or modification is facilitated at a later date. At the same time, gas barrier insulators ensure minimum disturbance to neighbouring equipment modules. True-to-scale plastic models of switchgear modules and equipment simplify application engineering. Three-dimensional comparisons facilitate the engineering evaluation of alternative layouts. When applying a layout of this type of GIS, the same standard electrical arrangements as for conventional AIS installations can be applied. Single and double busbar arrangements alternatively with transfer bus - as well as bus

SF6 gas system Each feeder is divided into independent gassegregated enclosures. A typical double-busbar arrangement has four separate gas compartments: compartment 1 • Busbar compartment 2 • Busbar Circuit breaker compartment • Feeder terminating compartment • 87

5.5

87

97-03-05, 13.42

INDUSTRIAL POWER SYSTEM High voltage switchgear sections, and bus couplers are realised using standard equipment modules. Similarly, other solutions not so common in Europe, such as duplex, one-and-a-half circuit breaker or ring-bus arrangements can be accomplished. Standardised flange dimensions provide for flexibility in combining switchgear modules. Application engineering and planning of an installation are thus simplified.

Busbar with combined disconnector and earth switch The busbar is built from modular bus sections, each 1200 mm long corresponding to the feeder spacing. The phase conductors are fixed to the gas barrier insulator of each feeder section. Each insulator is combined with a telescopic transverse assembly cover, which simplifies switchgear extension or rearrangement. The spring loaded flange coupling serves to compensate forces generated by the internal gas pressure and axial movement caused by temperature change. Split, multifinger tulip contacts connect the phase conductors at the assembly cover. They absorb axial movement caused by temperature change. Mechanical stress on the insulators from temperature difference between individual phases and enclosure is thus avoided. A combination of busbar disconnector and maintenance earth switch for maintenance, etc., is an integral part of each busbar module. The common operating mechanism for the combined disconnector and earth switch is mounted at the front. Depending on the direction of movement the contacts act either as a disconnector or an earth switch.

Fig. 6. Exterior view of a 145 kV SF6 switchgear bay.

SF6 gas system The SF6 gas has a dual function as arc extinction and insulating medium. Therefore the circuit current extinguishing chambers are differentiated from the insulating gas compartments of busbars, disconnectors, load break switches, etc. The gas compartments are segregated by gas barrier insulators, and the gas pressure is monitored by temperature compensated pressure relays. During commissioning, each compartment receives a final filling of gas, allowing for an extremely small leakage over a lengthy period of time. All gas compartments have their own automatic vacuum coupling, so that all maintenance jobs, such as gas sampling, conditioning or filling up can be carried out during normal operation.

Fig. 7. Cross-section of double busbar 145 kV feeder. 1 Busbar with combined disconnector/ earthing switch 2 Circuit-breaker 3 Current transformer 4 Potential transformer 5 Cable end unit with combined disconnector/earthing switch 6 Fast acting earthing switch 7 Control cubicle

88

5.5

88

97-03-05, 13.43

INDUSTRIAL POWER SYSTEM High voltage switchgear 145 kV application example This type of modular switchgear concept lends itself ideally to the usual switchgear configurations. A feeder spacing of 1.2 m is typical, and the required building depth is normally 7 m, the building height being less than 6 m. Although installation of a crane is recommended for this size of switchgear, erection and maintenance on the factory-assembled feeders and equipment can be performed effectively even without a permanent crane. Small industrial substations are often of the single busbar arrangement. A bus section disconnector or circuit breaker gives operational flexibility. In this way it is possible to extend or modify the switchgear with part of the substation still in service. A load break switch is recommended for bus sectionalising, with the distinct advantage of switching under load. The single busbar layout closely resembles that of the double busbar since only the top or bottom busbar is omitted. If the appropriate connecting flanges have already been fixed to the circuit breakers in the initial extension stage, then subsequent upgrading to a double busbar version is easy to implement.

89

5.5

89

97-03-05, 13.43

INDUSTRIAL POWER SYSTEM Fault Control

FAULT CONTROL Contents page 1 Protectiv relayng and Co-ordination

90

2 Motor protection

91

3 Transformer protection overview

94

4 Generator protection

96

5 Selectivity in industrial power systems

99

Protectiv relayng and Co-ordination

Introduction Although ”Fault Control” may sound like a new expression to some, the protective relaying and co-ordination in a power system is indeed a ”control” system with an inherent intelligence which senses faults and abnormal conditions, makes decisions and carries out remedial action to isolate as small a part of the system as possible (selectivity). Today this type of ”control” is more and more being entrusted to microprocessors. Industrial plants vary greatly in the complexity of their power distribution systems. A small plant may have a simple radial design with low voltage fuse protection only, whereas a major industrial process complex may incorporate an intricate network of high, medium and low voltage substations, uninterruptible power sources, and co-generation required to operate in parallel with and/or isolated from local power company networks. At an early design stage, the electrical engineering project team will meet with local power company staff to review and resolve the common protection requirements. The need for increased production from and availability of xindustrial plants has created demands for greater industrial power system reliability. Meshed networks and parallel operation with power companies have produced extremely high short-circuit capacities during fault condi-

tions. The high costs of power distribution equipment and the time required to repair or replace damaged equipment such as motors, transformers, cable, high voltage circuit breakers, etc., make relay protection design a very important consideration. The losses associated with an electrical power interruption due to equipment or system failures vary widely with different types of industries. For example, a service interruption in a machining operation means loss of production, loss of tooling, and loss from damaged products. Likewise, an interruption in a chemical plant can cause loss of product and create major clean-up and restart problems. To avoid a disorderly shutdown which can be both hazardous and costly, it may be necessary to tolerate a short-time overload condition and the associated reduction in life expectancy of the affected electrical apparatus. Other industries such as oil refineries, paper mills, automotive plants, offshore oil and gas plants, textile mills, steel mills, and food processing plants are similarly affected, and losses can represent substantial expenses. For some types of processes, even a momentary voltage dip can shut down the entire plant. Thus the nature of the industrial operation will determine the degree of protection which can be justified. 90

5.6

90

97-03-05, 13.43

INDUSTRIAL POWER SYSTEM Fault Control cable, and associated switchgear or controlgear. This excessive load may be a gradual increase from overloading, voltage drop, faulty bearings, or other reasons. In any case, it is a slowly built up type of overcurrent, whereas the short circuit is a sudden burst of current. Protection of both types must be considered.

MOTOR PROTECTION General aspects The mandatory electrical safety regulations in any country usually offer minimum motor protection. Therefore, a number of additional considerations have to be evaluated. The equipment being driven by the motor must be taken into account as well as whether running the motor to destruction is preferable to “instant” shutdown. Further, the anticipated areas of failure, such as bearing failure, winding failure, ventilation failure, phase reversal, winding turn failure, vibration, field failure, etc., must be considered. Last, but not least, comes the cost of the protection itself. The totality of all this is to provide “optimum” protection, which means that total costs of shutdown, loss of production, repair, reinstallation and recommissioning is weighed against the costs for varying degrees of protection. These decisions will all be different, depending on who decides what. The chemical engineer may decide that the process could become unstable and that the motor should be run to destruction. An accountant might think that extra money is good insurance if it shortens downtime. A few extra seconds running time may provide valuable purging time for gas process systems where contamination of gas without the purging could involve costs, representing money which could have purchased many replacement motors. What all this means is that there are no easy answers. However, some specific advice is presented in the following sections. Synchronous motors resemble generators, and therefore some generator protection schemes may also be used for synchronous motors.

SMALL THREE-PHASE ASYNCHRONOUS MOTORS (MAX. 200 kW) Three-phase induction motors stand for the bulk of application work in an industrial plant. Up to 200 kW, all motors are normally low voltage. Usually protection complying with local wiring regulations is adequate, unless special highinertia loads exist. The setting of the motor circuit overcurrent device is primarily to protect the cable and starter from short-circuit stresses and temperatures. The motor running overload protection is usually provided by three thermal heaters located in the motor starter. These are fixed-rated units, and the correct one should be installed with the starter to meet the allowed overcurrent. This is listed in most mandatory electrical safety regulations, but 115% is a generally acceptable value. Allowances should be made for motors with switched capacitors. In dual-speed motors the windings provide two different power outputs. Therefore, two different sizes of thermal overloads are required, each set of windings being considered separately. LARGE THREE-PHASE MOTORS (ABOVE 300 kW) Three-phase motors above 300 kW are normally medium voltage, that is 3 to 11 kV. This makes everything, including the motor, more expensive than the equivalent at low voltage. Each medium voltage motor is considered as an individual unit with its own specific application problems and cost. First, it has to be established whether the motor is essential or non-essential to the process. If it is essential, then the basic short-circuit protection will be installed and all other overload devices will actuate alarms rather than initiate shutdown. This will allow the operator time to reduce overload conditions or start a shutdown procedure before closing down the motor.

TWO TYPES OF OVERCURRENT PROTECTION Whether at low or medium voltage, a motor circuit is supplied either via a circuit breaker or a fused contactor. This is the “last” overcurrent device in a series of overcurrent devices. There are two types of protection required for a motor circuit. First of all, the motor and feeder cables are protected against a short circuit by the circuit breaker or fuses. Secondly, protection is required to prevent an increase in load causing excessive current and heating in the motor, 91

5.6

91

97-03-05, 13.43

INDUSTRIAL POWER SYSTEM Fault Control setting to prevent operation due to the starting current.

f the motor is non-essential, the overload and other protective devices will be set to shut the motor down as fast as possible. Large motors vary in size, cost and characteristics. The following is therefore a typical menu of protection schemes.

Stator overheating For large motors stator overheating is much more serious than for smaller motors. Apart from insulation damage, stator overheating can cause frame distortion and, in some cases, bearing damage. In small motors a bimetallic heater or electronic replica type relay monitors overcurrent and thus estimates temperature as a consequence of this current. In large motors RTDs (resistance-temperature detectors) are installed together with the winding in the slot to detect actual hot-spot temperatures. Mandatory electrical safety regulations allow this method of overload protection. The RTDs operate a temperature relay which first gives an alarm and then trips the circuit breaker when a certain temperature is reached; consequently, no other protection is required except for shortcircuit protection. However, in some process industries the RTDs are wired to the process control, giving central control room operators temperature information instead of providing a protection function which trips the motor. Further protection may be provided with current-balance relays, which are essentially protection for single-phase conditions. This can also include a phase sequence combination if desired. For large motors (≥1500 kW) the overload protection is a choice between the inverse-time overcurrent relay and the thermal or electronic replica (motor heating curve) relay. The thermal replica type relay tries to imitate the motor heating curve. At light loads and long time overload it gives best performance. At heavy overloads it tends to be slow. The inverse overcurrent relay works just the opposite, giving the best protection at heavy loads while remaining unsure and therefore not very sensitive at light loads. A combination of both gives the best protection, but RTDs should always be included for large motors. Both the replica and the inverse overcurrent relays can be equipped with instantaneous trips for short-circuit protection. Modern static or microprocessor type replica relays are much more versatile and may be set to imitate the motor heating and cooling curves much more closely than earlier thermal or electromechanical relays. These newer ver-

Short circuit protection This protection applies to the feeder cable, the stator winding and the motor starter itself (circuit breaker or fused contactor). One protective device for each phase is required. Where a circuit breaker is used, mandatory electrical safety regulations usually permit sensing the fault by integral or external sensing elements which disconnect all phase conductors simultaneously. Where fuses are used, obviously a single fuse could open, leaving the other two intact. This would cause the motor to keep running, but at a reduced output, eventually leading to overheating and overloading. Disconnecting the motor in time to prevent damage depends on the setting of the overload trips. The limiting factor for overload heating is sometimes the stator and at other times the rotor. A more serious single-phase condition will exist if an earth fault on a single phase of a primary feeder to a star-delta transformer blows the fuse, leaving only two phases feeding the transformer. This means that the three-phase secondary side will be essentially single-phased with currents in each motor branch circuit of 115, 115, and 230%, respectively, for the three phases. This means that every motor connected to such a transformer would run the risk of being overheated and seriously damaged. Phase fault protection Phase fault protection can be provided by current-limiting fuses specially designed for motor protection. Alternately, instantaneous overcurrent relays can be used with settings higher than the starting current and feedback current in case of external faults. Instantaneous overcurrent devices affected by the DC component of the current must be set well above the symmetrical value of the starting and fault feedback currents. Modern electronic relays have an instantaneous overcurrent function which is practically unaffected by the DC component of the current. Another alternative is to use an inverse time overcurrent relay with a 92

5.6

92

97-03-05, 13.43

INDUSTRIAL POWER SYSTEM Fault Control The rate of decrease (decrement) of the DC component is a function of the X/R ratio of the circuit and will probably not be fully asymmetrical. Therefore, the setting of the instantaneous devices should be around 12 to 15 times rated full load current. The setting of the time delay devices is based on normal symmetrical starting currents. Acceleration and deceleration time The motor accelerating time depends on the inertia Wk2 of the motor and the load. This must be taken into consideration when setting inverse time overcurrent relays and replica type thermal time constant relays. A motor usually has a guarantee from the manufacturer of, say, ”3 starts from cold” and ”2 starts from running hot condition”. A motor with water cooling may have as many as three different time constants for heating up when starting, and cooling down when cruising down or standing still; one time constant for the copper winding, one time constant when iron is counted in and one more when the body of the cooling water is included. If the cooling water pump is still running after standstill, this will also affect the cooling curve without the replica relay ”knowing” it. Calculation of the accelerating time is fairly simple, providing the loads are known; alternately, the motor manufacturer can be contacted to obtain acceleration times. However, protecting the motor from being ”started to death”, or preventing shutdown due to spurious trips from a thermal relay that is too conservatively set or selected, requires a considerable amount of attention.

sions depend on motor thermal time constants which are sometimes difficult to obtain from the motor manufacturers where international standards for specification of such time constants is still lacking. Rotor overheating In squirrel-cage motor rotors it is not economically possible to install detector devices. The methods used for stator overheating detection will in most cases reflect general overload conditions, but in the event of locked rotor and attempted single-phase starting, the current sensing thermal devices should be used rather than RTDs. The time lag associated with stator RTDs during starting may not reflect the rotor heating at all. As rotor heating on subsequent starts can damage the rotor without the stator current or temperature giving a true reflection of the condition, it is extremely important to agree with the manufacturer on the rotor protection if it is ”rotor-limited”. Particularly during the commissioning phase, motors are often subject to excessive starting stresses. See also ”Accelerating time” below. In synchronous motors a field current is supplied to the rotor, and a connection thus exists to monitor what is going on in the rotor. By using a replica thermal relay in the field current, protection can be provided if the motor fails to start. A further protection is provided by an out of step device, which will shut down the motor for an out-of-step condition (where the motor is out of synchronism with the network). Starting current considerations The symmetrical value of the motor starting current can be anywhere between 3 to 9 times rated current. The asymmetrical (offset) current can vary between 6 to 16 times, or approximately I.8 of the symmetrical value. The asymmetrical current with its DC component is highest during the first cycle and then gradually decays. The rate of decay (decrement) of the DC component is a function of the ratio X/R of the circuit Since the asymmetrical current with its DC component is highest in the first cycle and then gradually decays, the instantaneous part of the relay will react to the DC component, while the inertia in an induction relay or thermal element will only recognise the symmetrical current.

Feedback of fault current into the power system During a fault, all motors will supply current into the fault. The amount of feedback is an inverse function of the subtransient reactance (X”d) which is around 15 to 25% for induction motors, 10 to 25% for high-speed synchronous motors, and 25 to 45% for low-speed synchronous motors. During the few cycles while the subtransient reactance is in effect, the symmetrical value of the feedback currents can be around 4-7 times the full-load current for induction motors, 4-10 times for high-speed synchronous motors, and 2-4 times for low-speed synchronous motors. The setting of the instantaneous overcurrent relay must consider the 93

5.6

93

97-03-05, 13.44

INDUSTRIAL POWER SYSTEM Fault Control The alarm option is usually better for continuous process plants. If the voltage drops to a level where contactors are dropping out, then a major plant problem exists anyway.

sensitivity of the relay to the DC component of the feedback fault current. Phase-to-earth fault protection The most common type of electrical fault in the stator circuit is the phase-to-earth fault. In solidly earthed or low impedance earthed systems an earth fault will result in heavy damage to the stator iron. Therefore the best protection (in addition to fast relaying) is a high impedance system earthing method which will reduce the earth fault current to a no-damage value. A simple method of detecting phase-to-earth faults is installing all three phase conductors through a ”window” type current transformer. Normal load currents are self-cancelling due to 120 degree phase shift; however, during an earth fault, zero-sequence currents will flow. These are all in phase and therefore combine with together, producing an output from the current transformer which is connected to an instantaneous overcurrent relay. It offers sensitive protection and eliminates false tripping due to starting and high inrush currents. An alternate method is a residual connection with a time overcurrent relay in the neutral of star-connected current transformers. This uses a relay with a setting of not more than 10% of the maximum earth fault current. If the maximum earth fault current is more than four times the motor rated current, an additional instantaneous trip could be included with a setting of 3 to 10 times the rated motor current, depending on inrush and feedback criteria.

Differential relays Phase-to-phase short circuits in a motor seldom occur, but if they do, they generally involve large currents and can cause severe motor damage. Phase overcurrent relays must have high settings or long time delays to override starting or fault feed-back currents. For motors above, say, 500 kW, a differential relay is a sensible consideration. For each phase the differential relay compares the currents on the line side with the neutral side. In solidly earthed or low impedance earthed networks, the differential relay also operates for phase-to-phase faults. Bearing temperature protection Some motors are equipped with bearings that are force-lubricated by a lubrication pump. A failure could occur in the oil line rather than in the pump motor. Therefore, the only way of preventing overheating or detecting a fault in the lube oil system is by installing an RTD in the bearing and running the conductors back to a dial thermometer with adjustable trip switch. This switch can operate an alarm or shut down the motor when a certain temperature is reached. An option is the use of a sensor connected to a relay located at the motor. However , this means running control wires and power to the motor, which could represent an obstacle, necessitating an explosion-proof enclosure for the relay if it is in a hazardous area.

Undervoltage protection Undervoltage protection in a large motor is a standard consideration. The torque of asynchronous motors is reduced by the square of the voltage, and by the time contactors drop out at about 65-70% voltage, motor available torque is reduced to about 40%. If the voltage is reduced during starting in a direct-on-line application, the rotor could overheat faster than the stator. Depending on the motor design and winding connection of the motor, the stator could also overheat first. The undervoltage relay or trip device can also have time-delay functions for transient or voltage dip override. The connection can be made as an alarm only, or it can automatically disconnect the motor from the line.

TRANSFORMER PROTECTION OVERVIEW In selecting the right transformer protection one must first consider the transformer’s role in the power system: 1. The size and importance of the transformer. Two possibilities: It is an expensive and important part of the ystem. A transformer failure would result in loss of production. A spare is not available. It is of moderate size or importance. There is an alternative supply or a spare transformer available.

• • 94

5.6

94

97-03-05, 13.44

INDUSTRIAL POWER SYSTEM Fault Control device for the tap-changer compartment. Selective earth fault protection is normally provided for the primary side and selective or differential earth fault protection for the secondary side. Delayed overvoltage protection is provided for transformers with a risk of elevated voltage, which can cause core magnetic saturation and overheating damage if permitted to last.

2. The type of transformer insulation: oil or dry insulated 3. The voltage level and the type of system earthing on the primary and secondary side. In some cases even a tertiary winding earthing has to be considered. Transformer protective relaying is first of all provided to limit the consequences of faults and failures such as a short circuit inside the transformer and in the connecting leads. Such faults are very rare, but if such a failure should occur, it may develop very fast, such that the protection cannot save the transformer from permanent damage. Fast disconnection will, however, limit the results preventing a devastating fire or explosion. Some kind of short-circuit back-up protection is generally provided further out in the supplying network. In an isolated neutral system or with high impedance earthing the earth fault current will be small, and therefore sensitivity and selectivity are more important than high speed clearing. Another internal transformer failure is a broken circuit or a bad conductor joint, which can cause gas build-up or localised overheating with the risk of explosion. A transformer should also be protected against inadvertent overload and high temperatures in the winding insulation.

GAS DETECTION OR BUCHHOLZ RELAY This type of protection is used with oil-insulated transformers with conservator for oil expansion. The gas relay is installed in the connecting pipe between the tank and the conservator. A slow accumulation of gas in the oil will rise through the connecting pipe to the detector relay, and a float will actuate an electrical contact to operate an alarm. The gas can be released and tested. Flammable gas is a serious sign of oil or paper decomposition, while ordinary air is less alarming. For a serious internal fault involving arcing there will be a sudden pressure rise and a surge of oil and gas which will be detected by the Buchholz surge detector. It is a fast and efficient detector which trips the transformer circuit breaker instantaneously. Transformers with a nitrogen cushion instead of conservator are common in North America and can be fitted with a sudden pressure relay instead of the Buchholz type.

Below 1000 kVA, transformer protection is generally provided by primary fuses as shortcircuit protection and overload and earth fault relays as stipulated in most national wiring regulations and electric safety codes. Oil-insulated transformers above 1000 kVA generally require gas detector devices, usually referred to as Buchholz relays, having two levels of gas detection: gas warning and surge tripping. Depending on size and voltage, overcurrent relays or fuses provide additional short-circuit protection.

WINDING TEMPERATURE PROTECTION This device is more of an overload sensor rather than a fault detector. It measures top oil temperature, but has a compensating device which reflects the hot-spot temperature rather than the oil temperature. This also means that the temperature-rise curve is closely followed by the device. It can be made to operate an alarm or a cooling device. This fairly inexpensive protection pays for itself by allowing operation up to full thermal capacity.

Larger transformers have further protective arrangements such as redundant or duplicated short-circuit protection, underimpedance and differential protection. In the case of on-load tap-changers, there may be an overpressure

TRANSFORMER DIFFERENTIAL RELAY This relay operates instantaneously for faults in the zone between the two sets of current transformers (for a two-winding power transformer). 95

5.6

95

97-03-05, 13.44

INDUSTRIAL POWER SYSTEM Fault Control manufacturer’s recommendations. Therefore, it becomes a matter of understanding what the generator manufacturer recommends, rather than simply making one’s own selection. The following gives an overview of the basic areas of generator protection.

In order to be stable for external faults and unsymmetrical events like inrush current during energising, the relay uses a current stabiliser. Harmonic restraint by second and higher harmonics is often used. There is also a need to compensate for possible current transformer differences and for the phase shift in the transformer to obtain current balance for load and external fault currents. This may be solved by interposing current transformers or adjusted for internally in the relay.

STATOR WINDING PROTECTION If a fault occurs within the stator winding, opening of the main circuit breaker only removes the fault current supplied from the network. The fault still exists within the generator and current continues to flow into it. As long as the field is energised, the generator is producing an e.m.f. (electromotive force) which maintains the fault current. If the field supply is removed, then the time to decay the flux will dictate the current reduction in the fault. This is a problem related to winding protection.

SPECIAL EARTH FAULT PROTECTION According to local preferences and practice the following types of earth fault protection may be used: A restricted earth fault relay is an instantaneous differential earth fault relay, useful in solidly earthed neutral systems where an earth fault will represent a high and destructive current. A so-called ”tank protection” is similarly (in a solid neutral earthing system) a current relay connected to a current transformer between the transformer tank and earth.

Phase-to-phase fault This fault requires fast detection and instant shutdown. Even with the generator breaker opened, the generator will still attempt to feed the fault if the field is still connected or the magnetic flux is still decaying. Relaying must therefore detect the fault, disconnect the load, open the field breaker and shut down the prime mover. Phase overcurrent or impedance relays must have a setting above the maximum load current, as well as a time setting which gives selectivity for external faults. Consequently, this relay does not provide an adequate first line of defence for internal phase faults, only as back-up protection. Instead, a sensitive differential relay is applied, except for very small generators.

GENERATOR PROTECTION OVERVIEW A generator is probably the most important item in an industrial electrical power system, being either a co-generator running in parallel with the power company, a standby unit or an emergency generating set. Without it the rest of the system may be useless, or in great trouble. Therefore, the reliability and protection of generators are of paramount importance. A short repair time is also essential, and this means that fast shutdown is required, as well as early indication of an impending fault. The economic justification for maximum possible protection must be weighed against the effect on the system of generator shutdown. If the fault involves iron damage or requires hard-to-get replacement parts in remote areas, then the maximum protection may still be relatively inexpensive. The IPS designer will rarely be called upon to apply protection for generators without the

Turn-to-turn fault This is a short circuit between two coil turns in one of the phases. This type of fault is not detected by phase overcurrent or generator differential relays. For generators with split neutrals the interturn protection consists of a time-lag low-set overcurrent relay sensing the current in the connection between the stator neutrals. The time delay is short, as the interturn fault current can be high, but must also prevent operation due to external fault currents being unbalanced. One method is to use a low-burden ”very inverse” overcurrent relay with specially designed split-phase current transformers. 96

5.6

96

97-03-05, 13.44

INDUSTRIAL POWER SYSTEM Fault Control large unbalance would occur in the magnetic flux. Because of the small air gap in some highspeed turbogenerators with long shafts, distortion could cause the rotor to rub on the stator, resulting in major damage. Again, this problem and the selection of appropriate relaying should be co-ordinated with the generator manufacturer.

Phase-to-earth fault Neutral earthing and stator earth faults The phase-to-earth high current fault is the most serious of the stator winding faults because of possible damage to the stator laminated steel core. Most generators are Y-connected with an earthed neutral. The neutral may be solidly or impedance-earthed, often resistance earthed on low voltages 500-690 V or on medium voltage 3-11 kV generators. Where the earthing is done through low resistance (or direct neutral earthing), then a high fault current exists and differential relaying can be used, protecting about 80% of the winding. High resistance earthing reducing fault currents to some 5-10 A, would preclude differential relaying, but also reduces the danger of iron damage. In this case a separate earth fault relay must be considered. When high-resistance earthing is used, then a current relay may have trouble distinguishing between third-harmonic currents and fault currents.

GENERATOR MOTORING When a generator prime mover cannot supply sufficient power to its own losses, then the electric power system will supply energy to the prime mover via the generator. This is known as ”motoring”. A reverse-power relay can protect against motoring. It can be designed for sensitive operation and will detect reverse power down to about 0.5%. A timer in the relay eliminates spurious tripping due to transient surges, which usually occur during synchronising and switching operations. The losses and motoring power for prime movers vary within a wide range for gas turbines, diesel units and steam turbines.

One alternative method of generator earthing (instead of connecting the 5-10 A resistor directly between the star point and earth) is to insert a small distribution transformer in the neutral with a resistor connected across the secondary. In both cases a voltage relay with third harmonic filter - measuring the voltage across the resistor - will provide reliable stator earth fault protection. If the resistor is selected to limit the earth current to 10 A, the damage to the iron will be limited and therefore high speed is not as essential as if the neutral was solidly earthed. Even so, it is good practice to have the generator shut down as fast as possible on earth faults, taking into consideration the option of the relay instead sounding an alarm in the case of a standby or emergency generator.

NEGATIVE PHASE-SEQUENCE PROTECTION ”Negative sequence” is a term used in symmetrical-component analysis. It is, in effect, a three-phase system of vectors but rotating in the opposite direction to the normal “positive sequence.” The generator produces positive sequence currents of 50 or 60 Hz. A phase-tophase fault or unbalanced loading of the phases would produce negative sequence currents. This negative sequence current produces an additional ampere-turn wave which rotates backwards. This means that it rotates relative to the rotor at twice synchronous speed, producing double frequency eddy currents in the rotor causing excessive heating in the rotor surface, but also in the potential damper winding. Phase-to-phase faults only allow a few seconds before damage can occur. Since the negative sequence currents decrease with distance, they are greatest at the fault location and decrease approaching the generator. This means that the closer the fault is to the generator, the less time is available for clearing it. Phase-tophase faults should be cleared in less than 5 seconds.

FIELD EARTH FAULTS The generator field is DC and is normally unearthed. Therefore, if one earth fault occurs, nothing much will happen. If a second earth fault occurs, part of the field winding will be short-circuited. The field magnetic flux becomes unbalanced. If the short circuit occurred in close proximity, the unbalance could be minimal. If it occurred at two extreme points, a 97

5.6

97

97-03-05, 13.45

INDUSTRIAL POWER SYSTEM Fault Control types rather than anti-friction bearings. As such the bearing metal is subject to “flow” at high temperatures. Various methods are available to measure these temperatures, such as contact-making thermometers, bulb-type thermometers, RTDs (resistance temperature detectors), and thermocouples. All these options are acceptable, but lowmass items such as RTDs and thermocouples should be protected from cold and incorrect locations which could affect their reading.

GENERATOR OVERVOLTAGE Overvoltage at the generator output can be produced by overspeed or by a faulty voltage regulator. Both problems are serious enough to warrant instant shutdown. Therefore, the relaying should ignore transients, accept some voltage rise within limits, and shut down instantly on large increases. A straight overvoltage relay will provide protection against overspeed due to a faulty governor on the prime mover, or a faulty voltage regulator. The time-delay setting should be around 10% above normal, while the instantaneous setting should be slightly above the expected transient voltage increase due to load dumping and prior to the regulator correcting the voltage.

VIBRATION Vibration can be caused by electrical as well as mechanical faults. Therefore, vibration can also be a “back-up” indication of an electrical failure causing unbalanced magnetic forces. The vibration due to the stator becoming “eggshaped,” or asymmetrical, is caused by unbalanced magnetic forces. Loss of dynamic balance of the rotor can produce vibration along with other possible mechanical faults. Where the generator unit is a high-speed turbine, vibration recorders with alarm contacts can be used. For other low-speed types of prime movers, vibration protection is not usually provided.

FIELD WINDING OVERVOLTAGE Overvoltage protection of the field winding is usual on self-excited, slow-running hydro generators, which are mostly power company type installations. Therefore, the average industrial power system designer will not encounter this problem. LOSS OF EXCITATION Loss of excitation will cause the generator to operate as an induction generator. In order to do this, it must lose synchronism. This can cause oscillating and temporary instability. There is time to sound an alarm and allow reapplication of the field, providing the system stability is not affected and the time to correct the problem did not exceed a few seconds, or come up to half a minute. Normally, installing a timer will provide an alarm to give the operator the chance of corrective action. After this, the unit automatically shuts down. The loss of excitation results in numerous changes to reactive power, power factor, system stability, and real power output, and the protection can be accomplished by a directional distance relay operating from alternating current and potential transformers at the generator. The relay is a field-loss relay and inevitably a discussion of field loss will involve the generator manufacturer, who will also have his own recommendations, purely as self-defence against warranty claims.

OTHER TYPES OF PROTECTION Cooling water for bearings can sometimes leak into the bearing oil. An oil level indicator can give an alarm if the level suddenly increases. Creep indicators are limited to the hydro-type generators and are used to prevent bearing damage. A certain speed is required before a film of oil covers the bearing. If the generator is allowed to slowly “creep,” bearing damage could occur due to lack of an oil film. This creep indicator allows the operator to see if the generator rotor has actually stopped rotating. Potential transformer fuse protection will act when fuses blow due to ageing and transient overvoltages. EMERGENCY GENERATOR PROTECTION Emergency or essential generators are usually smaller diesel or combustion engine driven units and may have survival value for many people (say on an offshore oil or gas platform). Such generators are started and load-tested

BEARING TEMPERATURE Bearings in large generators are usually sleeve 98

5.6

98

97-03-05, 13.45

INDUSTRIAL POWER SYSTEM Fault Control Low voltage cable feeders are protected by fuse and switch combinations, or by moulded case circuit breakers (MCCBs) providing short-circuit and earth fault trips.

fairly regularly, mostly every week, and run for a couple of hours. It is important to recognise that the protection requirements during test are quite different from a situation where the generator is called upon in an emergency. Hence it may be correct to equip the generator with a fairly comprehensive set of protective relays which are active only during testing, whereas many of these are blocked off with only shortcircuit protection active during an emergency. Most important for a small generator is to have sufficient short-circuit capacity to operate selectively all connected fuses and circuit breakers in an emergency. Otherwise, the unit could simply shut down due to an uncleared remote system fault. (Faults are very likely to happen in a catastrophic situation where fire or explosions can occur, but these should not affect the operation of the emergency power supply.)

Overhead line circuits Overhead lines are normally not part of industrial complexes, but where distances are cableprohibitive, medium or high voltage lines may be the solution. Lines are protected by circuit breakers with phase short-circuit and earth fault protective relays. When distance protection is used, it is provided in conjunction with overcurrent and earth fault protection, the latter serving as back-up protection. Three-phase auto-reclosing schemes may be considered if the geographical location and lightning incidence require this.

SELECTIVITY IN INDUSTRIAL POWER SYSTEMS

Capacitor banks High voltage capacitor banks normally have their capacitor units individually fused. Large capacitor banks exceeding 1 MVAr are usually connected in double star. A current relay with third harmonic filter connected to a current transformer in the interconnection between the two neutrals can be used to trip the bank before fuses disconnect too many individual units. The unbalance protection can also be a voltage relay set to operate when the neutral voltage of the capacitor bank exceeds a certain value. Individual capacitors may be switched by contactors, circuit breakers or, for low voltage applications, by fused switches. They have to be rated for at least 1.5 times capacitor rated current, and must withstand transient inrush currents up to 100 times rated current. Capacitor bank feeders are normally protected by fused contactors or switches. In the case of circuit breakers, phase fault and earth fault protection are provided.

The following is a short description of what a selectivity analysis is, and a set of rules to use in a relay co-ordination study. This guide is also a check-list for use when auditing selectivity work done by others, performed by hand or by computer. Selectivity means that series-connected overcurrent protection should operate successfully during short-circuit and earth faults and isolate as small a part of the system as possible. The rest of the power system should remain operative. In British terminology “Discrimination” is sometimes used instead of Selectivity. In U.S. terminology “Coordination of overcurrent devices” or “Relay coordination” is also used. SETTING THE SELECTIVITY STAGE A preliminary selectivity study is often carried out early in the power system planning. This is followed up during subsequent stages of design and operation. The analysis consists of drawing time-current curves on log-log paper, either by hand or computer, in order to establish type and settings of overcurrent protective devices. The selectivity study also includes calculating and determining how the devices will operate on available short-circuit currents, as well as giving a graphical presentation of the degree of protection provided.

Cable feeders Plain cable feeders for high voltage distribution are protected by phase short-circuit and earth fault protective relays. Differential protection is provided on all high voltage feeders which can be operated in parallel, or where instantaneous fault clearance is required.

99

5.6

99

97-03-05, 13.45

INDUSTRIAL POWER SYSTEM Fault Control Further, let-through energy (I2t) during the first half cycle of the short circuit must be considered for current-limiting fuses and circuit breakers. Characteristic operating curves of overcurrent devices are usually published by manufacturers on log-log paper with time as ordinate and current as abscissa. To be useful in selectivity work, such diagrams should contain full information on all time and current tolerances and ranges of adjustment and settings. There is at present no international standard stating how such data should be presented, and therefore such diagrams vary considerably.. This is one of the major challenges for anyone engaged in selectivity work. The time-current characteristics for electromechanical, thermal or electronic overcurrent releases are usually published in the U.S.A./ Canada as operating bands including all tolerances. In Europe, these characteristics are often published as lines with tolerances given in per cent. The term ”primary relay” is often used for a direct-acting trip device directly mounted on a low voltage circuit breaker. A ”secondary relay” is energised by the current or voltage derived from an instrument transformer and is installed separately from the CB and considered as a ”switchgear relay” or simply ”relay”. It is important to realise the difference between these terms, as these devices are all part of the selectivity analysis and co-ordinated with each other at different voltage levels. Inverse or constant time/current relays used for overload or short-circuit protection, may be directional or with voltage restraint. When a ”primary relay” is found to be insufficient for a low voltage circuit breaker, separate ”secondary” relays may be added. An example: generator circuit breakers in low voltage installations.

paper, but new computer-aided design simplifies this process. This preliminary analysis is normally part of the conceptual study, and the short-circuit and fault protection philosophy at this stage is reviewed together with network layout and reliability of supply required by the industrial process. An offshore or remote area plant may have a much higher priority for protection compared with selectivity. This is due to the time and expense of bringing in new equipment if it is damaged, as compared with an industrial plant where transport and access are easy. In addition, a purely radial network is easy to make selective, whereas a complicated interconnected network may be difficult to coordinate using ordinary overcurrent devices. DETAILED SELECTIVITY STUDY FOR A NEW PLANT. When all protective devices have been chosen and time-current characteristics have been determined, a detailed selectivity plan is established. The detailed selectivity plan is the basis for setting of relays and overcurrent releases during commissioning. Commissioning also includes calibration and testing of characteristics as part of the hardware quality control. These checks and calibrations may also take place as part of QA witnessing at the manufacturer’s premises. SELECTIVITY FOR EXISTING PLANTS The selectivity plan for a new plant is an effective diagnostic tool for operating personnel when troubleshooting after faults and unscheduled outages. The graphical representation on timecurrent charts is the key to understanding the built-in intelligence in the protection during overload or fault conditions. The familiarity of operating personnel with the selectivity plan is of vital importance to the safety of both people and plant. It therefore becomes essential to update the selectivity plan at regular intervals or whenever changes to the electrical system take place. In older plants so many changes may have occurred that a new selectivity study must be carried out. This usually falls within the responsibility of preventive maintenance and is often combined with other switchgear service during the yearly audit and shutdown in process plants.

PRELIMINARY SELECTIVITY STUDY FOR A NEW PLANT. In order to select correct protection which may later be co-ordinated, an early selectivity evaluation is necessary. This evaluation is based on practical experience of how protective devices best fit together, and at the same time give good protection. Such an evaluation may be difficult without the drawing of time-current curves on log-log 100

5.6

100

97-03-05, 13.45

INDUSTRIAL POWER SYSTEM Fault Control the system neutrals are earthed via • When impedances, the selectivity of earth fault

A TYPICAL SELECTIVITY REPORT A selectivity report may typically contain the following sections:

devices is shown on separate earth fault charts. Each voltage level is considered a separate circuit.

1. Scope of work. Describing the extent of, and criteria for, the analysis. 2. Single-line diagram. Overall diagram for the whole plant 3. General criteria for settings and margins 4. Short-circuit calculations 5. Time-current charts. 6. Tables of device settings 7. Comments on the time-current charts 8. Summary and conclusions

WRITTEN COMMENTS (see under Item 7 above) Each time-current chart is accompanied by separate written comments including: Explanation of each setting chosen (which is not covered by some general criteria). It is important for future revisions and changes to know what were the original reasons for selecting the settings. Comments are made when adequate selectivity or protection cannot be obtained with existing equipment. Alternative solutions are to be proposed (such as replacing equipment and the practical aspects of each alternative evaluated). This applies first of all to old installations, but new plants also sometimes suffer from difficulties with regard to selective co-ordination. Full selectivity is sometimes impossible to achieve, and may not be necessary either. These comments are therefore very important in order to explain aspects which may not be obvious from just looking at the charts. The possibility of operating a device or melting a fuse with the minimum available shortcircuit current must be evaluated and commented on. This is especially important where short-circuit currents are so small that tripping may not occur at all, such as in emergency systems, UPS systems or vital auxiliary low voltage systems . When the network layout causes difficulties in respect of obtaining selectivity, this is to be commented on. In meshed networks it may be difficult to see from the time-current curves and the single-line diagram how the protection is supposed to operate. A functional description which includes interlocks is then required. When, as a by-product of the selectivity analysis, equipment is discovered which may be underrated or misapplied, this is to be commented on. This applies particularly to old circuit breakers which may have become incapable of coping with the current shortcircuit level.

•

TIME-CURRENT CHARTS (see under Item 5 above) These contain the characteristic curves with associated settings of each protective device plotted (or drawn) on transparent A3 log-log paper, 4.5 x 5 decades, 5.6 cm per decade. Although there is no international standard for these graphs, this is a frequently encountered and also practical size to ensure sufficient clarity and legibility.

•

time-current chart includes a single• Each line diagram of the electrical system covered

• • • •

•

by the chart (for ease of reading). Each time-current chart also includes a table of settings for each protective device on the chart, with associated current transformer ratio (where applicable). Each protective device characteristic should be clearly identifiable, both in the singleline diagram and in the table. There should not be so many protective devices on a chart that overview and clarity are jeopardised. (A small number of characteristics should be aimed at.) Maximum and minimum available symmetrical short-circuit currents for three-phase faults are shown on each chart. These currents apply for the network covered by the single-line diagram on the chart. When necessary, the time-dependent variation of the current is shown. When several voltage levels are represented on the same chart, the current scale in amps is marked for each voltage level on the abscissa. Alternatively, multiplication factors are given for converting current to the other voltage levels.

•

•

•

101

5.6

101

97-03-05, 13.46

INDUSTRIAL POWER SYSTEM Fault Control

•

older installations) should be noted in the table. These tables are primarily intended for operating personnel who can then check that all settings are correct. The tables must be updated along with the rest of the selectivity plan.

•

SINGLE-LINE DIAGRAM (see under Item 2 above) An overall single-line diagram of the plant subject to analysis is to be included in the report. When only part of the electric power system is subject to analysis, this is to be clearly indicated in the single-line diagram.

CRITERIA FOR SETTINGS AND MARGINS (see under Item 3 above) The report also requires a section where general criteria for the selectivity work is explained, such as: Time and/or current margins between relay curves of inverse time characteristic. This usually applies to systems for medium or high voltage where relays and current transformers are used. The relay may be of different manufacture from the circuit breaker and have different operating times which must be accounted for in the lower time decades. Time and/or current margins used in the analysis between low voltage characteristics. These margins may be accounted for in two ways: 1. The devices are drawn with their tolerance bands including opening or melting times. The margins are thus inherent in the charts themselves. (This is of course easiest to understand by operating personnel.) 2. The devices may be drawn as single lines, requiring margins to be specified elsewhere.

•

SHORT-CIRCUIT CALCULATIONS (see under Item 4 above) These are the basis for all selectivity work, and are carried out in advance or as part of the study. A summary of short-circuit calculations applicable to the protection is to be presented as a separate section of the report. All impedances and time constants used in the calculations are to be included.

North American and some Japanese manufacturers usually provide time-current graphs as TOTAL operating bands including all tolerances. European and other manufacturers may provide graphs drawn as lines only, tolerances being found elsewhere in their publications. The margins of the device settings are given with respect to: a) Motor rated current and starting current as well as locked rotor withstand time. b) Transformer rated inrush current and thermal withstand current due to secondary short circuit. c) Cable thermal withstand current or I2t curve.

DATA REQUIRED In order to carry out a selectivity analysis, the following data are required: A single-line diagram of the electrical system involved showing: Power, voltage, impedance and connections of all transformers. Normal and emergency switching conditions. Name-plate ratings and subtransient reactance of all major motors and generators. Transient reactances of synchronous motors and generators. Synchronous reactances of generators. Current forcing of smaller generators during short circuit. When emergency supply is part of the study, short-circuit capability of capacitor banks and UPS converters must be given. Conductor sizes, types and configurations. Current transformer ratios. Identification of all overcurrent devices. Ratings, time-current characteristics and ranges of adjustment of all relays, overcurrent releases and fuses under consideration. Complete short-circuit study for both firstcycle and interrupting duties. This includes maximum and minimum expected currents as well as available short-circuit current from all sources.

•

•

TABLES OF DEVICE SETTINGS (see under Item 6 above) For each switchgear assembly (MCC, panel, etc.) a table covering all overcurrent devices should include the following:

or number of feeder, or circuit device • • Name identification number/tag number. of device (relay, fuse, MCCB, MCB, • Type etc., or combination of these) manufacturer, • current transformer ratio, setting ranges and recommended settings.

change to a previous setting or com• Any plete change of protective device (e.g. for 102

5.6

102

97-03-05, 13.46

INDUSTRIAL POWER SYSTEM Fault Control maximum loading on any circuit • Expected considered. special overcurrent protective device • Any setting requirements stipulated by the supply company

EX(e) motors are included in the • When study, it is necessary to include rated current

•

and starting current as well as thermal time limit Te. When direct-started motors and their protection are part of the analysis, starting current, actual starting time and maximum allowable starting time are to be given.

103

5.6

103

97-03-05, 13.46

INDUSTRIAL POWER SYSTEM Voltage and frequency control

INDUSTRIAL POWER SYSTEM CONTROL Contents page 1 General

104

2 Voltage and frequency control

104

3 Fault control

105

4 Motor control

105

5 System events recorder (SER)

105

6 Central control room (CCR) aspects

106

7 Load management system (LMS)

107

8 Load shedding system

108

9 Reacceleration (RA) and restart (RS) system

108

10 Black start system

109

11 Operation control (where to operate, monitor and alarm)

109

General Frequency If the plant is interconnected with a power company, frequency is not generally of much concern, since it is wholly looked after by the generator prime movers in the main grid. However, if the industry has co-generation, or is running its generators in island operation, frequency must be corrected to 50 or 60 Hz, with some small acceptable tolerance. The generator(s) follow the rotation of the prime movers, and the speed is controlled by the turbine or diesel engine governor, which comes with the unit as part of the package delivery. Hence frequency is wholly a slave of the prime mover speed.

There are frequently misconceptions amongst project design engineers as to what is to be ”controlled” and by whom or what in an industrial power system. Unfortunately, the word ”control” has many meanings, and with the advent of fast integrated circuits, microcomputers these days can ”control” just about anything in a large plant. In the following sections some of the most common power system parameters subject to ”control” will be presented. This overview will hopefully assist the electrical and instrument/automation disciplines in agreeing on what method and type of equipment will be best suited to supervise, regulate, monitor, give alarm and execute action in the power system.

Voltage The voltage normally swings continually within acceptable limits as loads are connected and disconnected inside the plant and outside in the power company network. In-plant generator voltage regulators will attempt to bring the voltage back to a predetermined value when it falls outside the set values. Otherwise, the large transformers may be equipped with on-load tapchangers (OLTCs), which also adjust the volt-

Voltage and frequency control During normal operation an industrial electrical system has only two parameters which are subject to regulation, namely frequency and voltage. However, both these parameters are controlled by regulators which are delivered with and are part of the power equipment itself, and which it is not generally considered suitable to purchase separately. 104

5.7

104

97-03-05, 13.46

INDUSTRIAL POWER SYSTEM Voltage and frequency control an assembly of such units is likewise given the name ”Motor Control Centre” or MCC. Starting and stopping may only require manual operation; however, MCCs are normally under the management of a computer, which may execute the start and stop actions without interference from operators. Sometimes the process operator may wish to overrule the computer and start or stop motors manually, providing it is safe for the process to do so. Of course, motor control may be more sophisticated and include the variation of speed, traditionally done with DC motors, but more and more often by adjusting the frequency to either cage-type induction motors or, for larger units, using synchronous motors all the way up to 40 MW for large compressor drives. For such large ASDs (adjustable speed drives) it is essential to recognise that the speed controller (or frequency converter) is an integral part of the motor package, where all parts are finely tuned to each other. The interface with process control is basically only to provide a protocol for an input signal to the frequency converter in order to increase or reduce speed.

age according to a predetermined value. None of this equipment is able to cope with fast variations that are due to disturbances such as starting of large motors in a weak network. Here it is up to the designer to foresee all ”normal” disturbances and engineer the power system such that it can ride through the larger voltage swings without connected loads being interferred with.

Fault control By far the largest and most complex intelligent supervisory system in any power supply is the fault control system. It is generally not thought of in this way as it is usually termed ”relay protection” or simply ”protection”. However, as microprocessors are becoming steadily faster and cheaper, relays are gradually incorporating more and more chips to become like small individual processors with a great number of protection functions, which previously required many individual relays to accomplish. However, there is still some way to go before microprocessor-based relays take over industrial protection altogether, particularly in the low voltage fuse field where a current-limiting fuse will take 2 milliseconds to complete the entire fault sensing, decision making and fault interrupting cycle. Control and instrument engineers are generally not aware of the speed and fast operation required in a power system to protect people and plant against harm and damage when short circuits and other faults occur. Therefore, fault control or power system protection still remains much like a separate electrical discipline with its own engineering rules, sometimes being more of an art than an exact technology. However, this has been presented in rather more detail in the previous section.

System events recorder (SER) When faults or severe disturbances occur in the industrial power system, it may be of advantage to know the sequence of events afterwards in order to diagnose the source of the fault or disturbance. With the extremely fast behaviour of power systems, as many as 50 actions may have taken place within a 100 millisecond period. This is usually too fast for normal process control since the industrial process which it is designed to handle is much slower. Here are some reasons for installing an SER to look after the high and medium voltage systems:

Motor control

1. When medium voltage motors fail, they may still be under some form of guarantee. In order to prove to the manufacturer, for instance, that the failure was due to low quality stator winding insulation and not maltreatment of the motor, the recorded sequence of temperature, protective relay operation and circuit breaker action is a sound basis for further discussions.

The starting and stopping of constant speed asynchronous induction motors, as required by the manufacturing or plant process, is the most common control function in any industry. For this reason, a fused switch combined with a contactor and some minor protective and auxiliary relaying is, in some parts of the world, given the name ”motor controller”. Similarly, 105

5.7

105

97-03-05, 13.46

INDUSTRIAL POWER SYSTEM Voltage and frequency control 2. Although a power company should know its availability at the point of delivery, it may well happen that it does not include in such statistics short voltage dips or even complete outages of less than say 0.2 seconds. These may cause complete plant shutdown if the plant has not been designed to ride through them. A recording of all important power system events is the only solid proof in contractual discussions with the power company in such cases. 3. Some faults or disturbances are very hard to get to the roots of, especially in the area of earth faults. A recording of the sequence of events just prior to and following such a fault may give valuable clues. 4. Spurious (nuisance) tripping without any obvious cause sometimes happens, especially in a commissioning or early operational phase. Even if such trips often have their causes in small wiring or control circuitry not being monitored by the events recorder, a record of the main events may indirectly point to the cause.

Fig. 1. Traditional mimic where the IPS is represented on a sheet steel panel with physical symbols, operating ”handles”, monitoring instruments and indicating lights.

Central control room (CCR) aspects A process type industry will normally have a central control room where all process-related decisions are taken and executed from, either by the operators themselves or by some computer-based logic assisting the operators. There may be one single or many separate control systems collected in the CCR, with their associated mimic panels or equivalent video screen graphical pictures. However, the power supply control and the electrical operators have traditionally had their location elsewhere, outside the CCR. The reason for this is the traditional duties and education of electrical staff, along with the electrical safety codes or regulations in most industrial countries having a 100-yearold history. However, the electric power supply has a strong interface with all that is going on in the CCR.

Fig. 2. Single-line diagram, monitoring, indication and operating functions represented on a modern video display unit (TV screen).

arising which will require partial or total shutdown of the entire plant. Gas release in a chemical plant would be such a situation. And if the gas, in addition, is flammable, all electrical equipment that is not explosion-proof may be subject to shutdown. The ESD is normally a stand-alone control system located in a CCR and is heavily interfaced with all electrical supply circuits. However, there is not much influence from the electrical discipline, except in one respect: the ESD is itself powered from one or several UPS feeders. (If UPS supply is lost, the ESD usually shuts down as a ”failsafe” mode, and nuisance trips of the UPS can therefore be rather embarrassing.)

Interface with emergency shutdown system (ESD) The safety philosophy of any industry has to evaluate the possibility of hazardous situations 106

5.7

106

97-03-05, 13.47

INDUSTRIAL POWER SYSTEM Voltage and frequency control Plants in petroleum refining have carried out integration programmes where all CCR operators have to have full electrical and instrumentation competence (in addition to being familiar with the plant process). Such programmes have invariably been in conjunction with installing an entirely new CCR and process control. In this manner the CCR has become the integrated control centre that control engineers have dreamed about for years. Some of the issues involved are:

Interface with fire detection All electrical rooms usually have smoke and/or fire detectors feeding into an autonomous control system often located in CCR. This control system will sometimes automatically release some fire extinguishing agent after smoke or fire is detected. This could be detrimental to electrical switchgear and control electronics, as an electrical ”fire” is seldom anything but a smouldering in cable insulation, which is best taken care of by interrupting the faulty circuit. It may be hard to convince safety engineers that there is not much point in pouring fire extinguisher onto a burning arc, as this has only the outer appearance of a ”fire”, but is altogether a different natural phenomenon. A burning arc consumes no air and will burn in spite of no oxygen present, contrary to all other fires. The best arc ”extinguisher” is again disconnection of the circuit. Fresh water, with its cooling effect, has proved to be just about the only medium which has any real purpose as a fire extinguisher in electric power equipment rooms.

of electrical and instrument staff is • Training required to make all CCR operators multi-

• • •

disciplinary. This may involve courses and on-the-spot training of two years´ duration. Integrating the operations and maintenance functions Reorganising of the electrical and instrument disciplines, and setting up an advisory engineering department Doing away with the earlier engineering and maintenance manager positions

Load management system (LMS)

Integration of the electrical and instrumentation/automation disciplines The impact of modern control equipment is not so much a technical question as one which addresses old and new ways of organising work. The continuous discussion over many years about what is required to integrate the entire electric power system into process control has an answer which is closely related to what education and training is mandatory in each country for electrical and instrument operators. The answer to this question will also decide how the power system (traditionally run by electricians) can now be operated by ”any” CCR operator. Such integration makes the CCR into the true plant nerve centre from where ALL plant operational commands are executed, AND from where maintenance work permits are issued. Additionally, the CCR may at the same time be made into a command headquarters from where necessary electrical operations would also be carried out in an emergency or catastrophic situation (similar to the way the CCR functions in a nuclear power station). Most manufacturing industries may not need an integrated CCR of this nature, but for large process industries and offshore petroleum installations, there are real benefits to be obtained by such integration.

This is a type of control aimed at saving energy and costs, and is therefore also termed Energy Management. It attempts to optimise the running of all loads such that they will consume a minimum amount of power at all times, without interfering with plant production. A typical load management system (LMS) run by a central processing unit (CPU) and a number of field processing units (FPUs) for a manufacturing plant may incorporate: (heating, ventilating and air condi• HVAC tioning) loads are generally serious energy

•

wasters if not closely monitored. The highest cold-duct temperatures and lowest hotduct temperatures should be set by the CPU to maintain zone comfort. Weekly and holiday scheduling can be set so as to match the running of equipment to expected occupancy in various buildings. With a large number of buildings, and variations of occupancy, the switching on and off the correct amount of lighting, HVAC and other energy consuming services is a difficult manual task, even with semi-automatic devices scattered throughout the complex.

107

5.7

107

97-03-05, 13.47

INDUSTRIAL POWER SYSTEM Voltage and frequency control chilled and hot water monitoring • Process includes tight control of temperatures and

frequency and for setting the disconnection times. 3. The fault duties of switchgear may be exceeded as a result of plant expansion. Rather than purchasing and installing new equipment, one can choose to disconnect fault current sources (such as large motors) prior to attempting to open a circuit breaker in the faulty area in the case of a short circuit. Thus the fault current is reduced to an acceptable level before the breaker begins to interrupt it. 4. Generator overloading may occur if the power company supply is lost. Existing inplant generators will attempt to take on the entire connected load, which causes:

cycling in chiggers, heat pumps, cooling towers, heat exchangers under widely varying seasonal temperatures. Manual control can waste a great deal of energy. By controlling the above parameters, a typical manufacturing plant reduced its 3 MUSD electric energy bill by 10-15% in the first year of operation of an LMS. It is obvious that if a plant process control system is to be designed and installed, it is sensible to incorporate load management as well, rather than purchase a separate CPU just for this purpose.

Load shedding system

drop, resulting in: • Frequency drop on station auxiliaries, which • Speed may be critical on steam turbines, where

Disconnecting loads during faults, or other abnormal circumstances such as too high a load or reduced supply, sometimes also falls under the category of ”load management”. However, it is treated separately here since its main purpose is not to reduce costs in the long run, but rather to avoid total system collapse, or avoid penalty clauses in power contracts. Automatic load shedding equipment is sometimes manufactured under that name by specialist manufacturers. But due to its generally ”slow” operation, it may also be integrated in some central process control scheme. The following situations may call for automatic load shedding in a typical large pulp and paper mill with hydro and steam co-generation:

pump or fan output is proportional to speed. Transformer overfluxing, if voltage can be maintained. Steam turbine blading resonance, which can shorten blade life to as little as 10 seconds. Detrimental effects on process control or computers. Appropriate load shedding will prevent these results from happening.

• • •

Reacceleration (RA) and restart (RS) system It is important to distinguish between on the one hand reacceleration, which takes place after a short voltage dip has occurred and all plant motors have slowed down and are now all trying to regain speed, and on the other hand restart, which assumes that the voltage dip or outage has lasted for so long that RA is no longer possible and an automatic sequential restart has to take place. In the RA case, there may not be enough strength (short-circuit capacity) in the system to manage the large reactive demand from induction motors, which are all drawing starting current at the same time. The system voltage would simply collapse if some load shedding as described above were not applied. In other words, RA is normally incorporated with a load shedding scheme. Similarly, automatic RS may also be combined in a load shedding scheme.

1. Limiting demand may be required if the mill´s own generation is disconnected and demand exceeds the power company contract value. Tripping of load inevitably means restarting, which in case of large motors may represent a large voltage drop and which may be a nuisance to the rest of the plant. This may influence the selection of the loads to be shed. 2. A system instability situation may develop during a sudden loss of generation or power supply, or during fault conditions. Only dynamic fault studies will indicate if there is a latent instability which requires disconnection of loads in order to save the entire system from collapse and total shutdown. Such studies will also provide the necessary time margins for recovery of voltage and 108

5.7

108

97-03-05, 13.47

INDUSTRIAL POWER SYSTEM Voltage and frequency control

Black start system

The locations are: CRC = central control room Gen CP = Generator control panel located close to or adjacent to the generator HV SWGR = high or medium voltage switchroom or a local control room adjacent to the outdoor switchyard LOC = local (this may be on the motor skid or generator skid) ER = system events recorder (located adjacent to switchgear or in a local electrical control room) PMCP = prime mover control panel at the skid or in adjacent local control room LOCANN = local annunciator at the switchboard or in an adjacent local control room LOC CP = local switchgear control panel at the main intake substation

This is normally not only a control scheme, but also a manual or semi-automatic procedure to follow when a complete black-out has occurred. It includes the possible depletion of UPS batteries and necessary fuel tanks for emergency or essential power generation. In other words, all plant is now completely dead, including all control systems and emergency lighting. This may not be an actual possibility for a land-based plant, but an offshore petroleum plant which in an gas explosion or other emergency may have evacuated all staff has to be designed for such a possibility. Also, if land plants go through the exercise of envisaging how to start up after a complete and total outage at any time of the year, there may be some unforeseen consequences in store. For instance, if a chemical plant in a winter climate comes to a complete halt, how are all the frozen pipes going to be thawed out after some hours of standstill?

Other abbreviations are: BS = bus section CB = circuit breaker G/T = generator transformer OLTC = on-load tap changer.

Operation control (where to operate, monitor and alarm) The following is a typical listing of controls, instruments and meters, status indications and alarms, both locally and remotely, for some common types of large electric power equipment. The situation reflected in these tables represents a plant with a normal autonomous system of electrical discipline. This is not an integrated situation as described above in the CCR section ”Integration of electrical and instrument disciplines”. Therefore, a number of local control rooms and locations are assumed. Although this may be typical of a chemical or petroleum type of industry, the lists may also be applied to other types of plant. The requirements at each location in the tables are as follows: M = mandatory MA = mandatory if applicable GA = grouped alarm IA = individual alarm O = optional T = close circuit breaker in test position only

109

5.7

109

97-03-05, 13.48

INDUSTRIAL POWER SYSTEM Voltage and frequency control MAIN GENERATOR with PRIME MOVER CCR Generating set controls: - normal start - auto start - fast loading - normal stop - emergency stop - local/remote (control selector switch) Isochronous/droop control (selector switch) Generating set - base/peak (load selector switch) Governor setpoint control Generator CB control G/T OLTC - tap raise/lower G/T OLTC - local/remote selector switch Field switch - on/off Voltage regulator - auto/manual selector switch Voltage regulator - voltage/P.F. selector switch Voltage setpoint control - auto Voltage setpoint control - manual P.F. setpoint control Synchronising selector switch Generator status indications: Generating set - local/remote control Generating set - base/peak load operation Isochronous/droop operation Generator CB - open/closed Field switch - open/closed Voltage regulator - voltage/P.F. Generator instruments and meters: voltage frequency current real power reactive power power factor real energy summated reactive energy summated Synchronising instruments Field voltage Field current Stator temperature Hours-run meter

HVSWB

Gen CP

M M MA M M

MA M M M

O

MA

MA

MA

M O MA

XT MA MA

M M

110

M

M M MA

MA

M M MA O

M M MA M

M MA MA M O MA M M M M M M

M MA MA M M

M MA

M M M MA

M M M M M M

M M O

M M M M M M

110

5.7

PMCP

97-03-05, 13.48

INDUSTRIAL POWER SYSTEM Voltage and frequency control Generator alarms: Master trip relay(s) Stator temperature high alarm Coolant temperature high alarm G/T temperature high alarm G/T Buchholz gas alarm Excitation system alarm Rotor earth fault Auxiliary systems All protection relays (trip) Mechanical non-trip alarms Mechanical trip alarms

M M M MA MA M MA M

M M M MA MA M MA M IA GA GA

IA IA

POWER COMPANY INTAKE AND MAIN HIGH VOLTAGE SUBSTATION CCR Controls: Incoming feeder CB CB control sync relay & selector switch Incoming transformer OLTC tap raise/lower local/remote selector switch AVR setpoint control auto/manual selector switch Bus tie or Bus coupler CB CB control sync relay & selector switch Outgoing feeder CB CB control

O MA

HV SWBD

XT

O

LOC CP

M M MA MA MA MA

O O MA

XT

M MA

O

XT

M

Status indications: All CBs - open/closed

O

M

M

Instruments and meters: Busbars (per section) voltage frequency

O O

M M

M M

Incoming feeder voltage current real power reactive power power factor real power summated reactive power summated transformer tap position

O O O O O O MA O

M M M M M M MA MA

M M

MA

111

5.7

111

97-03-05, 13.48

M O O O O O O O O GA GA

INDUSTRIAL POWER SYSTEM Voltage and frequency control Outgoing feeder current real power reactive power real energy

O O O O CCR

Alarms: All circuits - master trip relay(s) Switchgear tripping supplies battery/charger Switchgear closing supplies battery/charger (each) Trip circuit supervision (per busbar section) Loadshed (per stage) Annunciator repeat alarms Annunciate fault HVAC failure Substation temperature too high/too low

IA

MA GA GA GA

M O O O HV SWBD

LOCANN

ER

IA

IA

IA

IA

IA

O

IA IA

IA IA

O O

MA

IA IA IA

GA

IA

MEDIUM VOLTAGE SWITCHBOARDS CCR Controls: Incoming feeder CB close/open Bus section CB close/open Distribution feeder CB close/open Motor CB close/open Status Indications: Incoming and bus section CBs open/closed Distribution feeders - open/closed Motor feeders - open/closed Motor feeders - operations counter Instruments and meters: Busbar voltage (per section) Incoming line voltage Incoming line current Bus section current Distribution feeder current Motor feeder current Motor running hours

HVSWBD

LOCANN

M M M XT

O

O O O O

M

O

M M M

O

M M M O M M M

M

112

5.7

112

ER

97-03-05, 13.49

INDUSTRIAL POWER SYSTEM Voltage and frequency control

Alarms: All non-motor circuits master trip relay Motor circuits - protection operated Switchgear trip supply battery/charger (each) Switchgear closing supplies battery/charger (each) Trip circuit supervision (per busbar section) Annunciator repeat alarms Annunciator fault Ventilation failure Substation temperature too high/too low

GA

IA IA

IA IA

GA

IA

IA

O

IA

IA

O

IA

IA

GA

GA GA GA

IA IA

GA

IA

LARGE MOTORS (Medium Voltage) CCR Manual controls: Normal start Normal stop Emergency stop Local/remote selector switch CB control Excitation setpoint control (sync. mach.)

M M

Status indications : Drive stopped Drive running CB - open/closed

M M M

Instruments and meters: Stator current Real power M Reactive power Power factor Field voltage (sync. mach.) Field current (sync. mach.) Stator temperature Hours-run meter

M M M

M

HVSWBD

M

M M M M

M M M M

M M M M M M M

M M

Alarms: Master trip relay Stator temperature high alarm Coolant temperature high alarm Excitation system alarm (sync. motor) Protection relays on switchgear Electrical non-trip alarms Mechanical non-trip alarms Mechanical trip alarms

LOC CP

M M M M M M M

M M M M IA

IA IA IA

IA GA GA

113

5.7

113

97-03-05, 13.49

INDUSTRIAL POWER SYSTEM Fault Control

INDUSTRIAL COGENERATION OF POVER AND HEAT Contents page 1 Introduction

114

2 The steam turbine cycle

114

3 The steam turbine

115

4 The VAX steam turbine family

116

5 The gas turbine cycle

117

6 A gas turbine example

120

7 The combined cycle

121

8 Cogeneration with steam or gas turbines

122

9 Electric generators

123

10 Control system

124

Introduction Power generation by thermal processes in gas turbines and steam turbines involves unavoidable losses. These losses are discharged in the form of low grade heat to the environment as hot exhaust air or warm cooling water in normal power stations. In industry, however, as well as in areas where heat can be distributed to local heat users by district heating, this low grade heat can often be used as a full replacement for heat from separate heating boilers. The various thermal cycles will be explained in some detail in the following.

continue all the way down to nearly the cooling water temperature. Thus, the steam is expanded down to a pressure far below atmospheric pressure. It is then condensed into water again in order to reduce the volume of working fluid to be put under pressure again for a new cycle. High pressure and high steam temperature increases the efficiency of the steam cycle. Additional efficiency can be achieved by introducing multiple stage preheating of the feed water.

The steam turbine cycle In a conventional steam turbine cycle, water is used as the working fluid. The water is put under high pressure by a feed pump in order not to boil at a too low a temperature. In industrial systems the pressure may be 100 bar or even higher. The water is heated in a boiler by burning of fuel. It evaporates into steam, which is subsequently superheated in a separate section of the boiler. The resulting superheated steam is expanded in a turbine where mechanical power is generated. The expansion may

KOPIA

Fig. 1. Elementary diagram of the steam cycle. 114

5.8

114

97-03-05, 13.49

INDUSTRIAL POWER SYSTEM Fault Control At partial arc admission, steam is introduced at full pressure through several separate valves and nozzles covering part of the first rotating blade row, the governing stage. The load is controlled through opening or closing these valves. At full arc admission, the steam enters the turbine through one control valve and over the full arc of the first turbine stage. The load is controlled through sliding live steam pressure. This arrangement is common for steam turbines operating in combined cycles. Steam for process purposes can be extracted from the turbine at suitable pressure levels. At the stage of extraction, additional space is provided for between the turbine discs. Holes in the turbine casing allow steam to flow into a collection chamber integral with the casing and further to the extraction steam pipe. The steam flow from the extractions may be controlled by means of internal or external devices. The steam turbines can be equipped with a stage of variable stator vanes for extraction control. By closing the stator vanes downstream of the extraction, more steam will flow into the extraction pipe. External control is accomplished by a throttle valve in the extraction pipe. In district heating turbines the steam flows to heating condensers, which operate under some pressure. Often, two-stage heating is used to increase the overall cycle efficiency. Steam then leaves the turbine at two different pressure levels close to each other and condenses in two separate heating condensers. Steam turbines are tailored to their purpose, i.e., the steam channel is designed uniquely for the individual customer’s needs. Modern de-

Steam is then extracted from the turbine at suitable temperature levels and fed to heat exchangers in the feed water stream. Steam cycles operate in the range of 40 % efficiency as condensing power plants. The limiting temperature for the steam cycle depends on the maximum operating temperature of low- to medium-cost construction steels used in the boiler and turbine or on other economic restrictions. The temperature is often in the range 450 to 540oC. Industrial steam power plants often operate in the pressure range 60 to 120 bar. Process related limitations may reduce these values, e.g. in waste incineration boilers, where hot corrosion is a major problem at high temperature. The combustion of fuel is separate from the working steam, and practically any fuel can be used. Coal is used frequently around the world for steam power plants due to its attractive cost in comparison with other fossil fuels available.

The steam turbine The steam turbine transforms the thermal energy of the superheated steam into mechanical power driving the electric generator. Modern designs feature high efficiency of the steam expansion in order to maximise the useful energy. Steam enters the turbine through an emergency stop valve and one or more inlet control valves. The valves control the steam flow during operation. Depending on the design, the steam admission can either be full or partial arc.

Fig. 2. Typical configuration of a two-cylinder VAX steam turbine. 115

5.8

115

97-03-05, 13.49

INDUSTRIAL POWER SYSTEM Fault Control signs can meet these demands and yet have many standard parts and components. The VAX steam turbine manufactured by ABB STAL is a typical example. For each module the casing, bearings, bearing distances, shaft glands, blade clearances and inlet valves are the same. Also the auxiliaries come in modules, including the lube oil, hydraulic oil, gland steam and control systems.

see Fig. 3. The stator parts are kept together by means of rings, which build up the package. The HP turbine is geared to the generator, which can be of a four-pole or two-pole design. Steam is introduced through four symmetrically located control valves with integral cast nozzle segments. Alternatively, a single inlet valve can be used. This is an efficient arrangement for sliding pressure operation or for turbines running with small load variations.

The VAX steam turbine family

LP modules The LP modules normally have an axial exhaust with the condenser bolted directly to the turbine casing. It allows for a simple foundation and a low and compact layout. The blading has a roof in all stages except in the last two, which have free-standing blades. The LP modules are either geared or directly coupled to the generator. If required, the steam exhaust can also be arranged downwards, upwards or to one side. The casing is horizontally split and fabricated from steel plate. Steam up to 20 bar and 440oC can be used.

To get the best possible efficiency from the VAX steam turbine, expansion has been split over two turbine casings when admission conditions are high and the steam expands to vacuum. The following modules are available in the VAX series: HP modules The high pressure turbines are of a barrel design with a vertically split casing. They accept steam up to 140 bar and 540oC. Designs are also available for 180 bar and 560oC. The radially uniform casing allows for thermal flexibility and smaller clearances, which contributes to a high efficiency. The rotor and diaphragms are assembled into a package, which is put into the casing horizontally using a special tool,

MP modules The MP modules are extremely compact. High efficiency is secured by using advanced exhaust blading developed for large power plant steam turbines. The smallest MP-modules are

Fig. 3. VAX HP turbine. Assembly of rotor/stator package with casing. 116

5.8

116

97-03-05, 13.50

INDUSTRIAL POWER SYSTEM Fault Control nally controlled extraction. Moreover, they can have pass-in of steam from external sources or from a multi-pressure steam boiler in a combined cycle. The MP modules can have a maximum of five different extractions. Control system The VAX control system comprises a built-in turbine governor and electro-hydraulic valve control, turbine protection and man-machine communication. The man-machine communication and automatic equipment for controlling auxiliary equipment can be connected to the corresponding system of other parts of a plant. The control system is based on wide experience from demanding power plant applications. It has been delivered as a stand-alone system as well as for integration with the overall decentralised control system for the plant.

Fig. 4. Medium pressure module MP24 of the VAX series. of high speed design and is geared to the generator. Larger modules, however, are directly coupled. The MP modules can accommodate steam up to 90 bar and 510°C. MP modules can accommodate steam up to 90 bar and 510°C. MP modules are used in combined cycle plants and in waste-to-energy plants, where the admission data are lower.

Starting The thermal flexibility of the VAX results in short start-up times compared to other designs. Typically, less than one hour is required from cold machine to full load. A hot start can be made in less than 20 minutes. For plants operating in dispatch mode short starts contribute additional power.

Extractions and admissions The VAX modules are designed for multiple admissions and extractions with internal or external control. Two locations are available in the HP turbine. In two-casing arrangements, steam can be extracted from the cross-over pipe and an additional three extractions can be arranged in the LP turbine. The LP and MP turbine modules can be furnished with interNatural gas

71.9 MJ/s 245,3 MBtu/h

48 815 kJ/kg 20 987 Btu/Ib 1.013 bar 14,69 psis

15°C 59°F 77,7kg/s 171 lb/s

14,2 Bar 206 psia

The gas turbine cycle In a gas turbine the power is generated by compressing air up to a pressure level of ten to

1.013 bar 14,69 psia

534°C 993°F 79,2 kg/s 174,4 lb/s

Combustion chamber

7700/1500 rpm 7700/1800 rpm

1112°C 2034°F

Gas generator

10 Stage axial compressor

2 Stage compressor turbine

2 Stage power turbine

ISO cond., Nat. gas fuel, Base load operation 24,6 Output, MW 34,2 El. eff., % 10 256 (9 979) Heat rate, kJ/kWh (Btu/kWh) 71,9 (245,3) Fuel flow, MJ/s 8MBtu/h) 79,2 (174,4) Exhaust flow, kg/s (pps) 534 (993) Exhaust temperature, °C (°F) 44,7 (152,7) Exhaust heat, MJ/s (MBtu/h)

Fig. 5. Elementary flow diagram. GT10 gas turbine manufactured by ABB STAL. 117

5.8

117

97-03-05, 13.50

INDUSTRIAL POWER SYSTEM Fault Control power than the net power output of the gas turbine. Efficient axial compressor designs were developed after World War II, which made jet engines a realistic alternative to piston engines. One of the reasons for the success was the considerably reduced number of parts compared with a piston engine. This is accompanied by higher reliability and lower maintenance costs. Modern computer-based calculation has improved the efficiency and reliability of the compressor designs to very high levels. The compressor is usually made from 12% Cr steels and the temperature is around 400 to 500 oC at the exit of the compressor. A compressor based on aerodynamic effects is sensitive to surge in contrast to displacement compressors. Surge occurs when the real pressure ratio is higher than the pressure ratio the compressor is able to deliver at a certain mass flow. This can occur at start-up or during transients. In order to handle these difficulties movable guide vanes are used to modify the compressor performance. Extraction of air from the compressor is often necessary in order to safely pass some difficult operating points during start-up. The compressor transports very large volumes of air There is always some particulate present, which can deposit on the blades in the air channel. Deposits will make the compressor less efficient and may lead to increased risk of

40 bar. The compressed air is further heated by combustion of gas or liquid fuel. The resulting hot combustion gases then expand through turbines, which drive the compressor and the generator. All combustion products - including ashes - get in contact with the metal surfaces of the turbine stages. In order to avoid hot corrosion, the range of fuels is limited to gas and relatively clean liquid fuels. As air has a larger volume at higher temperatures it will be possible to extract more energy during expansion than during compression of the air. The surplus energy is used to drive the generator. As a by-product, hot exhaust gas is emitted. Typically, the temperature of the exhaust gas will be 375-600 oC. The thermodynamic efficiency of gas turbines will be in the range 28 to 42 %. Modern industrial machines will have an efficiency of some 35%. Gas turbines should be seen as standard products. Contrary to steam turbines, they are seldom changed to fit a process due to the large development costs involved. The main components of the gas turbine are the compressor, the combustor and the turbine. The compressor The compressor design is in most cases of the axial flow type with 10 to 20 stages. The efficiency is critical as the compressor uses more

Fig. 6. Cut-away of the GT10 gas turbine. 118

5.8

118

97-03-05, 13.51

INDUSTRIAL POWER SYSTEM Fault Control surge. They have to be removed on a regular basis by washing. Washing is accomplished by spraying cleaning liquid into the compressor during low load operation or by stopping the machine and soaking the compressor with water for a short period. It will remove most of the dirt and restore the compressor to its original performance. Some deterioration is not possible to correct, however. The reason may be corrosion causing rough surfaces, or erosion of particles which changes the blade geometry to a less efficient form. Another cause of performance deterioration is increasing blade tip clearances, which may occur if rubbing takes place, i.e., contact between the rotating and stationary parts during operation. Rubbing results from rapid transients when the thermal expansion of the rotor and the stator reduces the blade tip clearances to zero. In normal operation it results in increased leakage over the blade tips and lower efficiency. This is one of the reasons for keeping the transients at reasonably low levels. The compressor controls the air flow and thereby the available gas turbine power. The effect of air temperature on performance is quite marked, as a result of higher air density at low temperatures. Thus, the gas turbine output is higher at low ambient air temperature. A 15 oC increase in temperature may reduce the available power by some 5%.

stability and requires extra arrangements for some operating points, such as low part load. In practice most of the air is mixed with fuel before combustion. Cooling of the combustor by leaking cold air films along its walls to reduce metal temperature is no longer possible. The combustor has to be cooled by leading combustion air for cooling in channels in the combustor.

Combustion air

Gas fuel stage 2

Flame

Gas fuel stage 1 Liquid fuel Gas fuel stage 2 Atomisation nozzle

Gas injection ports

Fig. 7. ABB’s burner for dry low emission (DLE). Combustor design is a compromise between different requirements: low NOx formation, fuel flexibility and turbine inlet temperature at various operation points. Limiting factors are the maximum metal temperature, permissible pressure drop, etc. Today, dry low emission (DLE) combustors dominate the market now that gas is the main fuel. DLE burners for liquid fuel will be based on the same principles, but they are not yet available on the market. ABB has developed a DLE burner, which is based on the principle of lean premix combustion. The burner is a very simple design with a split cone where the two halves are displaced to form two slots where air can enter. Fuel gas is introduced in the centre as well as through small holes along the slots to achieve the desired premix. The gas-air mixture forms a vortex, which breaks down at the end of the cone and combustion takes place. Nox formation is reduced to some 25 ppm at 15% O2.

The combustor The combustor is an advanced device for combustion of fuel. The energy density is very high. A major requirement on the combustion process is to keep Nox production at a minimum to reduce the effects on the environment. The major gas turbine manufacturers are striving to reduce Nox formation by developing so-called lean premix combustors. In a traditional combustor the fuel is introduced as a cone of concentrated fuel, which burns in contact with the surrounding air flow. Thus, the temperature passes the theoretical maximum during combustion and dilution of the gases. In a lean premix burner the fuel is mixed with the combustion air before it is ignited. The maximum temperature can be kept at a lower level close to that required at the turbine inlet, and the NOx-producing peak temperatures are eliminated. Unfortunately, this results in lower flame

The turbine. The turbine parts operate at very high temperatures. In fact, the operating gas temperature may exceed the melting point of the materials used. To master the situation the metal parts 119

5.8

119

97-03-05, 13.51

INDUSTRIAL POWER SYSTEM Fault Control have to be efficienly cooled. This is done by film cooling and internal convective cooling with air bypassing the combustor. The cooling air is not used for power generation and represents a loss. Optimum use of small flows of cooling air would give a competitive edge. Due to the high temperatures, the relative expansion of different turbine components is critical, especially during transients. A good design results in low clearances during operation and low risk of rubbing.

the grid. It could also be a limit of heat production if it is a cogeneration unit or any other control parameter from the outside or from design limitations of the installation. A simple cycle gas turbine or a condensing combined cycle is usually limited by the rated maximum power or some grid limitation. A cogeneration system is usually limited by the rated maximum power or the need for heat.

Gas turbine operation The gas turbine must be started by a cranking motor, often electric or diesel. Other means of starting are also available, such as gas expanders or by compressed air. When the air flow through the combustor is sufficient, the fuel valve opens and the fuel is ignited by a spark plug. The engine is accelerated to idling speed by the external power in combination with the power it generates on its own. Blow-off valves on the compressor are opened during part of the start-up process to avoid surge due to to low mass flow. When the turbine is running it will be influenced by at least one limitation in the control system. It is left to increase its power until it reaches a limit. Such limits could be the maximum generator electric output or a limit from

The ABB STAL GT10 is a modern industrial gas turbine. The machine has a power of 24,600 kW at continuous load. The pressure ratio is 13.6, i.e., the compressor raises the pressure almost14 times the ambient pressure. The air mass flow is 79.2 kg/s. The high exhaust temperature of 536 oC makes the unit most suitable for combined cycle operation. Further technical particulars are shown in Fig. 5. The GT10 is an industrial gas turbine designed and built for industrial operation. There are two main machine types on the market, the aeroderivative and the industrial gas turbine. Both are used in power plants, but they have different characteristics. The aeroderivative, as the designation implies, is a jet engine originally developed for powering aircraft but suitably modified for

A gas turbine example

Fig. 8. A GT10 gas turbine in the assembly shop at ABB STAL, Finspong. 120

5.8

120

97-03-05, 13.52

INDUSTRIAL POWER SYSTEM Fault Control stationary duty. It features compact design, light weight and high efficiency, but requires good fuels and frequent maintenance. The industrial gas turbine has been developed for industrial use and high continuous load. Weight is normally not an issue, so more rugged designs and materials are applied to ensure reliability in operation. A wider range of fuels can be burnt, but the simple cycle efficiency is slightly lower. Maintenance intervals are longer and on-site overhauls can be made. While the design of a modern jet engine departs from the optimum path for cogeneration and combined cycle, the industrial design is narrowing the efficiency gap in such installations. Both the GT10 and the 17 MW GT35 industrial gas turbines are equipped with a DLE burner for NOx abatement. The GT10 has been been used with a DLE burner since 1991.

Fig. 10. The interior of a KA10-1 combined cycle power plant with the steam and gas turbine driving the same generator, providing simplicity and compactness.

Steam Turbine 15% 11%

WHR Unit

Combined cycle power plants have gained popularity for their compactness, short construction time and environmental friendliness when built around modern gas turbines with DLE capability. Based on a given heat demand, the com-

80°C 40%

33%

55°C

100% Fuel

Fig. 9. Elementary combined cycle flow chart. Waste heat recovery unit Steam turbine

The combined cycle From the cycle descriptions above it appears that the gas turbine cycle will emit hot gas at a temperature close to the temperature of live steam in the steam turbine cycle. It would be natural to use the hot exhaust gas from a gas turbine as the heat source for a steam turbine cycle. Such cycles are called combined cycles. The exhaust gas is used for steam production in an exhaust boiler and the total effect will be that a gas turbine with 33 % efficiency will produce another 40 % of the recoverable heat as electricity. The thermal efficiency may reach 50 % or more if the steam turbine runs in condensing mode. The steam end of the combined cycle can be arranged in the same way as a conventional steam cycle for extraction or back pressure operation.

Fig. 11. Combined cycle power plant of type KA10-2 in Borculo, Holland. 121

5.8

121

97-03-05, 13.54

INDUSTRIAL POWER SYSTEM Fault Control If the process requires a fixed temperature, the steam flow is normally throttled in the turbine downstream of the extraction. This is a controlled extraction. The remaining steam, not bled off to the process, can be used in a low pressure turbine, where it expands down to the same pressure level as in a condensing turbine. A pulp mill is a typical example, where the digesters will need steam at 12 bar and the dryers at 2 to 3 bar. The plant may also need heating of buildings in the winter with heat at 80 oC. A similar application is district heating, where the heat load usually is at lower temperatures than in a process industry. Often, the peak temperature of the hot water is 120 oC and normal operation is at 80 oC. In all these applications the steam turbine has to provide the flexibility, be it in a steam turbine cycle or a combined cycle. In a gas turbine cycle where the heat is recovered directly from the exhaust gas, the temperature level is usually not changed. A rare exception is when fuel is burnt in the exhaust to increase the temperature of the gas. More common, however, is to burn fuel in the exhaust boiler to get enough heating energy at extreme conditions or to follow heat load variations.

bined cycle will produce more electricity thanks to its high efficiency. The KA10-1 from ABB STAL built around the GT10 gas turbine can serve as an example of a modern combined cycle plant. The output is about 35 MW in condensing operation. The gas turbine and the steam turbine drive the common generator from opposite ends. Steam is raised in a two-pressure boiler to maximise the efficiency of the total plant. The machinery and boiler are arranged in a compact common building. Depending on customer demand, the gas turbine and the boiler may be arranged for vertical or side exhaust. The steam turbine and gas turbine are connected to the generator by means of flexible couplings to allow for flexibility in operation.The KA10-1 combined cycle plant may reach a thermal efficiency of close to 50% in condensing mode. If higher power is required, two GT10 gas turbines may be combined with one larger steam turbine to produce up to 70 MW, forming the KA10-2 model.

Cogeneration with steam or gas turbines

14% Losses

Process industries often have large heat demands as well as demands for power. They may also generate by-products which can be used as fuels, such as gas from refineries or black liquor from sulphate pulp mills. These fuels form the basis for the application of cogeneration. In industrial applications where heat is used the temperature is set by process requirements, and these can vary considerably. The steam cycle is adapted to such requirements by extracting steam at a pressure where condensation corresponds to the temperature required by the process. The condensation heat of the extracted steam flow is then used to replace heat generated directly by some fuel in a separate boiler. Several extractions are often needed to supply steam at the correct pressure levels set by the process. In some applications, the total steam flow passing the turbine is used for process heat demands. The turbine is then of the back pressure type. In most cases, the condensate is recovered and pumped back to the boiler, and the residual heat goes back into the system.

Waste Heat Recover Unit

53% Heat

33% Electric Power

100% Fuel

Fig. 12. Gas turbine with cogeneration. Waste heat recovery unit Electric power Economy of cogeneration The normal thermal efficiency of a steam turbine cycle is approximately 40 % and for a gas turbine cycle some 30 to 40 %. The combined cycle reaches approximately 50% in the size range used by industries. These values could be compared with the efficiency of a cogeneration system, where the total efficiency of power and heat may be in the order of 85 %. 122

5.8

122

97-03-05, 13.54

INDUSTRIAL POWER SYSTEM Fault Control The loss of power by heat recovery is negligible for a gas turbine plant. The power loss in steam turbines operating in process applications depends on the pressure and flow at the extraction points. In order to use the heat at the temperature needed, it is necessary to sacrifice part of the power production. In a district heating turbine the losses are some 30 % of the power attainable in condensing mode. The extra production of heating energy is often twice the production of electrical energy. The ratio between electrical and heating energy of a cogeneration plant is called the alpha value. Steam cycles have an alpha value around 0.5, while combined cycle plants may reach 1.0 or even higher. The loss of power has to be evaluated against the value of the additional heat produced and the incremental investment for cogeneration. In most cases it is very profitable to use cogeneration when replacing heat produced by commercial fuels. The structure of the power sector differs widely between countries, and the success of a cogeneration project also depends on how the benefits can be shared between the power user, the heat user and the power supplier.

Figure 13: ABB air-cooled turbo-generatortype GTL. Typical arrangement for industrial cogeneration with top-mounted coolers, overhung exciter, main terminals on the sides and shield bearings. speed turning of the turboset, the generator can be equipped with a turning gear/ motor if the drive arrangement is a single-end drive. The exciter machine is of the overhung type. Air/water coolers are normally located on the top and the generator terminals are sidemounted. This arrangement has been developed to suit the specific requirements of industrial cogeneration where low foundations are required in order to minimise civil works.

Electric generators Introduction Generators for industrial cogeneration are either of 2-pole or 4-pole type. The 2-pole generator, a so-called turbo-generator, has speeds of 3000 and 3600 rev/min for 50 and 60 Hz, respectively. The 4-pole generator has corresponding speeds of 1500 and1800 rev/min. The output range is 20-150 MVA, with 2-pole generators covering the entire range and 4-poles covering 20-60 MVA. The voltage range is 6 - 20 kV, with standard voltages being 10.5 - 11.5 kV for 50 Hz and 13.8 kV for 60 Hz operation. The efficiency is > 98 %. The generators comply with the international standards IEC and ANSI.

Cooling For industrial cogeneration applications the generators used are predominantly air-cooled. Even though other cooling media than air are used at outputs as low as 70 MVA, air cooling is today the predominant method of cooling. The advantages lie in lower unit cost, less space required, system simplicity, higher safety and lower maintenance requirements. In terms of size, the air-cooled technology is today used for above 300 MVA, and even higher outputs are being contemplated. There are, in principle, two ways of arranging air cooling: 1. Air is circulated in the generator internally, dissipating the heat collected in the generator active parts in air/water coolers mounted on the generator body. The coolers can be arranged as top or side mounted.

Arrangement The turbo-generator is a synchronous machine with a horizontal shaft and a cylindrical rotor supported in shield bearings. The shaft can be connected to a turbine at one end (single-end drive) or at both (double-end drive). For low 123

5.8

123

97-03-05, 13.54

INDUSTRIAL POWER SYSTEM Fault Control imity (shaft displacement) probes. Temperatures in windings, cooling air and bearings are monitored by means of resistance temperature detectors (RTDs). To ensure that fluids (oil, water) do not collect in the interior of the generator, leakage detectors are supplied.

2. Air is supplied from outside through filter arrangements, and after collecting heat from the active parts, it leaves the generator through an exhaust. Inside the generator the air is circulated by means of axial fans mounted on the generator shaft. The cooling air collects heat from the stator winding end turns, the rotor winding and the stator core/ winding.

Excitation The generator is usually equipped with a brushless excitation system. In order to guarantee a black start feature, the excitation energy is generated in a permanent magnet generator (PMG) mounted on the generator shaft. The PMG feeds a thyristor bridge, which feeds the main exciter with field voltage. The main exciter armature voltage is converted to generator rotor DC voltage in a rotating diode bridge. The diodes are over-sized in order to guarantee trouble-free operation with minimum maintenance requirements. The voltage regulation equipment controls the thyristor bridge and thereby the generator voltage.

The rotor The cylindrical rotor has radial slots in which the rotor (field) winding of silver-alloyed copper is located. The winding is secured in the rotor slots by means of wedges. The winding is cooled with air flowing axially in the rotor slots. The stator The stator housing has three functions: 1. Support for the rotor via the shield bearings 2. Support for the stator active parts, e.g. core and winding 3. Directing of the cooling air

Control and protection A generator control and protection cubicle (GCP) can be supplied with a choice of equipment to be installed in it. It normally contains the voltage regulating equipment, excitation thyristors, protection relays, synchronising equipment, temperature monitoring and various instruments. The generator voltage is regulated with an automatic voltage regulator (AVR), see above, Excitation). The regulator is of a digital type, based on the ABB Advant Controller ® process control system and can comprise additional functions such as synchronising. Other features in the AVR are a power system stabiliser, diode monitoring and rotor earth fault detection. Protective relays of ABB type REG can be supplied for various protection tasks.

The stator core consists of silicon-alloyed electroplate which is stamped into segments and stacked inside the stator housing. The core is arranged in a number of axial packages with cooling air flowing radially between the packages. The core is held together by means of two preasure plates mounted in the stator housing. The winding insulation system used is predominantly of the pre-impregnated, resin-rich type. ABB generators use the Micarex® system. Terminals The generator neutral leads are terminated in a neutral terminal enclosure, equipped with neutral grounding devices. The line leads are conveyed through a line terminal enclosure which contains surge protection and connection points for cables or busbars. Both enclosures have transformers for generator protection and control.

Control systems For the control of cogeneration systems, i.e., boiler, turbine generator, gas turbine and auxiliary equipment, microprocessors have replaced the old systems based on mechanical relays. A boiler control system handles the fuel transport system, burners, boiler control, safety system and auxiliary systems. For the turbine a turbine

Instrumentation The generator is equipped with instruments to monitor the operation of the unit. Shaft vibrations are monitored with either seismic or prox124

5.8

124

97-03-05, 13.54

INDUSTRIAL POWER SYSTEM Fault Control regulator and automatic control are included, as well as the turbine-related safety systems. The auxiliary systems are also protected by relays for transformers, intermediate voltage and low voltage systems. Control and monitoring of the unit are based on man-machine communication where selection of process pictures and operation is made directly on a display. Large units usually have a common control room where all the separate parts of the power plant are controlled. The computer-based system includes printout facilities for alarm list reports, etc. The industrial process and the cogeneration unit can be integrated in common control systems for increased efficiency of the operators in the control room. High energy costs and more stringent environmental regulations require more complex control systems, which impose additional responsibility on the operator. By using microprocessor systems for control and supervision the operator is better equipped to meet these increasing demands.

125

5.8

125

97-03-05, 13.54

INDUSTRIAL POWER SYSTEM Standby and uninterrupted power supply (UPS)

STANDBY AND UNINTERRUPTED POWER SUPPLY (UPS) Contents page 1 General

126

2 Standby versus uninterrupted power

126

3 Engine driven generating sets as standby power

127

4 Diesel engine versus gas turbine (GT) generating sets

127

5 Generator current ”forcing”

129

6 Uninterrupted power supply (UPS)

132

7 Dublicated invertes

137

General operating speed, voltage and frequency before they can be connected to the load.

Many of today’s activities, industrial, commercial and governmental, require power supply in addition to, or, of a quality that is better than normal. Anyone planning to install or operate such equipment should carefully review power requirements and determine in advance the quality of electric power he or she will require to operate the plant successfully, efficiently and economically.

Uninterrupted power implies continuous voltage and frequency within the limits of the equipment that is using it. One type of equipment might supply what could be termed uninterrupted power, even though there might be a perceptible change in voltage and frequency, but for a very short period of time. In another system, there may be little or no discernible change in voltage or frequency. It is extremely important to realise that there is no perfect continuous power from any source, including the most modern and best-regulated UPS units available in the market. There will always be instances, although there may be no discontinuity of power, when there will be changes in voltage or frequency unacceptable to the equipment being supplied. Even though power is continuous, intermittent changes in voltage of 10% may occur. Such a change can be acceptable to most equipment but not acceptable to some. Spikes occurring on the voltage, or over-harmonic voltages and currents generated by other loads would cause no problem for most equipment, but could be disastrous to other equipment.

Standby versus uninterrupted power. An understanding is essential of the difference between these two types of equipment and supply systems. Standby power is power which is standing by and is not necessarily immediately available, usually from an engine-driven generator. In some cases, the standby power may actually be running idle, but must be switched into the system after main power has failed. This operation requires a time interval resulting in an interruption. Generally speaking, all standby systems are idle and must receive a signal to start up and accelerate to 126

5.9

126

97-03-05, 13.55

INDUSTRIAL POWER SYSTEM Standby and uninterrupted power supply (UPS) cost (USD 150 to USD 200 • ItperhaskWlowin initial the 100-1000-kW rating range —

Engine driven generating sets as standby power Generating sets, where a combustion engine is driving an electric generator, are self-contained and independent from the mains supply. This makes them particularly suitable where:

•

electric supply would not otherwise • another • be available, or would be available only at unjustifiable cost. priced power supply is required • ainreasonably • addition to the mains supply for the pur-

• •

pose of covering peak loads. a standby power supply is needed to cover vital or emergency loads during power company supply failures. the equipment and its power requirements have been analysed and can tolerate a power interruption of l0 to 15 seconds for diesels and 4-5 minutes for gas turbines, either an idle/standby system or a continuously running system can be selected.

•

this is roughly 15-30% less than a turbinedriven set). It is capable of operating in extreme environments — high altitudes and high temperatures, dust, and even extreme cold (if proper low temperature provisions are made). A good record has been shown for prompt and competent service facilities almost everywhere in the world. Frequent opportunities are present to use the engine’s waste heat in the jacket and stack cooling water when the unit is used for longterm peak-shaving service. There is reduced fuel consumption — roughly 30% less under full load as compared to the single-cycle gas turbine.

Diesel engine disadvantages

engine is heavy, requiring solid, earth• The based support or expensive structural sup-

In principle, generating sets can be driven by steam turbines, gas turbines, gas-engines, gasoline (petrol) engines and diesel engines as the prime mover for the electric generator. In practical use, however, the diesel-engine and gasturbine-driven units have gained almost exclusive acceptance in the power ratings from about 50 to 5000 kVA. Diesels are most common in the lower range and gas turbines become competitive at the high end of the scale. For ratings below 50 kVA, alternative power supplies such as gasoline engines, fuel cells, solar or wind powered generators combined with batteries are in common use.

• •

• Diesel engine versus gas turbine (GT) generating sets The following is an extract from an IEEE evaluation, where the two types of units are compared on a broad basis:

•

Diesel engine advantages Experience has been gained from vast numbers of sets in service The unit has the ability to start within 10 sec. and accept load while being brought up to speed.

• •

port if it is above earth. Moving or modifying such an installation later can be an expensive project. Engine noise and vibration are in the low frequency range, which is difficult and expensive to isolate acoustically from nearby occupied areas because of the substantial air volume required for both cooling and combustion. Maintaining the starting reliability for emergency or standby service requires the engine to be started and run under at least 30% load at least twice a month. Generally this energy is wasted if the unit is not equipped with synchronising equipment such that it can be run in parallel with the main power system. Starting in extremely cold weather is reliable only if lube oil or water jacket or both are kept heated. In cold climates, this supplementary heat source, and its maintenance, can be expensive. Speed regulation for isolated machines under load is inferior to that of gas turbine units. Typically, sudden addition or removal of half the rated load will change the frequency by about 3 Hz before the governor reacts and can compensate.

Gas turbine advantages

• Weight of a gas-turbine generator package is 127

5.9

127

97-03-05, 13.55

INDUSTRIAL POWER SYSTEM Standby and uninterrupted power supply (UPS)

• • • • •

• •

greater time required for start-up is so • The much more (2-4 minutes) that the larger sets

typically only about half that of an equivalent diesel unit. Thus, it costs less to mount it on the roof or similar locations, or to move it if production or building-utilisation plans change. When recommended clearance space is included, a gas turbine set may take less than half the volume of a diesel unit. Less air is required. All the turbine’s air is for combustion, whereas diesels require a great volume of air for cooling. Turbine vibration and noise are generally easier to handle. The familiar turbine ”whine” is almost all in the higher frequency ranges, which are easy to isolate from the building and to attenuate within the air intakes and exhaust ducts. Starting reliability is extremely high, even in the coldest weather. Tens of thousands of turbine starts are made daily, with failures almost unknown, in jet aircraft all over the world. Speed regulation is inherently superior because of the high speed and high inertia characteristics of the turbine. For instance, once full speed is reached, full load can be instantaneously connected or disconnected on a turbine generator set, and the frequency deviation will remain within 2 Hz. This frequency stability is particularly important where the emergency load consists of large motors (e.g. compressors or fans) which are connected on line intermittently. Routine maintenance requirements are simpler. Manufacturers typically recommend that a turbine for emergency standby be operated unloaded once a month and under load only at three-month intervals. Gas turbines offer greater fuel flexibility, burning lighter fuel oils, including highly stable kerosene, aviation jet fuel, or residential fuel oils.

•

cannot be used as the sole emergency power source (such as in hospital facilities) where life-support needs call for start-up times around 10 seconds or less. However, in a battery-supported UPS combination that carries the no-break loads over the starting time of the gas turbine, this may be an excellent combination. Initial costs are generally 20-30% higher than for a diesel unit. (Much of this premium may be offset by reduced installation costs.)

Operating diesel and gas turbine sets electrically in parallel A practical example of operating diesel and gas turbine generator sets electrically in parallel in an emergency power system manifests the remarkably different mechanical and control characteristics as stability problems such as severe transient overloads, voltage drop, and frequency drop, which are likely to cause catastrophic shutdowns during emergency conditions. To illustrate such a situation, a land-based oil producing facility has two emergency generators consisting of a 2.5 MW diesel unit and a 3.5 MW gas turbine unit connected electrically in parallel. Emergency loads includes HVAC loads, fire pumps, freeze protection, emergency lighting and life support loads. The majority of these loads can switch on automatically upon process demands or safety requirements. Some normal loads (also automatic) are mixed with emergency loads in the same switchgear. Thus very large load steps, including motor starting as well as static loads, can suddenly be applied to the generators. Because of its heavy weight, the diesel engine generator set is quite stable; the inertia WK2 is almost ten times the inertia of the gas turbine. However, the diesel is also less sensitive to system changes. This creates a stability problem on the lighter and more sensitive gas turbine unit during large step loading. The gas turbine, having lower inertia and higher governor gain constant than the diesel unit attempts to assume much of the load while the diesel engine hardly reacts to a load change. Loadsharing devices used to distribute steady-state load proportionally between the two machines have an insignificant effect during the first one

Gas turbine disadvantages

higher fuel costs — typically more • Inherently than 30% greater than that of a diesel unit of the same capacity.

availability of trained service per• Limited sonnel. Estimates are that there are only about 5-10% as many standby and emergency power turbine sets in service as there are diesel units. 128

5.9

128

97-03-05, 13.55

INDUSTRIAL POWER SYSTEM Standby and uninterrupted power supply (UPS) ers in case of fault. Therefore, standby generators are mostly compound-excited from current and voltage transformers on the output from the generator, see Fig. 1. Another way to produce the necessary short-circuit current is to provide a separate exciter, as in Fig. 2. The requirement in many standby generator specifications (and even some mandatory government regulations) call for the generator to produce 3 times its rated current for at least 10 seconds. Fig. 2 shows a comparison between a self-excited machine and current-forcing machines.

or two seconds of the transient duration. Transiently exceeding the shaft torque limit of the aeroderivative gas turbine may damage the shaft or reduce its useful life significantly. The diesel generator unit is designed to start and assume load within 10 seconds after the loss of normal power. The gas turbine unit is a very lightweight aeroderivative type, compact and low in cost and designed to take load 120 sec. after the loss of normal power. The difference in starting time means that the diesel unit can become heavily overloaded during the period before the gas turbine unit is ready to assume load. The two generators operate with an isochronous (constant speed) control mode. They start, synchronise, and pick up load automatically after disruption of normal power. The governor systems of the generator units are also dissimilar. The gain of the gas turbine governor is much higher than the gain of the diesel engine governor. The voltage regulator exciter system of the gas turbine generator has a faster response than that of the diesel-generator unit, providing fast response under short-circuit or large motor starting conditions, when the voltage is low but the current is high. The conclusion in this particular case is first of all that modified load-sequencing and loadshedding controls have to be provided to avoid transient stability problems. Secondly, system stability in general is an important issue in all planning and design of small emergency power systems with step loading or intermittent motor starting. This practical case shows the necessity for parallel emergency generator sets to have compatible dynamic responses to system transient disturbances. The factors which significantly influence the dynamic response of a generator set are: mechanical inertia, governor system, excitation system, generator subtransient and transient reactance. Parallel generators with compatible dynamic responses provide a more reliable emergency power system and much simpler controls.

Fig. 1. Compound excited generator.

Generator current ”forcing” One disadvantage of small self-excited generators is that they may have a problem supplying a sustained short-circuit current which is sufficient to operate fuses, relays and circuit break-

Fig. 2. Separately excited generator. 129

5.9

129

97-03-05, 13.56

INDUSTRIAL POWER SYSTEM Standby and uninterrupted power supply (UPS) exhaust gases travel at high speed and are inflexible to rapid changes in direction of flow. Back pressure then builds up in the exhaust system with a resultant loss of engine power.) The first section of exhaust from the engine manifold should be a flexible section of pipe or bellow. The next section must be supported to allow the flexible pipe movement without putting weight on the engine manifold. The support brackets on the exhaust should allow for pipe expansion, and short flexible sections should be used on long runs between fixing brackets. Where the exhaust pipe passes through walls, a sleeve or wall plate should be fitted, or a clearance hole left. Care should be taken when deciding on the exhaust point to atmosphere to ensure that there are no air inlets or windows which could allow exhaust gas to re-enter the building. On long exhaust runs a drain point or condensation trap should be fitted near the engine. Where more than one engine is installed, each should have its own independent exhaust system. More than one exhaust into a common pipe can be dangerous and could cause damage to various parts of the engine. On long exhaust runs it is nearly always necessary to fit a secondary silencer near the end of the run.

Fig. 3. Comparison between available shortcircuit current. Mechanical hints when designing for standby sets (mostly diesels) Space Where space is a problem, the set should be sized physically before it is sized electrically. Ideally, the set should be positioned alongside two external walls so that the cooling and discharge air can be easily vented.

Cooling and ventilation Ventilation of the engine room is important, and provision should be made for adequate airflow through the room to replace the air consumed by the engine and the air pushed out of the room by the engine cooling system. Various types of cooling can be adopted. The main ones are set-mounted radiator, remotelypositioned radiator, heat exchanger cooling or cooling tower. The large volume of air required by a diesel engine for cooling and combustion is not always appreciated, and the total area of incoming air vents should be at least double that of the engine radiator outlet at the opposite end of the engine room. In cold climates where sets are employed on standby duty the room must be kept warm; air inlets and radiator outlets should be closed when the set is not in use. When using a remote radiator or heat exchanger cooling system, allowance must still be made for cooling the engine room to remove radiated heat from the engine/generator. Air for

Foundation To overcome the problem of vibration with diesel engines, machines are built on heavy bedplates. These must be bolted down on to substantial (and expensive) concrete foundations. With the unit construction method — where the rigidly coupled engine and generator sit on a lightweight subframe attached to a mainframe through resilient mountings — less substantial foundations are needed. Maintenance clearances It is important to allow at least 1 metre working clearance at the sides and 1.5 metres at the generator end of the machine for maintenance. The set can be placed on a level floor, but maintenance is facilitated if it is raised on two longitudinal plinths. Exhaust systems The exhaust should be as short as site conditions and regulations will permit. Bends should be few in number and of a long radius. (Hot 130

5.9

130

97-03-05, 13.56

INDUSTRIAL POWER SYSTEM Standby and uninterrupted power supply (UPS) aspiration in the engine must also be included in this air flow requirement figure. Water used in all engine cooling should preferably be treated irrespective of the quality of the water available. The cost of doing this is small compared with the cost of damage resulting from lack of adequate treatment.

sufficient maintenance and regular ”exercising” - under load - have not been practised. There are four methods of interconnecting and operating standby generators in parallel with main supply to facilitate this routine of exercising under load: 1. Generators continuously running take up inhouse loads without the usual momentary interruption associated with a transfer switch. 2. If synchronising equipment is installed, any magnitude of exercising load can be applied to the generators without the need to select actual loads for transfer. 3. Generators can double as peak-shaving equipment, either for the power company’s peak or for the customer’s. 4. In the case of large generators, income from power sold back into the power company network may be used to offset the cost of exercising..

Fuel oil system With all fuel oil installations, local regulations have to be checked. When locating the daily service tank away from the set, care must be taken that the engine fuel lift pumps can supply the fuel pump. For reliability, gravity fuel supply with the tank at a higher level is to be preferred. Where bulk storage is required, the simplest and most economical method is to install the tank adjacent to the engine room. A typical fuel oil arrangement is shown in Fig. 4. Authorities who should be consulted during the planning of a standby generating set installation include the local building authorities, the fire and environmental authorities, and local power company.

Most emergency generating systems are operated at low voltage and connected to the emergency loads, when needed, by transfer switching. The transfer switches are carefully designed to eliminate the possibility of the emergency generators ever being paralleled with the power company or main supply. This arrangement has been dictated partly by the power companies´ traditional resistance to interconnection, and partly by the need to keep the emergency system totally separate. Some national safety regulations or codes even require

Paralleling standby emergency power with main supply When lights go out, emergency generators sometimes fail to start. They may have been in the basement for months, even years, representing thousands of dollars in capital investment. But when the time comes, they may fail to perform their vital function. The answer is often that

Fig. 4. Typical fuel oil system. 131

5.9

131

97-03-05, 13.56

INDUSTRIAL POWER SYSTEM Standby and uninterrupted power supply (UPS) need emergency generation also have multiple feeder networks to ensure maximum reliability of power company supply, even before the emergency generator is considered. It is unlikely that all of the power company feeders will fail simultaneously. Individual feeders may fail due to problems at local substations, and their loads can usually be taken up by the other feeders in the event of a total power company blackout; the feeders usually go dead one after the other as the power company system goes down.

that no transformer be interposed between the emergency generator and its loads. One of the problems with the transfer method, whether done with a transfer switch or with interlocked circuit breakers, is that one supply must be dropped before the other is connected. This causes a brief power interruption to vital systems. The result is that hospital administrators, and industry managers concerned with the continuous operation of their facilities, may actually resist regular exercising of emergency generators. It’s interesting to note that power company thermal generating stations, which must maintain emergency power supply to lubrication pumps and lighting in the event of a total blackout, never interrupt the prime power source in order to load the emergency system. They exercise emergency generators regularly by dumping the power generated into dedicated resistive loads. However, as power costs continue to increase, more and more emergency generating systems are being built for parallel operation with main power. But the cost is high. Fully protective relays used with a continuous interconnected generator are required, as well as additional circuit breakers. So the emergency generating system must either be large, or important enough to justify the interconnection.

UNINTERRUPTIBLE POWER SUPPLY (UPS) As previously stated, standby power takes some time to come on line after a main power outage has occurred. This does not satisfy the requirements of vital or essential equipment in numerous industrial applications, which may require AC or DC no-break supply, such as: emergency lighting and escape route lighting public address and status alarm SOLAS (safety of life at sea) radio for offshore plants fire and gas protection systems instrument safeguarding/emergency shutdown systems power supply to process instruments and analysers power supply to process computers and data processing machines lube oil pumps for large generators and motors switchgear and relay protection in substations

• • • • • • • • •

Peak shaving may very well be the cost-cutter that justifies interconnection. Of course, one can peak-shave by switching selected loads over to the emergency generator as total plant load nears its peak, but this is cumbersome and depends on having switchable loads of the right size. Ideally, the load shaved should be carefully selected to minimise generator running time for maximum reduction in peak load. An interconnected generator can be automatically loaded to keep the total plant load below a predetermined level, whatever the total plant load.

Direct current uninterruptible supply. This is a power supply derived from a batteryrectifier combination (DC UPS unit) or from rectifiers energised from one or more AC uninterruptible supply sources (AC UPS unit). Such supplies should be considered for applications where the load is relatively small and closely concentrated, or to supplement AC UPS systems (e.g. for switchgear auxiliary circuitry, fire alarm or communications systems). When designing DC systems, due consideration should be given to permissible voltage drop at the load terminals.

Power company paralleling adds security Because an emergency generator operates only when power company supply fails, it may be thought that there is not much opportunity for operating the systems in parallel when an emergency actually occurs. But this is not the case. Most industrial and commercial facilities that 132

5.9

132

97-03-05, 13.56

INDUSTRIAL POWER SYSTEM Standby and uninterrupted power supply (UPS) Common causes of failure Before starting to design DC (and also AC) uninterruptible power supply, it may be useful to review some of the most common causes of failure:

Although there are differences from one manufacturer to another, most chargers are designed for approximately 40°C maximum ambient temperature and 1000 m altitude. Deviations exceeding these values should be avoided. Derating of 4% per ambient degree above 40°C is not uncommon. Therefore, between 40 and 50°C, the battery charger may require a derating of 40%. Altitude is a problem that can also affect the battery charger application, while not adversely affecting the battery. A typical derating of approximately 2% applies per 100 metres above 1000 metres. In sizing the DC current rating of the charger, the following applies: A = (Ich + L) X 1/C1 X 1/C2, where

maintenance and insufficient system • Poor supervision is one of the most frequent causes.

•

• •

•

This includes battery failures or too low capacity due to: Incorrect charging voltage Battery circuit interruption, such as single-cell failure Too high temperature Incorrect design of the distribution system Non-selective short-circuit protection can result in total or partial shutdown of the system Insufficient tripping can result in fire or low voltage Long cable runs result in high voltage drops which can render sensitive loads inoperative Incorrect battery or charger sizing Unsuitable or non-existing redundancy For certain loads like switchgear relay protection and emergency shutdown systems with main and back-up functions, it is necessary to have a duplicated supply with dual batteries, chargers (and inverter for AC) Incorrect design of charger AC supply (such as feeder protection that is set too low, which inadvertently may trip the charger and deplete the battery)

charger rating in amps. • AIch==thethebattery charging according to the • battery manufacturercurrent for specified recharg-

• • • • • •

ing time at given charging voltage. (As an approximation Ich = 1.1 x Ah/T.) 1.10 = typical charger conversion factor Ah = the ampere-hours of recharge T = recharge time in hours, L = DC continuous load current in amps C1 = temperature correction factor C2 = altitude correction factor

DC system supervision Although a multitude of options are available as indications and alarms, the following are useful and will assist in diagnostic work and maintenance: 1. Loss of AC alarm. Most causes of loss of AC are due to temporary power failures affecting the entire plant. But it is also possible that a fuse or breaker on the battery charger circuit has operated. Without proper alarm, the loss of AC can result in the loss of the entire DC UPS system. 2. Low DC voltage alarm (U< and U<<). Numerous conditions exist which could create a low DC battery voltage. One of the most common is loss of the battery charger. Normal settings for U< is 1% below float charge voltage. U<< is normally set at 85 % of float charge voltage. 3. Battery earth fault indication and alarm. Most critical DC UPS systems are unearthed. Therefore, a single earth fault will not result

Where an AC UPS is available and the DC load does not exceed 15% of the AC supply capacity, the DC may be derived via a rectifier unit fed from the AC UPS distribution switchboard. Sizing the battery charger While the size of the battery will provide definite constraints on the battery charger, the following should all be considered when designing the charger (or DC UPS rectifier): AC supply, three-phase or single-phase source and voltage battery voltage and capacity normal DC continuous load current battery discharge levels and recharging times ambient temperature altitude

• • • • • •

133

5.9

133

97-03-05, 13.57

INDUSTRIAL POWER SYSTEM Standby and uninterrupted power supply (UPS)

4.

5.

6. 7.

in operating a breaker or melting a fuse. However, a second earth fault on another ”phase” may result in the loss of several DC circuits. An earth fault indicator is normally used to detect a battery earth in order to locate and remove the earth fault before a second one occurs. High DC voltage alarm (U> and U>>). Like U<, the U> is set 1% above the float charge voltage. Although not common, chargers may occasionally “run away” into an overvoltage condition. Such a condition can result in damage to connected loads, severe gassing from the battery, loss of water to the battery, and ultimately a battery failure. To avoid nuisance alarms, the voltage high alarm (U>>) should be set above the high rate equalising charge. High rate equalising timer. Most batteries require a high rate equalising charge on a periodic basis. While a selector switch from ”float” to ”equalise” is available on most chargers, it does require that a person remembers to return the selector switch back to ”float”. As usual, people are not always perfect, and an equalise rate can be left in the “on” position, resulting in possible damage to the battery. The use of an equalising timer will reduce the probability of this occurring. High temperature alarm Battery circuit failure

It is obvious with such varying times that each individual application has to be carefully engineered with respect to size of the loads and normal (and worst case) duty cycle. Battery types 1. Vented lead acid Sometimes this is called ”open type” lead acid, and is the most common type of battery and also the lowest price one. It requires few quick-charges. Some of its disadvantages are: Sensitive to high temperatures and ripple voltage (harmonics) generated by the charger (and inverter for AC UPS) as well as certain types of load. More maintenance is required than for the two other types. Wrong type of float charging influences its useful life. 2. Valve regulated lead acid This type of battery is a more modern lead acid type sometimes claimed to be ”maintenance-free” or ”sealed”. Neither of these claims are quite true. Because they have a ”solid” type of electrolyte, they require no water refill. Having no acids that may spill is an advantage in handling and promotes personnel safety. Since there is little generation of explosive gases during charging, because of the regenerative process, there is no need for a separate battery room as for conventional lead acid. The size is relatively compact. On the negative side are the same disadvantages as for freely ventilated lead acid. In addition, it is impossible to check the level of charging by measuring electrolyte density. The valve may become stuck, which may cause drying out or explosion. Further, there are limitations on charging voltage and current. The battery cannot be stored without loss of lifetime as the gel is included during manufacturing. Finally, there is considerable spread of cell voltage, which makes monitoring difficult. 3. Nickel cadmium This battery has the longest lifetime (approximately 20 years). It is a mechan-i cally robust battery which can tolerate

Battery capacity requirements The batteries of DC or AC UPS units are sized to supply loads for greatly varying time intervals. Assuming that ”normal” load is present during the entire interval, the following are some examples of how to size battery amperehours (if load is less than ”normal”, the batteries would take longer before depletion): for process plant shutdown • 301-8minutes for substations and power gene• ratinghours plants minutes for non-process computer • 10-20 installations hours for offshore plant shutdown and • 3abandon platform procedures for fire fighting, fire alarm systems • 8andhours telecommunication systems. 36 hours equipment or naviga• tional aidsfor(onSOLAS offshore plants) 134

5.9

134

97-03-05, 13.57

INDUSTRIAL POWER SYSTEM Standby and uninterrupted power supply (UPS) High end voltage implie low utilisation of the battery, and therefore high cost. In order to achieve a low end voltage, the highest possible system voltage should be aimed at, normally 110% of nominal voltage. In addition, the lowest possible float charge and the lowest possible system voltage (80-90% of nominal voltage) will achieve this result.

many deep discharges as well as out-oftolerance charging, which does not affect battery life. There is no risk of so-called ”sudden death”. On the negative side there is the high cost - approximately 2-3 times the cost of lead acid. So-called ”voltage depression” means that the battery must be oversized or many quick charges may have to be made. The electrolyte has a tendency to carbonise.

Duty cycle calculation

Battery sizing Sizing the battery means: 1. Determining the number of cells 2. Determining the rated Ah capacity The following guidelines may offer some assistance when determining the battery bank size: IEEE Recommended Practice for Sizing Large Lead Storage Batteries for Generating Stations and Substations ANSI/IEEE Std 485-1983

•

Fig. 5. Typical basis for battery duty cycle sizing.

Recommended Practice for Sizing • IEEE Nickel-Cadmium Batteries for Stationary

The above figure illustrates, for a substation battery, how the capacity must take into account the base load (such as relay protection), intermittent load (such as motor operation), peak load (busbar trip) over the entire standby period.

Applications ANSI/IEEE Std 1115-1992 While the calculations for sizing a battery can be performed by hand, computer programs are normally used. The battery manufacturer can assist in choosing and sizing the battery. Here are some simple rules for estimating battery size:

The continuous current = load/min. system voltage (I = P/Umin) Required Ah capacity = continuous current x standby period (Ah = A x t) This assumes all loads of constant power type and battery capacity data based on constant current load.

Maximum system voltage This is the highest acceptable voltage for the equipment to be connected to the battery, normally not less than 110% of nominal voltage.

Average discharge current for intermittent load I = S(In x tn)/t where In = intermittent load during time interval tn t = required discharge time (standby period) Required Ah capacity for intermittent load = I x t (to be added to Ah for continuous load) For peak loads shorter than 1 minute, the battery capacity will not be affected. For worstcase conditions it is assumed that all peak load

Minimum system voltage This is the voltage designated by the user to define the lowest acceptable voltage for the equipment connected to the battery. Number of cells = max. system voltage/float voltage per cell End voltage/cell = 0.85-0.90 x Vnom/number of cells

135

5.9

135

97-03-05, 13.57

INDUSTRIAL POWER SYSTEM Standby and uninterrupted power supply (UPS) insufficient charging. Further, towards the end of the battery life, the capacity will begin to drop, and to obtain an approved battery test requires 80% of remaining battery capacity. If it is below this level, the battery should be replaced. To allow for these capacity variations over the life of the battery, a life margin of another 20% should be considered. This means altogether a design factor in the sizing of the battery of 35-40%, which may appear high. But, it is important to consider the application of the battery and the consequences of failure. In most cases, the added margins are well worth the added cost.

takes place at the end of the discharge period. The voltage drop caused by the peak load will influence the rated capacity in the following way: Voltage drop per cell DU = Ip x Ri where Ip = peak load Ri = internal resistance given at the actual state of charge Then Ubatt + DU ≥ Umin Where Ubatt = Battery voltage per cell after continuous and intermittent load discharge. Umin = minimum system voltage per cell. Fig. 6 shows how a large battery can accommodate a peak load at the end of the discharge period without voltage dropping to an unacceptable level (battery 1)

Alternating current uninterruptible supply The principal difference between DC and AC UPS is that the AC system basically consists of a DC system with an added inverter to it. In other words, AC UPS = DC UPS + inverter. Therefore, all that has previously been said about rectifier and battery also applies to the AC UPS. Over the past two decades, the process control industry has become one of the major customers for static AC UPS systems, the main reason being that the UPS provides for a better quality power supply with less pollution of the AC sinus wave than the power company can provide. In some parts of the world a computer UPS supply is therefore called a ”power conditioner”. This is, of course, in addition to the non-interruptability of a UPS. Further, with the increasing use of real-time, on-line data processing, together with associated data communication, the demand for static AC UPS systems is growing consistently. As the technology has developed, various system configurations have emerged catering for various levels of operational security. All static UPS systems (as opposed to previously more common rotating UPS units) are based on the same fundamental principles. Normal AC supply is used to feed a rectifier which provides DC at battery voltage. This DC feeds a static inverter which generates the required AC voltage and frequency for the load, see Fig. 7. A static switch between the inverter and load is normally included. This static switch incorporates detection and sensors of any out-oftolerance situation, such as an inverter fault, and switches the load over to an alternate AC

Fig. 6. Effect of peak load applied at the end of the discharge period. Min. system voltage

Temperature correction Under ideal conditions the battery should be maintained at a temperature near 20°C, which is normally accomplished with HVAC equipment. At low temperatures the battery capacity is reduced significantly Since most industrial rooms are electrically heated, the ambient temperature of the battery room could become quite cold in winter if main power is lost. Therefore, the minimum temperature of the battery room must be considered in battery sizing calculations. During an 8-hour outage in extreme frost conditions, the battery room could easily drop to less than 0°C. Correction factors A margin of 15-20% is recommended to be added when sizing the battery due to unforeseen additional loads, poor maintenance and 136

5.9

136

97-03-05, 13.58

INDUSTRIAL POWER SYSTEM Standby and uninterrupted power supply (UPS)

Dublicated Invertes

source. This alternative could be the normal main supply or some other source of sufficient quality.

The two inverters may also be operated paralleled, with each sharing 50% of the full load. In the event of a fault in either inverter, the other one takes over the full load. This configuration allows a faulty inverter component or unit to be replaced without disabling the whole system. Advanced inverter/rectifier designs have dynamic response regulation to enable switching 0-100% load changes within acceptable voltage variation. Where even greater security is required, the main power supply is used as a third power source. In addition, a redundant rectifier, or even duplicate batteries, may be included. These steps would be taken only where exceptionally high security is required, or where the site location is remote and unmanned. Further, even a by-pass maintenance switch will permit the UPS to be taken out of service, where a reliable by-pass source is available.

Fig. 7 Simple AC Uninterruptible Power Supply

This basic UPS configuration is widely used. The chance of a static inverter fault coinciding with a main power failure is extremely small. However, in some applications, even this small risk may be unacceptable, and various more sophisticated configurations have been engineered to higher degrees of redundancy.

Limited short-circuit capacity A criticism often levelled at the static UPS inverter as opposed to rotating machine types is its low short-circuit capability. While rotating generators supply momentarily 5-10 times its rated current to melt fuses or trip circuit breakers promptly, static UPS can only deliver about 1.5 times its rated current into a short circuit. This represents much tighter restrictions on the protection equipment and selectivity than if a rotating generator were used, sometimes requiring an oversized static UPS just to produce sufficient short-circuit current. There is a popular misconception about this which goes as follows: ”The static switch will switch to mains supply to blow fuses or trip a circuit breaker in case of a short circuit, and there is little probability that a mains supply and short circuit will occur at the same time”. This is correct for most UPS applications, but for UPS systems also designed to operate during a catastrophe situation, the previous statement is not true. This is because it is exactly during calamities (fires, earthquakes, gas explosions, etc.) that short circuits will occur. Therefore, if the UPS system is supposed to operate in real emergencies too, the inverters must be designed to deal with short circuits without help from the static switch.

Increased reliability One of the more popular arrangements for additional reliability is shown in Figure 8 where the inverter is duplicated, with the two inverters run in a duty/standby mode, one supplying 100% of the load and the other on standby.

Fig. 8. High reliability uninterruptable power supply.

137

5.9

137

97-03-05, 13.58

INDUSTRIAL POWER SYSTEM Standby and uninterrupted power supply (UPS) haviour, as well as careful protection and selectivity analysis, are essential if surprise outages are to be insured against. Such surprise outages unfortunately have a tendency to reveal themselves at the most inconvenient times. There are numerous accounts of failures of vital supply to important equipment, including even the total and permanent breakdown of entire power stations, which could have been avoided with only a little attention to the issues mentioned above.

Another drawback of the static UPS related to the small short-circuit capacity is its poor ability to cope with harmonics created by nonlinear loads. Such harmonics may be created by certain types of loads such as switch-mode power supplies (see Fig. 9), and if these are of large size or several in number, they can create havoc in a system which should be completely free from disturbances.

Fig. 9. Non-linear load - switch-mode power supply. Typical video display terminal Summarised load from 100 VDTs A combined standby and UPS power ystem There is more to vital and essential supply than just the generators and converters/batteries. In a large industrial complex there may be many UPS units, both AC and DC, normally fed from high priority distribution boards supplied by standby or emergency generation. One of the important questions here is whether to centralise some or all UPS units into one or two, or to provide a distributed UPS supply with many small units. Secondly, the possibility of fire hazard and other calamities has to be considered. Important loads usually have two independent feeder cables from the UPS distribution board(s), and separate paths have to be found such that process control or vital communication systems are not knocked out because both power supply cables happened to lie on the same cable rack. Thirdly, the dynamic behaviour of the standby or UPS power system is no different from large power systems, and may suffer from the same disturbances if motors are started, loads are switched, or when faults happen. Therefore, system study and calculations of dynamic be138

5.9

138

97-03-05, 13.58

INDUSTRIAL POWER SYSTEM Prefabricated and mobile substations

PREFABRICATED AND MOBILE SUBSTATIONS Contents page 1 General

139

2 Secondary substations

139

3 Main or primary substations

141

4 Modular substation buildings (docking type)

141

5 Integral transformer substation

142

6 SF6 compact substation

143

General countries). Although the low voltage fusegear is typical of power company tradition, these substations are nowadays also customised for industrial applications. Up to now, transformers have normally been purchased separately and installed after the substation frame and walls have been erected at site. This is primarily because the substation frame need not then be made so rigid in order to stand up to the heavy transport weight. But more important, procurement tradition has considered distribution transformers as bulk items and purchased them separately. Access to the equipment is through outside doors or louvres. Fig. 1 shows a modern typical mini-substation being hoisted into place. Over the years the demand has increased for larger, prefabricated, indoor-operated ”walkin” type substations, especially in colder climates, and this has led to the development of the medium-sized substation. The philosophy behind this factory-assembled unit was more or less the same as for the earlier ”mini” type: Simple transportation Flexibility in size through the use of different standard sizes of housing. For larger substations two or more housing-modules are coupled into one large substation at site. Minimum civil works at site. Minimum erection and commissioning at site. Easy to relocate. Combining housing and foundation makes it simple to move the substation from one site to another.

Substations are assemblies of primary switchgear, transformers, and secondary distribution equipment. Many different arrangements are used, with the equipment normally closely connected, although primary or secondary equipment may also be separated from the transformer. Traditionally, the various components of a substation are shipped as separate items and erected, wired and tested at site in custommade buildings or rooms by local contractors. In recent years the trend towards using prefabricated or even mobile units and buildings have led the way to savings in costs, made available by efficient factory assembly methods. In the following section the essence of this modular and prefabrication trend is described using the substation definitions and terms as stated in section 5.1 IPS Design.

Secondary substations Mini and kiosk type substations The European development of factory-assembled substations started in the 1960s with small outdoor ”mini” or ”kiosk” type substations primarily for the electric power companies. Distribution transformers in the range of 3001500 kVA, together with medium voltage and low voltage fusegear are installed inside the same factory-made metal enclosure. Since it is often used in 10-20 kV ring networks, the medium voltage T-connection is sometimes called a ”ring main unit” (in British-influenced

• • • • • 139

5.10

139

97-03-05, 13.59

INDUSTRIAL POWER SYSTEM Prefabricated and mobile substations A recent development in the prefabricated line of substations is the ”container” type. This has become particularly popular in various process industries, not only as a substitute for conventional transformer and switchgear rooms, but also for housing of computer and control equipment, pneumatic and hydraulic auxiliaries and diesel generator sets.

Fig. 1. Typical mini-substation for outdoor operation. Front page photo from Swedish brochure 1WAF 201 (M-Stationen) The housing itself consists of a painted steel frame with insulated walls and roof. The wall cladding may be sheet steel, concrete or hard pressed reinforced fibre glass with a surface cover of crushed natural stone. The substation can accommodate optional equipment such as:

Fig. 2 . Prefabricated ”container”-type substation installed in a metal processing industry.

voltage indoor switchgear 12-36 kV • Medium voltage indoor switchgear • Low power transformers • Distribution transformers • Auxiliary AC and DC supply from batteries • Auxiliary Relay and control equipment • Forced ventilation conditioning • Substation heatingorandairlighting • Different layouts may be accomplished almost without any limitation of the possible combinations of the above equipment. This means that these prefabricated units may be placed both outdoors and indoors in industrial environments. Particularly in dusty or dirty atmospheres, the overpressure ventilation or purging will prevent harmful particles or gas entering the substation and interfering with sensitive relay or control equipment. This is an interesting alternative to the conventional way of arranging substations in permanently built rooms for this purpose, with the added work of co-ordinating the design between many different disciplines.

Fig. 3. Layout of a substation consisting of four interconnected container modules. Medium volt-age switchgear (1), transformers (2), low voltage switchgear (3) and supervisory control (4).

Unit substations The ”unit” concept for substations was conceived of during the enormous industrial expansion that took place in the U.S.A. during and after the Second World War. In order to reduce engineering work, standardise manufacturing and simplify erection, the transformer and as-

140

5.10

140

97-03-05, 13.59

INDUSTRIAL POWER SYSTEM Prefabricated and mobile substations sociated primary and secondary switchgear were adapted to each other in such a manner that they could be bolted together into a single unit. Most often the substation was enclosed in such a manner that it was installed in factory environments without a separate room being required. This concept was both cost- and space-saving, introducing the so-called ”load centre” principle, where the substation was located more or less in the centre of a heavy electrical load area. Since the transformer was manufactured with a bolt-on facility on the primary and secondary side, it became a special transformer, only to be used in the context of a unit substation. This concept of the ”unit” substation was not widely accepted in other parts of the world until many years later. Today there are not such large differences around the world;in the way indoor substation equipment are arranged; however, European units more often use cables instead of buswork to connect between switchgear and transformer.

areas where the cost of sending • Insiteremote personnel is high and where it is difficult

Main or primary substations

Modular substation buildings (docking type)

to accommodate them.

areas with difficult terrain and/or climatic • Inconditions a relocation or extension of the sta• Where tion may be necessary in future. an emergency where a new station is • Inrequired with short notice due to, say, the total failure of an old station.

standard building modules, design costs • With are reduced quality is achieved due to good factory • High working conditions, skilled and experienced personnel and effective quality control

transport and handling • Easy and minimised design, trans• Co-ordinated port, civil works, erection and electrical com-

•

Primary or main industrial substations may be wholly or partly owned by the electric power company supplying the industrial plant. This is due to the need of the power company to switch in the high voltage network, in which the industrial substation may be located in a strategic position. In some cases the industrial customer may locate his metering on the low side of the transformer if there is more than one consumer connected to the substation low side. If the plant is located at the end of transmission line(s) and no other customers in the future are likely to connect to the substation, the industrial plant may also own the transformer and high side switchgear, which usually results in a better tariff contract. Due to a restrictive economic climate, both electric power and industrial companies may have reduced their engineering capacity due to high personnel costs, and at the same time may need to reduce the total cost for putting a substation into commercial operation. The following are some advantages of modular, factory-assembled substations and buildings in general, pointing out where they may be particularly useful:

missioning With only one supplier responsible for the entire substation, co-ordination between subsuppliers is eliminated

The concept of prefabricated or modular substations is easily extended from the smaller secondary substations to the larger main or primary substations. Concrete or steel enclosed factory-assembled substation buildings have been in use in Europe and elsewhere for many years. A range of substations has been developed where the prominent features have been simplicity and flexibility. Simplicity is important not only to reduce costs, but also because trained operating and maintenance personnel may be scarce in the country of application. Flexibility means that different electrical safety codes and standards must be met without increased costs. Dimensions are kept within the transport limitations by overland truck and assuming that local hoisting facilities are generally available. Other requirements are: mechanical strength, corrosion resistance, personnel safety, aesthetic appearance. The building module consists of a steel frame with insulated roof and walls. The walls can consist of: 141

5.10

141

97-03-05, 13.59

INDUSTRIAL POWER SYSTEM Prefabricated and mobile substations polyester panels with • Fibre-glass-reinforced insulation and steel plate inside, or painted steel panels with • Plastic-covered insulation and steel plate inside The advantage of factory assembly versus site assembly is basically the same as for smaller units and can be summarised as follows: 1. Less co-ordination at site 2. Integrated civil and electrical engineering 3. Equipment protected against weather and theft during transport and at site 4. Less application and installation costs 5. Short completion time 6. Routine testing at the factory before delivery 7. Minimum of civil work.

Fig. 4b. Layout of the 4 modules showing switchgear room to the right (2 modules), control room (1 module) and pantry/WC in the left module.

Integral transformer substation

A large substation or switchgear building is divided into modules after the substation and the building layout (according to certain guidelines) have been designed electrically and mechanically. Each module is pre-fabricated and pre-assembled in the workshop before shipment. The range of electrical equipment that can be erected in this building is similar to that described for medium-sized substations. Each module is made of concrete and has a cable trench under a fireproof false floor. The modules are fitted to each other at site using a mobile crane, and afterwards all cables, busbars and other equipment are installed exactly as they were before being disconnected before shipment. This design requires no concrete foundation, with civil work reduced to a compacted and drained ground area. The modules can be used for different purposes for housing switchgear, control, battery banks, workshop and office. Complete assemblies with a floor area of 150-200 m2 are not uncommon.

By combining a modular building as described in the above section with conventional outdoor equipment mounted directly on the power transformer, an ”integral” transformer substation (ITS) becomes a unique design compared with a conventional substation. The cable between the transformer and the medium voltage switchgear can be provided with either dead break elbow connectors at both ends (for fast disconnection) or with conventional cable terminations. ITS concept is ideal for small allotted •areasTherequiring less civil work and less space for high voltage switchgear, this being erected either on top of the power transformer or, on the substation building factory-assembled building unit is ready • The for immediate hook-up of outgoing cables. of erection work at site • Minimum time span between site delivery and • The commercial operation is much shorter than

•

for conventional substations Conventional high voltage apparatus and indoor switchgear are used

Civil work is limited to:

clearing and levelling • Site Concrete plinths for power transformer and • building supports. Alternatively, skid mounting eliminates plinth requirements.

Fig. 4a. Site assembly of a 4-module substation building with 10 kV switchgear in a doublebus, front-to-front arrangement.

fencing • Security • Earth grid or rods 142

5.10

142

97-03-05, 14.00

INDUSTRIAL POWER SYSTEM Prefabricated and mobile substations Each substation has the following features:

Common designs are: One or two transformers each fed from separate high voltage line circuits. Any busbar system can be supplied Provisions for future busbar arrangements can be made at the design stage Flexibility in cable connection between transformer with switchgear and the substation allow for considerable ground movement, i.e., clay soil conditions, without detrimental effect on the equipment.

to meet current demand • Designed extendible or may be removed and • Easily replaced by a conventional type substation when justified by demand.

circuit identification as all equipment • Easy is mounted on its associated power trans-

•

former. Maintenance easily and safely performed. Disruption of supply may occur unless two incoming feeds to the medium voltage switchgear are available. voltage 36-145 kV • Primary voltage 10-36 kV • Secondary • Transformer rating 5-40 MVA

SF6 compact substation

Another compact concept for the primary side of a main industrial substation is to use SF6 switchgear inside a modular or custom-built building. Owing to the compact cubicle shape and the light weight of SF6 equipment, there is no need for an overhead crane in the building. A low building profile of 3.5 m height using 72 kV switchgear may be achieved due to the absence of any overhead crane. This is shown in Fig. 6. Future extension of the substation may be carried out by providing the addition of a 72 kV feeder at either end of the switchboard. Although the building layout shows a cable basement, this may not be really necessary, as the HV terminations are at a suitable height above the floor. This type of substation arrangement is typical of requirements in industrial zones. Although the 72.5 kV switchgear is shown located in a separate room, it could also be placed together with the 11 kV switchgear. Omission of a full cable basement economises on building costs, but makes cable repair or reinstallation more difficult. In such a case the 72.5 kV cables are brought in at floor level and the 11 kV cables are run in a shallow trench. Except for the transformer concrete box, all rooms in the building have a ceiling clearance of only 3.5 m, resulting in a low building profile and lower costs.

Fig. 5. Example of an 86 kV portal switchgear arrangement for two incoming lines and single busbar. The transformer and distribution unit are here located at the end of the portal arrangement. The single-line diagram also shows from right to left: receiving unit, transformer unit and distribution unit making up a complete substation.

The installation work at site is reduced: complex items are prewired and factory• All tested prior to shipment minimum of specialist staff is required • AErection time is reduced due to the elimina• tion of equipment support structures. earthing and earthing quantities • Secondary are considerably reduced. 143

5.10

143

97-03-05, 14.00

INDUSTRIAL POWER SYSTEM Prefabricated and mobile substations A feeder outage, or even worse the outage of a whole substation, can lead to blackouts with very serious economic consequences. In order to maintain acceptable service it is often necessary to install standby feeders in each substation. Such an arrangement may be too heavy a financial burden and instead it may be possible to bring in to the site a strategically located emergency mobile substation. This may be an interesting alternative for both industrial plants and power companies. Shared ownership could also be a viable solution, with some agreed contract of user priority in case of failure. A mobile substation may contain a combination of the following equipment: voltage switchgear • High • Transformer voltage feeder switchgear • Medium Relay and control equipment • Trailer or semi-trailer • Depending on the configuration of the incoming line, the primary and secondary voltage, the number of medium voltage cubicles involved and the size of the transformer unit, the substation may consist of one or several trailers.

Fig. 6. Layout of a 72.5 kV SF6/ 11/0.4 kV load centre substation. 1. 2. 3. 4. 5. 6. 7. 8.

Transformer Auxiliary transformer 72.5 kV switchgear 11 kV switchgear LV distribution board Battery room Mess room W.C.

Fig. 7. Mobile substation 84/13.8 kV (??) mounted on a single trailer.

The outdoor line receiver switchgear can be up to 132 kV and indoor up to 36 kV. Medium voltage feeder switchgear can be either for indoor or outdoor use. Switchgear for indoor use, relay and control equipment are built into cubicles or small prefabricated kiosks with insulated roof and walls.

Mobile substations Interruptions in electric supply, no matter how short, are a menace to industrial plants which then experience a total shutdown and lost production, unless some reserve feeder or substation from an alternative source of supply is available. 144

5.10

144

97-03-05, 14.01

INDUSTRIAL POWER SYSTEM Prefabricated and mobile substations These factors result in considerable overall savings for the mobile solution. Erection and commissioning costs are also much lower for the mobile plant, since the mobile substation is entirely factory-assembled and pre-tested.

The total weight and overall dimensions finally depend on local traffic regulations in each country, but a weight of 50 tonnes and a length of 17 metres are not unusual. The limits in power output and voltage level installed on a single trailer are determined mainly by the maximum dimensions and the maximum load per axle imposed by the local traffic rules. These limitations do not vary much from one country to another. The following are some typical industrial applications where the advantages of mobile substations are useful: In the initial stages of production of a new plant, the power company may not yet have completed its final supply with acceptable reliability. The availability of electric power may be substantially increased by using one or more mobile substations during this first period. In order to maintain acceptable power availability, it is often necessary to install standby feeders in substations. Such permanent arrangements are very costly. A technically and economically attractive alternative is to locate one or several emergency mobile substations which can be brought to site at short notice. Earthquakes, landslides and other natural catastrophes are typical events when interruption in electricity supply may occur, and which in some countries it may be useful to safeguard against. Temporary power supply to construction sites, oil drilling fields, mines, construction camps, etc.

Semi-trailer substation In this case the substation equipment is installed on a platform coupled to front and rear with removable rubber-tyre supports for carrying the unit to the various sites. Raising and lowering the platform to the ground is performed by a hydraulic system. Connecting and releasing the support of the platform is a quick and simple operation, involving the connecting couplings between supports and platform only.

•

The main advantages offered by this solution are: a single set of rubber-tyre supports for several substations the rubber-tyre supports can be stored separately, away from the influence of sun, sandstorms, etc., when the substations is in service in tropical or desert areas

•

• •

•

Trailer substation Selection of the most appropriate trailer is determined by the total weight and size of the equipment and the length of the whole plant. The heaviest substations (over 40 tonnes and over 15 m long) are most conveniently installed on trailers because of: better weight distribution and axle loading the possibility of using steerable wheels at the rear to permit travel on roads with sharp bends by using a large number of wheels, substations of considerable weight and length will give a lighter weight distribution on soft roads.

•

Economic advantages The overall cost of a mobile substation is usually lower than the installed cost of a similar, conventionally built substation. For road trailers consisting of a main platform carried at both ends by removable rubbertyre supports, a single set of axles and wheels is sufficient for several mobile units, and thus total cost may be further reduced. The cost of the equipment is the same for both mobile and conventional arrangements. However, the cost of the platform itself is less than that of the supporting structures and foundations of a permanently built substation, and the plot area required is also greatly reduced.

Skid-mounted substation A special application of a mobile substation is the skid-mounted unit. Fig. 8 shows an example of a fully equipped unit for tropical environments. Skid-mounted substations are particularly suitable for movable applications in quarrying and open-cast mining. The fully enclosed robust sheet steel superstructure is divided into different compartments, mounted on a solid foundation raft. The rigid 145

5.10

145

97-03-05, 14.01

INDUSTRIAL POWER SYSTEM Prefabricated and mobile substations raft enables the unit to be easily repositioned, either by crane or by means of towing gear or tractor. The unit may be erected outdoors on practically any suitable surface without special foundations. Owing to the size of the raft, the specific load-carrying capacity of the supporting ground need not be particularly high. The factory-assembled unit contains all electrical equipment required and it is ready for hook-up. A special elevated roof protects against sunlight and prevents overheating in tropical areas. Incoming and outgoing cable transits are sealed against vermin, dust, etc.

Fig. 8. Skid-mounted substation fully equipped for tropical mining environments.

146

5.10

146

97-03-05, 14.01

INDUSTRIAL POWER SYSTEM Prefabricated and mobile substations

Bibliography [ABBDSI1316 87E] Switchgear Manual, 8th Edition, Mannheim 1987, ISBN 3-59080841-1 [Laz80]

Irwin Lazlar. Electrical Systems Analysis and Design for Industrial Plants, McGraw-Hill , New York 1980, ISBN 0-07-036789-2

[IEEE 141]

Electric Power Distribution for Industrial Plants IEEE Red Book - 1986

[IEEE 242]

Protection & Coordination of Industrial and Commercial Power Systems IEEE Buff Book - 1986

[IEEE 739]

Energy Conservation and Cost-Effective Planning in Industrial Fascilities IEEE Bronze Book - 1984

[IEEE 241]

Electric Power Systems in Commercial Buildings IEEE Gray Book - 1983

[IEEE 142]

Grounding Industrial and Commercial Power Systems IEEE Green Book - 1982

[IEEE 399]

Industrial and Commercial Power System Analysis IEEE Brown Book -1980

[IEEE 493]

Design of Reliable Industrial and Commercial Power Systems IEEE Gold Book - 1980

[IEEE 446]

Emergency and Standby Power for Industrial and Commercial Applications IEEE Orange Book - 1980

147

5.11

147

97-03-05, 14.02

BOOK No 11 Version 0

Power Systems for Industry

ABB Transmission and Distribution Management Ltd BA THS / BU Transmission Systems and Substations P. O. Box 8131 CH - 8050 Zürich Switzerland

ReklamCenter AB (99311)

Printed in Sweden, ABB Support 1999-06

BU TS / Global LEC Support C/o ABB Switchgear AB SE - 721 58 Västerås Sweden