Maintenance Of Electrical Plant

HYDROELECTRIC POWER PLANTS Selected Topics for Consultation Engineers

Maintenance of Electrical Plant SCOPE and PURPOSE Maintenance recommendations are based on industry standards and experience. The electrical plant within a power station, by the duty it performs, must have high reliability and thus a regime of planned maintenance

Linz 2007 Maintenance of Electrical Plant

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Content MAINTENANCE OF ELECTRICAL PLANT........................................................................................... 4 INTRODUCTION ...................................................................................................................................... 4 Preventive Maintenance............................................................................................................... 4 CONDITION-BASED MAINTENANCE .......................................................................................................... 5 COMBINATION OF CONDITION-BASED AND PREVENTIVE MAINTENANCE ..................................................... 5 MAINTENANCE AND TEST PROCEDURES ........................................................................................ 6 GENERAL .............................................................................................................................................. 6 INFRARED SCANNING ............................................................................................................................. 6 FAULT AND LOAD FLOW STUDIES/EQUIPMENT RATINGS........................................................................... 7 MAINTENANCE SCHEDULES AND DOCUMENTATION .................................................................................. 7 ELECTRICAL EQUIPMENT MAINTENANCE ....................................................................................... 8 ANNUNCIATORS ..................................................................................................................................... 8 ARRESTERS .......................................................................................................................................... 8 SWITCH-GEAR ....................................................................................................................................... 9 High voltage switch-gear ............................................................................................................. 9 Medium and low voltage switchgear ........................................................................................ 11 Incoming and bus-section circuit breakers ............................................................................. 11 Contators ..................................................................................................................................... 11 Isolators ....................................................................................................................................... 12 Control indication and protection equipment.......................................................................... 12 Function checks ......................................................................................................................... 12 Switchboard inspection and overhaul...................................................................................... 12 PROTECTION EQUIPMENT TESTING ........................................................................................................ 13 Primary injection tests ............................................................................................................... 13 Secondary injection tests .......................................................................................................... 14 Automatic voltage regulator (AVR)........................................................................................... 15 Supervisory and protection equipment.................................................................................... 15 ELECTRIC MOTORS.............................................................................................................................. 15 ROUTINE MAINTENANCE (ON THE SITE)................................................................................................. 16 ROUTINE MAINTENANCE - FULL WORKSHOP OVERHAUL .......................................................................... 18 Bearings....................................................................................................................................... 20 Slipring, brushgear, commutator .............................................................................................. 21 Cooling circuit............................................................................................................................. 21 Airgaps......................................................................................................................................... 21 REPAIR FOLLOWING BREAKDOWN ......................................................................................................... 21 MOTOR TESTING .................................................................................................................................. 22 Insulation resistance measurement ......................................................................................... 22 Polarisation index measurement .............................................................................................. 22 Loss angle testing ...................................................................................................................... 24 Squirrel-cage rotor testing......................................................................................................... 26 TRANSFORMERS ................................................................................................................................ 26 FLUID-FILLED TRANSFORMERS.............................................................................................................. 26 Fluid ............................................................................................................................................. 27 Breather ....................................................................................................................................... 28 Transformer tank and compound ............................................................................................. 28 Neutral earthing resistors .......................................................................................................... 28 Bushings and connections........................................................................................................ 28 Off-load tapchanger.................................................................................................................... 29 On-load tapchanger.................................................................................................................... 29 Winding temperature indicators ............................................................................................... 29 Buchholz relay ............................................................................................................................ 30 Maintenance of Electrical Plant

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Pressure relief diaphragm ......................................................................................................... 30 Pressure relief valve................................................................................................................... 30 Cooling equipment ..................................................................................................................... 30 Marshalling kiosk........................................................................................................................ 30 Dry air-cooled and cast-in-resin air-cooled transformers ...................................................... 30 ANCILLARY EQUIPMENT ................................................................................................................... 31 BATTERY SYSTEMS AND CHARGERS ...................................................................................................... 31 LEAD-ACID BATTERIES ......................................................................................................................... 31 NICKEL-CADMIUM ALKALINE BATTERIES ................................................................................................ 33 BATTERY CHARGERS ........................................................................................................................... 34 SECURE INSTRUMENT SUPPLY SYSTEMS (UPS)..................................................................................... 34 Rotary converters ....................................................................................................................... 34 Inverters....................................................................................................................................... 34 Maintenance Schedule – Flooded, Wet Cell, Lead Acid Batteries......................................... 35 Maintenance Schedule – Battery Chargers ............................................................................. 35 CABLING AND EARTHING ................................................................................................................. 36 CABLING SYSTEMS ............................................................................................................................... 36 EARTHING SYSTEMS ............................................................................................................................ 36 STANDARDS AND RECOMMENDED PRACTICES........................................................................... 37

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Maintenance of Electrical Plant Introduction Maintenance recommendations are based on industry standards and experience. However, equipment and situations vary greatly, and good engineering and management judgment must be exercised when applying these recommendations. Several sources of information must be consulted (e.g., manufacturer’s recommendations, unusual operating conditions, personal experience with the equipment, etc.) in conjunction with these maintenance recommendations. The electrical plant within a power station, by the duty it performs, must have high reliability and thus a regime of planned maintenance, rather than breakdown maintenance, must be adopted. Such maintenance can be on the basis of: • • • • •

Elapsed time, Running hours. Number of operations. Performance monitoring. Condition monitoring.

For rotating plant, the last two would seem ideal, but monitoring is usually manpower intensive and so can only be applied to certain prime items. Even the checking of running hours or number of operations would be a major exercise if carried out on the multitude of electrical plant items within a station, and thus is again only of limited application. Preventive Maintenance Preventive maintenance (PM) is the practice of maintaining equipment on a regular schedule, based on elapsed time, run-time meter readings, or number of operations. The intent of PM is to “prevent” maintenance problems or failures before they take place by following routine and comprehensive maintenance procedures. The goal is to achieve fewer, shorter, and more predictable outages. Some advantages of preventive maintenance are: • It is predictable, making budgeting, planning, and resource leveling possible. • When properly practiced, it generally prevents most major problems, thus reducing forced outages, “reactive maintenance,” and maintenance costs in general. • It gives managers a level of assurance that equipment is being maintained. • It is easily understood and justified. Preventive maintenance does have some drawbacks: • It is time consuming and resource intensive. • It does not consider actual equipment condition when scheduling or performing the maintenance. • It can cause problems in equipment in addition to solving them (e.g., damaging seals, stripping threads). Maintenance of Electrical Plant

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Traditionally, preventive maintenance has been the standard maintenance practice in power stations. The maintenance recommendations - in this lecture series - are based on a preventive philosophy and should be considered as “baseline” practices to be used when managing a maintenance program. Whether utilizing a preventive maintenance (PM), reliability centered maintenance (RCM,) or condition based maintenance (CBM), or a combination of these, the primary focus of the in-house maintenance staff should be scheduled maintenance. This will reduce reactive (emergency and corrective) maintenance. Scheduled (planned) maintenance should have a higher priority than special projects. Scheduled maintenance should be the number one priority. Elapsed time planned maintenance is the most practical, routine method for the majority of electrical plant. Considerable effort is needed to ensure that the time periods are correctly chosen and there must be a willingness to change them in the light of experience. The maintenance planning system used must therefore have a flexibility, which allows such changes to be made simply as one of its main features. Condition-Based Maintenance This program relies on knowing the condition of individual pieces of equipment. Some features of CBM include: • Monitoring equipment parameters such as temperatures, pressures, vibrations, leakage current, dissolved gas analysis, etc. • Testing on a periodic basis and/or when problems are suspected, introduce double testing, vibration analysis and infrared scanning. • Careful monitoring of operator-gathered data. • Results in knowledgeable maintenance decisions, which would reduce overall costs by focusing only on equipment that really needs attention. Combination of Condition-Based and Preventive Maintenance A combination of condition-based maintenance and preventive maintenance is perhaps the most practical approach. Monitoring, testing, using historical data, and preventive maintenance schedules may provide the best information on when equipment should be maintained. By keeping accurate records of the “as found” condition of equipment when it is torn down for maintenance, one can determine what maintenance was really necessary. In this manner, maintenance schedules can be lengthened or perhaps shortened, based on experience and monitoring.

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Maintenance and Test Procedures General Electrical maintenance activities fall into three general categories: • Routine Maintenance – Activities that are conducted while equipment and systems are in service. These activities are predictable and can be scheduled, staffed, and budgeted. Generally, these are the activities scheduled on a timebased, run-time-meter-based, or a number of operations schedule. Some examples are visual inspections, infrared scans, cleaning, functional tests, measurement of operating quantities, lubrication, oil tests, governor, and excitation system alignments. •

Maintenance Testing – Activities that involve the use of test equipment to assess condition in an offline state. These activities are predictable and can be scheduled, staffed, and budgeted. They may be scheduled on a time, meter, or number of operations basis but may be planned to coincide with scheduled equipment outages. Since these activities are predictable, some offices consider them “routine maintenance” or “preventive maintenance.” Some examples are Doble testing (insulation test), meggering, relay testing, circuit breaker trip testing, alternating current (AC) high-potential (Hipot) tests, high voltage direct current (HVDC) ramp tests, battery load tests.

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Diagnostic Testing – Activities that involve use of test equipment to assess condition of equipment after unusual events such as faults, fires, or equipment failure/repair/replacement or when equipment deterioration is suspected. These activities are not predictable and cannot be scheduled because they are required after a forced outage. Each office must budget contingency funds for these events. Some examples are Doble testing, AC Hipot tests, HVDC ramp tests, partial discharge measurement, wedge tightness, core magnetization tests, pole drop tests, turns ratio, and core ground tests.

Infrared Scanning Infrared (IR) scanning is recommended as a regular maintenance procedure. Infrared scanning and analysis have become an essential diagnostic tool throughout all industries and have been used in power industry to detect many serious conditions requiring immediate corrective action. Several forced outages already have been avoided. Infrared scanning is non-intrusive and is accomplished while equipment is in service. It can be used not only for electrical equipment but also to detect mechanical and structural problems. Therefore, infrared scanning is HIGHLY recommended as a regularly scheduled maintenance procedure. Effective infrared scanning and analysis require the following: • The scanning equipment (IR camera and accessories) must be high quality and correctly maintained and calibrated. Maintenance of Electrical Plant

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The IR camera operator must be trained to use the equipment and deal with complicating issues such as differing emissivities of surfaces and reflectivity. Certified Level 1 Thermographer (e.g., Academy of IR Thermography) credentials, or higher, are recommended.

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The IR system operator must be able to analyze results using state-of-theart software critical to successful interpretation of problems.

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Experiences in the field have shown that technical knowledge of the equipment being scanned is highly desirable.

Fault and Load Flow Studies/Equipment Ratings Electrical power systems change as new generation and transmission lines are added or modified. Changes also occur as new equipment is added or upgraded inside the power plant. This may mean that load ratings of various equipment and interrupting ratings of breakers and fuses are no longer adequate. Underrated or misapplied electrical equipment can be hazardous to personnel, to the integrity of the power plant and power system, and to the equipment itself. Therefore, it is necessary to periodically conduct fault and load studies and to review equipment ratings for adequacy (continuous current, momentary current, momentary voltage, basic impulse insulation level [BIL], current interrupting ratings, etc.) and for coordination of protective relays, circuit breakers, and fuses to ensure safe and reliable operation. Maintenance Schedules and Documentation Complete, accurate, and current documentation is essential to an effective maintenance program. Whether performing preventive, predictive, or reliability centered maintenance, keeping track of equipment condition and maintenance — performed and planned — is critical. The maintenance record keeping system must be kept current so that a complete maintenance history of each piece of equipment is available at all times. This is important for planning and conducting an ongoing maintenance program. Regular maintenance and emergency maintenance must be well documented, as should special work done during overhauls and replacement. The availability of up-to-date drawings to management and maintenance staff is extremely important. Accurate drawings are very important to ongoing maintenance, testing, and new construction; but they are essential during emergencies for troubleshooting. In addition, accurate drawings are important to the continued safety of the staff working on the equipment.

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ELECTRICAL EQUIPMENT MAINTENANCE Annunciators Annunciators provide essential plant condition status information to operators and maintenance personnel. Two aspects must be considered: (1) correct operation of the annunciator itself (2) integrity of the alarm devices and interconnected wiring. Annunciator operation is easily tested using the “Test” button provided on most annunciators and is considered an “operations” activity. Verifying integrity of the alarm devices and interconnecting wiring requires a “functional test” of these circuits. Functional testing is accomplished by (1) resetting the annunciator, (2) closing (or opening) contacts at the alarm device, (3) verifying that the correct annunciator window is activated. It is recommended that the alarm device actually be triggered, where possible, for best assurance; however, it may be necessary to simulate contact operation with a “jumper” (or lifted lead) when device activation is not possible. Caution: Operating the alarm device may trigger unwanted control or protection actions as well as annunciation. Know what “should happen” by consulting up-to-date drawings before triggering alarms. Arresters Lightning or surge arresters provide protection for important equipment from highenergy surges. These arresters are static devices, which require fairly infrequent maintenance. Most maintenance must take place while the associated circuit is deenergized. However, crucial visual inspections and infrared (IR) scans can take place while energized. Maintenance schedule for arresters Maintenance or Test Review equipment rating Visual inspection Clean insulator and check connections Doble test (power frequency dielectric loss, direct current [DC], insulation resistance, power factor) Replace all silicon carbide arresters with metal oxide varistor (MOV) type Infrared scan Maintenance of Electrical Plant

Recommended interval 5 years Quarterly to semiannually 3-6 years Ambient dependent 3-6 years Ambient dependent As soon as possible Every year 8

Switch-gear High voltage switch-gear The high voltage (HV) switch-gear used within the works power system on a modern power station usually operates at 11 kV or 3.3 kV and is one of the foIlowing types: • Air circuit-breaker • Oil circuit-breaker • Vacuum circuit-breaker. It is not intended to give a detailed description of the maintenance of each type of switch-gear but rather to outline the essential aspects which must be borne in mind on any maintenance schedule. The duty cycle of some of the switch-gear is arduous and the potential fault levels that the switch-gear may be required to interrupt are high (typically, 750 MVA on an 11 kV switchboard). The aim of all maintenance activity must therefore be not only to ensure reliable operation in normal circumstances but also that the switch-gear remains capable of safe operation under fault conditions throughout its life. Setting the periodicity of switchgear maintenance is not an easy task. Some devices operate many times per day and are thus subject to wear; others may only operate a few times a year and are therefore unlikely to cause operational difficulties due to mechanical problems. Usually an annual overhaul is planned, with an additional check if a switch has operated under fault conditions or if it performs a particularly erroneous duty cycle. All HV switchgear overhaul specifications must cover the following points (where applicable to the particular design): • Pre-overhaul check of insulation resistances (e.g. with IR camera) between phases and phase-to-earth, and measurement of the resistance of each phase. • Lifting and inspection of arc chutes (air circuit-breakers). • Removal of oil tank, replacement of oil with clean tested oil (oil circuit-breakers). • Cleaning and inspection of moving and fixed main contact assemblies. • Checking of contacts for wear, clearances, wipe and alignment (phase-to-phase and between phases). • Checking of flexible connections for wear. • Inspection and checking of 'close' and 'trip' mechanisms, carrying out all specified measurements to ensure correct operation. • Checking the operation of isolating and earthing mechanisms. Maintenance of Electrical Plant

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• Examination of all auxiliary electrical equipment and overhaul of closing and tripping solenoids, contactors and anti-pump devices. • Checking the operation and integrity of air-blowing systems. • Checking all locking devices. • lnspection of all insulation for cracks, discoloration, or other signs of distress. • Checking of mechanicaI interlocks. • Lubrication of moving parts. • Final checks for 'hot spots' and measurement of resistance of each phase. • Cleaning and checking cubicle - check cubicle heater.

Throughout the maintenance, all electrical checks should be recorded on a checksheet. As experience is gained on each particular type of switch-gear, particular fauIts and weaknesses will be discovered and additional checks will be added to the maintenance specification. Following the overhaul of a circuit-breaker, a full function test must be carried out before returning it to service. The following checks must be included: • Operation from all positions, taking account of circuit interlocks and group interlocks. • Operation from each protection system. • Alarm checks • Trip circuit supervision checks. In addition to the overhaul and function checking of each circuit, the switchboard itself must be overhauled periodically. The planning of such maintenance can be difficult, since a total board outage will be required; however, since the board contains no moving an overhaul every five years will be usually sufficient with interim short outages for bus bar spout cleaning and incoming circuit-breaker maintenance. Areas of work covered by a switchboard outage are: • • • • • •

Cleaning of busbar spouts and checking of associated interlocks. Cleaning of busbars, checking of busbar joints and support arrangements. Busbar insulation and conductivity checks. Checking the security of all covers. Overhaul of board auxiliary wiring and supplies. lnspection of earthing.

Protection testing is also often carried out following HV switchgear overhaul work. This is discussed later in lecture series. Maintenance of Electrical Plant

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Medium and low voltage switchgear Medium and low voltage (MV and LV) switchgear is usually in a modular board arrangement, each module being the switchgear associated with a particular circuit. This is not invariable; sometimes contactors, isolators, etc., are part of a control panel which is the total control package for a plant item, e.g. a shoot-blower control panel for coal or oil fired systems or an auxiliary boiler control panel. The comments below are in the context of the component parts of a multi-circuit 0.4kV switchboard of modular design, but the particular maintenance details for each item apply equally to the components used in other MV and LV switchgear applications. It must be remembered that a 0.4kV board in a power station is subject to a high potential fault level (about 30 MVA). There have been many instances of electrical flashovers within 0.4kV switchgear which have been attributed to dirt, poor connections, loose fuses and bad condition of contacts. The consequences of such faults can be serious so, whilst the overhaul and maintenance of such switchgear can appear not a real pleasure and repetitive, a high standard of work must be encouraged. Incoming and bus-section circuit breakers These are similar in many aspects to the HV automatic circuit breakers; previously discussed and do not need to be dealt with separately. Contators The duty performed by the contactors in power station switchgear is often very complicated, with many operations a day. Such difficult duty is reflected in mechanical wear and tear, electrical contact wear. Therefore particular attention must be given to these aspects when undertaking a contactor overhaul. Maintenance instructions for a contactor must include, where applicable: • • • • • • • • • • •

Cleaning. Resurfacing (dressing) or renewal of main and auxiliary contacts. Note that some contacts are plated and cannot be dressed successfully. Checking of contact aIignment, wipe and spring loading. Mechanical operation of armature, alignment of pole faces. Checking of the mechanical interlocking between reversing starters and the latching mechanism on latched starters. Trip mechanism inspection. Electrical checks on main and trip coils (resistance and insulation). Check tightness and condition of all connections. Arc chute inspection. Lubrication.

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Isolators The maintenance of isolators consists mainly of cleaning, checking contact condition and tightness of connections, overhaul and checking of the operating mechanism, and confirming the tightness and continuity of fuses, if fitted. Isolators are usually interlocked with the cubicle door and often with their associated contactor. These interlocks are important safety features, which must be checked. Control indication and protection equipment The contactor cubicle may contain control relays, control selectors, fused supplies, mechanical and electrical indication equipment, protection relays, counters and timers, ammeters, etc., all of which are included in the maintenance routine. The cubicle heater is also checked. DC starters may have a series of timed contacts to switch starting resistances. Such starting resistances are usually only short-time rated, so that the correct timing of the contact operation is important. Function checks At the completion of the overhaul of each circuit, a function check is performed, as outIined for HV switchgear. Switchboard inspection and overhaul The complete MV or LV board is isolated and overhauled periodically. The frequency depends on the environment. The main areas of work are the busbars, common control supplies, auxiliary wiring, security of panel fixings and inspection of earthing.

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Protection equipment testing The electrical protection equipment fitted as an integral part of power station HV and MV switch-gear requires little routine maintenance apart from occasional careful cleaning and visual inspection. To ensure correct and reliable operation each device needs to be tested periodically, the results being entered into a record system containing the details of the device, its settings, and a record of tests performed on the device. Additionally, it is advisable to insert a relay-setting record card in each relay case. This enables a cross-check to be made after testing to ensure that the relay setting has not been inadvertently left at an incorrect value. Station: Panel: Date

Current setting

Time setting

Relay: C.T. Ratio: Initials Remarks

Central Workshop of Electricity Generating Co. Table 1. Typical relay-setting record card. Two types of protection testing are normally undertaken: • primary injection tests • secondary injection tests Primary injection tests This test is used to check the operation of the total system associated with a protection device. Connections are made to the main primary conductors of circuit under test and a current is injected in such a way as to check the operation of the protection system under normal and fault current conditions. The test set used for this is a single-phase transformer, fed by a 0 - 415 V variable transformer on its HV side and rated to suppIy about 2000 A at a few volts at its secondary terminals. Particular test procedures need to be prepared for each type of protective device. The means of connecting the heavy current test supply into the circuit should be specified; some HV switch-gear is supplied with connection devices for this purpose.

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Figure 1. Primary injection testing of 11 kV switch-gear The figure above shows a typical arrangement for 11 kV metal clad switch-gear. Primary injection testing is time consuming and would normally only be undertaken on a few occasions during the life of the equipment, say, every 5 years, unless evidence justified more frequent testing, or a fault was suspected on a particular circuit. Secondary injection tests This is used to check the operation of the protective relay alone and can be undertaken on the site or, for plug-in relays, with the relay plugged into a purpose built test rig. A test current from a secondary injection test set is fed into the relay circuit in such a way as to check the performance of the relay over its range of operation, in accordance with a specified test procedure. The secondary-injection test set will include: • A low voltage single-phase variable output up to, typically, 100 A. • A harmonic filter for use with relays sensitive to harmonic interference. • Accurate current indication • A timer initiated from the test set 'start' button and stopped from the relay trip contacts. • 0-240 V AC and DC low power variable-voltage outputs. The frequency of secondary injection testing should initially be set at every two years and then modified as a result of the experience gained on each type of device. Maintenance of Electrical Plant

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Automatic voltage regulator (AVR) Off-load maintenance of the AVR consists initially off cleaning the equipment, using a soft brush and vacuum cIeaner to remove all accumulations of dust, followed by a thorough examination of all components and wiring for signs of damage, overheating, loose fixings, etc. Relays should be checked and cleaned, if necessary. Motorised potentiometers and their cam-operated switches should be checked and lubricated, followed by a check of their traverse times. Correct operation of all tripping functions must also be checked. Fault finding and setting-up the AVR are specialised tasks, for which detailed procedures need to be prepared. Supervisory and protection equipment A schedule, identifying each supervisory and protection device, should be prepared. The schedule should give reference to particular test and calibration procedures for each device and thus form the basis of a routine maintenance system which ensures that the accuracy and reliability of this equipment is maintained. ln addition to the testing of individual devices in protection equipment, overall protection system checks must be carried out at regular intervals (typically annually).

Electric Motors The high number of electric motors employed in a power station are so varied in size, type, application and environment that it would be meaningless to stipulate a fixed periodicity of maintenance for all motors. A good basic approach is to plan regular on site maintenance and an occasional full workshop overhaul for each motor, the frequency being based on a consideration of all factors influencing the wear and tear on the particular machine. As a guide, most machines would need annual in situ maintenance, with perhaps a full workshop overhaul in every 3-rd year. The activities comprising in situ maintenance are outlined below, followed by a more detailed description of the work to be included in a workshop overhaul. The comments obviously do not apply to all machines; similarly, the complexity of each task varies between HV and MV motors and between different designs. Sufficient information is given to identify the activities in the maintenance schedule for a particular machine.

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Routine maintenance (on the site) • GeneraIly clean down the motor, check for obvious mechanical damage, check holding-down bolts, security of guards, etc. Inspect for oil leaks, coolant leaks and effectiveness of weather protection. • Tests From terminal box or switchgear, check insulation resistance of winding and supply cable; check resistance balance of windings. • Terminal box Clean terminal box internally; check connections; check insulation condition; ensure that cable glanding and earthing arrangements are satisfactory. Ensure that the terminal box is adequately sealed after maintenance. • Cooling circuit For open motors, remove covers (where possible) and blow out winding with dry compressed air. For enclosed motors, with air-to-air coolers, clean heat exchanger tubes. lnspect water-cooled machine heat exchangers for leaks, build-up of scale, or any other defect which may impair the cooling efficiency. • Windings It is not normally necessary to inspect the windings of an enclosed machine. Open machines should be thoroughly inspected at every accessible point for signs of serious contamination, winding damage, etc.

Figure 2. 11kV 10MW wound-rotor induction motor • Bearings 1. Ball and roller. The in situ check of the bearings is made by the best practicable method. For bearings fitted with end caps, the outer cap is removed, having first supported the inner cap with guide studs. The bearing can then be inspected for scoring, roughness, cracking, metallic debris or any other sign of distress. It may be feasible to assess bearing wear by checking the lift on the shaft. Where the bearings are inaccessible, it may be possible to run the motor to listen for bearing noise, or at least to turn the shaft to try to detect any Maintenance of Electrical Plant

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roughness or rubbing that may indicate wear. Finally, the bearing should be lubricated. Care is needed, over-greasing can cause the bearing to overheat due to churning of the grease. 2. Pad or sleeve bearings. Examine bearing housing for Ieaks from joints, shaft seaIs, drain plugs or level indicators, including (if possible) a check for internal oil leakage into motor. Drain the bearing oil and examine its condition, Iooking for signs of sludge, metallic debris or water. If unsatisfactory, further investigation of the bearing will be required. Check that bearing clearances are within tolerance by measuring shaft lift or by the gap between shaft and bearing top, as applicable. • Airgaps On motors provided with access points for air-gap readings, measure the air-gaps, using Iong, cIean feeler gauges, and check that they are within tolerance. Where the access points are placed at 120O around the stator, three sets of three readings should be taken, turning the shaft through 120O at each set. The three readings at each position are then averaged. Where the points are at 90O around the stator, take two sets of readings, turning the rotor through 180O between readings. • Coupling Remove guards and examine for excessive play. Excessive play in coupling gear requires checks of the spring and teeth for wear or damage. The grease on a coupling gear should also be checked and replaced, if necessary. Excessive play in a rubber-bushed coupling needs checks for rubber-bush wear, elongation of coupling bush holes, bolt waisting and bolt tightness. • Slip-rings (variable-speed AC motors) The overhaul of slip-rings and brush-gear has already been outlined in the generator maintenance section of this chapter. The principles described therein apply equally to slip-ring motors. On completion of maintenance, particular care must be taken to ensure that the assembly is clean and free of contamination from oil, grease or carbon dust. • Commutators (DC machines and AC commutator machines). Whilst the same general comments on sIip-ring and brushgear rnaintenance apply to commutator machines, particular care has to be taken to ensure that correct brush position is maintained. A good commutator eIectrical surface must be achieved. The surface must be uniform in appearance and irregularities should be regarded as indicators of electrical or mechanical faults requiring further investigation.

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Routine maintenance - full workshop overhaul The main stages of a full motor overhaul are now described. It is assumed that the motor has been checked electrically and any evidence of faults has been noted for investigation during the overhaul: Cleandown and strip After a thorough external clean, normal dismantling techniques are used, whereby each item is marked, where necessary, to show its position relative to other parts. All non-maintainable parts, such as covers, bearing housing, baffles, etc., should be cleaned after removal and inspected for cracks, distortion or other damage. Rotor removal The larger the machine, the higher the probability that special equipment will be required to enable the rotor to be removed. A typical method, employing a trolley, an extension shaft (a steel tube machined such that it just slips over the protected journal surface) and an overhead beam or crane. Stator The method of cleaning the stator depends upon the extent and nature of contamination. Common methods are: • • • •

Dry dusting and blow-out. Wiping clean, using grease solvent. Spraying with electrical solvent. Washing with distilled water (followed by dry-out).

When cleaning the windings of HV motors, particular care must be taken to ensure that contamination is not driven into the windings, thus becoming the focus of a future failure. Insulation resistance readings of the stator are taken before and after cleaning. A thorough inspection of the stator is now undertaken and any remedial work carried out. The following points should be covered: • Condition of winding insulation. Is the taping satisfactory? Is re-varnishing required? • End-winding integrity. Inspect for evidence of looseness, defective lashings, bracing or spacers. • Slot conductors. Check for any evidence of discharge, loose or damaged wedges. • Winding connections. To cable box and star point (if accessible). • Stator laminations. Evidence of hotspots or looseness, rubbing or scoring. Looseness of the stator pack may be indicated by evidence of brown powder from the insulation coating on the laminations. Maintenance of Electrical Plant

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Figure 3. Usual technique for large rotor removal A stator in poor overall condition may be referred to a specialist repair contractor for overhaul. General deterioration can often be reduced by through cleaning and a treble dipping/baking in epoxy resin varnish, a service offered by most repairers. Rotor Cleaning methods described in earlier paragraphs are also applicable to the rotor. The condition of the rotor is determined with regard to the following points: • • • • •

Loose, cracked or broken rotor bars. Cracked rotor bar-to-endring joints. Cracked or broken end-rings. Blockage of ventilation ducts. Laminations - signs of rubbing or scoring. Evidence of hot spots. • Rotor fan damage. Non-destructive testing (NDT) techniques may be used to inspect rotor bar-toendring joints, if cracking is suspected. Maintenance of Electrical Plant

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An indication of poor rotor bar-to-endring joints may be obtained by using a 'growler', an electromagnetic test instrument which produces a change of resonant sound when passed over a defective bar. Bearings 1. Ball and roller bearings. Remove the bearing from shaft using a hydraulic bearing puller. Clean all grease from bearing and inspect for: • • • • • • •

Scored or worn outer journals. Cracked or distorted cages. Pitting of balls or rollers. Excessive play. lndications of slack fit between bearing and housing. Slack fit to shaft. Rough running (when spun after light oiling).

Unsatisfactory bearings should be scrapped and replaced. To enable the bearing to be refitted to the shaft it should be expanded by heating to about 80OC using either a thermostatically controlled oil bath or a purpose made induction heater. Great care must be taken throughout the reassembly to ensure that no dirt enters the bearing. The bearing itself should be packed with the correct grade of grease and the housing about half-filled with grease. Overgreasing will lead to churning and subsequent overheating of the bearing. 2. Pad and sleeve bearings. The essential points on the overhaul of pad and sleeve bearings are given below; the techniques used vary with the particular design of bearing: • Pad type bearings. Inspect for pivot wear and bearing surface scoring, cracking or overheating. Measure oil clearances by taking leads. Take care to ensure that offset type pads are fitted in the correct relationship to the direction of rotation. • Sleeve type bearings. Inspect the bearing surface for heavy scoring, cracking, or signs of overheating, or polishing marks where the shaft has worn the sleeve. Check oil pick-up rings for wear or distortion. Check bearing bore against shaft dimension in several positions. If new sleeves are being fitted, check sleeve-to-casing fit, using feeler gauges. Check shaft-to-bearing contact arc by engineers' blue (ink paper to indicate contacting surfaces). Arc of contact should typically be 60O for high speed shafts (3000 r/min) and 100O for low speed shafts (500 r/min). Remove high spots on bearing surface. Check bearing clearance, using leads.

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Slipring, brushgear, commutator Comments made under the section on in situ maintenance give an outline to the overhaul of these items. Additionally, the opportunity may be taken to mount the rotor in a Iathe for grinding or machining the slip-ring or commutator. Cooling circuit A thorough clean and check of the cooling circuit should be carried out. Water coolers need a pressure test to ensure that they are Ieak-free. Airgaps After reassembling the motor, the rotor-to-stator airgap requires checking. If necessary, the bearing positions should be adjusted to ensure that the airgap error does not exceed 10%. The gaps between the shaft and oil seals, and between the air baffles, then also require checking and adjustment.

Repair following breakdown It is common practice to employ the services of motor repair workshops, either at the motor manufacturers or at specialist repair firms, for all major repair work on electric motors. Usually a fast turnaround time can be achieved. One of the major delays on large motor repairs is the availability of the correct size of copper conductor. Consideration should be given to stocking sufficient material within the station stores in order to avoid this delay for critical machines. lt is also feasible to stock a complete set of stator windings ready for installation by a contractor in the event of a serious fault. Now that extensive standardisation has been achieved on 0.4kV machines, it is often cheaper and quicker to scrap a small machine needing a rewind and to buy a replacement ex-stock. To ensure that a high standard of motor repair work is maintained, the relevant standards are already prepared and specified for all repairs. ln addition to the basic repair of the motor, consideration is also given to whether it may be advantageous to: • Up-rate the motor, by winding to a higher insulation class. • Improve the protection on particularly troublesome drives by embedding thermocouples within the stator. • Improve damp resistance by improved sealing.

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Motor testing Some of the main techniques used for fault finding on a motor, or for checking its fitness for duty are outlined below. Much of the detail associated with these techniques has already been given in the generator maintenance section to which reference is made. Insulation resistance measurement Insulation resistance measurement is used to measure the DC resistance of a motor winding to earth, or the resistance between windings. The test is carried out using a battery- or mains-powered, or hand-operated, megohmmeter (megger). Typically, a 500 V DC test is used on 0.4kV machines (care must be taken to disconnect diodes) HV motors are usually tested at 1 kV, for all windings up to 6.6 kV, and at 5 kV for windings of 11 kV or more. While this resistance measurement is a very quick method of assessing insulation integrity, it must be realised that the value of the information obtained is limited. On open motors, insulation resistance values can be affected by atmospheric humidity. When comparing readings with previous results, a temperature correction should be made, using the nomogram given in below. Where insulation systems are open to the atmosphere, the insulation resistance (IR) reading can be greatly affected by humidity. Water cooled windings, when drained down, need careful blowing through to remove water from all hoses if satisfactory IR readings are to be obtained. The manifold should be connected to the guard terminal on the megohmmeter so that a true reading of winding resistance to earth is obtained. Insulation resistance tests on filled winding will give a low reading due to the number of parallel paths to earth via the water circuit. To assess whether readings so obtained are reasonable, an equivalent circuit will need to be drawn which takes account of the resistance of the water (of known conductivity) in each hose. Polarisation index measurement Polarisation index (PI) measurement is an extension of insulation resistance (IR) testing an other method of assessing the surface contamination of insulation. The method of applying the test is outlined here, but the details can be found in the measurement section of this Course book. The voltages used are the same as those for insulation resistance testing, indicated above. The DC voltage is applied to the insulation for 10 minutes. The PI is the ratio of the IR reading obtained after 10 minutes to the reading obtained after 1 minute. As a general rule, a value of PI greater than 2 is desirable. Low PI is usually an indication of surface contamination of the end winding insulation by dirt or moisture, although, when accompanied by a low one-minute IR reading, more serious degradation of the Maintenance of Electrical Plant

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bulk of the insulation may be indicated. This can only be determined by loss angle measurement

Figure 4. Nomogram for correcting insulation resistance measurements to a standard temperature A polarisation index test should be made on any machine that has been out of service for some time to determine whether energisation or testing with AC is advisable. Maintenance of Electrical Plant

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Table 2 gives a guide to the interpretation of results obtained from insulation resistance (IR) and polarisation index (PI) testing. If values at or below those listed in the table are obtained after correction to 20OC, serious moisture or other contamination is indicated. Further investigation of the cause of this condition, and possibly a dry-out, will be required prior to AC energisation or testing. Minimum acceptance value of insulation resistance (IR), complete winding at 20OC [Megohms] PI > 1.6 PI < 1.6 IR1min IR10min IR1min IR10min any value 15 30 any value any value 30 50 any value any value 50 100 any value

Line voltage of machine [kV] 3.3 6.6 11.0

Table 2. Guide to determine acceptable values of insulation resistance and polarisation index testing If a constant-potential (high voltage) megohmmeter is available, a ratio of the 1-min ground-insulation-resistance reading to that obtained after 10 min is a very useful guide to insulation dryness. Values of 1.5 and over are obtained from a clean, dry winding (Figure 5.). The lower the ratio the greater the leakage path of the test voltage to ground. This method in effect measures the capability of the insulation to hold a capacitive charge.

Figure 5. Change in 1- and 10-min insulation resistance during drying process of Class B insulated AC armature winding.

Loss angle testing Loss angle testing (or tangent delta testing) is a method of assessing the condition of the bulk of the insulation of an HV motor. Maintenance of Electrical Plant

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The test is made on each phase, with the other two phases earthed, where the neutral can be split, or on the complete winding, where the neutral is solid. A maximum test voltage equivalent to line voltage is recommended, although a lower maximum voltage may be used where insulation is suspect. The Loss angle test should not be applied until satisfactory insulation resistance and polarisation index test results have been obtained. As it is stated earlier the Loss angle testing (or tangent delta testing) is a method of assessing the condition of the bulk of the insulation. As insulation deteriorates, it develops voids which in turn, cause a change in the capacitance of the winding and hence a change in the loss angle (delta) The test method is as follows: • Check that the winding has a satisfactory insulation resistance (IR) by means of a DC test. • Apply an AC voltage to the winding in steps of (0.2 x line voltage V1) from 0.2 to 1.0 V1 (The condition of the insulation may dictate that 0.8V1 should not be exceeded.) • At each step, record the capacitance and tangent of the Loss angle of the insulation, using an AC bridge. • On completion of the test, take great care to discharge the winding. Tangent delta is plotted against line voltage. Actual results will be affected by factors such as whether or not cooling water was present in the winding, the type of gas in the generator frame and its pressure and the amount of surface contaminant on the windings. By comparison with other tests on the same and similar machines, assessment can be made of the condition of the insulation.

Figure 6. Insulation loss angle The insulation may be considered as a capacitor and resistor in parallel. The ratio of resistive leakage current to capacitive leakage current is represented by the tangent of the loss angle.

Interpretation of the results is more straightforward than for a generator, where the effects of the cooling media have to be taken into account.

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Squirrel-cage rotor testing Defects, such as broken rotor bars or high resistance bar-to-endring joints, in squirrel cage rotors can be detected on-load by detection of the current, flux or speed fluctuations caused by the fault. A particularly sensitive device for the early identification of developing rotor faults is based on the stroboscopic effects. The device works on the principle that the rotational period of a motor with an electrically defective rotor bar will fluctuate slowly at twice the slip frequency. The device measures this fluctuation by processing the information from a speed pick-up on the motor shaft.

Transformers For maintenance purposes, the power transformer in use in a modern power station can be devided into three categories: • Fluid-filled. • Dry air-cooled. • Cast-in-resin air-cooled. In each, reliability is very high and normal maintenance consists of routine cleaning, checking and testing to ensure that the electrical integrity of the transformer is sustained. Fluid-filled transformers The majority of HV and some MV power transformers are fluid-filled. The fluid acting as both an insulation and cooling medium. The fluid may be cooled by • natural convection, • forced air, • water-cooled heat exchangers. Mineral oil based insulating oil is the most commonly used fluid. In high risk areas, fire-resistant fluids may be used, such as silicone fluid or a phosphate-ester. Polychlorinated biphenyIs (PCBs) are being phased out because of their high toxicity and non-biodegradability. The main routine activities are outlined below and consist of regular checks on fluid and on air breather systems, together with outage checks of other equipment. Normally, the outage checks should be yearly, but some flexibility in this may be needed to fit in with main unit outage dates. If, at any time, it becomes necessary to open up the transformer tank, extreme care must be taken to impose clean conditions and to ensure that no foreign matter is allowed to enter the tank. Those working on an open transformer should use onepiece pocketIess overalls and wear no loose items.

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All tools must be accounted for and taped to the operative so that they can be recovered, if accidentally dropped. Carelessness in the application of these measures can turn a simple task into a major, time consuming and costly exercise. Fluid Regular checks of fluid levels must be made and recorded to ensure that an adequate level of fluid is maintained. Where applicable, a note of winding temperature should also be made, since the level will obviously vary with fluid temperature. A particular danger applicable to transformers fitted with Buchholz relays is that, if the fluid level is allowed to become too low in cold weather it may fall below the relay during a period of light load and thus cause a spurious trip. Fluid added to top-up a transformer must be clean and tested immediately prior to use. The top-up is best carried out off-load, with a period of time allowed after filling for air bubbles to detrain from the oil. On-load topping-up is possible on some transformers. Care must be taken to entrain a minimum of air and, for a period after filling, the Buchholz relay will need regular bleeding to remove air and to prevent spurious operation. Fluid testing must be carried out at least annually and following a transformer trip, or gas or temperature alarm. The fluid test check for: • appearance, • moisture, • electric strength, • acidity, • resistivity • dissolved gases. The results from these tests may indicate that the fluid needs changing or reconditioning. Some sites purchase their own conditioning unit through which the fluid is circulated and then returned to the transformer. Other sites use specialist contractors for this work, who come to the location with all the necessary pumping, filtration and test equipment to deal with any size of transformer. Fluid test results may also indicate a possible incipient fault within the transformer, requiring monitoring by more frequent sampling. The method adopted to collect the sample must be designed to ensure that the sample is truly representative of the fluid in the tank and that it is not contaminated in the collection process. In circumstances where a fault is suspected or following a Buchholz gas alarm, gas samples should be collected from the Buchholz relay for analysis. The results can give a guide to the nature of the fault: in oil-filled transformers, these are as follows: • Presence of ethane, methane or carbon monoxide indicates a hot spot. Maintenance of Electrical Plant

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• Presence of hydrogen and /or acetylene indicates the presence of arcing or discharge within the oil. The sample may, of course, just prove to be air. A comprehensive guide to the maintenance of insulating oil is found in BSI Code of Practice BS5730. The tests described above are not very effective in detecting the low-temperature overheating of paper insulation in oil-filled transformers. Such overheating is known to have been a major factor in two generator transformer failures. A method has now been developed, where furfuraldehyde, a product produced exclusively by thermal degradation of paper at temperatures from as low as 110OC, may be detected from the analysis of an oil sample. It seems likely that, on certain transformers, this test will be included with those already carried out on routine oil samples. Breather The inspection of the breathing equipment should be associated with regular fluid level checks. Silicagel crystals are renewed if seen to be changing from blue to pink. At the same time, the breather oil seal is checked and, if necessary, topped-up to the correct level to prevent diffusion of moisture into the silica-gel when no breathing is taking place. The silica-gel may be recharged by drying in and oven at about 130OC until the crystals have regained a dark blue appearance. Transformer tank and compound Transformer compounds must be kept free of litter and oil drains, bund walls, cables, cable ducts and fire fighting equipment examined. A thorough clean down of the transformer tank and compound should be carried out as part of the outage work. Where spray-water protection is fitted to oil-filled transformers, it may be decided to apply solvent to the dirty areas of the tank and compound and then to test discharge the fire protection, which will in turn wash down the transformer. Other work is to inspect the tank, radiator/heat exchangers, conservator, pipe-work valves, etc., for leaks, and to ensure that all valves and cocks are secure and locked. Neutral earthing resistors The electrolyte level should be checked and its resistance measured, using a low voltage AC supply. Where fitted, heaters should be checked for correct operation. Bushings and connections Bushings are cleaned and checked for leaks, chips or cracks. A fluid level check/topup is carried out on filled bushings.

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Tightness and resistance checks of all connections, including the transformer earth connection, must be made. Off-load tapchanger The following procedure should be carried out at an outage to clean the contacts and prevent contact overheating defects due to pyrolytic growth. Unlock the tap-changer, note its position, operate the tap-change, switch it through its range several times, return it to its original position, and lock. On-load tapchanger The manufacturer's instruction must be followed for the particular design. Outage maintenance is usually confined to the diverter switch and operating mechanisms. Basic steps covered will be: • Thorough cleaning and flushing with insulating fluid. • DetaiIed inspection of all components, especially moving parts and diverter resistors. • Contact overhaul. • Check of all connections. • Overhaul of operating mechanisms. • Complete function check. • Refill with clean, tested fluid. Winding temperature indicators During the off-load examination, the winding temperature indicator should be checked to ensure that the reading is correct and that the alarm operates at the specified temperature. This is carried out by removing the indicator bulb from its pocket, in the top of the transformer and placing it into a temperature test oil-bath. The test unit is switched on to raise the temperature to 120 OC and the reading of the indicator checked against the test unit thermometer. A test meter connected across the winding temperature alarm contacts will check the operation of the alarm against the reading of the winding temperature indicator: any necessary adjustments are made and recorded. The indicator bulb pocket is then refilled with transformer oil and the bulb replaced. An injection test is then applied at the test links to the heating coil circuit and the current adjusted to the required test figure. The temperature rise is noted on the winding temperature indicator and compared with the calibration data.

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Buchholz relay The Buchholz relay is important in the transformer protection system and should be tested annually. This is carried out with a special test set in which a cylinder is charged with air at 2.1 bar gauge and connected to the test cock on the relay. Air from the cylinder is then admitted slowly to the relay until the gas-float contacts 'make': the volume of air required to achieve this is recorded. The surge float can then be tested by admitting air rapidly into the relay until the surge-float contacts 'make'. Contact operation is checked with a test meter, or with the secondary trip or alarm circuits alive. Care must be taken after the test to ensure that all air is bled off from the Buchholz chamber. Pressure relief diaphragm This is inspected during a maintenance outage for splits or leakage. A split diaphragm can allow the transformer to breath other than through the breather and thus cause unwanted moisture ingress. Pressure relief valve The valve is removed during an outage checked for correct mechanical and electrical operation on a purpose built test rig. Cooling equipment Off-load maintenance consists of an overhaul of pumps, fans, heat exchangers, etc., using normal maintenance techniques. It is particularly important to ensure that water-cooled heat exchangers have no water-to-oil leakage path and this should be checked. Marshalling kiosk The following outage work should be undertaken: • Clean and check connections. • Overhaul control equipment and check operation • Check heater. • Ensure that the weather protection is effective. Dry air-cooled and cast-in-resin air-cooled transformers The maintenance of these transformers is far simpler than for the fluid-filled type and consists of outage work where the main activities will be: • Cleaning of transformer and compound. • Checking of tightness and resistance of connections. • Testing of protective devices.

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Ancillary equipment Much could be written about the maintenance of the ancillary electrical equipment within a power station. Most of this equipment performs a vital role in the operation of the main plant. The notes below are limited to the main features, which need to be incorporated into the maintenance programme for these items. Battery systems and chargers Battery systems provide “last resort” power for performing communication, alarm, control, and protective functions when other sources of power fail. Battery system maintenance should have highest priority. Computerized, online battery monitoring systems can greatly reduce maintenance required on battery systems and actually improve battery reliability and increase battery life. Battery chargers, important to the health and readiness of battery systems, require regular maintenance as well. Lead-acid batteries The number of battery maintenance presents The work is by nature probably best confined of this nature.

systems in a modern power station is such that battery cell a considerable workload to the maintenance department. very repetitive yet must be carried out conscientiously: it is to a few operatives who are temperamentally suited to tasks

A typical approach to the maintenance of a large 120-cell Plante-type lead-acid battery would be to carry out the following routines: Two weeks routine Check electrolyte levels on all cells and top-up any cells approaching or below the minimum marks with distilled water. Note 'pilot cell' voltage, specific gravity and temperature readings. 'Pilot cells' are, say, 12 out of the 120 cells, selected so as to be evenly distributed over the length of the battery; they are used as indicators of the general battery condition. Inspect condition of connections - clean and re-grease with petroleum jelly, as necessary. Ensure that the battery room is clean, that the ventilation system is satisfactory and that the safety equipment is available. Note the battery charger current and voltage.

Three monthly routine Similar to above, except that all cell voltage and specific gravity readings are noted.

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A thorough inspection of all cells must be made and defects noted. Particular care to be taken to look for: • Terminal and connection corrosion. • Cells over-gassing. • Flaking of internal connections • Plate distortion. • Leakage of electrolyte. • Cracking of cell lids. Annual work Similar to the three monthly work, but in addition: • Remove and clean petroleum jelly from connections. • Check-tighten (using insulated tools), inspect and re-apply petroleum jelly. • Clean cell casings and support framework. The check sheet reading and notes from the routine maintenance activities should be independently inspected in order to identify any problems developing in the battery. The state-of-charge of a cell may be assessed from the specific gravity reading. An example is as follows: If a typical 240 V 1300 Ah battery were fully discharged, the specific gravity would drop from 1.260 to 1.120. The rate-of-change of specific gravity with charge is, for practical purposes, linear, and thus the state-of-charge of the above battery may be determined by the points drop from 1.260 as a proportion of 90; for example, if the specific gravity is 1.180, the battery is 30/90, or one-third discharged. For accuracy, the specific gravity readings must be corrected for any temperature variation from 15OC, as follows: + 1 point for each 1.5 OC of electrolyte temperature above/below 15 OC. The actual specific gravity values and points drop with charge will vary with type of cell and must be found from manufacturers' information. It would normally be expected that, on a standby battery, all cells would indicate a near-full state of charge. Those cells, which show readings out-of-step with the rest of the battery, require investigation. Low specific gravity, which remains low and constant over a period of charging, could indicate that the electrolyte has become weakened by some means, the strength should be checked and adjusted by addition of dilute sulphuric acid of specific gravity 1.840. But important to note that this problem is very rare. Low specific gravity may indicate sulphated cells. Usually this is the case, when the battery set were out-of-work and left uncharged for an extended period. Low specific gravity and low cell volts, which do not respond to charging, could indicate an internal cell short. It may be possible to clear a short by disturbing the Maintenance of Electrical Plant

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electrolyte by gently blowing air into the solution. (This means temporary solution only, the cell should be replaced.) Excessive gassing is most undesirable and can be caused by attempting to charge a near fully-charged battery at too high a rate. The excessive current charge (over and above that which the plate material can accept) decomposes the water content in the electrolyte into hydrogen and oxygen. If this is too violent, it lowers the electrolyte level, produces undesirable heat and causes active material to be scrubbed from the surface of the positive plates, which falls to the bottom as a deposit.

Figure 7. Temperature and float charging voltage effecting battery life-time

Figure 8. Relationship between specific gravity and charge

Nickel-Cadmium alkaline batteries These need less maintenance than lead-acid batteries. The cells are kept clean and dry, connections are kept lightly greased with petroleum jelly and are checked for tightness periodically. The electrolyte level should be regularly checked and topped-up with pure distilled water as necessary. The specific gravity of the electrolyte in a nickel-cadmium (Ni-Cd) battery does not vary with state of charge.

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Battery chargers Solid state battery chargers are usually reliable items of plant which require very little maintenance. Nevertheless, because they play such a key role in maintaining the capacity of the vital DC supplies, they must not be neglected in the overhaul programme. An annual inspection will include the following activities: • Clean and inspect all components, checking for signs of overheating, loose connections, corrosion or mechanical damage. • Overhaul switches, contactors, control relays, etc., using standard techniques. • Check fuses. • Check cooling fans, if fitted. • Check operation of the charger fail and high voltage' relays; ensure that the remote alarms are initiated. • Carry out running checks to ensure correct operation in both 'float' and 'boost charge' selections.

Secure instrument supply systems (UPS) Most modern power stations have a secure instrument supply system fed from batteries via a DC/AC converter. The DC/AC conversion is carried out by either a rotary converter or a solid state inverter. Rotary converters A rotating converter is basically a DC motor coupled to an AC generator, the motor being fed from a secure battery supply. Maintenance for the system thus follows the principles already laid down for these items. A typical installation consists of three machines per unit, two running in parallel and one on standby. Correct and reliable operation of synchronising and load-sharing circuits often proves difficult to achieve: it is important that expertise is built up within the station in order to ensure maximum reliability from the particular system installed. Inverters Normal maintenance of an inverter system consists of an annual overhaul, which follows a very similar pattern to that already described for a solid state battery charger. Particular features that may require regular checking are: • Stability and accuracy of frequency and voltage over the load range. • The automatic changeover switching (be it static or contacter switching) between running and standby inverters, or between inverters and the raw supply. Maintenance of Electrical Plant

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It is important to ensure that any interruption to the output from the system during such switching is within the specified limits, so as to have no effect on the equipment being supplied. Maintenance Schedule – Flooded, Wet Cell, Lead Acid Batteries Maintenance or Test

Recommended Interval

Visual inspection

Monthly

Battery float voltage Cell float voltage

Every shift: (charger meter) Monthly: overall battery voltage with digital meter compare with charger meter Monthly: pilot cells with digital meter Quarterly: all cells

Temperature

Monthly, pilot cells Quarterly, 10 percent (%) of cells Annually, all cells Monthly (pilot cells) Quarterly (10% of all cells), (use IR camera)

Connection resistance

Every year, all connections, (use IR camera)

Specific gravity

Capacity testing Safety equipment inspection Infrared scan cells and connections Battery monitoring system

3 years, Every year, if capacity less than 90% Monthly, test all sensor devices and inspect all safety equipment Every year, all connections According to manufacturer’s recommendations in "User Manual"

Maintenance Schedule – Battery Chargers Maintenance or Test Recommended Interval Preventive maintenance Infrared scan cables and connections if visible

Maintenance of Electrical Plant

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Cabling and earthing There is a great temptation to neglect the maintenance of cabling and earthing systems totally. Whilst this may have no adverse effect on operation for many years, such a policy will almost certainly eventually contribute to a major electrical failure. Inspections should therefore by undertaken to check for the following potential trouble spots: Cabling systems • Debris in cableways, which could present a fire hazard or could damage cable sheaths. • Sheath faults. Power cables may carry sufficient potential on their armouring to initiate a cable fault if the sheath breaks down and an armour-to-earth fault develops. Inspection of sheaths and, in some cases, periodic sheath insulation tests may be considered advisable. • Cable supports. Electrical faults can exert high forces between single core power cables even though safely cleared by protection. Cable support arrangements and cleats must be maintained in good order and inspected after faults. • Glanding. Ensure that gIand fixings are secure and that glands are correctly earthed, or insulated, according to the cable design. • lnsulation. A representative number of power cables should be tested periodically for the integrity of their insulation system to enable a continuous assessment to be made of the general condition of the cable system. For 0.4kV and 3.3 kV cables, such testing will normally consist of a DC insulation resistance test between phases and between phase and earth. For 11 kV extruded solid insulation cables a periodic partial discharge-voltage test should be carried out. Details of this test are given in the relevant Standards or Recommended Practices. Earthing systems Equipment grounding is an essential part of protecting staff and equipment from high potential caused by electrical faults. Equipment grounding conductors are subject to failure due to corrosion, loose connections, and mechanical damage. Grounding may also be compromised during equipment addition and removal or other construction-type activities. Periodically verifying grounding system integrity is an important maintenance activity. Copper conductor earthing systems have over recent years proved to be particularly vulnerable to theft of lengths of the earthing strap, particularly from unfrequented areas of the power station. This dangerous and criminal practice can cause items of plant to be effectively unearthed.

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Regular checks may be necessary to ensure that the earth circuits are complete. Where theft proves to be a serious problem, consideration should be given to replacing missing copper with aluminium of equivaIent resistance. If this policy is adopted, care must be taken to ensure that correct aluminium-to-copper jointing techniques are used. More recent power stations have earthing systems constructed from aluminium cables. The terminations of both copper and aluminium earthing systems should be periodically checked to ensure that they remain tight and free from corrosion. The individual earth electrodes must also be checked on a planned basis to ensure that they remain an effective earth path. A common way of measuring electrode resistance in a multiple-electrode earthing system uses a comparison method, which involves disconnecting the electrode under test from the earthing system and measuring its resistance to the main earthing system, using a null-balance earth test megger.

Standards and recommended practices A detailed description of establishing earth-electrode resistance is given in British Standard Code of Practice CP1013.1965 'Earthing'. Other important standards: IEC 364-5-54 Electrical installations of buildings, Part 5. Selection and erection of electrical equipment. Chapter 54: Earthing arrangements and protective conductors. IEC 621-2A Electrical installations for outdoor sites under heavy conditions. Part 2 General protection requirements. DIN VDE 0151/6.86 Materials and minimum dimensions of earth electrodes with regard to corrosion.

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