Power Quality by DSTATCOM

1

CONTENTS TOPIC NAME

PAGE NUMBER

List of Figures

i

List of Tables

ii

Abstract

9

1. Introduction

2.

1.1.

Introduction

11

1.2.

Objective of the project

12

1.3.

Overview of the thesis

12

1.4.

Literature Survey

12

Power quality 2.1.

Introduction

15

2.2.

Need For PQ Improvement

17

2.3.

Power Quality Standards

18

2.4.

Power Quality Terminology

19

2.5.

About PQ disturbances

21

2.6.

Improving techniques

22

3. Active Power Filters 3.1.

Introduction

31

3.2.

Classification

32

2 3.3.

Three phase Active Filters

33

4. DSTATCOM 4.1.

Introduction

43

4.2.

Principle and operation

44

4.3.

Control Scheme for the DSTATCOM

45

4.4.

Phasor diagram

48

4.5

Mathematical Modeling

49

5. MATLAB Simulation 5.1.

Introduction

52

5.2.

Proposed circuit-Single Line Diagram

52

5.3.

Circuit description

54

6. Results and Future Scope 6.1.

Case Studies

66

6.2.

Conclusion

77

6.3.

Future Scope

78

References

3

LIST OF FIGURES 2.1. Improving power quality by distortion elimination

23

2.2. Principle of input converter to eliminate distortion loads

23

on the power network.

3.1. Current fed type AF

33

3.2 .Voltage fed type AF

33

3.3. Shunt-type AF

34

3.4 .Series-types AF

34

3.5 Hybrid filter

35

3.6 Unified Power Quality Conditioner

35

3.7 Configuration of the three phase, three wire Active filtering system

39

3.8 Control block of Sample and Hold circuit's harmonic reference template

40

3.9 Method used to capture IP.

41

4.1 Schematic Diagram of a DSTATCOM

44

4.2 Indirect PI controller

46

4.3 Phase-Modulation of the control signal

46

4.4 Phasor diagram for shunt voltage controller

48

5.1 single line diagram of the test system for DSTATCOM

53

5.2 Simulink model of D-STATCOM test system

54

5.2.1 3-phase Functional Block

55

5.2.2 Simulink model of D-STATCOM

56

5.2.3 Simulink model of CONTROLLER

56

4 5.2.4 TRIGGER BLOCK Simulation Model

57

5.2.5 Functional Block of Discrete PI Controller

58

5.2.6 Functional Block of 3-ph Breaker

59

5.2.7 Functional Block of DC Voltage Source

59

5.2.8 Functional Block of #-ph Sequence Analyser

60

5.2.9 Functional Block of Unit Delay

60

5.2.10 Functional Block of 3-ph Transformer

61

5.2.11 Functional Block of Breaker

62

5.2.12 Functional Block of Discrete PWM Generator

63

5.2.13 Functional Block of Universal Bridge

64

6.1 Simulation model for SAG Formation

66

6.2 Voltage Vrms at the load point without DSTATCOM

67

6.3 Simulation Model for SAG Mitigation

68

6.4 Voltage Vrms at the load point with DSTATCOM

69

6.5 Simulation model for SWELL Formation

70

6.6 Voltage Vrms at the Load point without DSTATCOM

71

6.7 Simulation model for SWELL mitigation

72

6.8 voltage Vrms at the load point with DSTATCOM

73

6.9 Simulation model for voltage interruption formation

74

6.10 Voltage Vrms at the Load point without DSTATCOM

75

6.11 Simulation model for Voltage interruption mitigation

76

6.12 Voltage Vrms at the load point with D-STATCOM energy storage of 40.7KV

77

5

LIST OF TABLES 2.1 Voltage Characteristics as Published by Goteborg Energi

18

3.1. IEEE 519 Voltage Limits

31

6

ABSTRACT A Power quality problem is an occurrence manifested as a nonstandard voltage, current or frequency that results in a failure or a mis-operation of end user equipments. Utility distribution networks, sensitive industrial loads and critical commercial operations suffer from various types of outages and service interruptions which can cost significant financial losses. With the restructuring of power systems and with shifting trend towards distributed and dispersed generation, the issue of power quality is going to take newer dimensions. In developing countries like India, where the variation of power frequency and many such other determinants of power quality are themselves a serious question, it is very vital to take positive steps in this direction .The present work is to identify the prominent concerns in this area and hence the measures that can enhance the quality of the power are recommended. This work describes the techniques of correcting the supply voltage sag, swell and interruption in a distributed system. At present, a wide range of very flexible controllers, which capitalize on newly available power electronics components, are emerging for custom power applications. Among these, the distribution static compensator and the dynamic voltage restorer are most effective devices, both of them based on the VSC principle. A DVR injects a voltage in series with the system voltage and a D-STATCOM injects a current into the system to correct the voltage sag, swell and interruption. Comprehensive results are presented to assess the performance of each device as a potential custom power solution. In this project we are discussing the effects of using DSTATCOM in the power system during fault conditions. DSTATCOM means Distribution Static Compensator. It 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. In this paper we are mitigating the faults like voltage sag during single line to ground fault and voltage swell. STATCOM is a static VAR generator, whose output is varied so as to maintain or control specific parameters of the electric power system.

7

INTRODUCTION

INTRODUCTION

8

1.1 Introduction With the advent of power semiconductor switching devices, like thyristors, GTO's (Gate Turn off thyristors), IGBT's (Insulated Gate Bipolar Transistors) and many more devices, control of electric power has become a reality. Such power electronic controllers are widely used to feed electric power to electrical loads, such as adjustable speed drives (ASD's), furnaces, computer power supplies, HVDC systems etc. The power electronic devices due to their inherent non-linearity draw harmonic and reactive power from the supply. In three phase systems, they could also cause unbalance and draw excessive neutral currents. The injected harmonics, reactive power burden, unbalance, and excessive neutral currents cause low system efficiency and poor power factor. In addition to this, the power system is subjected to various transients like voltage sags, swells, flickers etc. These transients would affect the voltage at distribution levels. Excessive reactive power of loads would increase the generating capacity of generating stations and increase the transmission losses in lines. Hence supply of reactive power at the load ends becomes essential. Power Quality (PQ) has become an important issue since many loads at various distribution ends like adjustable speed drives, process industries, printers; domestic utilities, computers, microprocessor based equipments etc. have become intolerant to voltage fluctuations, harmonic content and interruptions. Power Quality (PQ) mainly deals with issues like maintaining a fixed voltage at the Point of Common Coupling (PCC) for various distribution voltage levels irrespective of voltage fluctuations, maintaining near unity power factor power drawn from the supply, blocking of voltage and current unbalance from passing upwards from various distribution levels, reduction of voltage and current harmonics in the system and suppression of excessive supply neutral current. Conventionally, passive LC filters and fixed compensating devices with some degree of variation like thyristor switched capacitors, thyristor switched reactors were employed to improve the power factor of ac loads. Such devices have the demerits of

9 fixed compensation, large size, ageing and resonance. Nowadays equipments using power semiconductor devices, generally known as active power filters (APF's), Active Power Line Conditioners (APLC's) etc. are used for the power quality issues due to their dynamic and adjustable solutions. Flexible AC Transmission Systems (FACTS) and Custom Power products like STATCOM (Static synchronous compensator), DVR (Dynamic Voltage Restorer), etc. deal with the issues related to power quality using similar control strategies and concepts. Basically, they are different only in the location in a power system where they are deployed and the objectives for which they are deployed.

1.2 Objective of the project To improve the power quality of a distribution system by injecting the required amount of currents to the distribution system from the storage element through DSTATCOM . . 1.3 Overview the Thesis The complete project thesis is divided into six chapters as follows. Chapter 1 provides the introduction of the project and defines the objective of the project. Chapter 2 provides a brief summary of power quality, assets and improvement techniques. Chapter 3 deals with active power filters. Chapter 4 deals with operational concepts of DSTATCOM. Chapter 5 provides the simulations diagrams and results. Chapter 6 deals with conclusion of the thesis and future scope for this project. 1.4 Literature Survey Power electronic based power processing offers higher efficiency, compact size and better controllability. But on the flip side, due to switching actions, these systems behave as non-linear loads [1-3]. Therefore, whenever, these systems are connected to the utility, they draw non-sinusoidal and/or lagging current from the source. As a result these systems pose themselves as loads having poor displacement as well as distortion

10 factors. Hence they draw considerable reactive volt-amperes from the utility and inject harmonics in the power networks. Until now, to filter these harmonics and to compensate reactive power at factory level, only capacitor and passive filters were used. Passive filters have been widely used for the harmonic and reactive power mitigation in the power lines earlier. They are suitable for only eliminating only few harmonics, large size, ageing and resonance. More recently, new PWM based converters for motor control are able to provide almost unity power factor operations. This situation leads to two observations: on one hand, there is electronic equipment which generates harmonics and, on the other hand, there is unity power factor motor drive system which doesn't need power factor correction capacitor. Also, we cannot depend on this capacitor to filter out those harmonics. This is one of the reasons that the research is being done in the area of APF and less pollutant drives. Loads, such as, diode bridge rectifier or a thyristor bridge feeding a highly inductive load, presenting themselves as current source at point of common coupling (PCC), can be effectively compensated by connecting an APF in shunt with the load [46]. On the other hand, there are loads, such as Diode Bridge having a high dc link capacitive filter. These types of loads are gaining more and more importance mainly in forms of AC to DC power supplies and front end AC to DC converters for AC motor drives. For these types of loads APF has to be connected in series with the load [4, 7]. The voltage injected in series with the load by series APF is made to follow a control law such that the sum of this injected voltage and the input voltage is sinusoidal. Thus, if utility voltages are non-sinusoidal or unbalanced, due to the presence of other clients on the same grid, proper selection of magnitude and phase for the injected voltages will make the voltages at load end to be balanced and sinusoidal. The shunt APF acts as a current source and inject a compensating harmonic current in order to have sinusoidal, in-phase input current and the series APF acts as a voltage source and inject a compensating voltage in order to have sinusoidal load voltage. The developments in the digital electronics, communications and in process control system have increased the number of sensitive loads that require ideal sinusoidal supply voltage for their proper operation. In order to meet limits proposed by standards it is necessary to include some sort of compensation. In the last few years, solutions based

11 on combination of series active and shunt active filter have appeared [8-9]. Its main purpose is to compensate for supply voltage and load current imperfections, such as sags, swells, interruptions, imbalance, flicker, voltage imbalance, harmonics, reactive currents, and current unbalance [10-16]. This combination of series and shunt APF is called as Unified Power Quality Conditioner (UPQC). In most of the articles control techniques suggested are complex requiring different kinds of transformations. The control technique presented here is very simple and does not require any transformation.

12

POWER QUALITY

13

POWER QUALITY 2.1 INTRODUCTION Power quality is defined as the concept of powering and grounding sensitive equipment in a matter that is suitable to the operation of that equipment. There are many different reasons for the enormous increase in the interest in power quality. Some of the main reasons are: •

Electronic and power electronic equipment has especially become much more sensitive. Equipment has become less tolerant of voltage quality disturbances, production processes have become less tolerant of incorrect of incorrect operation of equipment, and companies have become less tolerant of production stoppages. The main perpetrators are interruptions and voltage dips, with the emphasis in discussions and in the literature being on voltage dips and short interruptions. High frequency transients do occasionally receive attention as causes of equipment malfunction.

•

Equipment produces more current disturbances than it used to do. Both low and high power equipment is more and more powered by simple power electronic converters which produce a broad spectrum of distortion. There are indications that the harmonic distortion in the power system is rising, but no conclusive results are obtained due to the lack of large scale surveys.

•

The deregulation of the electricity industry has led to an increased need for quality indicators. Customers are demanding, and getting, more information on the voltage quality they can expect.

•

Also energy efficient equipment is an important source of power quality disturbance. Adjustable speed drives and energy saving lamps are both important sources of waveform distortion and are also sensitive to certain type of power quality disturbances. When these power quality problems become a barrier for the large scale introduction of environmentally friendly sources and users’ equipment, power quality becomes an environmental issue with much wider consequences than the currently merely economic issues.

14 2.2 NEED FOR POWER QUALITY IMPROVEMENT 1. Equipment has become less tolerant of voltage quality disturbances, production processes have become less tolerant of incorrect of incorrect operation of equipment, and companies have become less tolerant of production stoppages. Note that in many discussions only the first problem is mentioned, whereas the latter two may be at least equally important .All this leads to much higher costs than before being associated with even a very short duration disturbance. The main perpetrators are interruptions and voltage dips, with the emphasis in discussions and in the literature being on voltage dips and short interruptions. High frequency transients do occasionally receive attention as causes of equipment malfunction but are generally not well exposed in the literature. 2. Equipment produces more current disturbances than it used to do. Both low and high power equipment is more and more powered by simple power electronic converters which produce a broad spectrum of distortion. There are indications that the harmonic distortion in the power system is rising, but no conclusive results are obtained due to the lack of large scale surveys. 3. The deregulation of the electricity industry has led to an increased need for quality indicators. Customers are demanding, and getting, more information on the voltage quality they can expect. Some issues of the interaction between deregulation and power quality are discussed. 4. Also energy efficient equipment is an important source of power quality disturbance. Adjustable speed drives and energy saving lamps are both important sources of waveform distortion and are also sensitive to certain type of power quality disturbances. When these power quality problems become a barrier for the large scale introduction of environmentally friendly sources and users’ equipment, power quality becomes an environmental issue with much wider consequences than the currently merely economic issues.

15 2.3 POWER QUALITY STANDARDS 2.3.1 PURPOSE OF STANDARDIZATION Standards that define the quality of the supply have been present for decades already. Almost any country has standards defining the margins in which frequency and voltage are allowed to vary. Other standards limit harmonic current and voltage distortion, voltage fluctuations, and duration of an interruption. There are three reasons for developing power quality standards. 2.3.2THE EUROPEAN VOLTAGE CHARATERISTICS STANDARD European standard 50160 [80] describes electricity as a product, including its shortcomings. It gives the main characteristics of the voltage at the customer's supply terminals in public low-voltage and medium-voltage networks under normal operating conditions. Some disturbances are just mentioned, for others a wide range of typical values are given, and for some disturbances actual voltage characteristics are given. Voltage variation: Standard EN 50160 gives limits for some variations. For each of these variations the value is given which shall not be exceeded for 95% of the time. The measurement should be performed with a certain averaging window. The length of this window is 10 minutes for most variations; thus very short time scales are not considered in the standard. The following limits for the low-voltage supply are given in the document: • Voltage magnitude: 95% of the 10-minute averages during one week shall be within ±10% of the nominal voltage of 230 V. TABLE 2.1 Voltage Characteristics as Published by Goteborg Energi Phenomenon

Basic Level

Magnitude variations

Voltage shall be between 207 and 244 V

Voltage unbalance

Up to 2%

Voltage fluctuations

Not exceeding the flicker curve

Frequency

In between 49.5 and 50.5 Hz

16 2.4 Power quality Terminology DSTATCOM means Distribution Static Compensator. STATCOM is a static VAR generator, whose output is varied so as to maintain or control specific parameters of the electric power system.

SAG is a decrease in rms voltage or currents to between 0.1 to 0.9 p.u at the power frequency for duration of from 0.5 cycles to 1 minute. Balanced Sag is an equal drop in the rms value of voltage in the three-phases of a three-phase system or at the terminals of three-phase equipment for duration up to a few minutes. Voltage dip is sudden reduction in the supply voltage by a value of more than 10% of the reference value, fallowed by a voltage recovery after a short period of time. Unbalanced Fault is a short circuit or open circuit fault in which not all three phases are equally involved. Voltage Tolerance it is the immunity of a piece of equipment against voltage magnitude variations (Sags, Swells and Interruptions) and short duration over voltages. Duration (of Voltage Sag) it is the time during which the voltage deviates significantly from the ideal voltage. Critical Distance is the distance at which a short-circuit fault will lead to a voltage sag of a given magnitude for a given load position. Current Disturbance it is a variation of event during which the current in the system or at the equipment terminals deviates from the ideal sine wave.

17

Voltage Disturbance it is a variation of event during which the voltage in the system or at the equipment terminals deviates from the ideal sine wave. Power Quality it is the study or description of both voltage and current disturbances. Power quality can be seen as the combination of voltage quality and current quality. Interruption is the voltage event in which the voltage is zero during a certain time. The time during which the voltage is zero is referred to as the “duration” of the interruption. (OR) A voltage magnitude event with a magnitude less than 10% of the nominal voltage. Over Voltage is an abnormal voltage higher than the normal service voltage, such as might be caused from switching and lightning surges. (OR) Abnormal voltage between two points of a system that is greater than the highest value appearing between the same two points under normal service conditions.

Under Voltage is a voltage event in which the rms voltage is outside its normal operating margin for a certain period of time. (OR) A voltage magnitude event with a magnitude less than the nominal rms voltage, and a duration exceeding 1 minute. Swell it is a momentary increase in the rms voltage or current to between 1.1 and 1.8pu delivered by the mains, outside of the normal tolerance, with a duration of more than one cycle and less than few seconds. Recovery Time is the time interval needed for the voltage or current to return to its normal operating value, after a voltage or current event.

18

Fault is an event occurs on the power system and it effects the normal operation of the power system. Voltage Fluctuation is a special type of voltage variation in which the voltage shows changes in the magnitude and/or phase angle on a time scale of seconds or less. Severe voltage fluctuations lead to light flicker. Voltage Source Converters (VSC)

A voltage-source converter is a power electronic device, which can generate a sinusoidal voltage with any required magnitude, frequency and phase angle. Voltage source converters are widely used in adjustable-speed drives, but can also be used to mitigate voltage dips. The VSC is used to either completely replace the voltage or to inject the ‘missing voltage’. The ‘missing voltage’ is the difference between the nominal voltage and the actual. 2.5 About power Quality Disturbances 2.5.1 Voltage sags

Major Causes: Faults, Starting of large loads, and brown-out recovery. Major Consequences: Shorts, accelerated aging, loss of data or stability, Process interrupts, etc. 2.5.2 Capacitor Switching Transients

Major Causes: A power factor correction method Major Consequences: Insulation breakdown or spark over, semi conductor device damage, shorts, accelerated aging, loss of data or stability. 2.5.3 Harmonics

Major Causes: Power electronic equipment, arcing, transformer saturation.

19

Major Consequences: Equipment Over heating, high voltage/current, Protective device operations. 2.5.4 Lightning Transients

Major Causes: Lightning strikes Major Consequences: Insulation breakdown or spark over, semi conductor device damage, shorts, accelerated aging, loss of data or stability. 2.5.5 High Impedance faults

Major Causes: Fallen conductors, trees (fail to establish a permanent return path)

Major Consequences: Fire, threats to personal safety. 2.6.(i) Principles for improving power quality From the discussion already presented, it is evident that for improving power quality, the steps given in fig (4) have to be taken. As also pointed out, the appropriate decomposition of power for purposes of both identification and control of the distortion elimination by filters has to be achieved. Since it is essential to use clear and consistent terminology, the term non-active power filter will be used for equipment that eliminates non-active power. The actual types of these filters are to be discussed in a further chapter of this paper.

20

Identify distortion by using Appropriate power theory

Decide on method of Distortion elimination

Equipment with appropriate power electronic input converters. (Dynamic input filters)

Tuned impedance filters

Dynamic filters for distortion elimination

Type of filter: SVC, PWM (Series or parallel) hybrid, undefined

Figure 2.1: Improving power quality by distortion elimination. The non-active power filters to be used can be divided into the classes of input converters, dynamic filters and tuned impedance filters. Theses principles and the control requirements will now be discussed shortly.

Figure 2.2: Principle of input converter to eliminate distortion loads on the power network. 2.6 IMPROVEMENT TECHNIQUES To improve the power quality, some devices need to be installed at a suitable location. These devices are called custom power devices, which make sure that customers get pre specified quality and reliability of supply. The compensating devices compensate a load, i.e its power factor, unbalance conditions or improve the power quality of

21 supplied voltage, etc. some of the power quality improving techniques are given as below. 2.6.1 HARMONICS Harmonic Filters may be used to mitigate, and in some cases, eliminate problems created by power system harmonics. Non-linear loads such as rectifiers, converters, home electronic appliances, and electric arc furnaces cause harmonics giving rise to extra losses in power equipment such as transformers, motors and capacitors. They can also cause other, probably more serious problems, when interfering with control systems and electronic devices. Installing filters near the harmonic sources can effectively reduce harmonics.

For large, easily identifiable sources of harmonics, conventional filters

designed to meet the demands of the actual application are the most cost efficient means of eliminating harmonics. These filters consist of capacitor banks with suitable tuning reactors and damping resistors. For small and medium size loads, active filters, based on power electronic converters with high switching frequency, may be a more attractive solution. Benefits •

Eliminates harmonics

•

Improved Power Factor

•

Reduced Transmission Losses

•

Increased Transmission Capability

•

Improved Voltage Control

•

Improved Power Quality

Other applications •

Shunt Capacitors

2.6.2 VOLTAGE FLICKERS Voltage flicker can become a significant problem for power distributors when large motor loads are introduced in remote locations. Installation of a series capacitor in the feeder strengthens the network and allows such load to be connected to existing lines, avoiding more significant investment in new substations or new distribution lines.

22 The use of the MiniCap on long distribution feeders provides self-regulated reactive power compensation that efficiently reduces voltage variations during large motor starting. Benefits •

Reduced voltage fluctuations (flicker)

•

Improved voltage profile along the line

•

Easier starting of large motors

•

Self-regulation

2.6.3 BOTTLENECKS Bottlenecks may be relieved by the use of Series Compensation. Longer lines tend to have stability-constrained capacity limitations as opposed to the higher thermal constraints of shorter lines.

Series Compensation has the net effect of reducing

transmission line series reactance, thus effectively reducing the line length.

Series

Compensation also offers additional power transfer capability for some thermalconstrained bottlenecks by balancing the loads among the parallel lines. The power transfer between two-area interconnected systems is limited to 1500MW due to stability constraints. Additional electricity can be delivered between them if Series Compensation is applied to increase the maximum stability limits. Benefits •

Increased Power Transfer Capability

•

Additional flexibility in Grid Operation

•

Improved Grid Voltage Control

•

Lower Transmission Losses

•

Improved Transient Stability

Other applications •

Power Flow Control

•

Transient Stability Improvement

23 2.6.4 SHUNT CAPACITORS Regulation of the power factor to increase the transmission capability and reduce transmission losses. Shunt capacitors are primarily used to improve the power factor in transmission and distribution networks, resulting in improved voltage regulation, reduced network losses, and efficient capacity utilization. Figure shows a plot of terminal voltage versus line loading for a system that has a shunt capacitor installed at the load bus. Improved transmission voltage regulation can be obtained during heave power transfer conditions when the system consumes a large amount of reactive power that must be replaced by compensation. At the line surge impedance loading level, the shunt capacitor would decrease the line losses by more than 35%. In distribution and industrial systems, it is common to use shunt capacitors to compensate for the highly inductive loads, thus achieving reduced delivery system losses and network voltage drop. Benefits •

Improved power factor

•

Reduced transmission losses

•

Increased transmission capability

•

Improved voltage control

•

Improved power quality

Other applications •

Harmonic Filters

2.6.5 SHUNT REACTOR The primary purpose of the shunt reactor is to compensate for capacitive charging voltage, a phenomenon getting more prominent for increasing line voltage. Long highvoltage transmission lines and relatively short cable lines (since a power cable has high capacitance to earth) generate a large amount of reactive power during light power transfer conditions which must be absorbed by compensation. Otherwise, the receiving terminals of the transmission lines will exhibit a “voltage rise” characteristic and many types of power equipment cannot withstand such over voltages. A better fine tuning of the reactive power can be made by the use of a tap changer in the shunt reactor. It can be possible to vary the reactive power between 50 to 100 % of the needed power.

24 Benefits •

Simple and robust customer solution with low installation costs and minimum maintenance

•

No losses from an intermediate transformer when feeding reactive compensation from a lower voltage level.

•

No harmonics created which may require filter banks.

2.6.6 SVC Static VAR Compensators are used in transmission and distribution networks mainly providing dynamic voltage support in response to system disturbances and balancing the reactive power demand of large and fluctuating industrial loads. A Static VAR Compensator is capable of both generating and absorbing variable reactive power continuously as opposed to discrete values of fixed and switched shunt capacitors or reactors. Further improved system steady state performance can be obtained from SVC applications. With continuously variable reactive power supply, the voltage at the SVC bus may be maintained smoothly over a wide range of active power transfers or system loading conditions. This entails the reduction of network losses and provision of adequate power quality to the electric energy end-users. The traction system is a major source of unbalanced loads. Electrification of railways, as an economically attractive and environmentally friendly investment in infrastructure, has introduced an unbalanced and heavy distorted load on the three-phase transmission grid. Without compensation, this would result in significant unbalanced voltage affecting most neighboring utility customers. The SVC can elegantly be used to counteract the unbalances and mitigate the harmonics such that the power quality within the transmission grid is not impaired. Static Var Compensators are mainly used to perform voltage and reactive power regulation. However, when properly placed and controlled, SVCs can also effectively counteract system oscillations. A SVC, in effect, has the ability to increase the damping factor (typically by 1-2 MW per Mvar installed) on a bulk power system which is experiencing power oscillations. It does so by effectively modulating its reactive power

25 output such that the regulated SVC bus voltage would increase the system damping capability. SVC is used most frequently for compensation of disturbances generated by the Electrical Arc Furnaces (EAF). With a well-designed SVC, disturbances such as flicker from the EAF are mitigated Flicker, the random variation in light intensity from incandescent lamps caused by the operating of nearby fluctuating loads on the common electric supply grid, is highly irritating for those affected. The random voltage variations can also be disturbing to other process equipment fed from the same grid. The proper mitigation of flicker is therefore a matter of power quality improvement as well as an improvement to human environment. Benefits •

Increased Power Transfer Capability

•

Additional flexibility in Grid Operation

•

Improved Grid Voltage Stability

•

Improved Grid Voltage Control

•

Improved Power Factor

Other applications •

Power Oscillation Damping

•

Power Quality (Flicker Mitigation, Voltage Balancing)

•

Grid voltage support

2.6.7 STATCOM STATCOM, when connected to the grid, can provide dynamic voltage support in response to system disturbances and balance the reactive power demand of large and fluctuating industrial loads. A STATCOM is capable of both generating and absorbing variable reactive power continuously as opposed to discrete values of fixed and switched shunt capacitors or reactors. With continuously variable reactive power supply, the voltage at the STATCOM bus may be maintained smoothly over a wide range of system operation conditions. This entails the reduction of network losses and provision of sufficient power quality to the electric energy end-users. STATCOM® is an effective method used to attack the problem of flicker. The unbalanced, erratic nature of an electric arc furnace (EAF) causes significant fluctuating

26 reactive power demand, which ultimately results in irritating electric lamp flicker to neighboring utility customers. In order to stabilize voltage and reduce disturbing flicker successfully, it is necessary to continuously measure and compensate rapid changes by means of extremely fast reactive power compensation. STATCOM® uses voltage source converters to improve furnace productivity similar to a traditional SVC while offering superior voltage flicker mitigation due to fast response time. Similar to SVC, the STATCOM can elegantly be used to restore voltage and current balance in the grid, and to mitigate voltage fluctuations generated by the traction loads. Benefits •

Increased Power Transfer Capability

•

Additional flexibility in Grid Operation

•

Improved Grid Voltage Stability

•

Improved Grid Voltage Control

•

Improved Power Factor

•

Eliminated Flicker

•

Harmonic Filtering

•

Voltage Balancing

•

Power Factor Correction

•

Furnace/mill Process Productivity Improvement

Other applications •

Power Quality (Flicker Mitigation, Voltage Balancing)

•

Grid Voltage Support

27

ACTIVE POWER FILTER

28

Shunt Active Filters 3.1 Introduction The various nonlinear loads like Adjustable Speed Drives (ASD’s), bulk rectifiers, furnaces, computer supplies, etc. draw non sinusoidal currents containing harmonics from the supply which in turn causes voltage harmonics. Harmonic currents result in increased power system losses, excessive heating in rotating machinery, interference with nearby communication circuits and control circuits, etc. It has become imperative to maintain the sinusoidal nature of voltage and currents in the power system. Various international agencies like IEEE and IEC have issued standards, which put limits on various current and voltage harmonics. The limits for various current and voltage harmonics specified by IEEE-519 for various frequencies are given in Table 3.1 and Table 3.2. Table 3.1 IEEE 519 Voltage Limits Bus Voltage

Minimum Individual Harmonic Components (%)

Maximum THD (%)

69 kV and below

3

5

115 kV to 161 kV

1.5

2.5

Above 161 Kv

1

1.5

The objectives and functions of active power filters have expanded from reactive power compensation, voltage regulation, etc. to harmonic isolation between utilities and consumers, and harmonic damping throughout the distribution as harmonics propagate through the system. Active power filters are either installed at the individual consumer premises or at substation and/or on distribution feeders. Depending on the compensation objectives, various types of active power filter topologies have evolved, a proper briefing provided in further.

29 Table 3.2 IEEE 519 Current Limits SCR=Isc/Il

h