Automatic Bell

1. INTRODUCTION Power distribution system should provide with an uninterrupted flow of energy at smooth sinusoidal voltage at the contracted magnitude level and frequency to their customers. PS especially distribution systems, have numerous non linear load s, which significantly affect the quality of power. Apart from nonlinear loads, events like capacitor switching, motor starting and unusual faults could also inflict power quality (PQ) problems. PQ problem is defined as any manifested problem in voltage, current or leading to frequency deviations that result in failure or mal operation of customer equipment. Voltage sags and swells are among the many PQ problems the industrial processes have to face. Voltage sags are more severe. During the past few decades, power industries have proved that the adverse impacts on the PQ can be mitigated or avoided by conventional means, and that techniques using fast controlled force commutated power electronics (PE) are even more effective. PQ compensators can be categorized into two main types. One is shunt connected compensation device that effectively eliminates harmonics. The other is the series connected device, which has an edge over the shunt type for correcting the distorted system side voltages and voltage sags caused by power transmission system faults.

The STATCOM used in distribution systems is called DSTACOM (DistributionSTATCOM) and its configuration is the same, but with small modifications. It can exchange both active and reactive power with the distributio n system by varying the amplitude and phase angle of the converter voltage with respect to the line terminal voltage. Installation of a large number of SVCs and experience gained from recent STATCOM projects throughout the world motivates us to clarify certain aspects of these devices. The FACTS devices offer a fast and reliable control over the transmission parameters, i.e. Voltage, line impedance, and phase angle between the sending end voltage and receiving end voltage. On the other hand the custom power is for low voltage distribution, and improving the poor quality and reliability of supply affecting sensitive loads. Custom power devices are very similar to the FACTS. Most widely known custom power devices are DSTATCOM, UPQC, DVR among them DSTATCOM is very well known and can provide cost effective solution for the compensation of reactive power and unbalance loading in distribution system.

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In recent days, modern industrial devices are mostly based on the electronic devices such as programmable logic controllers and electronic drives. The electronic devices are very sensitive to disturbances and become less tolerant to power quality problems such as voltage sags, swells and harmonics. Voltage dips are considered to be one of the most severe disturbances to the industrial equipment’s. Voltage support at a load can be achieved by reactive power injection at the load point of common coupling. D-STATCOM injects a current into the system to correct the voltage sag and swell. The power quality devices are power electronic converters connected in parallel or series with the lines and the operation is controlled by a digital controllers. The modeling of these complex systems that contains both power circuits and control systems can be done different bases. One of the Power electronic solution to the voltage regulation is the use of a D-STATCOM.D-STATCOM is a class of custom power devices for providing reliable distribution power quality. They employ a shunt of voltage boost technology using solid state switches for compensating voltage sags and swells. The D-STATCOM applications are mainly for sensitive loads that may be drastically affected by fluctuations in the system voltage. The performance of the DSTATCOM depends on the control algorithm i.e. the extraction of the current components. Among these control schemes instantaneous reactive power theory and synchronous rotating reference frame are most widely used. Increase in such non-linearity causes different undesirable features like low system efficiency and poor power factor. It also causes disturbance to other consumers and interference in nearby communication networks. The effect of such non-linearity may become sizeable over the next few years. Hence it is very important to overcome these undesirable features. Classically, shunt passive filters, consist of tuned LC filters and/or high passive filters are used to suppress the harmonics and power capacitors are employed to improve the power factor. But they have the limitations of fixed compensation, large size and can also exile resonance conditions. Active power filters are now seen as a viable alternative over the classical passive filters, to compensate harmonics and reactive power requirement of the non- linear loads. The objective of the active filtering is to solve these problems by combining with a much-reduced rating of the necessary passive components. Various topologies of active power filters have been developed so far. The shunt active power filter based on current controlled voltage source converter has been proved to be effective even when the load is highly non- linear. Most of the active filters developed are based on sensing harmonics and reactive volt-ampere requirements of 2

the non- linear load and require complex control. A new scheme has been proposed in which the required compensating current is determined by sensing load current which is further modified by sensing line currents only. An instantaneous reactive volt-ampere compensator and harmonic suppressor system is proposed With the deregulation of the electric power energy market, power quality has become an important issue of concern for both electric utilities companies and consumers. In this environment, electric utilities are expected to compete with each other for the customer. The power quality has serious economic implications for consumers, utilities and electrical equipment manufacturers. Modern industry involves the use of nonlinear and electronically switched devices in distribution systems. Integration of non-conventional generation technologies such as fuel cells, wind turbines, and photo-voltaic with utility grids often requires power electronic interfaces. These devices contribute the problems of power quality such as voltage fluctuations, flicker, harmonics, and asymmetries of volta ges. Moreover, modern hightech industrial equipments are more sensitive to these power quality problems. Therefore, there is a stringent need of better quality power supply because acceptable power quality levels if not achieved, may result in costly downtimes. In view of this, the concept of DSTATCOM is introduced. The power electronic devices are one of the common loads in many industries like the steel industry, labs and colleges. The problem with the power electronic devices is that they deteriorate the power quality by introducing harmonic distortion in the power system network. The unbalance load currents with large reactive components results in voltage fluctuations and imbalance due to the system impedance. Their presence also affects the performance of other electric equipment connected in the power system network. It is due to the fact that it produces very high levels of harmonic distortion in the power system network. So, it is of utmost necessity to take proper measures to reduce this power quality problem. The one possible solution to the above- mentioned problem is the application of a DSTATCOM, which is a distribution static synchronous compensator. The load compensation by DSTATCOM helps to maintain unity power factor load while balancing the load.

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2. POWER QUALITY Our technological world has become deeply dependent upon the continuous availability of electrical power. In most countries commercial power is made available via nationwide grids, interconnecting numerous generating stations to the loads. The grid must supply basic national needs of residential, lighting, heating, refrigeration, air conditioning and transportation as well as critical supply to

governmental,

industrial,

financial, commercial, and

medical and

communications communities. Commercial power literally enables today’s modern world to function at its busy pace. Many power problems originate in the commercial power grid, which with its thousands of miles of transmission lines is subject to weather conditions such as hurricanes, lightning storms, snow, ice and flooding along with equipment failure, traffic accidents and major switching operations. Also power problems affecting today’s technological equipment are often generated locally within a facility from any number of situations such as local construction, heavy startup loads, faulty distribution components and even typical background electrical noise. Widespread use of electronics in everything from home electronics to the control of massive and costly industrial processes has raised the awareness of power quality. Power quality or more specifically a power quality disturbance is generally defined as any change in power (voltage, current or frequency) that interferes with the normal operation of electrical equipment. The study of power quality and ways to control it is a concern for electric utilities, large industrial companies, businesses and even home users. The study has intensified as equipment has become increasingly sensitive to even minute changes in the power supply voltage, current, and frequency.

2.1 D EFINITION : “Power Quality is the degree to which both the utilization and delivery of electric power affects the performance of electric equipment”. In general there is no unique definition of power quality. The power quality problem can be viewed from two different angles related to each side of the utility meter, namely the Utility and Consumer. An alternative definition of PQ is adopted.

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2.2 POWER QUALITY AND RELIABILITY: Power quality and reliability cost the industry large amounts due to mainly sags and short-term interruptions. Distorted and unwanted voltage wave forms, too. And the main concern for the consumers of electricity was the reliability of supply. Here we define the reliability as the continuity of supply. As shown in Fig.2.2, the problem of distribution lines is divided into two major categories. First group is power quality, second is power reliability. First group consists of harmonic distortions, impulses and swells. Second group consists of voltage sags and outages. Voltage sags is much more serious and can cause a large amount of damage. If exceeds a few cycle, motors, robots, servo drives and machine tools cannot maintain control of process. Both the reliability and quality of supply are equally important. For example, a consumer that is connected to the same bus that supplies a large motor load may have to face a severe dip in his supply voltage every time the motor load is switched on. In some extreme cases even we have to bear the black outs which is not acceptable to the consumers. There are also sensitive loads such as hospitals (life support, operation theatre, and patient database system), processing plants, air traffic control, financial institutions and numerous other data processing and service providers that require clean and uninterrupted power. In processing plants, a batch of product can be ruined by voltage dip of very short duration. Such customers are very wary of such dips since each dip can cost them a substantial amount of money. Even short dips are sufficient to cause contactors on motor drives to drop out. Stoppage in a portion of process can destroy the conditions for quality control of product and require restarting of production. Thus in this scenario in which consumers increasingly demand the quality power, the term power quality (PQ) attains increased significance.

Fig 2.2: Power Quality and Reliability 5

Transmission lines are exposed to the forces of nature. Furthermore, each transmission line has its load ability limit that is often determined by either stability constraints or by thermal limits or by the dielectric limits. Even though the power quality problem is distribution side problem, transmission lines are often having an impact on the quality of the power supplied. It is however to be noted that while most problems associated with the transmission systems arise due to the forces of nature or due to the interconnection of power systems, individual customers are responsible for more substantial fraction of the problems of power distribution systems.

2.3 POWER QUALITY PROBLEMS & ISSUES: A recent survey of Power Quality experts indicates that 50% of all Power Quality problems are related to grounding, ground bonds, and neutral to ground voltages, ground loops, ground current or other ground associated issues. Electrically operated or connected equipment is affected by Power Quality. Determining the exact problems requires sophisticated electronic test equipment. The following symptoms are indicators of Power Quality problems: • Piece of equipment misoperates at the same time of day. • Circuit breakers trip without being overloaded. • Equipment fails during a thunderstorm. • Automated systems stop for no apparent reason. • Electronic systems fail or fail to operate on a frequent basis. • Electronic systems work in one location but not in another location. The commonly used terms those describe the parameters of electrical power that describe or measure power quality are Voltage sags, Voltage variations, Interruptions Swells, Brownouts, Blackouts, Voltage imbalance, Distortion, Harmonics, Harmonic resonance, Interharmonics, Notching, Noise, Impulse, Spikes (Voltage), Ground noise, Common mode noise, Critical load, Crest factor, Electromagnetic compatibility, Dropout, Fault, Flicker, Ground, Raw power, Clean ground, Ground loops, Voltage fluctuations, Transient, Dirty power, Momentary interruption, Over voltage, Under voltage, Nonlinear load, THD, Triplens, Voltage dip, Voltage regulation, Blink, Oscillatory transient etc.

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2.3.1 Voltage Sags and Swells: •

Reduction in the ac voltage, at the power frequency, for durations from a half-cycle to a few seconds.

•

Voltage Sag is Characterized by two parameters – Magnitude and Duration

•

Power Electronics Loads are Sensitive to Voltage Sags

Fig 2.3.1: Voltage Sags and Swells

2.3.1.1 Causes for Sag: 

Motor Starting



Transformer Energization



Transmission Faults

2.3.1.2 Causes for Swell: 

Single line to ground fault



Removing a large load / adding a large capacitor bank

2.3.2 Voltage Unbalance: In a balanced sinusoidal supply system the three line-neutral voltages are equal in magnitude and are phase displaced from each other by 120 degrees. 2.3.2.1 Causes for Unbalance: 

Unequal system impedances



Unequal distribution of (a) Single-phase loads (b) Phase to Phase loads (c) Unbalanced Three phase loads 7

2.3.3 Voltage Flicker: Repetitive or random variations of the voltage envelope modulated at frequencies less than 25 Hz, which the human eye can detect as a variation in the lamp intensity of a standard bulb due to sudden changes in the real and reactive Power drawn by a load.

Fig 2.3.3 Voltage Flicker

2.3.3.1 Causes: (1) Induction Motor drive •

Arc furnaces

•

Arc welders

•

Frequent motor starts

2.3.3.2 Effect: Lamp flicker - Human eye is most sensitive to voltage waveform modulation around a frequency of 6-8Hz.

2.3.4 Voltage Interruption: Voltage interruption is nothing but the supply voltage goes close to zero that means lower than 10% of its nominal voltage. Interruptions can results the power system faults, equipment failure, and control system malfunction.

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2.3.5 Harmonics: It is a sinusoidal component of a periodic wave having a frequency that is an integral multiple of the fundamental frequency. A non- linear element in power systems such as power electronic devices, static power converters, arc discharge devices etc creates harmonics in system. Harmonics cause communication interference, heating, and malfunction of equipments. The voltage variation, sag, swell, harmonics causes ma lfunctioning of electronics equipments namely microprocessor based system, programmable logic controller, adjustable speed drives etc. Due to all this problems it may cause tripping of contractors, protection devices, also stoppage of sensitive equipments like computer, programmable logic control system and may be damage the sensitive equipments. Due to all this problems the whole system will be derated.

The issue of electric power quality is gaining importance because of several reasons: • The society is becoming increasingly dependent on the electrical supply. A small power outage has a great economical impact on the industrial consumers. A longer interruption harms practically all operations of a modern society. • New equipments are more sensitive to power quality variations. • The advent of new power electronic equipment, such as variable speed drives and switched mode power supplies, has brought new disturbances into the supply system.

2.4 EFFECT OF POWER QUALITY PROBLEMS IN EQUIPMENTS: The first sign of a power-quality problem is a distortion in the voltage waveform of the power source from a sine wave, or in the amplitude from an established reference level, or a complete interruption. The disturbance can be caused by harmonics in the current or by events in the main voltage supply system. In addition, the problems can be caused by the equipment supplied with electric power for example, power-electronic converters. Redundancy at all levels of the electricpower system reduces the incidence and duration of line-voltage disturbances. A. Effect of power quality problems in different equipments Some of the equipment affected by power-quality problems are discussed under the following heads: 9

a. Aircraft electrical System b. Personal computers c. Controllers d. Adjustable speed drives e. Contactors and Relays B. A perfect power supply would be one that is always available always within voltage and frequency tolerances and has a pure noise- free sinusoidal wave shape. Power Quality means the ability of utilities to provide electric power without interruption. Mainly the seven types of Power Quality problems are there. They are 1. Transients. 2. Interruptions. 3. Voltage Sag. 4. Voltage Swell. 5. Waveform distortion. 6. Voltage fluctuations. 7. Frequency variations.

2.5 SOURCES OF POWER QUALITY PROBLEMS: 

Large motor starting



Different faults

 Lightning The above problems cause high current and large drops in lines and leads to voltage sag. 

Capacitive Loads



Open circuits

These problems lead to voltage swell.

2.6 CAUSES OF POWER QUALITY: The causes of power quality problems are generally complex and difficult to detect. Technically speaking, the ideal AC line supply by the utility system should be a pure sine wave of fundamental frequency (50/60Hz).Different power quality problems, their characterization 10

methods and possible causes are discussed above and which are responsible for the lack of quality power which affects the customer in many ways. We can therefore conclude that the lack of quality power can cause loss of production, damage of equipment or appliances or can even be detrimental to human health. It is therefore imperative that a high standard of power quality is maintained. This project demonstrates that the power electronic based power conditioning using custom power devices like DSTATCOM can be effectively utilized to improve the quality of power supplied to the customers.

2.6.1 Major causes of power quality problems: 

Rural location remote from power source.



Unbalanced load on a three phase system.



Switching of heavy loads.



Long distance from a distribution transformer with interposed loads



Unreliable grid systems.



Equipment’s not suitable for local supply.

2.7 SOLUTIONS TO POWER QUALITY PROBLEMS: There are two approaches to the mitigation of power quality problems. The solution to the voltage quality can be done from customer side or from utility side First approach is called load conditioning, which ensures that the equipment is less sensitive to power disturbances, allowing the operation even under significant voltage distortion. The other solution is to install line conditioning systems that suppress or counteracts the power system disturbances. Currently they are based on PWM converters and connect to low and medium voltage distribution system in shunt or in series. Series active power filters must operate in conjunction with shunt passive filters in order to compensate load current harmonics. Shunt active power filte rs operate as a controllable current source and series active power filters operates as a controllable voltage source. However, with the restructuring of power sector and with shifting trend towards distributed and dispersed generation, the line conditioning systems or utility side solutions will play a major role in improving the inherent supply quality; some of the effective and economic measures can be identified as following. 11

2.7.1 Lightning and Surge Arrester: Arrester is designed for lightning protection of transformers, but is not limited to sufficient voltage limiting for protecting sensitive electronic control circuits from voltage surges.

2.7.2

Thyristor Based Static Switch:

The static switch is a versatile device for switching a new element in to the circuit when the voltage support is needed. It has a dynamic response time of about one cycle. To correct quickly for voltage spikes, sags or interruptions, the static switch can used to switch one or more devises such as capacitor, filter, alternate power line, energy storage systems etc. The static switch can be used in the alternate power line applications.

2.7.3

Energy Storage Systems:

Storage system can be used to protect sensitive protection equipment from shutdowns caused by voltage sags or momentary interruptions. These are usually dc storage systems such as UPS, batteries, superconducting magnet energy storage (SMES), storage capacitors or even fly wheels driving dc generators. The output of these devices can be supplied to the system through an inverter on a momentary basis by a fast acting electronic switch. Enough energy is fed to the system to compensate for the energy that would be lost by the voltage sag or interruption.

2.8 POWER QUALITY STANDARDS: Power quality is a worldwide issue, and keeping related standards current is a never-ending task. It typically takes years to push changes through the process. Most of the ongoing work by the IEEE in harmonic standards development has shifted to modifying Standard 519-1992. A. IEEE 519 : IEEE 519-1992, Recommended Practices and Requirements for Harmonic Control in Electric Power Systems, established limits on harmonic currents and voltages at the point of common coupling (PCC) or point of metering.

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The limits of IEEE 519 are intended to: 1) Assure that the electric utility can deliver relatively clean power to all of its customers; 2) Assure that the electric utility can protect its electrical equipment from overheating, loss of life from excessive harmonic currents, and excessive voltage stress due to excessive harmonic voltage. Each point from IEEE 519 lists the limits for harmonic distortion at the point of common coupling (PCC) or metering point with the utility. The voltage distortion limits are 3% for individual harmonics and 5% THD. All of the harmonic limits in IEEE 519 are based on a customer load mix and location on the power system. The limits are not applied to particular equipment, although, with a high amount of nonlinear loads, it is likely that some harmonic suppression may be necessar y. a. IEEE 519 Standard for Current Harmonics: • General Distribution Systems [120V- 69 kV] Below current distortion limits are for odd harmonics. Even harmonics are limited to 25% of the odd harmonic limits [1,3,5]. For all power generation equipment, distortion limits are those with ISC/IL<20 .ISC is the maximum short circuit current at the point of coupling “PCC”.IL is the maximum fundamental frequency 15-or 30- minutes load current at PCC. TDD is the Total Demand Distortion (=THD normalized by IL. • General Sub-transmission Systems [69 kV-161 kV] The current harmonic distortion limits apply to limits of harmonics that loads should draw from the utility at the PCC. Note that the harmonic limits differ based on the ISC/IL rating, where ISC is the maximum short circuit current at the PCC, and I is the maximum demand load current at the PCC

b. IEEE Standard For Voltage Harmonics : • IEEE-519 - Voltage Distortion Limits .The voltage harmonic distortion limits apply to the quality of the power. For instance, for systems of less than 69 kV, IEEE 519 requires limits of 3 percent harmonic distortion for an individual frequency component and 5 percent for total harmonic distortion. 13

B. IEC 61000-3-2 and IEC 61000-3-4 (formerly 1000-3-2 and 1000-3-4): a. IEC 61000-3-2 (1995-03): It specifies limits for harmonic current emissions applicable to electrical and electronic equipment having an input current up to and including 16 A per phase, and intended to be connected to public low- voltage distribution systems. The tests according to this standard are type tests. b. IEC/TS 61000-3-4 (1998-10): It specifies to electrical and electronic equipment with a rated input current exceeding 16 A per phase and intended to be connected to public low-voltage ac distribution systems of the following types: • Nominal voltage up to 240 V, single-phase, two or three wires. • Nominal voltage up to 600 V, three-phase, three or four wires. • Nominal frequency 50 Hz or 60 Hz. These recommendations specify the information required to enable a supply authority to assess equipment regarding harmonic disturbance and to decide whether or not the equipment is acceptable for connection with regard to the harmonic distortion aspect. The European standards, IEC 61000-3-2 & 61000-3-4, placing current harmonic limits on equipment, are designed to protect the small consumer's equipment. The former is restricted to 16 A, the latter extends the range above 16 A.

C. IEEE Standard 141-1993, Recommended Practice for Electric Power Distribution for Industrial Plants: A thorough analysis of basic electrical-system considerations is presented. Guidance is provided in design, construction, and continuity of an overall system to achieve safety of life and preservation of property; reliability; simplicity of operation; voltage regulation in the utilization of equipment within the tolerance limits under all load conditions; care and maintenance; and flexibility to permit development and expansion.

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D. IEEE Standard 142-1991, Recommended Practice for Grounding of Industrial and Commercial Power Systems: This standard presents a thorough investigation of the problems of grounding and the methods for solving these problems. There is a separate chapter for grounding sensitive equipment.

E. IEEE Standard 446-1987, Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications: This standard is recommended engineering practices for the selection and application of emergency and standby power systems. It provides facility designers, operators and owners with guidelines for assuring uninterrupted power, virtually free of frequency excursions and voltage dips, surges, and transients. The limits of 

Sag (dip) - a decrease to between 0.1 pu and 0.9 pu in rms voltage or current at the power frequency for durations of 0.5 cycles to 1 minute.



Swell - an increase to between 1.1 pu and 1.8 pu in rms voltage or current at the power frequency durations from 0.5 to 1 minute.

F. IEEE Standard 493-1997, Recommended Practice for Design of Reliable Industrial and Commercial-PowerSystems: The fundamentals of reliability analysis as it applies to the planning and design of industrial and commercial electric power distribution systems are presented. Included are basic concepts of reliability analysis by probability methods, fundamentals of power system reliability evaluation, economic evaluation of reliability, cost of power outage data, equipment reliability data, and examples of reliability analysis. Emergency and standby power, electrical preventive maintenance, and evaluating and improving reliability of the existing plant are also addressed.

G. IEEE Standard 1100-1999, Recommended Practice for Powering and Grounding Sensitive Electronic Equipment: Recommended design, installation, and maintenance practices for electrical power and grounding (including both power-related and signal-related noise control) of sensitive electronic processing equipment used in commercial and industrial applications. 15

H.IEEE Standard 1159-1995, Recommended Practice for Monitoring Electric Power Quality: As its title suggests, this standard covers recommended methods of measuring power-quality events. Many different types of power-quality measurement devices exist and it is important for workers in different areas of power distribution, transmission, and processing to use the same language and measurement techniques. Monitoring of electric power quality of AC power systems, definitions of power quality terminology, impact of poor power quality on utility and customer equipment, and the measurement of electromagnetic phenomena are covered.

I.IEEE Standard 1250-1995, Guide for Service to Equipment Sensitive to Momentary Voltage Disturbances: Computers, computer- like products, and equipment using solid state power conversion have created entirely new areas of power quality considerations. There is an increasing awareness that much of this new user equipment is not designed to withstand the surges, faults, and reclosing duty present on typical distributions systems..Momentary volta ge disturbances occurring in ac power distribution and utilization systems, their potential effects on this new, sensitive, user equipment, and guidance toward mitigation of these effects are described. Harmonic distortion limits are also discussed.

J. IEEE Standard 1346-1998 Recommended Practice for Evaluating Electric Power System Compatibility with Electronic Process Equipment: A standard methodology for the technical and financial analysis of voltage sag compatibility between process equipment and electric power systems is recommended. The methodology presented is intended to be used as a planning tool to quantify the voltage sag environment and process sensitivity.

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K. Standards related to Voltage Sag and Reliability: The distribution voltage quality standard i.e. IEEE Standard P1564 gives the recommended indices and procedures for characterizing voltage sag perfor mance and comparing performance across different systems. A new IEC Standard 61000-2-8 titled “Environment —Voltage Dips and Short Interruptions” has come recently. This standards warrants considerable discussion within the IEEE to avoid conflicting methods of characterizing system performance in different parts of the world.

L. Standards related to Flicker: Developments in voltage flicker standards demonstrate how the industry can successfully coordinate IEEE and IEC activities. IEC Standard 61000-4-15 defines the measurement procedure and monitor requirements for characterizing flicker. The IEEE flicker task force working on Standard P1453 is set to adopt the IEC standard as its own.

M. Standards related to Custom Power: IEEE Standard P1409 is currently developing an application guide for custom power technologies to provide enhanced power quality on the distribution system. This is an important area for many utilities that may want to offer enhanced power quality services.

N. Standards related to Distributed Generation: The new IEEE Standard P1547 provides guidelines for interconnecting distributed generation with the power system.

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3. FACTS CONTROLLERS With the rapid development of power electronics, Flexible AC Transmission Systems (FACTS) devices have been proposed and implemented in power systems. FACTS devices can be utilized to control power flow and enhance system stability. Particularly with the deregulation of the electricity market, there is an increasing interest in using FACTS devices in the operation and control of power systems with new loading and power flow conditions. A better utilization of the existing power systems to increase their capacities and controllability by installing FACTS devices becomes imperative. Due to the present situation, there are two main aspects that should be considered in using FACTS devices. The first aspect is the flexible power system operation according to the power flow control capability of FACTS devices. The other aspect is the improvement of transient and steady-state stability of power systems. FACTS devices are the right equipment to meet these challenges.

3.1 FACTS DEFINITION: According to IEEE, FACTS, which is the abbreviation of Flexible AC Transmission Systems, is defined as follows: Alternating current transmission systems incorporating power electronics based and other static controllers to enhance controllability and power transfer capability. The basic applications of facts-devices are: • Power Flow Control. • Increase of Transmission Capability. • Voltage Control. • Reactive Power Compensation. • Stability Improvement. • Power Quality Improvement. • Power Conditioning. • Flicker Mitigation. • Interconnection of Renewable and Distributed Generation and Storages.

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Fig 3.1 shows the basic idea of facts for transmission systems. The usage of lines for active power transmission should be ideally up to the thermal limits. Voltage and stability limits shall be shifted with the means of the several different facts devices. It can be seen that with growing line length, the opportunity for facts devices gets more and more important. The influence of facts-devices is achieved through switched or controlled shunt compensation, series compensation or phase shift control. The devices work electrically as fast current, voltage or impedance controllers. The power electronic allows very short reaction times down to far below one second.

Fig 3.1: Operational limits of Transmissions Lines for different voltage levels

The development of facts-devices has started with the growing capabilities of power electronic components. Devices for high power levels have been made available in converters for high and even highest voltage levels. The overall starting points are network elements influencing the reactive power or the impedance of a part of the power system. Fig 3.1 shows a number of basic devices separated into the conventional ones and the facts-devices. For the facts side the taxonomy in terms of 'dynamic' and 'static' needs some explanation. The term 'dynamic' 19

is used to express the fast controllability of facts-devices provided by the power electronics. This is one of the main differentiation factors from the conventional devices. The term 'static' means that the devices have no moving parts like mechanical switches to perform the dynamic controllability. Therefore most of the facts-devices can equally be static and dynamic.

Table 3.1.1 Overview of Major FACTS-Devices:

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The left column in table 3.1.1 contains the conventional devices build out of fixed or mechanically switch able components like resistance, inductance o r capacitance together with transformers. The facts-devices contain these elements as well but use additional power electronic valves or converters to switch the elements in smaller steps or with switching patterns within a cycle of the alternating current. The left column of facts-devices uses thyristor valves or converters. These valves or converters are well known since several years. They have low losses because of their low switching frequency of once a cycle in the converters or the usage of the thyristors to simply bridge impedances in the valves.

The right column of facts-devices contains more advanced technology of voltage source converters based today mainly on insulated gate bipolar transistors (IGBT) or insulated gate commutated thyristors (IGCT). Voltage source converters provide a free controllable voltage in magnitude and phase due to a pulse width modulation of the IGBT’s or IGCTS. High modulation frequencies allow to get low harmonics in the output signal and even to compensate disturbances coming from the network. The disadvantage is that with an increasing switching frequency, the losses are increasing as well. Therefore special designs of the converters are required to compensate this.

3.2 POWER FILTER TOPOLOGIES: Depending on the particular application or electrical problem to be solved, active power filters can be implemented as shunt type, series type, or a combination of shunt and series active filters (shunt-series type). These filters can also be combined with passive filters to create hybrid power filters. The shunt-connected active power filter, with a self-controlled dc bus, has a topology similar to that of a static compensator (STATCOM) used for reactive power compensation in power transmission systems. Shunt active power filters compensate load current harmonics by injecting equal-but opposite harmonic compensating current. In this case the shunt active power filter operates as a current source injecting the harmonic components generated by the load but phase-shifted by 180°.

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Series active power filters were introduced by the end of the 1980s and operate mainly as a voltage regulator and as a harmonic isolator between the nonlinear load and the utility system. The series-connected filter protects the consumer from an inadequa te supply. This type of approach is especially recommended for compensation of voltage unbalances and voltage sags from the ac supply and for low-power applications and represents an economically attractive alternative to UPS, since no energy storage (battery) is necessary and the overall rating of the components is smaller. The series active filter injects a voltage component in series with the supply voltage and therefore can be regarded as a controlled voltage source, compensating voltage sags and swells on the load side. In many cases, series active filters work as hybrid topologies with passive LC filters. If passive LC filters are connected in parallel to the load, the series active power filter operates as a harmonic isolator, forcing the load curre nt harmonics to circulate mainly through the passive filter rather than the power distribution system. The main advantage of this scheme is that the rated power of the series active filter is a small fraction of the load kVA rating, typically 5%. However, the apparent power rating of the series active power filter may increase in case of voltage compensation. The series-shunt active filter is a combination of the series active filter and the shunt active filter. The shunt active filter is located at the load side and can be used to compensate for the load harmonics. On the other hand, the series portion is at the source side and can act as a harmonic blocking filter. This topology has been called the Unified Power Quality conditioner. The series portion compensates for supply voltage harmonics and voltage unbalances, acts as a harmonic blocking filter, and damps power system oscillations. The shunt portion compensates load current harmonics, reactive power, and load current unbalances. In addition, it regulates the dc link capacitor voltage. The power supplied or absorbed by the shunt portion is the power required by the series compensator and the power required to cover losses. Hybrid power filters are a combination of active and passive filters. With this topology the passive filters have dynamic low impedance for current harmonics at the load side, increasing their bandwidth operation and improving their performance. This behavior is reached with only a small power rating PWM inverter, which acts as an active filter in series with the passive filter.

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Multilevel inverters are being investigated and recently used for active filter topologies. Three- level inverters are becoming very popular today for most inverter applications, such as machine drives and power factor compensators. The advantage of multilevel converters is that they can reduce the harmonic content generated by the active filter because they can produce more levels of voltage than conventional converters (more than two levels). This feature helps to reduce the harmonics generated by the filter itself.

3.3 CONTROL STRATAGIES: Most of the active filters developed are based on sensing harmonics and reactive voltampere requirements of the non- linear load and require complex control. In some active filters, both phase voltages and load currents are transformed into the α-β orthogonal quantities, from which the instantaneous real and reactive power. The compensating currents are calculated from load currents and instantaneous powers. The harmonic co mponents of power are calculated using high pass filters in the calculation circuit. The control circuit of the dc capacitor voltage regulates the average value of the voltage to the reference value. Reactive power compensation is achieved without sensing and computing the reactive current component of the load, thus simplifying the control circuit. Current control is achieved with constant switching frequency producing a better switching pattern. An active filter based on the instantaneous active and react ive current component in which current harmonics of positive and negative sequence including the fundamental current of negative sequence can be compensated. The system therefore acts as a harmonic and unbalanced current compensator. A comparison between the instantaneous active and reactive current component - method and the instantaneous active and reactive power method is realized.

A new scheme has been proposed in which the required compensating current is generated using simple synthetic sinusoid generation technique by sensing the load current. This scheme is further modified by sensing line currents only. An instantaneous reactive volt-ampere compensator and harmonic suppressor system is proposed without the use of voltage sensors but require complex hardware for current reference generator. The generated reference current is not a pure sine wave but stepped sine wave. Also, without the use of voltage sensors, the scheme generates balanced sine wave reference currents but do not compensate reactive power 23

completely (if source voltage is unbalanced/distorted) due to waveform difference between voltage and current). Control scheme based on sensing line currents. The 3-phase currents/voltages are detected using only two current/voltage sensors compared to three used in DC capacitor voltage is regulated to estimate the reference current template. Conventional solutions for controller requirements were based on classical control theory or modern control theory. Widely used classical co ntrol theory based design of PI family controllers requires precise linear mathematical mode ls. The PI family of controllers failed to perform satisfactorily under parameter variation, non linear ity, load disturbance, etc. During the past several years, fuzzy control has e merged as one of the most active and fruitful areas for research in the applications of fuzzy set theory, especially in the realm of industrial processes, which do not lend themselves to control by conventional methods because of a lack of quantitative data regarding the input-output relations. Fuzzy control is based on fuzzy logic-a logical system that is much closer in spirit to human thinking and natural language than traditional logical systems. The fuzzy logic controller (FLC) based on fuzzy logic provides a means of converting a linguistic control strategy based on expert knowledge into an automatic control strategy. Recently, fuzzy logic controllers (FLC’s) have generated a good deal of interest in certain applications. The advantages of FLC’s over the conventional controllers are: 1. It does not need accurate mathematical model; 2. It can work with imprecise inputs; 3. It can handle nonlinearity, and 4. It is more robust than conventional nonlinear controllers.

3.4 TYPES OF FACTS CONTROLLERS: In general, FACTS Controllers can be divided into four categories: 1. Series controllers 2. Shunt controllers 3. Combined series-series controllers 4. Combined series-shunt controllers

24

3.4.1 SERIES CONTROLLERS: A series controller could be variable impedance such as capacitor reactor or power electronics based variable source of main frequency, sub synchronous and harmonic frequencies (or a combination) to serve the desired need. In principle, all series controllers inject voltage in series with the line. Even variable impedance multiplied by the current flow through it, represents an injected series voltage in the line. Different types available are as follows: 

Static Synchronous Series Compensator (SSSC)



Interline Power Flow Controller (IPFC)



Thyristor- Controlled Series Reactor (TCSR)



Thyristor-Switched Series Reactor (TSSR)



Thyristor-Controlled Series Capacitor (TCSC)



Thyristor-Switched Series Capacitor (TSSC)

3.4.2 SHUNT CONTROLLERS: In principle all shunt Controllers inject current into the system at the point of connection. Even variable shunt impedance connected to the line voltage causes a variable current flow and hence represents injection of current into the line. Different types available are listed below. 

Static Synchronous Compensator (STATCOM).



Static Synchronous Generator (SSG).



Static VAR Compensator (SVC).



Thyristor Controlled Reactor (TCR).



Thyristor Switched Reactor (TSR).



Thyristor Switched Capacitor (TSC).



Static Var Generator (SVG).

3.4.3 COMBINED SERIES-SERIES CONTROLLERS: This could be a combination of separate series controllers, which are controlled in a coordinated manner in a multiline transmission system. The real power transfer ability of the unified series-series controller , referred to as interline power flow controller, makes it possible to 25

balance both the real and reactive power flow in the lines thereby maximize the utilization of the transmission system. 

Interline Power Flow Controller(IPFC)

3.4.4 COMBINED SHUNT-SERIES CONTROLLER: This could be a combination of separate series and shunt controllers, which are controlled in a coordinated manner or UPFC with the series and shunt elements. In principle, combined shunt and series controllers inject current into the system with the shunt part of the controller and voltage in series in the line with the series part of the controller. When series and shunt controllers are unified, there can be a real power exchange between the series and shunt controllers via the power link. Different types are listed below. 

Unified Power Flow Controller (UPFC).



Thyristor-Controlled Phase Shifting Transformer (TCPST).

3.5 SYMBOLS FOR FACTS CONTROLLERS:

(a)

(b)

(c)

(d)

(e) Fig.3.5: General symbols of FACTS controllers 26

(a) General symbol for a FACTS Controller (b) Series controller (c) Shunt controller (d) Combined series-series controller (e) Combined shunt-series controller

3.6 TYPICAL FACTS DEVICES AND THEIR FUNCTIONS: In these four typical FACTS devices are considered in detail: TCSC (Thyristor Controlled Series Capacitor), TCPST (Thyristor Controlled Phase Shifting Transformer), UPFC (Unified Power Flow Controller), SVC (Static Var Compensator).

27

Fig.3.6: Functional diagrams of FACTS devices

TCSC is a typical series FACTS device that is used to vary the reactance of the transmission line. Since TCSC works through the transmission system directly, it is much more effective than the shunt FACTS devices in the application of power flow control and power system oscillation damping control. The UPFC is the most powerful and versatile FACTS device due to the facts that the line impedance, terminal voltages, and the voltage angles can be controlled by it as well. Similar to the UPFC, TCPST is also a typical combined series-shunt FACTS device, which can be used to regulate the phase angle difference between the two terminal voltages. SVC is a shunt FACTS device that can be used to control the reactive compensation.

28

4. DISTRIBUTION STATIC COMPENSATOR (DSTATCOM) A D-STATCOM (Distribution Static Compensator), which is schematically depicted in Figure, consists of a two- level Voltage Source Converter (VSC), a dc energy storage device, a coupling transformer connected in shunt to the distribution network through a coupling transformer. The VSC converts the dc voltage across the storage device into a set of three-phase ac output voltages. These voltages are in phase and coupled with the ac sys tem through the reactance of the coupling transformer. Suitable adjustment of the phase and magnitude of the DSTATCOM output voltages allows effective control of active and reactive power exchanges between the D-STATCOM and the ac system. Such configuration allows the device to absorb or generate controllable active and reactive power. The VSC connected in shunt with the ac system provides a multifunctional topology which can be used for up to three quite distinct purposes: 1. Voltage regulation and compensation of reactive power 2. Correction of power factor 3. Elimination of current harmonics Here, such device is employed to provide continuous voltage regulation using an indirectly controlled converter. DSTATCOM is a voltage source converter (VSC) that is connected in shunt with the distribution system by means of a tie reactance connected to compensate the load current. In general, a coupling transformer is installed between the distribution system and the DSTATCOM for isolating the DSTATCOM from the d istribution system. In addition, the device needs to be installed as close to the sensitive load as possible to maximize the compensating capability. Being a shunt connected device, the DSTATCOM mainly injects reactive power to the system. The role of DSTATCOM is specifically appreciated in case of a weak AC system. The structure of DSTATCOM along with its operating modes is shown in Figure 4.

29

Fig4: Block diagram of D-STATCOM

4.1 CONSTRUCTION: The main components of DSTATCOM are – a VSC (voltage source converter), controller, filter, and energy storage device. The system scheme of DSTATCOM is shown in Figure 4.1. These are briefly described as follows: Isolation transformer: It connects the DSTATCOM to the distribution network and its main purpose is to maintain isolation between the DSTATCOM circuit and the distribution network. Voltage source converter: A voltage source converter consists of a storage device and switching devices, which can generate a sinusoidal voltage at any required frequency, magnitude and phase angle. In the DSTATCOM application, this temporarily replaces the supply voltage or generates the part of the supply voltage which is absent and injects the compensating current into the distribution network depending upon the amount of unbalance or distortion. DC charging unit: This unit charges the energy source after a compensation event and also maintains the dc link voltage at the nominal value. Harmonic filters: The main function of harmonic filter is to filter out the unwanted harmonics generated by the VSC and hence, keep the harmonic level within the permissible limit. It serves as the real power requirements of the system when DSTATCOM is used for compensation. In case, no energy source is connected to the DC bus, then the average power exchanged by the DSTATCOM is zero assuming the switches, reactors, and capacitors to 30

be ideal. Figure 4.1 represents the schematic scheme of DSTATCOM in which the shunt injected current

corrects the voltage sag by adjusting the voltage drop across the system impedance.

The value of

can be controlled by adjusting the output voltage of the converter.

Fig 4.1a: Schematic diagram of DSTATCOM

The shunt injected current

can be written as,

(1) (2) The complex power injection of the D-STATCOM can be expressed as,

(3) It may be mentioned that the effectiveness of the D-STATCOM in correcting voltage sag depends on the value of

or fault level of the load bus. When the shunt injected current

is

kept in quadrature with VL, the desired voltage correction can be achieved without injecting any active power into the system. On the other hand, when the value of

is minimized, the same

voltage correction can be achieved with minimum apparent power injection into the syste m. The control scheme for the D-STATCOM follows the same principle as for DVR. The switching frequency is set at 475 Hz.

31

It may be mentioned that the effectiveness of the DSTATCOM in correcting voltage sag depends on the value of

or fault level of the load bus. When the shunt injected current

is

kept in quadrature with VL, the desired voltage correction can be achieved without injecting any active power into the system. On the other hand, when the value of

is minimized, the same

voltage correction can be achieved with minimum apparent power injection into the system. The contribution of the DSTATCOM to the load bus voltage equals the injected current times the impedance seen from the device, which is the source impedance in para llel with the load impedance. The ability of the DSTATCOM to compensate the voltage dip is limited by this available parallel impedance. DSTATCOM is utilized to eliminate the harmonics from the source currents and also balance them in addition to providing reactive power compensation. It helps to reduce the voltage fluctuations at the PCC (point of common coupling). Voltage dips can be mitigated by DSTATCOM, which is based on a shunt connected voltage source converter. VSC with pulse-width modulation (PWM) offers fast and reliable control for voltage dips mitigation. The topology of the DSTATCOM connected at distribution level.

In the proposed model, the application of DSTATCOM to improve the power quality in a distribution network with induction furnace load is investigated. The SIMULINK model representing the compensation using DSTATCOM of a distribution network with induction furnace load is investigated in this work. PI controller is used for gate signal generating and to reduce the harmonic distortion due to the induction furnace. Output from the controller block is in the form of an angle  that is used to introduce an additional phase-lag/ lead in the three-phase voltages.

32

Fig 4.1b: Control algorithm of DSTATCOM with PI controller

The sinusoidal signal

is phase- modulated by means of the angle . i.e.,

VA = Sin (t + )

(4)

VB = Sin (t +  - 2/3)

(5)

VC = Sin (t +  + /3)

(6)

The modulating angle is applied to the Gate pulse in phase A. The angles for phases B and C are shifted by 240º and 120º, respectively. Load current is sensed and passed through a sequence analyzer. The controller used in the proposed test model is PI controller.

33

4.1.1 TEST SYSTEM: Figure shows the test system used to carry out the various D-STATCOM simulations.

Fig 4.1.1: Single line diagram of the test system for D-STATCOM

4.2 Voltage Source Converter (VSC): A voltage source converter (VSC) is a power electronic device, which can generate a three-phase ac output voltage is controllable in phase and magnitude. These voltages are injected into the ac distribution system in order to maintain the load voltage at the desired voltage reference. VSCs are widely used in adjustable speed drives, but can also be used to mitigate the voltage sags and swells. The VSC is used to either completely replacing the voltage or to inject the 'missing voltage'. The 'missing voltage' is the difference between the nominal voltage and the actual voltage. The converter is normally based on the some kind of energy storage, which will supply the converter with a dc voltage.

34

4. 3 DSTATCOM OF VOLTAGE REGULATION: 4.3.1 Voltage Regulation without Compensator: Voltage E and V mean source voltage and PCC voltage respectively. Without a voltage compensator, the PCC voltage drop caused by the load current, IL is as shown in Fig.4.3.1 as ΔV IS=IL+ IR

(7)

Where IR is the compensating current

Fig 4.3.1: The Equivalent Circuit of Load and Supply System

4.3.1.1 PHASOR DIAGRAMS:

Fig.4.3.1.1Phasor of uncompensated line

35

Fig.4.3.1.2.Phasor of the compensated line

ΔV = E -V = ZSIL

(8)

S = VI*, S*=VI

(9)

From above equation I L = (PL-jQL/V)

(10)

So that ∆V= (Rs+jXs)((PL-jQL)/V)

(11)

=∆Vr+∆Vx

(12)

The voltage change has a component ΔVR in phase with V and a component ΔVx, in quadrature with V, which are illustrated in Fig.4.3.1.2. It is clear that both magnitude and phase of V , relative to the supply voltage E, are the functions magnitude and phase of load current, na mely voltage drop depends on the both the real and reactive power of the load. The component ΔV can be written as ∆V=Is*Rs-jIs*Xs

(13)

36

4.3.2 Voltage Regulation Using the DSTATCOM: Fig.4.3.1.2 shows the vector diagram with voltage compensation. By adding a compensator in parallel with the load, it is possible to make |E| =|V| by controlling the current of the compensator.

IS=IL+ IR

(14)

Where IR is compensator current.

4.4 BASIC OPERATING PRINCIPLE: Basic operating principle of a DSATCOM is similar to that of synchronous machine. The synchronous machine will provide lagging current when under excited and leading current when over excited. DSTATCOM can generate and absorb reactive power similar to that of synchronous machine and it can also exchange real power if provided with an external device DC source. 4.4.1 Exchange of Reactive Power: If the output voltage of the voltage source converter is greater than the system voltage then the DSATCOM will act as capacitor and generate reactive power (i.e. provide lagging current to the system).

4.4.2 Exchange of Real Power: As the switching devices are not loss less there is a need for the DC capacitor to provide the required real power to the switches. Hence there is a need for real power exchange with an AC system to make the capacitor voltage constant in case of direct voltage control. There is also a real power exchange with the AC system if DSTATCOM id provided with an external DC source to regulate the voltage incase of very low voltage in the distribution system or in case of faults. And if the VSC output voltage leads the system voltage then the real power from the capacitor or the DC source will be supplied to the AC system to regulate the system voltage to the =1p.u or to make the capacitor voltage constant. Hence the exchange of real power and reactive power of the voltage source converter with AC system is the major required phenomenon for the regulation in the transmission as well 37

as in the distribution system. For reactive power compensation, DSTATCOM provides reactive power as needed by the load and therefore the source current remains at unity power factor (UPF). Since only real power is being supplied by the source, load balancing is achieved by making the source reference current balanced. The reference source current used to decide the switching of the DSTATCOM has real fundamental frequency component of the load current which is being extracted by these techniques.

A STATCOM at the transmission level handles only fundamental reactive power and provides voltage support while as a DSTATCOM is employed at the distribution level or at the load end for power factor improvement and voltage regulation. DSTATCOM can be one of the viable alternatives to SVC in a distribution network. Additionally, a DSTATCOM can also behave as a shunt active filter, to eliminate unbalance or distortions in the source current or the supply voltage as per the IEEE-519 standard limits. Since a DSTATCOM is such a multifunctional device, the main objective of any control algorithm should be to make it flexible and easy to implement in addition to exploiting its multi functionality to the maximum. The main objective of any compensation scheme is that it should have a fast response, flexible and easy to implement. The control algorithms of a DSTATCOM are mainly implemented in the following steps: 

Measurements of system voltages and current and



Signal conditioning



Calculation of compensating signals



Generation of firing angles of switching devices

4.5 ADVANTAGES: 1) It occupies small areas. 2) It replaces the large passive banks and circuit elements by compact converters. 3) Reduces site work and time. 4) Its response is very fast.

38

4.6 WIND TURBINES: Wind energy is a source of renewable power which comes from air current flowing across the earth's surface. Wind turbines harvest this kinetic energy and convert it into usable power which can provide electricity for home, farm, school or business applications on small (residential), medium (community), or large (utility) scales. Wind energy is one of the fastest growing sources of new electricity generation in the world today. These growth trends can be linked to the multi-dimensional benefits.

4.6.1 HORIZONTAL AXIS: Horizontal-axis wind turbines (HAWT) have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator.

4.6.1.1 The Advantages of Horizontal-Axis Wind Turbines: Wind turbines harvest the power of the wind to create electricity. The wind turns the blades, which spin a shaft, which connects to a generator that supplies an electric current. As of 2010, the two basic types of wind turbines are used in wind energy systems are the traditional farm styled, horizontal--axis turbines and the eggbeater- style vertical turbines. The more common, traditional styled horizontal--axis wind turbines offer some advantages. 1. Blade Adjustment o Horizontal-axis turbines offer the ability to adjust the pitch of the blades to catch the wind at just the right angle to collect the maximum amount of wind energy for the time of day and season. 1. Blade Adjustment - Horizontal-axis turbines offer the ability to adjust the pitch of the blades to catch the wind at just the right angle to collect the maximum amount of wind energy for the time of day and season. 2. Efficiency of Operation - Horizontal-axis wind turbines have blades that are designed perpendicular to the direction of wind. This efficient design increases wind power throughout the entire rotation. In contrast, vertical-axis wind turbines require airfoil surfaces to backtrack against the wind for part of the cycle in a less efficient manner.

39

4.6.1.2 The Disadvantages of Horizontal Axis Wind Turbines: 1. Produces 50% less electricity on an annual basis than vertical axis wind turbine with the same swept area. 2. Needs higher wind speeds to start generating electricity and less become less efficient at producing power in very hign winds over 90 MPH. 3. They cannot withstand extreme weather conditions due to frost, freezing rain or heavy snow plus heavy winds in excess of 110 MPH. 4. The gearboxes and excessive yawing reduce the lifespan of these HAW turbines. 5. Tend to be visual distraction to the human eye. 6. Birds are injured or killed by the propellers since they are not solid objects so the birds fly into the blades. 7. Cannot be installed in turbulent wind conditions because the flow of wind must be "smooth" to make the HAWT efficient. 8. Blades are too thin to promote graphics or signage for advertising. 9. The propellers on the HAWT design make more noise as the wind speeds increase. 10. More moving parts lead to more service and constant maintenance. 11. Structurally less stable as wind speeds increase do to eccentric loading.

4.6.2 VERTICAL AXIS: Vertical-axis wind turbines (or VAWTs) have the main rotor shaft arranged vertically. Key advantages of this arrangement are that the turbine does not need to be pointed into the wind to be effective. This is an advantage on sites where the wind direction is highly variable, for example when integrated into buildings. The key disadvantages include the low rotational speed with the consequential higher torque and hence higher cost of the drive train, the inherently lower power coefficient, the 360 degree rotation of the aerofoil within the wind flow during each cycle and hence the highly dynamic loading on the blade, the pulsating torque generated by some rotor designs on the drive train, and the difficulty of modeling the wind flow accurately and hence the challenges of analyzing and designing the rotor prior to fabricating a prototype. With a vertical axis, the generator and gearbox can be placed near the ground, using a direct drive from the rotor assembly to the ground-based gearbox, hence improving accessibility for maintenance. When a turbine is mounted on a rooftop, the 40

building generally redirects wind over the roof and this can double the wind speed at the turbine. If the height of the rooftop mounted turbine tower is approximately 50% o f the building height, this is near the optimum for maximum wind energy and minimum wind turbulence. It should be borne in mind that wind speeds within the built environment are generally much lower than at exposed rural sites, noise may be a concern and a n existing structure may not adequately resist the additional stress.

4.6.2.1 The Advantages of Vertical Axis Wind Turbines : 1. Produces up to 50% more electricity on an annual basis versus conventional turbines with the same swept area. 2. Generates electricity in winds as low as 4.5 mph and continues to generate power in wind speeds up to 150 mph based on the model. 3. Withstands extreme weather such as frost, ice, sand, salt, humidity, and very high wind conditions in excess of 140 mph. 4. Non-polluting through its sealed generator design. 5. Easy on the eyes with non-reflecting surfaces to eliminate shadow strobing effect. 6. Does not harm wildlife as birds can detect a solid object and can be seen on aircraft radar. 7. Can be installed in turbulent wind conditions such as between buildings, alleys, or even downtown urban rooftops with adjacent buildings.

4.7 Wind Energy Generating System: The wind energy is not a constant energy source. Its output is varies according to variation of wind, the electricity is produced by using the power of wind to rotate the induction generator. The wind turbine generation system is depending upon the constant speed with variable pitch angle. The induction generator is used in the proposed scheme because of its simp licity, it does not require a separate field circuit, it can accept constant and variable loads, and has natural protection against short circuit. The available power of wind energy system is presented as under in below equation

41

(15)

Where ρ (kg/m ) is the air density and A (m ) is the area swept out by turbine blade, Vwind is the wind speed in mtr/s. It is not possible to extract all kinetic energy of wind, thus it extract a fraction of power in wind, called power coefficient Cp of the wind turbine.

Fig 4.7: Grid connected system for power quality improvement

Wind Turbine is located where the power quality is highly influenced. Its operation and its influence on the power system depend on the structure of the network. But the self-excitation of wind turbine generating system arises a risk equipped with commutating capacitor. It provides the reactive power compensation to the induction generator. The self-excitation are the safety aspect and balance between real and reactive power.

42

4.7.1 Variable-Speed Wind Turbine:

Fig 4.7.1: Variable-Speed Wind Turbine

The system presented in Figure. 4.7.1 consists of a wind turbine equipped with a converter connected to the stator of the generator. The generator could either be a cage bar induction generator or a synchronous generator. The gearbox is designed so that maximum rotor speed corresponds to rated speed of the generator. Synchronous generators or permanent- magnet synchronous generators can be designed with multiple poles which imply that there is no need for a gearbox. Since this "fullpower" converter/generator system is commonly used for other applications, one advantage with this system is its well-developed and robust control.

4.7.2 Control Scheme: The Control scheme approach is based on injecting the currents in to the grid. The controller used in this is hysteresis current controlled technique. Using such control technique, the controller keeps the control system variable between boundaries of hysteresis area and gives correct switching signal for STATCOM operation. The control algorithm needs the measurements of several variables such as three phase source current, DC voltage, inverter current with the help of sensor. The current control block, receives an input of reference current actual current are subtracted so as to activate the operation of STATCOM in current control mode.

43

4.7.2.1 Grid Synchronization:

In three-phase balance system, the RMS voltage source amplitude is calculated at the sampling frequency from the source phase voltage (Vsa,Vsb,Vsc) and is expressed, as sample template Vsm, sampled peak voltage, as in (13).

(16)

The in-phase unit vectors are obtained from AC source—phase voltage and the RMS value of unit vector Usa, Usb, Usc as shown in (14),(15,)(16) (17) (18) (19)

The in-phase generated reference currents are derived using in-phase unit voltage template as in (17),(18),(19)

(20) (21) (22)

Where I is proportional to magnitude of filtered source voltage for respective phases. This ensures that the source current is controlled to be sinusoidal. The unit vectors implement the important function in the grid connection for the synchronization for STATCOM. This method is simple, robust and favorable as compared with other methods.

44

4.7.3 System Operation: The shunt connected STATCOM with battery energy storage is connected wit h the interface of the induction generator and non-linear load at the PCC in the grid system. The STATCOM compensator output is varied according to the controlled strategy, so as to maintain the power quality norms in the grid system. The current control strategy is included in the control scheme that defines the functional operation of the STATCOM compensator in the power system. A single STATCOM using insulated gate bipolar transistor is proposed to have a reactive power support, to the induction generator and to the nonlinear load in the grid system. The main block diagram of the system operational scheme is shown in Fig.4.7.3.

Fig 4.7.3: System Operational Scheme in grid system

45

5. SIMULINK MODEL AND RESULTS

5.1: Circuit Diagram:

Fig 5.1: Circuit Diagram

The system having one conventional source, wind turbine generating system, STATCOM with Battery Energy Storage System, IGBT pulse control subsystem and Non- linear load. The power factor correction capacitor is connected with wind generation s ystem shown in Figure 5.1. A wind turbine is a device that converts kinetic energy from the wind, also called wind energy, into mechanical energy; a process known as wind power. If the mechanical energy is used to produce electricity, the device may be called wind turbine or wind power plant.

46

5.2 Simulation Results: The simulation is performed on the test system with non-linear load using MATLAB SIMULINK. The simulations are performed for the cases: (i) without DSTATCOM and (ii) with DSTATCOM - in the circuit. The system performance is analyzed. These cases are summarized below: Case I: In the distribution network with non-linear load, the current initially takes time to have a steady value and is distorted. Moreover, the FFT analysis of the load current wave indicates the presence of current harmonics and its THD level observed is 0.35% at 50 Hz fundamental frequency which is relatively high. Case II: In the second feeder of the distribution network, the system is compensated using DSTATCOM. In this case, the current wave takes no time to attain its steady value and the distortion in the wave is negligible. Moreover, the FFT analysis of the load current wave indicates that the current harmonics are almost compensated and its THD level observed is only 3.04% at 50 Hz fundamental frequency. 5.2.1 Single Phase:

Fig 5.2.1: Single Phase

47

5.2.2 Three Phase:

Fig 5.2.2: Three Phase

5.2.3 Powe r Factor:

Fig 5.2.3: Power Factor

48

5.2.4 Source Current THD:

Fig 5.2.4: Source Current THD

5.2.5 Variable Wind Speed:

Fig 5.2.5: Variable Wind Speed

49

5.3 FFT Analysis: 5.3.1 With Filter:

Fig 5.3.1: With Filter

5.3.2 With STATCOM:

Fig 5.3.2: With STATCOM

50

5.3.3 Without STATCOM:

Fig 5.3.3: Without STATCOM

51

6. FUTURE SCOPE

Most of the power quality issues created in power systems are due to the non-linear characteristics and fast switching of power electronic equipment. Now obtaining a real-time example for a non-linear load in Simulink model helps us to control the issues. So we can further develop a Simulink model for Electrical Arc furnace as a load. Active power filters have been developed over the years to solve these problems to improve power quality. Among all this issues we can further develop interconnection of grid with available renewable energy source and can generate a quality power to the customer. In this work both PI controller based and DSTATCOM controller, the three-phase shunt active power filter is used to compensate reactive power and harmonics at nonlinear load which improves power quality for three-phase, three wire systems. Further the controller technique can be improvised for upgrading the current controlling technique with the compensation process based on sensing line currents only, an approach different from conventional methods which require sensing of harmonics or reactive power components of the load. Further interconnecting the motor speed with a feedback system can improve the characteristics of non- linear loads.

52

7. CONCLUSION This project represents the STATCOM-based control scheme for power quality improvement in grid connected wind generating system and non- linear load. It has a capability to cancel out the harmonic parts of the load current. It maintains the source voltage and current in-phase and support the reactive power demand for the wind generator and load. With this support of STATCOM unity power factor is achieved, such that current and power are in phase during the working of STATCOM. So, we get the output in the form of time functioning and variable speed ie. Output is presented with a particular time limit, where working output is shown with working of STATCOM and without STATCOM. During the change in speed also the disturbances are cleared with the help of control technique.

53

8. REFERENCES 1. “Grid Interconnection of Renewable Energy Sources at the Distribution Level with PowerQuality Improvement” IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 26, NO. 1, JANUARY 2011. 2. IEEE, “IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems,” IEEE Std. 519-1992, revision of IEEE Std. 519-1981. 3. J. P. Pinto, R. Pregitzer, L. F. C. Monteiro, and J. L. Afonso, “3-phase 4-wire shunt active power filter with renewable energy interface,” presented at the Conf. IEEE Renewable Energy & Power Quality, Seville, Spain, 2007. 4. F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, “Overview of control and grid synchronization for distributed power generation systems,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1398–1409,Oct. 2006. 5. SharadW.Mohod and Mohan V.Aware, “A STATCOM Control Scheme For Grid Connected Wind Energy System for Power Quality Improvement”, IEEE Systems Journal, Vol.4, No.3, September 2010. 6. N.SrinivasaRao and DrG.V.Siva Krishna Rao, “Modeling and Simulation of D-STATCOM for Power

Quality

Improvement”,

International

journal of

Engineering

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