1

CONTENTS

TOPIC NAME PAGE NUMBER

List of Figures i

List of Tables ii

Abstract 9

1. Introduction

1.1. Introduction 11

1.2. Objective of the project 12

1.3. Overview of the thesis 12

1.4. Literature Survey 12

2. 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

8

INTRODUCTION

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

9

employed to improve the power factor of ac loads. Such devices have the demerits of

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

10

these systems pose themselves as loads having poor displacement as well as distortion

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 [4-

6]. 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

11

is necessary to include some sort of compensation. In the last few years, solutions based

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 QUALITY2.1 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.

Major Consequences: Equipment Over heating, high voltage/current,

19

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

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

Identify distortion by using Appropriate power theory

Decide on method of Distortion elimination

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

Dynamic filters for distortion elimination

Tuned impedance filters

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

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 thermal-

constrained 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 high-

voltage 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 VoltageMinimum Individual

Harmonic Components (%) MaximumTHD (%)

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<11 11 to 17 17 to 23 23 to 35 35<hTHD

<20 4.0 2.0 1.5 0.6 0.3 5.020 - 50 7.0 3.5 2.5 1.0 0.5 8.050 -100 10.0 4.5 4.0 1.5 0.7 12.0

100 - 1000 12.0 5.5 5.0 2.0 1.0 15.0>1000 15.0 7.0 6.0 2.5 1.4 20.0

3.2 Classifications of Active Power Filters

3.2.1 Converter based classification

Current Source Inverter (CSI) Active Power Filter (Fig 3.1) and Voltage Source

Inverter Active Power Filter (VSI) (Fig 3.2) are two classifications in this category.

Current Source Inverter behaves as a nonsinusoidal current source to meet the harmonic

current requirement of the nonlinear loads. A diode is used in series with the self-

commutating device (IGBT) for reverse voltage blocking. However, GTO-based

configurations do not need the series diode, but they have restricted frequency of

switching. They are considered sufficiently reliable, but have higher losses and require

higher values of parallel ac power capacitors. Moreover, they cannot be used in

multilevel or multistep modes to improve performance in higher ratings.

The other converter used as an AF is a voltage-fed PWM inverter structure, as

shown in Fig 3.2. It has a self-supporting dc voltage bus with a large dc capacitor. It has

become more dominant, since it is lighter, cheaper, and expandable to multilevel and

multistep versions, to enhance the performance with lower switching frequencies. It is

more popular in UPS-based applications, because in the presence of mains, the same

Inverter bridge can be used as an AF to eliminate harmonics of critical nonlinear loads.

3.2.2 Topology based Classification

AF’s can be classified based on the topology used as series or shunt filters,

and unified power quality conditioners use a combination of both. Combinations of active

series and passive shunt filtering are known as hybrid filters. Fig 3.3 is an example of an

30

active shunt filter, which is most widely used to eliminate current harmonics, reactive

power compensation (also known as STATCOM, and balancing unbalanced currents. It is

mainly used at the load end, because current harmonics are injected by nonlinear loads. It

injects equal compensating currents, opposite in phase, to cancel harmonics and/or

reactive components of the nonlinear load current at the point of connection. It can also

be used as a static VAR generator (STATCOM) in the power system network for

stabilizing and improving the voltage profile.

Fig 3.1 Current fed type AF Fig 3.2 Voltage fed type AF

Fig 3.4 shows the basic block of a stand-alone active series filter. It is connected before

the load in series with the mains, using a matching transformer, to eliminate voltage

harmonics, and to balance and regulate the terminal voltage of the load or line. It has

been used to reduce negative-sequence voltage and regulate the voltage on three-phase

systems. It can be installed by electric utilities to compensate voltage harmonics and to

damp out harmonic propagation caused by resonance with line impedances and passive

shunt compensators.

Fig 3.4 shows the basic block of a stand-alone active series filter. It is connected before

the load in series with the mains, using a matching transformer, to eliminate voltage

harmonics, and to balance and regulate the terminal voltage of the load or line. It has

been used to reduce negative-sequence voltage and regulate the voltage on three-phase

systems. It can be installed by electric utilities to compensate voltage harmonics and to

31

damp out harmonic propagation caused by resonance with line impedances and passive

shunt compensators.

Fig 3.5 shows the hybrid filter, which is a combination of an active series filter

and passive shunt filter. It is quite popular because the solid-state devices used in the

active series part can be of reduced size and cost (about 5% of the load size) and a major

part of the hybrid filter is made of the passive shunt L–C filter used to eliminate lower

order harmonics. It has the capability of reducing voltage and current harmonics at a

reasonable cost.

Fig 3.3 Shunt-type AF Fig 3.4 Series-type AF

32

Fig 3.5 Hybrid filter Fig 3.6 Unified Power Quality Conditioner

Fig 3.6 shows a unified power quality conditioner (also known as a universal AF),

which is a combination of active shunt and active series filters. The dc-link storage

element (either inductor or dc-bus capacitor) is shared between two current-source or

voltage-source bridges operating as active series and active shunt compensators. It is used

in single-phase as well as three-phase configurations. It is considered an ideal AF, which

eliminates voltage and current harmonics and is capable of giving clean power to critical

and harmonic-prone loads, such as computers, medical equipment, etc. It can balance and

regulate terminal voltage and eliminate negative-sequence currents. Its main drawbacks

are its large cost and control complexity because of the large number of solid-state

devices involved.

3.2.3 Supply-System-Based Classification

This classification of AF’s is based on the supply and/or the load system

having single-phase (two wire) and three-phase (three wire or four wire) systems. There

are many nonlinear loads, such as domestic appliances, connected to single-phase supply

systems. Some three-phase nonlinear loads are without neutral, such as ASD’s, fed from

three-wire supply systems. There are many nonlinear single-phase loads distributed on

four-wire three-phase supply systems, such as computers, commercial lighting, etc.

Hence, AF’s may also be classified accordingly as two-wire, three-wire, and four-wire

types.

33

1)Two-Wire AF’s:

Two-wire (single phase) AF’s are used in all three modes as active series, active

shunt, and a combination of both as unified line conditioners. Both converter

configurations, current-source PWM bridge with inductive energy storage element

and voltage-source PWM bridge with capacitive dc-bus energy storage elements, are

used to form two-wire AF circuits. In some cases, active filtering is included in the

power conversion stage to improve input characteristics at the supply end.

2)Three-Wire AF’s:Three-phase three-wire nonlinear loads, such as ASD’s, are major applications of

solid-state power converters and, lately, many ASD’s, etc., incorporate AF’s in their

front-end design. A large number of publications have appeared on three-wire AF’s

with different configurations. All the configurations shown in Figs 3.1–3.6 are

developed, in three-wire AF’s, with three wires on the ac side and two wires on the dc

side. Active shunt AF’s are developed in the current-fed type (Fig 3.1) or voltage-fed

type with single-stage (Fig 3.2) or multi-step/multilevel and multi-series

configurations. Active shunt AF’s are also designed with three single-phase AF’s

with isolation transformers [18] for proper voltage matching, independent phase

control, and reliable compensation with unbalanced systems. Active series filters are

developed for stand-alone mode (Fig 3.4) or hybrid mode with passive shunt filters

(Fig 3.5). The latter (hybrid) has become quite popular to reduce the size of power

devices and cost of the overall system. A combination of active series and active

shunt is used for unified power quality conditioners (Fig 3.6) and universal filters.

3) Four-Wire AF’s:

A large number of single-phase loads may be supplied from three-phase mains with

neutral conductor. They cause excessive neutral current, harmonic and reactive power

burden, and unbalance. To reduce these problems, four-wire AF’s have been

attempted. They have been developed as: 1) active shunt mode with current feed and

voltage feed; 2) active series mode; and 3) hybrid form with active series and passive

shunt mode.

34

3.2.4 Compensated Variable Based Classification

(1) Harmonic Compensation

(2) Multiple Compensation

This is the most important system parameter requiring compensation in power

systems and it is subdivided into voltage- and current-harmonic compensation. The

compensation of voltage and current harmonics is interrelated.

Different combinations of the above systems can be used to improve the effectiveness of

filters. The following are the most frequently used combinations.

· Harmonic currents with Reactive power compensation.

· Harmonic voltages with Reactive power compensation.

· Harmonic currents and voltages.

· Harmonic currents and voltages with reactive-power compensation.

3.2.5 Voltage Type Vs Current Type APF’s

A clear trend for preferred type of APF’s does not exist. A choice depends on

source of distortion at the specified bus, equipment cost, and amount of correction

desired.

Voltage-type has an advantage in that they can be readily expanded in parallel

to increase their combined rating. Their combined switching rate can be increased if they

are carefully controlled so that their individual switching times do not coincide.

Therefore, using parallel voltage-type converters with out increasing individual converter

switching rates can eliminate higher order harmonics. Voltage type converters are lighter

and less expansive than current-type converters.

The main drawback of voltage-type converters lies in the increased complexity of

their control system. For systems with several connected in parallel, this complexity is

greatly increased.

Current-type converters have advantages of excellent current controllability, easy

protection and high reliability over Voltage source APF. More over CSI topology has

superior characteristics compared to VSI topology in terms of direct injected current,

which result in a faster response in time varying load environment and lower dc energy

35

storage requirement. The drawback of the current source APF is larger power losses of

the dc-link inductor. However, the current-type active power filter will become more

attractive when the super conducting coils are available in the future. Losses are less

important in low- power applications but very important in high power applications.

Since they are easily expandable, voltage type APF’s are likely to be used for

network wide compensation. Current type APF’s will continue to popular for single-node

distortion problems. In other words, electric utility interest will likely to be focused on

voltage type converters, while industrial users likely to use both type of converters.

3.3 Operation of Three Phase Active Power Filters

In recent years, the power quality of the AC main system has become a great

concern due to the rapidly increased number of electronic equipment. In order to reduce

the harmonic contamination in power lines and improve the transmission efficiency

Active power filters become essential. A current source is connected in parallel with

nonlinear load and controlled to generate the harmonic currents needed for the load.

The basic configuration of a three-phase three-wire active power filter is shown in

Fig 3.7. The diode bridge rectifier is used as an ideal harmonic generator to study the

performance of the Active filter. The current-controlled voltage-source inverter (VSI) is

shown connected at the load end. This PWM inverter consists of six switches with

antiparallels diode across each switch. The capacitor is designed in order to provide DC

voltage with acceptable ripples. In order to assure the filter current at any instant, the DC

voltage Vdc must be at least equal to 3/2 of the peak value of the line AC mains voltage.

36

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

Three aspects have to be considered in the design of APF.

a) The parameters of the inverter such as inverter switches and the values of the link

inductances.

b) Modulation method used and

c) The control method used to generate the harmonic reference template.

3.3.1 Sample and Hold circuit’s method for harmonic reference template

This method is simple, eliminates complicated transformations and mathematical

operations such as multiplications and divisions, and permits good transient response. Fig

3.8 shows implementation of Sample and Hold method.

37

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

The current in each phase of the load is filtered to get the fundamental phase

current. A "Sample and Hold" circuit, synchronized with the peak value of the phase-to-

neutral voltage, allows to get three dc signals, which are proportional to the amplitude of

the active component of the current for each phase. Three dc signals, with the information

of the total active power in the load, are averaged to balance the system. Then, by

multiplying the averaged dc signal for a set of balanced reference waveforms (in phase

with the mains voltages), three in phase balanced currents for each phase are obtained.

Finally, these currents are subtracted from the real load currents to get the compensation

currents. These harmonic are then able to correct the harmonic distortion, the power

factor and the unbalances of the load.

Let to assume that IL is the total load current in one phase. This current contains

basically three components.

IL = IP + IQ + IH

Where IP, IQ and IH are the fundamental active, reactive and harmonic currents

respectively. The APF will eliminate IQ and IH by subtracting IP from IL.

Extraction of IP

First the load currents sensed and filtered to eliminate the (I H) and then the total

fundamental currents (one for each phase) are obtained. These currents have to be

separated in their active and reactive components.

I = IP + IQ

38

Where

IP = I cos

However the angle "" does need to be known, because the term "I cos" can be

obtained from the time function of the fundamental when the main voltage reaches the its

maximum value. Fig 3.9 explains graphically the idea.

Fig 3.9 Method used to capture IP.

IP is captured and "stored" until the next sample of IP is obtained to replace the

old one. This action is executed with the help of "Sample and Hold" circuits, which are

synchronized with the synchronization pulses to trigger the S&H are generated through

the "zero-crossing" signals, obtained from the set of "in-quadrature voltages". These "in-

quadrature voltages" are generated in the control block with the DZ0 connection, signal

transformer.

The control circuit is also has the capability to avoid flickers and transient

phenomena in the source, produced by sudden changes in load current. To do this, control

system makes soft variation of IP during these moments. However, this action will require

having the energy storage components in the APF. Hence the design of the control

system has to take in account the characteristics of the power filter.

39

DSTATCOM

40

DSTATCOM

4.1 Introduction

Modern power systems are complex networks, where hundreds of generating

stations and thousands of load centers are interconnected through long power

transmission and distribution networks. Even though the power generation in most

countries is fairly reliable, the quality of power is not so reliable. Power distribution

system should provide their customers with an uninterrupted flow of energy at smooth

sinusoidal voltage at the contracted magnitude level and frequency. Power system

especially distribution systems have numerous non linear loads, which significantly affect

the quality of power supplies. This ends up producing many power quality problems.

Apart from non linear loads events like capacitor switching, motor starting and unusual

faults could also inflict power quality problems.

Power quality problem is defined as any manifested problem in voltage /current or

leading to frequency deviations that result in failure or mis-operation of customer

equipment. Voltage sags and swells are among the many power quality problems the

industrial process has to face. Voltage sags are most severe.

During the past few decades, power industries has proved that adverse impacts on

the power quality can be mitigated or avoided by conventional means, and that

techniques using fast controlled force commutated power electronics are even more

effective. Power quality compensators can be categorized into two main types. One is

shunt connected compensation device, which has dominates the most common

application-power harmonics elimination. The other is the series connected type, which

has an edge over the shunt type for correcting the distorted system side voltage and

voltage sags caused by power transmission system faults.

The DSTATCOM is a power quality device, which can protect these industries

against the sags and swells. Usually sags and swells are related to remote faults. A

DSTATCOM compensates for these voltage disturbances provided that supply grid does

41

not get disconnected entirely through breaker trips. It can exchange both active and

reactive power with the distribution system by varying the amplitude and phase angle of

the converter voltage with respect to the line terminal voltage.

4.2 Principle and Operation of DSTATCOM

A D-STATCOM (Distribution Static Compensator), which is schematically

depicted in Figure 4.1, 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 system through the reactance of the coupling transformer.

Suitable adjustment of the phase and magnitude of the D-STATCOM 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; and

3. Elimination of current harmonics.

Here, such device is employed to provide continuous voltage regulation using an

indirectly controlled converter.

Fig. 4.1 Schematic Diagram of a DSTATCOM

42

Figure-4.1 the shunt injected current Ish corrects the voltage sag by adjusting the voltage

drop across the system impedance Zth. The value of Ish can be controlled by adjusting

the output voltage of the converter. The shunt injected current Ish can be written as,

Ish = IL – Is = IL – (Vth – VL)/ Zth -------- 4.1

------- 4.2

The complex power injection of the D-STATCOM can be expressed as,

------- 4.3

It may be mentioned that the effectiveness of the D-STATCOM in correcting voltage sag

depends on the value of Zth or fault level of the load bus. When the shunt injected current

Ish 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 Ish is

minimized, the same voltage correction can be achieved with minimum apparent power

injection into the system.

4.2.1 Voltage Source Converter

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. The converter is normally based on some kind of energy

storage, which will supply the converter with a DC voltage. The solid-state electronics in

the converter is then switched to get the desired output voltage. Normally the VSC is not

only used for voltage dip mitigation, but also for other power quality issues, e.g. flicker

and harmonics.

4.3 Control Scheme for the DSTACOM The aim of the control scheme is to maintain constant voltage

magnitude at the point where a sensitive load is connected, under system disturbances.

43

The control system only measures the r.m.s voltage at the load point, i.e., no reactive

power measurements are required. The VSC switching strategy is based on a sinusoidal

PWM technique which offers simplicity and good response. Since custom power is a

relatively low-power application, PWM methods offer a more flexible option than the

Fundamental Frequency Switching (FFS) methods favored in FACTS applications.

Besides, high switching frequencies can be used to improve on the efficiency of the

converter, without incurring significant switching losses.

The controller input is an error signal obtained from the reference

voltage and the value rms of the terminal voltage measured. Such error is processed by a

PI controller the output is the angle δ, which is provided to the PWM signal generator. It

is important to note that in this case, indirectly controlled converter, there is active and

reactive power exchange with the network simultaneously: an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point. The PI

controller process the error signal generates the required angle to drive the error to zero,

i.e., the load rms voltage is brought back to the reference voltage.

Fig. 4.2 Indirect PI controller

The sinusoidal signal Vcontrol is phase-modulated by means of the angle .

i.e., VA = Sin (ωt +δ), --------- 4.4

VB= Sin(ωt+δ-2π/3), ---------- 4.5

VC = Sin (ωt +δ+2π/3). --------- 4.6

44

Fig. 4.3 Phase-Modulation of the control signal

The modulated signal Vcontrol is compared against a triangular signal in order to

generate the switching signals for the VSC valves. The main parameters of the sinusoidal

PWM scheme are the amplitude modulation index of signal, and the frequency

modulation index of the triangular signal. The amplitude index is kept fixed at 1 pu, in

order to obtain the highest fundamental voltage component at the controller output.

----------- 4.7

is the peak amplitude of control signal,

is the peak amplitude of triangular signal.

The switching frequency is set at 1080 Hz. The frequency modulation index is given by,

mf = fs/f1= 1080/60 = 18

Where f1 is the fundamental frequency.

The modulating angle is applied to the PWM generators in phase A. The

angles for phases B and C are shifted by 240 degrees and 120 degrees, respectively. It can

be seen in that the control implementation is kept very simple by using only voltage

measurements as the feedback variable in the control scheme. The speed of response and

robustness of the control scheme are clearly shown in the simulation results.

45

4.4 Phasor diagram of DSTATCOM

Fig 4.4 Phasor diagram for shunt voltage controller

Solid Lines Without phase angle jump

Dashed Lines with phase angle jump

The large increase in active power injected with increasing phase-angle jump is explained

in fig 4.4. The injected voltage is the required voltage rise at the load due to the injection

of a current into the source impedance. This injected voltage is the difference between the

normal operating voltage and the sag voltage as it would be without controller. The

injected current is the injected voltage divided by the source impedance. In phasor terms,

46

the argument (angle, Direction) of the injected current is the argument of the injected

voltage minus the argument of the source impedance. The source impedance is normally

mainly reactive. Incase of sag without phase-angle jump, the injected current is also

mainly reactive. A phase-angle jump causes a rotation of the injected voltage as indicated

in the fig 4.4. This leads to a rotation of the injected current away from the imaginary

axis. From the fig 4.4 it becomes obvious that this will quickly cause a serious increase in

the active part of the current (i.e. the projection of the current on the load voltage). The

change in the reactive part of the current is small, so is the change in current magnitude.

4.5 Mathematical Modeling

The load voltage during the sag can be seen as the superposition of the voltage

due to the system and the voltage change due to the controller. The former is the voltage

as it would have been without a controller. The former is the injected current.

Assume that the voltage with out controller is

Vsag = V cosӨ + jV sinӨ ---------- 4.8

The load voltage is again equal to 1 pu:

Vload = 1+j0 ---------- 4.9

The required change in voltage due to the injected current is the difference between the

load voltage and the sag voltage:

∆V = 1- V cosӨ - jV sinӨ ----------- 4.10

The change in voltage must be obtained by injecting a current equal to

I cont = P – jQ ------------ 4.11

With P the active power and Q the reactive power injected by the controller. The active

power will determine the requirements for energy storage. Let the impedance seen by the

shunt controller (Source impedance in parallel with load impedance) be equal to

Z = R+ jX ------------- 4.12

The effect of the injected current is a change in voltage according to

47

∆V = Icont Z = (R + jX)(P – jQ) ----------- 4.13

The required voltage increase (4.10) and the achieved increase (4.13) have to be equal.

This gives the fallowing expression for the injected complex power:

P - jQ = (1-VcosӨ - jVsinӨ) ------------ 4.14

R + jX

Splitting the complex power in a real and an imaginary part, gives expressions for active

and reactive power:

P = R(1 – V cosӨ) - VXsinӨ -------------- 4.15

R2 + X2

Q = RV sinӨ + X (1-V cosӨ) -------------- 4.16

R2 + X2

The Main limitation of the shunt controller is that the source impedance

becomes very small for faults at the same voltage level close to the load. Mitigting such

Sags through a shunt controller is impractical as it would require very large currents. We

therefore only consider faults upstream of the supply transformer. The minimum value of

the source impedance is the transformer impedance.

The current rating of the controller is determined by both active and reactive

power. From 4.15 and 4.16 we find for the absolute value of the injected current:

I cont = ((1-2VcosӨ+V2)/(R2+X2)). ------------ 4.17

48

MATLAB SIMULATION

49

MATLAB Simulation

5.1 Introduction

MATLAB® is a high-level technical computing language and interactive

environment for algorithm development, data visualization, data analysis, and numeric

computation. Using the MATLAB product, you can solve technical computing problems

faster than with traditional programming languages, such as C, C++, and FORTRAN.

You can use MATLAB in a wide range of applications, including signal and

image processing, communications, control design, test and measurement, financial

modeling and analysis, and computational biology. Add-on toolboxes (collections of

special-purpose MATLAB functions, available separately) extend the MATLAB

environment to solve particular classes of problems in these application areas.

MATLAB provides a number of features for documenting and sharing your

work. You can integrate your MATLAB code with other languages and applications, and

distribute your MATLAB algorithms and applications.

Key Features

High-level language for technical computing

Development environment for managing code, files, and data

Interactive tools for iterative exploration, design, and problem solving

Mathematical functions for linear algebra, statistics, Fourier analysis, filtering,

optimization, and numerical integration.

2-D and 3-D graphics functions for visualizing data

50

Tools for building custom graphical user interfaces

Functions for integrating MATLAB based algorithms with external applications

and languages, such as C, C++, FORTRAN, Java, COM, and Microsoft Excel.

5.2 Proposed Circuit Single Line Diagram

The below figure shows the test system used to carry out the various DSTATCOM

simulations.

Fig 5.1 single line diagram of the test system for DSTATCOM

Where Vs = 3-ph Voltage generator,

Zs = Source Impedance,

Vdc = DC voltage Source,

1,2 and 3 are known as feeders or buses to which we connect tharious electric

Equipments,

L1, L2 are the loads connected to the system,

S1 is the switch

Switch S1 is used to attain various situations in the power system, i.e. sags, swells.

In the actual practice we are using CB(Circuit breaker instead of switch), by varying the intial

positions of the CB . We are performing the tests by using DSTATCOM.

51

5.3 Circuit Description

Fig 5.2 Simulink model of D-STATCOM test system

52

Figure 5.2 shows the basic simulink model of D-STATCOM test system

which consists of 3-phase source it produce required 3-ph voltage for supplying to loads

which is having ph to ph 230 KV with phase angle of phase A is 0 deg and frequency 0

Hz and this generator internally connected as Yn, with source resistance(RS) 0.1 ohms

and source inductance 0.758H . Here the loads are also 3-ph series RL loads where we

neglect the capacitance (3-ph Source Function Block shown in fig 5.2.1).

Fig 5.2.1 3-phase Functional Block

DSTATCOM is the static var generator connecter in shunt to the line to

mitigate the faults that are occurring in power system in the distribution side. This is

connected through CB which is normally in OPEN position, when the fault occurs at that

53

time this CB closes and DSTATCOM comes into picture it mitigates the losses that are

occurred.

DSTATCOM consists of a DC voltage source of 19.5KV magnitude,

breakers normally in closed position, Universal bridge used to convert the DC stored

energy into 3-ph AC voltage and this is helpful to mitigate the faults occurred in the

Power system. And it consists of CONTROLLER, Discrete PWM generator and Delay

unit. The controller circuit also consist a trigger circuit. Figure 5.2.2 showa alla these

blocks that are in the DSTATCOM.

Fig. 5.2.2 Simulink model of D-STATCOM

54

Fig. 5.2.3 Simulink model of CONTROLLER

This CONTROLLER model consists of a 3-ph sequence analyzer which takes

instantaneous value as input and converts it into magnitude and phase parameters. The

phase parameter is terminated and the magnitude parameter is compared with the

constant signal whose p.u value is one. If there is any difference in this compared value,

it is given to the PI (proportional Integrator) and the output of this PI block is used as a

input to the TRIGGER and its output is used to activate PWM generator and finally it is

given to the UNIVERSAL bridge through unit delay block.

Fig. 5.2.4 TRIGGER BLOCK Simulation Model

The 3-ph series RL load is the general load that is connected to the supply

mains through 3-ph transformer. Among the two loads connected to the supply one is

connected through the 3-ph breaker which is may be closed/opened depends upon the

fault requirement i.e. SWELL / SAG. The Functional blocks that are used in this

simulation model are shown below.

55

Fig 5.2.5 Functional Block of Discrete PI Controller

56

Fig 5.2.6 Functional Block of 3-ph Breaker

Fig 5.2.7 Functional Block of DC Voltage Source

57

Fig 5.2.8 Functional Block of #-ph Sequence Analyzer

Fig 5.2.9 Functional Block of Unit Delay

58

Fig 5.2.10 Functional Block of 3-ph Transformer

59

Fig 5.2.11 Functional Block of Breaker

60

Fig 5.2.12 Functional Block of Discrete PWM Generator

61

Fig 5.2.13 Functional Block of Universal Bridge

62

RESULTS & FUTURE SCOPE

Results & Future Scope

63

6.1Case Studies

6.1.1 Simulation results of voltage sag

The first simulation contains no DSTATCOM and the CB (Circuit

breaker) connected to the load is initially in open position. Suddenly the CB is closed

so extra load is connected to the power system. So more amount of current is drawn

as compared to earlier case and the voltage is dropped at the instant where the CB is

closed. In the functional block of this CB already time is mentioned as 0.5ms to

0.9ms so during this period the CB is operated that is CB turned into CLOSING

position. Like this in the period specified earlier voltage drop takes place.

Fig 6.1 Simulation model for SAG Formation

64

In this case we are not connected DSTATCOM so the drop (Voltage SAG) is not

mitigated. This can be observed by seeing the scope of the Magnitude at 3-ph sequence

analyzer. We are observing a drop in the time period 0.5ms to 0.9 ms. And the graph is

like this

Fig 6.2 Voltage Vrms at the load point without DSTATCOM

If we are using DSTATCOM and connected this as shown in the fig 6.3 and carried out

the simulation again, we are observing the wave form as shown in the fig 6.4, where the

actual fault that is appeared in between the time specifications was vanished because of

the mitigation done by the DSTATCOM.

65

Fig 6.3 Simulation Model for SAG Mitigation

66

Fig 6.4 Voltage Vrms at the load point with DSTATCOM energy

storage of 20.9 KV.

67

6.1.2 Simulation results of voltage Swell

The first simulation contains no DSTATCOM and the CB (Circuit breaker) connected to

the load is initially in closed position. Suddenly the CB is opened so the load is removed

from the power system. So the amount of current drawn is less as compared to earlier

case and the voltage is increased at the instant where the CB is opened. In the functional

block of this CB already time is mentioned as 0.5ms to 0.9ms so during this period the

CB is operated that is CB turned into OPENING position. Like this in the period

specified earlier voltage Swell takes place.

Fig 6.5 Simulation model for SWELL Formation

68

In this case we are not connected DSTATCOM so the SWELL is not mitigated. This can

be observed by seeing the scope of the Magnitude at 3-ph sequence analyzer. We are

observing a drop in the time period 0.5ms to 0.9ms. And the graph is like this

Fig 6.6 Voltage Vrms at the Load point without DSTATCOM

If we are using DSTATCOM and connected this as shown in the fig 6.7 and carried out

the simulation again, we are observing the wave form as shown in the fig 6.8, where the

actual fault that is appeared in between the time specifications was vanished because of

the mitigation done by the DSTATCOM.

69

Fig 6.7 Simulation model for SWELL mitigation

70

Fig 6.8 voltage Vrms at the load point with DSTATCOM with energy storage of 16.8KV.

6.1.3 Simulation results of Voltage Interruption

The first simulation simulation contains no D-STATCOM and three phase fault is applied

via fault resistance of 0.001 OHM, during the period 500-900ms. The voltage at the load

point is 0%. With respect to the reference voltage is shown in fig 6.10. Similarly, a new

set of simulations was carried out but now with the D-STATCOM connected to the

system, the load voltage is shown in fig 6.12.

71

Fig 6.9 Simulation model for voltage interruption formation

72

Fig 6.10 Voltage Vrms at the Load point without DSTATCOM

73

Fig 6.11 Simulation model for Voltage interruption mitigation

74

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

6.2 Conclusion

This paper has presented the power quality problems such as voltage dips,

swells and interruptions, consequences, and mitigation techniques of custom power

electronic device D-STATCOM. The design and applications of D-STATCOM for

voltage sags, interruptions ands swells, and comprehensive results are presented.

75

A new PWM-based control scheme has been implemented to control the

electronic valves in the two-level VSC used in the D-STATCOM. As opposed to

fundamental frequency switching schemes already available in the MATLAB/

SIMULINK, this PWM control scheme only requires voltage measurements. This

characteristic makes it ideally suitable for low-voltage custom power applications. It was

also observed that the capacity for power compensation and voltage regulation of D-

STATCOM depends on the rating of the dc storage device.

6.3 Future Scope

In this paper we are discussing the various problems that are occurring in

the power system i.e. voltage sags, swells and voltage interruption. To solve these

problems we are using D-STATCOM modeling but in this process we are unable to

compensate the faults up to maximum level. There is some disturbances in the final

output and we are vary the DC energy storage in the D-STATCOM time to time for

different faults.

So, we need a new method in which the faults are compensated

completely i.e we get 1 pu value as the output after using the compensated device. And

the internal control voltage setting is should be changed by itself such that it compensates

all the faults that are occurring in the power system, for that we are developing a concept

of UPQC. In this the input voltage is taken as reference, and it is tested with the fault

voltage, the amount of voltage should be compensated is calculated time to time.

76

REFERENCES

77

References

Papers

[1] G. Yaleinkaya, M.H.J. Bollen, P.A. Crossley, “Characterization of voltage sags in

industrial distribution systems”, IEEE transactions on industry applications, vol.34, no. 4,

July/August, pp. 682-688, 1999.

[2] Haque, M.H., “Compensation of distribution system voltage sag by DVR and D-

STATCOM”, Power Tech Proceedings, 2001 IEEE Porto, vol.1, pp.10-13, Sept. 2001.

[3] Anaya-Lara O, Acha E., “Modeling and analysis of custom power systems by

PSCAD/EMTDC”, IEEE Transactions on Power Delivery, Vol.17, Issue: 1, Jan. 2002,

Pages:266 – 272

[4] Bollen, M.H.J.,” Voltage sags in three-phase systems” Power Engineering Review,

IEEE, Vol. 21, Issue: 9, Sept. 2001, pp: 8 - 11, 15.

[5] M.Madrigal, E.Acha., “Modelling of Custom Power Equipment Using Harmonic

Domain Techniques”, IEEE 2000.

[6] R.Mienski,R.Pawelek and I.Wasiak., “Shunt Compensation for Power Quality

Improvement Using a STATCOM controller: Modelling and Simulation”, IEEE Proce.,

Vol.151, No.2, March 2004.

Websites

www.poweronline.com

www.sandc.com

www.powerquality.com

www.mathworks.com

Books

1) Flexible AC Transmission Systems by HingoRani

78

2) Understanding Power Quality Problems by Math H.J. Bollen