voltage regulation using deltapwminverter fedseriescompensator

Voltage Regulation UsingDelta PWM Inverter Fed Series Compensator

A thesis submitted to the Department of Electrical and Electronics Engineering of

Bangladesh University of Engineering and Technology (BUET) Dhaka, Bangladesh in

partial fulfillment of the requirements for the degree of Master of Science in Electrical

and Electronics Engineering

Amzad Ali Sarkar (Roll 04030620 1P)

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Department of Electrical and Electronic EngineeringBangladesh University of Engineering and Technology, Dhaka

..February 2008

ii

"

Dedication

iii

To my beloved parents

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Declaration

"'This is to certify that this thesis is the outcome of the original work of the

undersigned student. No part of this work has been submitted elsewhere partially or fully

for the award of any degree or diploma" Any material reproduced in this thesis has been

properly acknowledged.

~ ..-------------- :_--------------( Amzad Ali Sarkar )

Roll 040306201 P

iv

The thesis titled "Voltage Regulation Using Delta PWM Inverter Fed SericsCompcnsator." submitted by Amzad Ali Sarkar Roll No. 040306201 Pol' EEE, SUET,session April 2003, has been accepted as satisfactory in partial fulfillment of therequirements for the degree of Master of Science in Electrical and Electronic Engineeringon February 27, 2008

BOARD OF EXAMINERS

.'}p .. 'uJ-uO(Prof. Dr. Mohammad Ali Choudhury)ProfessorDepartment of Electrical andElectronic Engineering, BUETDhaka-l 000, Bangladesh

(Prof. Dr. Mohammad Ali Choudhury)Professor & HeadDepartment of Electrical andElectronic Engineering, BUETDhaka-l 000, Bangladesh

~MH43 ~.olf............. ~ :..:..;-: .

(Prof. Dr. Saiful Islam)ProfessorDepartment of Electrical andElectronic Engineering, BUETDhaka-lOaD, Bangladesh

( Prof. Dr. Md. Bashir Uddin)ProfessorDepartment of Electrical andElectronic Engineering, DUETDhaka, Bangladesh.

v

(Supervisor)

(Ex-Officio)

Chairman

Member

Member

Member (External)

Acknowledgement

I would like to express my sincerest gratitude to my superVIsor, Prof. Dr.

Mohammad Ali Choudhury, Professor, Department of Electrical and Electronics

Engineering of Bangladesh University of Engineering and Technology, Dhaka

Bangladesh, for his friendly supervision, constructive suggestions and constant support

during this research.

I am thankful to him for introducing me to the area of power electronics.

I would like to thank Prof. Dr. Satya Prashad Majumder, Head and Professor,

Department of EEE, BUET, for his kind support for fulfilling my thesis.

My sincerest gratitude goes to Prof. Dr. Saiful Islam, Professor Department of

EEE, BUET, who encouraged and supported my research work and academic activities

throughout.

I am also grateful to Professor Prof. Dr. Md. Bashiruddin, for spending his

valuable time to make this work a successful.

I shall remain ever grateful to those teachers and friends who gave me very

valuable suggestion when I was in the middle of this research work.

vi

Abstract

In this thesis a method of generation of PWM signal, called Delta PWM has been

proposed amongst the many existing complicated methods to generate the same for use in

series voltage regulation of utility supply system. Using this Inverter in series, the line

voltage compensation in a supply system can be achieved in a relatively simpler way.

The delta modulation scheme is a self carrier generating and the carrier is stable which is

important in the designing of the filter of the rectifier inverter system used in series

compensation system.

A simple control technique for the senes voltage compensation scheme has been

proposed in this thesis. The scheme is based on phase addition and determination of

required phase angle is carried out using analogue feedback control circuit.

A microcontroller based system may be engaged to generate the delta PWM signal

directly shifted as required. Method of analogue computation is discussed. A theoretical

approach leading to the formulation of algorithm applicable for microcontroller based

PWM signal generation has been presented.

Simulation was carried out using both the series and parallel filters separately and the

results were presented. The Fourier analysis was carried out to observe the amplitudes of

all the harmonics in each case.

The results of both the series and parallel filter for eliminating of harmonics have been

studied.

This work provides an easy method of voltage regulations of a utility power supply with

improved performance, easy implementation and controls. It will also ensure reliable

operation with fast dynamic and steady state response of the control scheme and power

circuits for phasor addition of voltage are almost instantaneous operating in nature.

vii

Contents

Declaration

Acknowledgement

Abstract

Contents

List of figures

List of tables

List of symbols

List of Abbreviations

Chapter 1. Introduction

1.1 Introduction

1.2 Literature Review

1.2.1Voltage SAG Compensation

1.2.2 Reactive Power Compensation Principles

1.2.3 VAR Generators

1.2.4 Thyristorized VAR Compensators

1.2.5 Self -commuted VAR Compensators

1.2.6 New VAR Compensator Technology

1.3 Objective of the Thesis

IA Thesis outline

Chapter 2. Proposed Voltage compensation System

2.1 Introduction

2.2 System Topology

2.3 Basic Blocks of the Proposed Series Voltage Compensation System

2.4 Summary

viii

Page No.

IV

VI

VII

IX

XI

xv

XVI

XVII

2

2

4:-;:',

5

6

9

1 1

14

15

16

16

18

34

p••••

Chapter 3 System analysis and results

3.1 Introduction

3.2 Results of Phase Addition and

3.2. i Higher Line Resistance

3.2.2 System with variation in Line Voltage

3.3 Implementation

3.3.1 Rectifier

3.3.2 Inverter and Delta Modulation

3.3.3 The Filter

3.3.4 The Coupling Transformer

3.4 Phase Contro I

3.5 Results

3.5.1 Results for Series Filter

3.5.2 Results for Parallel Filter.

3.6 Filtering Harmonics

3.7 Summary

Chapter 4 Summary and Conclusions

4.1 Introduction

4.2 Conclusions

4.3 Recommendations for Future Works

References

ix

35

3636

37

40

4040424243

43

43

54

64

64

65

65

66

67

Figure Number.

List of Fig" res

Description of the figure Page Number.

Figure 2.1 The topology of the proposed line voltage compensating system. 17

Figure 2.2 The frequency response of the filter along with the load resistor. 19

Figure 2.3 The diagram of a coupling Transformer and its frequency response. 19

Figure 2.4 Simple series filter. 20

Figure 2.5 Frequency response of a series resonance filter. 20

Figure 2.6 Parallel stop-band filter. 21

Figure 2.7 Frequency responses of parallel filters. 21

Figure 2.8 A scheme of obtaining dc from ac source 22

Figure 2.9 Circuit diagram of the PWM Inverter 23

Figure 2.10 Output waveforms of the PWM inverter. 23

Figure 2. liThe Fourier analysis of the inverter output 24

Figure 2.12 The circuit diagram of the Delta PWM Modulator 26

Figure 2.13 Output waveforms of the Delta PWM Modulator 26

Figure 2.14 Fourier analysis of the output waveforms shows that

the carrier frequency lies within a spectrum 5.5 -6.5 KHz. 27

Figure 2. I 5 The analogue implementation of Line voltage sensing and

reference voltage generation 28

Figure 2.16 The triangular waveform oscillates between upper windows

and the lower window cutting the reference voltage at /i 29

Figure 2.17 Phasor addition, parallelogram theory 32

Figure 3.1 The voltage on the load resistance varies from 275 V to 200 V

as the load resistance changes from ssn to Isn corresponding

to SA and 20 A loads. 36

Figure 3.2 A system that supplies line voltage in range IISV to 345 V. 37

Figure 3.3 Line voltage VI is compensated by a fixed amplitude voltage V2

to obtain a rated output voltage in the system. 37

x

Figure Number. Description of the figure Page Number.

Figure 3.4 Circuit diagram for simulation and analysis using series filter. 38

Figure 3.5 Circuit diagram for simulation and analysis using parallel filter. 39

Figure3.6 The reference sinusoidal waveform at the input of the Delta PWM and the

generated rectangular waveform available at the output of the modulator.

This pulse has peak to peak amplitude 30V. and it is'" 15V 41

Figure 3.7 Waveforms in the compensation when line voltage is 115V (rms)

a) Available line voltage 115 V (172.5 peaks)

b) Corresponding reference voltage of the modulator (e = 90°)

c) Output waveform of the inverter (e = 0°)d) Spectrum of the compensated voltage. 44

Figure 3.8 Waveforms of the compensation when line voltage is 115V (rms)

a) The compensating series voltage of constant magnitude (series filtered)

of the inverter for angle addition. (e = 0°)b) The compensated output voltage 230V

c) Spectrum of the compensated voltage. 45

Figure 3.9 Waveforms in the compensation when line voltage is 172.5V (rms)

a) Available line voltage 172.5V (258.75V peaks)

b) Corresponding reference voltage of the modulator (e = 1650)

c) Output waveform of the inverter (e = 75.6°) 46

Figure 3.10 Waveforms of the compensation when line voltage is 172.5V (rms)


of the inverter for angle addition (e = 75.6°).b) The compensated output voltage 230V


•

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a) Available line voltage 230V (475V peaks)

b) Corresponding reference voltage of the modulator (e = 194.5°)




of the inverter for angle addition (e = 104.5°)b) The compensated output voltage 230V



a) Available line voltage 287V (431 .25V peaks)

b) Corresponding reference voltage of the modulator (e = 220.50)

c) Output waveform of the inverter (e = 130.53°) SO



of the inverter for angle addition (e = 130.53°).b) The compensated output voltage 230V

c) Spectrum of the compensated voltage. 5 I


a) Available line voltage 345V (517.5V peaks)


c) Output waveform of the inverter (e = 180°) 52



of the inverter for angle addition (e = 180°).b) The compensated output voltage 230V


xii

Figure Number. Description ofthe figure Page Number.


a) Available line voltage 115 V (172.5 peaks)




a) The compensating series voltage of constant magnitude (parallel

filtered) ofthe inverter for angle addition (e = 0°)b) The compensated output voltage 230V


Figure 3.19 Waveforms in the compensation when line voltage is 172.5V (rms)




Figure 3.20 Waveforms ofthe compensation when line voltage is 172.5V (rms)

a) ) The compensating series voltage of constant magnitude (parallel

filtered) of the inverter for angle addition (e = 75.7°).b) The compensated output voltage 230V





c) Output waveform of the inverter (e = 104.5° ) 58



filtered) of the inverter for angle addition (e = 104.5°).b) The compensated output voltage 230V

c) Spectrum of the compensated voltage. 59"

xiii


Figure 3.23 Waveforms in the compensation when line voltage is 287v (nns)

a) Available line voltage 287v (431.25v peaks)



Figure 3.24 Waveforms of the compensation when line voltage is 287v (nns)


filtered) of the inverter for angle addition (e = 130.53°).b) The compensated output voltage 230v


Figure 3. 25 Waveforms in the compensation when line voltage is 345v (rms)

a) Available line voltage 345v (517.5v peaks)



Figure 3.26 Waveforms of the compensation when line voltage is 345v (rms)


filtered) ofthe inverter for angle addition (e = 180°).b) The compensated output voltage 230v


xiv

Table No.

List of Ta bles.

Description of the Table. Page No.

Table.l:

Table.2:

Table.3:

The amplitudes of prominent lower and higher harmonics

ofthe PWM Inverter. 24

Comparison of the amplitudes of different harmonics

component at the output of Delta PWM modulator. 27

Calculated phase angle for different line voltage values 34

xv

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Symbol

+8

8"

t,

Vc

VL

VRC.V

(J

List of Sym boIs

Description of the symbol.

Inverter DC Voltage

Carrier frequency.

Rising Slope of the triangular waveform

Falling Slope of the triangular waveform

ith switching of the modulator.

(i + l)th switching of the modulator.

Compensating AC voltage

Available Line Voltage

Regulated or compensated Line Voltage.

Window margin of the delta Modulator.

Angular velocity of the sinusoidal waveform.

Phase angle of the sinusoidal waveform

xvi

• < • .....,-; •• "

\ c"",

Abbreviation

ASD

DVR

HID

IGBT

IPFC

PLC

PWM

SMES

SPWM

SSSC

STATCOM

SVC

THD

TSC

TCR

TCSC

UPFC

UPS

VAR

VSC

List of Abbreviations

Description

Adjustable Speed Drive.

Dynamic Voltage Restorer.

High Intensity Discharge Lamp.

Isolated Gate Bipolar Transistor.

Interline Power Flow Controller.

Programmable Logic Controller

Pulse Width Modulation.

Super conducting Magnetic Energy Storage.

Sinusoidal Pulse Width Modulation

Static Synchronous Series Compensator

Static Synchronous Compensator.

Static VAR compensator

Total Harmonic Distortion

Thyristor Switched Capacitor.

Thyristor Controlled Reactor.

Thyristor Controlled Series Compensator.

Unified Power Flow Control.

Uninterruptible Power Supply.

Volt-Ampere (VA) Reactive.

Voltage-Sourced Converter.

xvii

Introduction

1.1 IntroductionThe electricity supply often exhibits imperfections. Increased automations In home and

factories have increased power quality deviations. Power quality has been defined as any

problem manifested in voltage, current, or frequency deviations that result in failure or mal-

operation of utility or end user equipment. Devices like surge arrestor, Filter, isolation

transformer, constant voltage transformer, Dynamic Voltage Restorer, Back-up Generator,

Humidity control, Energy Storage (Off-line, Line-interactive, On-line) are capable of

protecting against some effects of power quality deviations. One major power quality

deviation is voltage sags in utility lines. It has been reported that High Intensity Discharge

Lamps (HID) used for industrial illumination get extinguished at voltage dips of 20%. Also

critical industrial equipments like Programmable logic Controllers (PLCs) and Adjustable

Speed Drives (ASDs) are adversely effected by voltage dips of about 10%. Studies have

been carried out to estimate the loss incurred for voltage sags to calculate the investment for

the protective equipments to protect the user's sensitive equipments. In general the problem

of reactive power compensation is viewed from two aspects: load compensation and load

voltage support. Traditionally shunt capacitor was first employed for power factor correction

in 1914. Leading current drawn by the capacitors compensate the lagging current drawn by

the load. Among many traditional methods, fixed or mechanically switched capacitors,

synchronous condensers were used mostly. Semiconductor switching based thyristorized

VAR compensators has also been used. They use standard reactive power shunt element,

which are controlled to provide rapid and variable reactive power. The most effective

solution is the application of self-commuted converters as means of compensating reactive

power. It uses the semiconductor switching devices like IGBT for faster on and off\¥nd

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efficient control systems. Static Synchronous Compensator (ST ATCOM), Unified Power

Flow Controller (UPFC) and Dynamic Voltage Restorer have been designed and developed.

The converters use PWM techniques to generate patterns that produce energy equivalent to a

sinusoidal waveform. Solution approaches to the voltage disturbance problem, using active

devices, can involve either (i) a series injection of voltage, or (ii) a shunt injection of

reactive current. The Static VAR Compensator (SVC) and the STATCOM are compensation

device. This research deals with the compensation of voltage sags and swells that appears in

the grid line by static power converter in series with the utility line.

In theory installing an UPS is the easiest way to protect sensitive processes against sags.

Cost benefit analysis often shows that installing a UPS is too expensive. On the other hand

Dynamic Voltage Restorer (DVR) can be very cost effective, because it adds the missing

voltage through a transformer installed in series with load. The load remains connected to

the grid and the DVR calculates the missing part of the voltage waveform and corrects it.

Thus the volume of required energy storage is low. The self-commutated converter used in

this purpose may have no energy storage and may remain continuously on line. When a dip

occurs, the extra energy required to cOmpensate the difference voltage is drawn from the

supply as increased current. The self-commutated converter may have small energy storage

in batteries, which are also charged on line. This type of DVR can mitigate more dips than

one without energy storage. This research investigates the application of Delta Pulse Width

Modulation as a modulation technique for switching the compensating converter. Delta

PWM approximates sinusoidal waveform in a simple way and it needs fewer components to

design the modulator.

1.2 Literature ReviewA large number of related works have been done in the field of voltage compensation. A

brief discussion of some of these works is given in the following paragraphs.

1.2.1 Voltage SAG Compensation

VAR compensation is the management of reactive power to improve the perfonnance of an

ac power system. The concept of VAR compensation embraces a wide and diverse field of

both system and customer problems related with power quality issues. Most of power

quality problems can be attended or solved with an adequate control of reactive power [tW\ i\/

3

In general, the problem of reactive power compensation is viewed from two aspects: load

compensation and voltage support. In load compensation the objectives are to increase the

value of the system power factor, to balance the real power drawn from the ac supply,

compensate voltage regulation and to eliminate current harmonic components produced by

large and fluctuating nonlinear industrial loads [2], [3]. Voltage support is generally required

to reduce voltage fluctuation at. a given terminal of a transmission line. Reactive power

compensation in transmission systems also improves the stability of the ac system by

increasing the maximum active power that can be transmitted. It also helps to maintain a

substantially flat voltage profile at all levels of power transmission; it improves conversion

terminal performance, increases transmission efficiency, controls steady-state and temporary

over voltages [4], and can avoid blackouts [5], [6]. Series and shunt VAR compensation are

used to modify the natural electrical characteristics of ac power systems. Series

compensation modifies the transmission or distribution system parameters, while shunt

compensation changes the equivalent impedance of the load [I], [7]. In both cases, the

reactive power that flows through the system can be effectively controlled improving the

performance of the overall ac power system.

In the past, rotating synchronous condensers and fixed or mechanically switched capacitors

or inductors have been used for reactive power compensation. However, in recent years,

static VAR compensators employing thyristor switched capacitors and thyristor controlled

reactors to provide or absorb the required reactive power have been developed [7], [8], [9].

Also, the use of self-commutated PWM converters with an appropriate control scheme

permits the implementation of static compensators capable of generating or absorbing

reactive current components with a time response faster than the fundamental power

network cycle [10], [11], [12]. Based on the use of reliable high-speed power electronics,

powerful analytical tools, advanced control and microcomputer technologies, Flexible AC

Transmission Systems, also known as FACTS, have been developed and represent a new

concept for the operation of power transmission systems [13], [14].ln these systems, the use

of static VAR compensators with fast response times play an important role, allowing to

increase the amount of apparent power transfer through an existing line, close to its thermal

capacity, without compromising its stability limits. These opportunities arise through the

ability of special static VAR compensators to adjust the interrelated parameters that govern

(I'..

4

the operation of transmission systems, including shunt impedance, current, voltage, phase

angle and the damping of oscillations [IS].

1.2.2 Reactive Power Compensation Principles.

In a linear circuit, the reactive power is defined as the ac component of the instantaneous

power, with a frequency equal to 100 I 120 Hz in a 50 or 60 I-Iz system. The reactive power

generated by the ac power source is stored in a capacitor or a reactor during a quarter of a

cycle, and in the next quarter cycle is sent back to the power source. In other words, the

reactive power oscillates between the ac source and the capacitor or reactor, and also

between them, at a frequency equals to two times the rated frequency (50 or 60 Hz). For this

reason it can be compensated using VAR generators, avoiding its circulation between the

load (inductive or capacitive) and the source, and therefore improving voltage stability of

the power system. Reactive power compensation can be implemented with VAR generators

connected in parallel or in series.

Shunt Compensation [l6] [18] [20]

Most of the load being inductive, they requires reactive power for proper operation and

hence, the source must supply it, increasing the current from the generator and through

power lines. If reactive power is supplied near the load, the line current can be reduced or

minimized, reducing power losses and improving voltage regulation at the load terminals.

This can be done in three ways: a) with a capacitor, b) with a voltage source, or c) with a

current source. The main advantages of using voltage or current source VAR generators

(instead of inductors or capacitors) is that the reactive power generated is independent of the

voltage at the point of connection.

Series Compensation 14)

VAR compensation can also be of the series type. Typical series compensation systems use

capacitors to decrease the equivalent reactance of a power line at rated frequency. The

connection of a series capacitor generates reactive power that, in a self-regulated manner,

balances a fraction ofthe line's transfer reactance. The result is improved functionality of the

power transmission system through:

i) increased angular stability of the power corridor,

ii) improved voltage stability of the corridor,

5

iii) optimized power sharing between parallel circuits.

Like shunt compensation, series compensation may also be implemented with current or

voltage source devices. However, the compensation strategy is different when compared

with shunt compensation. In this case, compensation voltage is added between the line and

the load to change the angle of voltage, which is now the voltage at the load side. With the

appropriate magnitude adjustment of compensating voltage, unity power factor can again be

reached at load side.

Independent of the source type or system configuration, different requirements have to be

taken into consideration for a successful operation of VAR generators. Some of these

requirements are simplicity, controllability, dynamics, cost, reliability and harmonic

distortion. The following sections describe different solutions used for VAR generation with

their associated principles of operation and compensation characteristics.

1.2.3VAR Generators

In general, VAR generators are classified depending on the technology used in their

implementation and the way they are connected to the power system (shunt or series).

Rotating and static generators were commonly used to compensate reactive power. In the

last decade, a large number of different static VAR generators, using power electronic

technologies have been proposed and developed [7]. There are two approaches to the

realization of power electronics based VAR compensators, the one that employs thyristor-

switched capacitors and reactors with tap changing transformers, and the other group that

uses self-commutated static converters. A brief description of the most commonly used

shunt and series compensators is presented below.

Fixed or mechanically switched capacitors

Shunt capacitors were first employed for power factor correction in the year 1914 [16]. The

leading current drawn by the shunt capacitors compensates the lagging current drawn by the

load. The selection of shunt capacitors depends on many factors, the most important of

which is the amount of lagging reactive power taken by the load. In the case of widely

fluctuating loads, the reactive power also varies over a wide range. Thus, a fixed capacitor

bank may often lead to either over-compensation or under-compensation. Variable VAR

compensation is achieved using switched capacitors [17]. Depending on the total VAR

.-,

6

requirement, capacitor banks are switched into or switched out of the system. The

smoothness of control is solely dependent on the number of capacitors switching units used.

The switching is usually accomplished using relays and circuit breakers.

However, these methods based on mechanical switches and relays have the disadvantage of

being sluggish and unreliable. Also they generate high inrush currents, and require frequent

maintenance [16].

Synchronous Condensers

Synchronous condensers have played a major role in voltage and reactive power control for

more than 50 years. Functionally, a synchronous condenser is simply a synchronous

machine connected to the power system. After the unit is synchronized, the field current is

adjusted to either generate or absorb reactive power as required by the ac system. The

machine can provide continuous reactive power control when used with the proper

automatic exciter circuit. Synchronous condensers have been used at both distribution and

transmission voltage levels to improve stability and to maintain voltages within desired

limits under varying load conditions and contingency situations. However, synchronous

condensers are rarely used today because they require substantia! foundations and a

significant amount of starting and protective equipment. They also contribute to the short

circuit current and they cannot be controlled fast enough to compensate for rapid load

changes. Moreover, their losses are much higher than those associated with static

compensators, and the cost is much higher compared with static compensators. Their

advantage lies in their high temporary overload capability [I].

1,2.4 Thyristorized VAR Compensators

As in the case of the synchronous condenser, the aim of achieving fine control over the

entire VAR range, has been fulfilled with the development of static compensators (SVC) but

with the advantage of faster response times [6], [7]. Static VAR compensators (SVC) consist

of standard reactive power shunt elements (reactors and capacitors), which are controlled to

provide rapid and variable reactive power. They can be grouped into two basic categories,

the thyristor-switched capacitor and the thyristor-controlled reactor.

7

i) Thyristor-Switched Capacitors

First introduced by ASEA in 1971 [16], the shunt capacitor bank is split up into

appropriately small steps, which are individually switched in and out using bi-directional

thyristor switches. Each single-phase branch consists of two major parts, the capacitor C and

the thyristor switches. In addition, there is a minor component, the inductor L, whose

purpose is to limit the rate of rise of the current through the thyristors and to prevent

resonance with the network (normally 6% with respect to Xc). The capacitor may be

switched with a minimum of transients if the thyristor is turned on at the instant when the

capacitor voltage and the network voltage have the same value. Static compensators of the

TSC type have the following properties: stepwise control, average delay of one half a cycle

(maximum one cycle), and no generation of harmonics since current transient component

can be attenuated effectively [16], [17]. Despite the attractive theoretical simplicity of the

switched capacitor scheme, its popularity has been hindered by a number of practical

disadvantages: the VAR compensation is not continuous, each capacitor bank requires a

separate thyristor switch and therefore the construction is not economical, the steady state

voltage across the non-conducting thyristor switch is twice the peak supply voltage, and the

thyristor must be rated for or protected by external means against line voltage transients and

fault currents.

An attractive solution to the disadvantages of using TSC is to replace one of the thyristor

switches by a diode. In this case, inrush currents are eliminated when thyristors are fired at

the right time, and a more continuous reactive power control can be achieved if the rated

power of each capacitor bank is selected following a binary combination, as described in

[13] and [18]. The advantages of this topology are that many compensation levels can be

implemented with few branches allowing continuous variations without distortion.

Moreover, the topology is simpler and more economical as compared with thyristor

switched capacitors. The main drawback is that it has a time delay of one complete cycle

compared with the half cycle ofTSC.

ii)Thyristor-Control1ed Reactor

Reactors may be both switched and phase-angle controlled [20], [21], [22]. When phase-

angle control is used, a continuous range of reactive power consumption is obtained. It

results, however, in the generation of odd harmonic current components during the control

8

pr6cess. Full conduction is achieved with a gating angle of 90°. Partial conduction is

obtained with gating angles between 90° and 180°. By increasing the thyristor gating angle,

the fundamental component of the current reactor is reduced. This is equivalent to increase

the inductance, reducing the reactive power absorbed by the reactor. However, it should be

pointed out that the change in the reactor current may only take place at discrete points of

time, which means that adjustments cannot be made more frequently than once per half-

cycle. Static compensators of the TCR type are characterized by the ability to perform

continuous control, maximum delay of one half cycle and practically no transients. The

principal disadvantages of this configuration are the generation of low frequency harmonic

current components, and higher losses when working in the inductive region (i.e. absorbing

reactive power) [20].

Combined TSC and TCR

Irrespective of the reactive power control range required, any static compensator can be built

up from one or both of the above-mentioned schemes (i.e. TSC and TCR). [n those cases

where the system with switched capacitors is used, the reactive power is divided into a

suitable number of steps and the variation will therefore take place stepwise. Continuous

control may be obtained with the addition of a thyristor-controlled reactor. If it is required to

absorb reactive power, the entire capacitor bank is disconnected and the equalizing reactor

becomes responsible for the absorption. By coordinating the control between the reactor and

the capacitor steps, it is possible to obtain fully step-less control. Static compensators of the

combined TSC and TCR type are characterized by a continuous control, practically no

transients, low generation of harmonics (because the controlled reactor rating is small

compared to the total reactive power), and flexibility in control and operation. An obvious

disadvantage of the TSC-TCR as compared with TCR and TSC type compensators is the

higher cost. A smaller TCR rating result in some savings, but these savings is more than'

absorbed by the cost of the capacitor switches and the more complex control system [16].

To reduce transient phenomena and harmonics distortion, and to improve the dynamics of

the compensator, some researchers have applied self-commutation to TSC and TCR. Some

examples of this can be found in [21], [22]. However, best results have been obtained using

self-commutated compensators based on conventional two-level and three-level inverters.

9

Thyristor Controlled Series Compensation (TCSq

TCSC. provides a proven technology that addresses specific dynamic problems in

transmission systems. TCSC's are an excellent tool to introduce if increased damping is

required when interconnecting large electrical systems. Additionally, they can overcome the

problem of Subsynchronous Resonance (SSR), a phenomenon that involves an interaction

between large thermal generating units and series compensated transmission systems.

There are two bearing principles of the TCSC concept. First, the TCSC provides

electromechanical damping between large electrical systcms by changing the reactance of a

specific interconnecting power line, i.e. the TCSC will provide a variable capacitive

reactance. Second, the TCSC shall change its apparent impedance (as seen by the line

current) for subsynchronous frequencies such that a prospective subsynchronous resonance

is avoided. Both these objectives are achieved with the TCSC using control algorithms that

operate concurrently. The controls will function on the thyristor circuit (in parallel to the

main capacitor bank) such that controlled charges are added to the main capacitor, making it

a variable capacitor at fundamental frequency but a "virtual inductor" at subsynchronous

frequencies. For power oscillation damping, the TCSC scheme introduces a component of

modulation of the effective reactance of the power transmission corridor. By suitable system

control, this modulation of the reactance is made to counteract the oscillations of the active

power transfer, in order to damp these out.

1.2.5 Self-com mutated VAR Compensators

The application of self-com mutated converters as a means of compensating reactive power

has demonstrated to be an effective solution. This technology has been used to implement

more sophisticated compensator equipment such as static synchronous compensators,

unified power flow controllers (UPFCs), and dynamic voltage restorers (OVRs) [15], [19].

With the remarkable progress of gate commutated semiconductor devices, attention has been

focused on self commutated VAR compensators capable of generating or absorbing reactive

power without requiring large banks of capacitors or reactors. Several approaches are

possible including current-source and voltage-source converters. The current-source

approach uses a reactor supplied with a regulated dc current, while the voltage-source

inverter uses a capacitor with a regulated dc voltage. voltage source converter. The principal

10

advantages of self-commutated VAR compensators are the significant reduction of size, and

the potential reduction in cost achieved from the elimination of a large number of passive

components and lower relative capacity requirement for the semiconductor switches [19],

[23]. Because of its smaller size, self-commutated VAR compensators are well suited for

applications where space is a premium. Self-commutated compensators are used to stabilize

transmission systems, improve voltage regulation, correct power factor and also correct load

unbalances [19], [23]. Moreover, they can be used for the implementation of shunt and

series compensators.

Multi-Level Compensators

Multilevel converters are being investigated and some topologies are used today as static

VAR compensators. The main advantages of multilevel converters are less harmonic

generation and higher voltage capability because of serial connection of bridges or

semiconductors. The most popular arrangement today is the three-level neutral-point

clamped topology.

Three-level converters [24] are becoming the standard topology for medium voltage

converter applications, such as machine drives and active front-end rectifiers. The advantage

of three-level converters is that they can reduce the generated harmonic content, since they

produce a voltage waveform with more levels than the conventional two-level topology.

Another advantage is that they can reduce the semiconductors voltage rating and the

associated switching frequency. Three-level converters consist of 12 self-commutated

semiconductors such as lGBTs or IGeTs, each of them shunted by a reverse parallel

connected pOWerdiode, and six diode branches connected between the midpoint of the dc

link bus and the midpoint of each pair of switches. By connecting the dc source sequentially

to the output terminals, the converter can produce a set of PWM signals in which the

frequency, amplitude and phase of the ac voltage can be modified with adequate control

signals. ~ ,

Multi-Level Converters with Carriers Shifted

Another technology that has been successfully proven uses basic "H" bridges. connected to

line through power transformers. These transformers are connected in parallel at the

converter side, and in series at the line side [25]. The system uses SPWM (Sinusoidal Pulse

Width Modulation) with triangular carriers shifted and depending. on the number of

11

converters connected in the chain of bridges; the voltage waveform becomes more and more

sinusoidal.

Optimized Multi-Level Couverter

The number of levels can increase rapidly with few converters when voltage scalation IS

applied.

1.2.6 New VAR Compensator Technology

Based on power electronics converters and digital control schemes, reactive power

compensators implemented with self-commutated converters have been developed to

compensate not only reactive power, but also to mitigate voltage regulation, flicker,

harmonics, real and reactive power, transmission line impedance and phase-shift angle. It is

important to note, that even though the final outcome is to improve power system

performance, the control variable in all cases is basically the reactive power. Using self

commutated converters the following high performance power system controllers have been

implemented: Static Synchronous Compensator (STATCOM), the Static Synchronous Series

Compensator (SSSC), the Dynamic Voltage Restorer (DVR), the Unified Power Flow

Controller (UPFC), the Interline Power Flow Controller (IPFC) and the Superconducting

Magnetic Energy Storage (SMES). The principles of operation and power circuit topology

of each one are described below.

Static Synchronous Compensator (STATCOM).

The static synchronous compensator is based on a solid-state voltage source, implemented

with an inverter and connected in parallel to the power system through a coupling reactor, in

analogy with a synchronous machine, generating balanced set of three sinusoidal voltages at

the fundamental frequency, with controllable amplitude and phase-shift angle. This

equipment, however, has no inertia and no overload capability. Examples of these topologies

are reported in [19], [26].

Static Synchronous Series Compensator (SSSC).

A voltage source converter can also be used as a series compensator. The SSSC injects a

voltage in series to the line, 90° phase-shifted with the load current, operating as a

controllable series capacitor. The basic difference, as compared with series capacitor, is that

I,

12

the voltage injected by an SSSC is not related to the line current and can be independently

controlled. [28].

Dynamic Voltage Restorer (DVR)

A DVR, is a device connected in series with the power system and is used to keep the load

voltage constant, independently of the source voltage fluctuations [27]. When voltage sags

or swells are present at the load terminals, the DVR responds by injecting three ac voltages

in series with the incoming three-phase network voltages, compensating for the difference

between faulted and pre-fault voltages. Each phase of the injected voltages can be controlled

separately (i.e., their magnitude and angle). Active and reactive power required for

generating these voltages are supplied by the voltage source converter, fed from a DC link

[26], [27], [28]. In order to be able to mitigate voltage sag, the DVR must present a fast

control response. Static Synchronous Series Compensators (SSSC) and Dynamic Voltage

Restorers (DVR) can be integrated to get a system capable of controlling the power flow of

a transmission line during steady state conditions and providing dynamic voltage

compensation and short circuit current limitation during system disturbances [28].

Unified Power Flow Controller (UPFC).

The unified power flow controller (UPFC) consists of two switching converters operated

from a common dc link provided by a dc storage capacitor. One connected in series with the

line, and the other in parallel [26], [30]. This arrangement functions as an ideal ac to ac

power converter in which the real power can freely flow in either direction between the ac

terminals of the two inverters and each inverter can independently generate (or absorb)

reactive power at its own ac output terminal. The series converter of the UPFC injects via

series transformer, an ac voltage with controllable magnitude and phase angle in series with

the transmission line. The shunt converter supplies or absorbs the real power demanded by

the series converter through the common dc link. The inverter connected in series provides

the main function of the UPFC by injecting an ac voltage with controllable magnitude and

phase angle, at the power frequency, in series with the line via a transformer. The

transmission line current flows through the series voltage source resulting in real and

reactive power exchange between it and the ac system. The real power exchanged at the ac

terminal, that is the terminal of the coupling transformer, is converted by the inverter into dc

13

power, which appears at the dc link as positive or negative real power demand. The reactive

power exchanged at the ac terminal is generated internally by the inverter.

The basic function of the inverter connected in parallel (inverter I) is to supply or absorb the

real power demanded by the inverter connected in series to the ac system (inverter 2), at the

common dc link. Inverter I can also generate or absorb controllable reactive power, if it is

desired, and thereby it can provide independent shunt reactive compensation for the line. It

is important to note that whereas there is a closed "direct" path for the real power negotiated

by the action of series voltage injection through inverter I and back to the line, the

corresponding reactive power exchanged is supplied or absorbed locally by inverter 2 and

therefore it does not flow through the line. Thus, inverter I can be operated at a unity power

factor or be controlled to have a reactive power exchange with the line independently of the

reactive power exchanged by inverter 2. This means that there is no continuous reactive

power flow through the UPFC.

Interline Power Flow Controller (lPFC)

An Interline Power Flow Controller (IPFC), consists of two senes VSCs whose DC

capacitors are coupled, allowing active power to circulate between different power lines

[33]. When operating bclow its rated capacity, the IPFC is in regulation mode, allowing the

regulation of the P and Q flows on one line, and the P flow on the other line. In addition, the

net active power generation by the two coupled VSCs is zero, neglecting power losses.

Superconducting Magnetic Energy Storage (SMES)

A superconducting magnetic energy storage (SMES) system, is a device for storing and

instantaneously discharging large quantities of power [32], [33]. It stores energy in the

magnetic field created by the flow of DC current in a coil of superconducting material that

has been cryogenically cooled. These systems have been in use for several years to improve

industrial power quality and to provide a premium-quality service for individual customers

vulnerable to voltage fluctuations. The SMES recharges within minutes and can repeat the

charge/discharge sequence thousands of times without any degradation of the magnet.

Recharge time can be accelerated to meet specific requirements, depending on system

capacity. It is claimed that SMES is 97-98% efficient and it is much better at providing

reactive power on demand. The first commercial application of SMES was in 1981 along the

500-kV Pacific Inter-tie, which interconnects California and the Northwest, USA.

\,.

14

The device's purpose was to demonstrate the feasibility of SMES to improve transmission

capacity by damping inter-area modal oscillations. Since that time, many studies have been

performed and prototypes developed for installing SMES to enhance transmission line

capacity and performance. A major cost driver for SMES is the amount of stored energy.

Studies have shown that SMES can substantially increase transmission line capacity when

utilities apply relatively small amounts of stored energy and a large power rating (greater

than 50 MW).

VAR Generation Using Coupling Transformers.

The power industry is in constant search for the most economic way to transfer bulk power

along a desired path. This can only be achieved through the independent control of active

and reactive power flow in a transmission line. Traditional solutions, such as shunt or series

inductor/capacitor and phase angle regulator affect both the active and the reactive power

flow in the transmission line simultaneously. With the use of Unified Power Flow Controller

(UPFC), which is based on Voltage-Sourced Converter (VSC), the active and the reactive

power flow in the line can also independently be regulated. However, a new concept using

proven transformer topologies is being investigated: The SEN Transformer [38].

1.3 Objective of the Thesis

The objective of this thesis work is to investigate the suitability of the application of the

Delta modulation technique in a PWM inverter that will work on the principle of phase

addition and work as a voltage compensator in a power supply line.

The Inverter will generate a semi-sinusoidal waveform with a variable phase angle with

respect to the line voltage. The line voltage compensation will take place through a series

transformer that will insert the voltage generated by the static VAR compensator. . CJThe line voltage is supposed to deviate by :t 50%. Angle compensation has been used as a

basic algorithm. The inverter voltage with proper phase angle with respect to the line voltage

will be added to yield required utility voltage of rated rms value. A simple feedback control

system which may apply an analogue computation or a microcontroller programmed

algorithm will be necessary to calculate the required phase angle of compensation at a

certain interval of time.

15

1.4 Thesis Outline

The thesis work is based on the simulation which was carried out using OrCAD software.

This soft\vare is suitable for professional design and research analysis to investigate

different aspects of any circuit.

Chapter I is the introduction and provides the review of voltage compensation techniques.

Chapter 2 presents the topology of the proposed line voltage compensation scheme that

utilizes the angle compensation. A summary of the modules that was investigated and the

function of each of the modules have been described. Brief analysis has been included

related to the function of the respective modules. Parameter or value selection and practical

limits are also discussed in chapter 2.

Chapter 3 presents the system simulation which was carried out using pspice software.

The results that focus the suitability of the angle compensation are presented.

Five cases have been presented.

Chapter 4 concludes the thesis with remarks on the work done and recommendations for

future investigations.

16

Chapter 2

Proposed Voltage Compensation System

2.1 Introduction

The Voltage compensation system has been modeled in general with a voltage source and a

load has been considered. This load is a linear one and has medium power rating. The supply

is assumcd to dclivcr 230 +/- I 15V ac and as a rcsult a voltagc sag and swell will take placc.

The proposed scheme, with the help of Dclta PWM Inverter, will compensate the line

voltages. This chapter describes the system and the modules that comprise the topology.

2.2 System Topology

Thc topology of the total compensating system is presented in figure 2.1. It consists of an ac.voltage source that supplies a line voltage of 230 +/- 115V ac. An Inverter feeds voltage in

series through a coupl ing transformer. Thc invertcr uses dclta modulation technique for its

control. The inverter consists of a Delta PWM modulator, and control system to shift the

reference voltage phase on the basis of line voltage amplitudes. The control system

implements the phase addition or subtraction method. The inverter uses the PWM signals

coming out of the Delta PWM controller. The output of the inverter is fed in series with the

source voltage to compensate any sag or swell that may occur in the linc. The magnitude of

this compensating voltage is constant, but the phase is shifted according to thc need of

compensation. Simulations have been carried out in five steps for line voltages that vary from

liS V ac to 345 Vac.

A scheme for micro-controller based system for voltage sensing and PWM signal generation

with shifted reference sine wave has to be used for this purpose to obtain required series

compcnsation scheme.

The principle of phase addition or vector addition is used. It relies on the fact that the

compensating voltage is ofa fixed magnitude (115 Volts ac, 50% of the ratcd voltagc).

17

Only the phase angle of the waveforms with respect to the supply voltage is changed or

adjusted in such a way that after addition (vector addition) the total voltage becomes equal to

the rated voltage i.e., 230V, 50 Hz.

CouplingTransformer

R-Line

sistor Sw it

R-Load

Q2

Transistor Sw it h

Inverter

Delta IVbdulator IF8J"LJ">- .

Figure 2.1: The topology of the proposed line voltage compensating system.

The secondary side of the coupling transformer receives semi sinusoidal waveforms from a

PWM Inverter. A filter placed either in series or in parallel with the secondary winding of the

transformer provides the sine wave necessary for the compensation. A modulator (Delta

PWM modulator) drives the PWM Inverter. The Input of this modulator is a sine wave signal

called reference signal.

The output of the PWM Inverter has exactly the same shape as the driving signal, only its

amplitude changes from :tIS V to :tE where B is the inverter dc voltage. The dc voltage

can either be storage energy system or a rectifier dc voltage obtained from the line source

itself.

18

2.3 Basic Blocks of the Proposed Series Voltage Compensation System

The full system can be split into following major blocks,

i) Line Voltage Source,

ii) A load with a smoothing Filter,

iii) A coupling transformer of transforming ratio I: I,

iv) Secondary passive ripple filter (series or parallel),

v) Line Voltage Rectifier and PWM Inverter and

vi) Control Circuit implementation.

vi a) Analogue Implementation

vi b) Microcontroller based control implementation.

i) Line Voltage Source: The source has the rated supply voltage 230 VACI 50 Hz. For the

thesis purpose it has been assumed that this supply provides 230V :t 50%, and hence it is not

constant. Different cases will be studied to see how the minimum normal and maximum

voltage is compensated in this scheme.

ii) A load resistor with a smoothing filter: AlOOf.! resistor has been used here as load

resistor. The resistor has medium power rating. A filter is used to filter out high frequency

components, generated by PWM inverter. Existence of some high frequency components is

possible on the load side. For this reason a smoothing filter, in the form of a single parallel

capacitor is used. This will yield a reasonably acceptable wave shape on the load that looks

nearly a sinusoid. The value of this capacitor has been selected 2f.iF.

Figure 2.2 shows the circuit diagram and the frequency response of the filter along with the

load resistor. It shows that the critical frequency for the low pass filter l = li71RC is equal

to 800 Hz.

R1A A A

V

19

R-C Filter

10m,!\'

SmA

OA10Hz

o I (R2)

== c

100Hz

Frequency

R Load

1.0KHz 10KHz

Figure 2.2 The frequency response of the filter along with the load ~esistor.

It has the significance that for the 50 Hz. components it shows the total resistance

1600Q, which is 16 times higher than the load resistance.

TX1

10mA

SmA

,

•'e.

. "'"

• " "' .........

"" .......

OA10Hz

o I(R2)100Hz 1.0KHz 10KHz

FrequencyFigure 2.3 The diagram of a coupling Transformer and its frequency response.

.(

20

iii) Coupling transformer: It has the transformation ratio I: I. This transformer plays an

important role in this compensation scheme. Figure 2.3 shows the diagram of the coupling

transformer and its frequency response. It is a low frequency module meant for low frequency

operation. Theoretically it works in the voltage and current ranges suitable for our

requirements.

iv) Secondary passive ripple filters: The scheme may use either the series filter or the

parallel filter. In case of the series filter the components are an Inductor of value 31.8 mH and

a Capacitor of value 318 uFo These values have been selected theoretically to obtain a good

selective fiIter to pass 50 Hz. component of the power signal.

L

31.85mH

C R

I---W'v---J318.5uF 1 Ohm

Figure 2.4 Simple series Filter.

1. OA

O.SA

.l\...., ,...•...................

,\ ...

....................

/ .......... "\;- ... "-

/" '-...........................

OA1.0Hz

D I (R2)10Hz 100Hz

Frequency

1.OKHz 10KHz

Figure 2.5: Frequency response ofa series resonance filter.

The components have been assumed to be ideal. In case of the parallel filter the filter stands

as a 50 Hz. Band-stop filter. The components are also Land C of the same value as in the

series filter. Figure 2.4 and Figure 2.5 show the two types of Filters along with their frequency

responses.

21

L----~C5f:l~~:Jvv-G~ 10hm

318.5 uF

Figure 2.6: Parallel stop-band filter.100mA

SOmA

OA1.0Hz

c I (R2)10Hz 100Hz

Frequency

1.0KHz

Figure 2.7 frequency responses of parallel filters.

v) The Line voltage rectifier and PWM inverter: The inverter of the module converts dc

voltage from a pair of storage energy device or a rectified dc obtained from source

rectification, into PWM waveform. The inverter waveform shows peak-peak value +B to ~B,

where B is dc voltage. Pulse Width Modulated Inverter output signals, representing a semi-

sinusoidal waveforms and its rms value has been selected to be I l5V. It is necessary that dc

voltage to inverter be 163 V for this purpose. For this reason the inverter input voltage has

been selected as 165V dc. This storage device is charged from the line itself as shown in

figure 2.8.

Two Transistors, with reversed biased diodes between the collector and the emitter, have been

used to implement half bridge inverter. As Switch S I is in its closed position while S2 is open

+ J 65 volts is applied across the primary winding of the transformer. In a similar way ~ 165

volts is applied across the primary winding of the transformer when S2 is closed and S1 IS

open.

(.\\.)

'\...I V Line

115-345V

Transformer D1

-=- GND

2

165 V de +_ Battery

L load

R Load

22

Figure 2_8 A scheme of obtaining dc from ac source.

The driving signals for these two switches come from the Delta PWM modulator. It is a

Pulse Width Modulated signals with peak values + ISv to -lSV. These signals are applied

directly on the inputs ofthe switches S 1 and S2 in such a way that S I operates for positive

value of the driving signal and S2 operates for the negative value of the driving signal.

Thus the PWM Inverter generates exactly the same pattern waveform as driving Delta"'PWM modulator signal. The waveforms are a PWM pattern with a peak value +16Sv to -

l6SV. DC input supplies the extra real and reactive energy needed.

Figure 2.8 depicts the process of charging circuit of the storage device. The inverter

output signal is filtered using either secondary ripple filter or a primary ripple filter. Those

filters are connected either in series or in parallel and are passive filters. The output

voltage is found 230V ac after the compensation_ In case of compensation of line voltage

sags the energy will flow from the battery to the load, but in case of compensation of line

voltage swells the energy will flow from the line to the battery and the battery will be

charged.

Signals from Dell PWMModulalor

Swilch-1

Switch-2

23

• Batlery165Vdc _

InlA:rter Output

~TX2

Batlery165Vdc'_

Figure 2.9 Circuit diagrams of the PWM Inverter

200V

Ov

-200VOs 5mso V(SI:4)

10ms 15ms

Time

20ms 25ms 30ms

Figure 2.10 Output waveform of the PWM invel1er

Figure 2.9 shows the Circuit diagram of the P\VM Inverter and figure 2.10 shows its

output waveforms of the PWM inverter.

,/J,.

24

Figure 2.11 shows the Fourier analysis of the inverter output. Table 1 in page 24 shows

the comparisons of the amplitudes of different harmonics of the waveforms received at the

output of the inverter.

.... ., T ~

..............: •.

• ...... .............. .

........ ..... . ......\ ....... ......

.l I•..• ,

200V

lOOV

OVOHzo V(Sl:4)

2.0KHz 4.0KHz

Frequency

6.0KHz 8.0KHz

Figure2.11: The Fourier analysis of the inverter output

frequency Amplitude frequency Amplitude

50Hz 135V 5800 Hz 28V

150 Hz llV 5900 Hz 35V

250 Hz 9.0V 6100 Hz 35V

350 Hz 2V 6200 Hz 28V

Tables I. The amplitudes of prominent harmonics of the PWM Inverter.

From the table, it is seen that the 50 Hz. components is l30V and the 150Hz. component

is I 1V. Thus the performance parameter, total harmonic distortion (THO) when the higher

order carrier frequency is removed.

iHO= 11'+92+2'135' +11' +9' +22

= 11%.

25

vi) Implementation of control system.The system may engage an analogue computation technique or digital computation technique

using the micro-controller.

a) Analogue implementation:i) Delta PWM modulator: Simple square waveforms generated in different Inverters

have the disadvantages of contamination of lower harmonics. In square wave Inverters

lower order harmonics are very prominent and performance parameter shows that THD is

as high as 35%. Single Pulse Width Modulation (SPWM) Techniques were applied and

influences of the lower harmonics were reduced. Many types of modulations have been

used in inverter switching to reduce the lower harmonics. The purpose of pwm is to make

the sequence of the square waves as effective as a single frequency (50 Hz. In this work)

sinusoidal waveforms. Delta PWM modulator can generate a sequence of square waves in

two levels between +B to -B, and contain less lower order harmonics. However it has a

carrier frequency. The carrier frequency of the delta PWM modulator is self-generating

and depends upon the value of resistor R and the capacitor C of the analogue modulator

circuit that comprises the integrating sub module of the Delta PWM modulator. The

carrier frequency is chosen around 6.5 KHz. showing a spectrum within 5.5 to 7 KHz. The

carrier frequency can be reduced to desired level by using a properly designed filter.

Figure 2.12 shows circuit diagram and the output waveforms of the Delta PWM

Modulator used in this thesis for simulation work. U 1 is the first stage which works as

Comparator. The + (non-inverting) input receives a reference sine wave and - (inverting)

input receives a triangular waveform coming out of the third stage called Integrator. The

pspice allows setting the phase angle of the reference voltage. U2-is the buffer stage that

outputs the properly shaped square waves generated by the comparator. These waveforms

are the actual output of this modulator and are taken to the PWM Inverter for driving the

power switches. The Operational Amplifier U3 along with one resistor and a capacitor

forms an integrator circuit that converts the square waves into triangular waves and those

triangular waves are led to - input of the comparator stage. '

26

VIA3

Della PWM Signal 10 Inverter

V ref

15VOC

V2AUJA -3

R2

Rl OUlk

10k0

'5V 0GC1

.39u

Figure 2. I2 Circuit diagram of Delta PWM modulator.

It can be mentioned that the output of the modulator is a sequence of rectangular

waveforms that resembles to a sine wave of prominent frequency 50 Hz. (same as of the

reference voltage) and leads the same by 90 degree in phase. Therefore the phase angle,

which is a very important factor in this thesis as the whole compensation scheme is based

on the phase addition. Hence proper tracing and calculation of this angle is vital to set the

reference voltage phase for proper compensation.

1111'S161'10;11>ns

•• ~ - - - t ••• ~ •• • ••••••••••••••.+..;...~.....;...;..+..

-: ..;.....;...;.. -:- ..

"':1 ::t::::l:l:t::

12M

...;...;.. -:- .... ,:-

:. ..: : ;. .. ...~; ...: .. : : :j ::r:: ::t::~::i::::t::t::t... { .. i---~..... i..of ••• :••••• of .. ; •• ~••.••• ~ •• .r ••• i +..j ••• t-. {--.i.--~-- i...j .. _;.••.•• i ~..~ _-~-. i-+ +_.~_.,._.!._+.' .. -f .. ~.. {._- '-{- .. ! ~ ~ f i..,!,.+_ !..+--j- ! ~..~ ~ ~ i- ..

::r:1:::;::

""

'"

'''''

-'''''Os: 2"..o U(S2:7)

Figure 2. I3 Output waveforms of the Delta PWM Modulator

."

27

,';u

"""c U("":2)

Figure 2.14 Fourier analysis of the output waveforms shows that the carrier frequency lies

within a spectrum 5.5 -6.5 KHz.

Frequency Amplitude Frequency Amplitude

50 Hz 11 V 5800 Hz 1.6 V

150 Hz IV 5900 Hz 2.8V

250 Hz 0.6 V 6100 Hz 2.8 V

350 Hz 0.2 V 6200 Hz 1.6 V

Table 2. Comparison of the amplitudes of different harmonics component of the output of

Delta PWM modulator.

Performance Analysis: Figure 2.14 shows the Fourier analysis of the output waveforms. Table

2 compares the amplitudes of different prominent frequencies both the lower order signal

frequencies and higher order carrier frequencies. The Total Harmonic distortion THO = 45%.

However high frequency components are easier to remove. Then the indicative performance

parameter will improve.

ii) Reference voltage generator: Figure 2.15 presents the method of reference voltage

generation. The circuit consists of a step down transformer of ratio 10: I. The primary winding

is connected in parallel with the line and secondary winding is placed in series with a voltage

source 23V.

10:1

Line voltage sense)

1l5V - 345V

23V

R1

1k

R2

1kU1A<O

au

AD648A

Phase Inversion

R3

1k90 degree Phase Sh"fting

C1 U2Aoo

R4

1k

28

Figure 2.15: The analogue implementation ofline voltage sensing

and reference voltage generation.

The phase of the voltage source can be varied until the vector sum becomes equal to 11.5

volts. At this stage the obtained voltage is inverted be by a phase inverter. Next a 90° shift

ahead is done with a gain I by a differentiator. The output of the differentiator is the reference

voltage.

b) Micro-controller based control scheme

The main function of the control module is to generate a reference sinusoidal waveform in

proper phase with .respect to the line voltage. Micro-controller will receive the rms value of

the line voltage and will calculate the necessary phase adjustment in order to carry out the

compensation.

The function of a delta modulator is to generate a pattern of variable width rectangular

waveform. The Micro-controller can use the algorithm which is formulated from the

following theoretical deduction.

Switching point determination of delta modulated sine wave.

29

12.5V

12.0V

11.5V

11.0V

10.5V3.5ms ~.OmJ

o Y(CI;2) ~ V(UIA:-)~. 5m3 10 Oms

Time

10. ~"'''' 11 0",:;, 11.

Figure 2.16 The triangular waveform oscillates between upper window and the lower window

cutting the reference voltage at Ii

Figure 2.16 shows the method of generation of the modulated square waveform. Here a

triangular waveform is used to determine the pulse width of the square wave that varies

proportionately to the amplitude of a sine wave.

The reference waveform is v = sin wi

The upper envelope is v + C.V = sin wi + C.V

And the lower envelope is v - C.v= ~C.v + sin wi

The triangular waveform cuts the reference sinusoidal waveform in times denoted by I, where

at iIh time the modulator is switched to +8 or -8 volts.

For a given value ofR and C in the integrator section of the delta modulator the slope is fixed.

We define the rising slope as S/I , and the falling slope as SF and practically they are of the

same value S [Volts per second]

The value of slope is proportional to the carrier frequency of the modulation scheme. The

carrier frequency being more than 5 KHz. That is 100 times the line frequency.

30

Therefore in surrounding of a given time Ii the reference voltage is assumed to be linear. For

2L',v

rising slope,

') _ (L',v +~" sin Wli+,) - (-Lw +V" sin OJI,)J. N -

11+1 -I;

Thus we get

S" (li+1 -I,) = 2L',v+ ~,,(sin Wli+1 - sin WI,) ;

From the rules of derivatives we know,

L',sinwl--- =- OJ cos OJ!

L',I

Therefore the equation (2) can be written as

S"L',I = 2L',v+ V" (L',sin WI)

Or, S" =[2L',v+~,,(L',sinwl)]IL',1

S" = 2L',v/ L',I + ~"WCOSWI

2L',vL',I=-----

Sf{ - v,nOJcosmt

2L',vOr, {HI-Ii =------S" -V",WCOSWI

2L',v(+1 = Ii +------

S" - V",WCOSWI

Similarly for falling slope it can be shown that

'i+1 = l; - SF - ~IOJCOS OJ!

In general assuming Silas positive and SF as negative.

............... (2.1)

............... (2.2)

............... (2.3)

i= 1,2,3, n Where n indicates number of crossing or switching points In one utility

cycle.

For carrier frequency above 6 KHz. n? 120

Times 1'+1 are calculated using rising and falling slopes alternatively.

For carrier frequency F;. more than 6 KHz.

31

1'..1; 80,118 that corresponds to 0.03 degree of the supply frequency.

The initial time point is calculated using the relation 1= 20 x 0 mili second, 360 ........ (24)

The control circuit will sense the rms value of the line voltage. The microcontroller either

calculates the required phase angle for vector addition or subtraction using a certain algorithm

or uses a look up table to determine the same.

Once the angle is determined the value is used to calculate switching times from the following

equation.

21'..vli+1 = ( +--------

8 - V;"WCOS(Wl, +0).................... (2.5)

Thus at the beginning of every cycle a new sequence of switching will take place.

For 3-phase application other switching points will be calculated simply by putting 0 +2~

in one phase and 0 - 2~ in another phase in stead of 0 in equation (5).

In fact the time sequences are stored in the memory of the microcontroller. The program

decides about from which time the pulses will start.

Calculation of required phase angle after sensing the rillS value of the line

voltage.

The rated line voltage of the supply system is 230V. The compensating voltage is J 15V. The

range through which the compensation will take place is I J 5V to 345V.

In figure 2.17 Vector AC = V" where V;,indicates the rated or regulated voltage 230 V, vector

AB the uncompensated line voltage 1';. and CB is the compensating voltage J 15 V.

An expression can be deduced to calculate the required phase angle eFrom the known parallelogram theory the resultant vector VR is calculated.

vn' = V/ + 11;' + 2T"JII; cose (2.6)

32

Figure 2.17 Vector addition, parallelogram theory ..

The vector addition method is being used in this thesis work where

~. indicates the line voltage to be compensated.

11;. indicates the compensating voltage

~, indicates the line voltage after compensation or regulated voltage.

() indicates the angle at which the vector addition will take place.

For our case~, is the regulated 230V

~. is the line uncompensated line voltage and is sensed by the control circuit.

II; is the compensation voltage of fixed magnitude 115 V.

e the angle which is determined by the control circuit.

The equation (2.6) can be re-written as follows

3V~ - V2cose- C I.

2T"JII;.. . .. .. . .. .. .. . .. .. .. .. .. ..... .. .. .. . .. .. (2.7)

••••

33

It has been assumed that the compensating voltage V;. is half the value of the rated regulated

voltage ~,.

The control circuit will calculate the value of cos ()and the value of ()

A microcontroller based control system may be used to work according to the following

algorithm. The algorithm is based on the above theoretical deduction.

The Algorithm

At zero crossing when a new cycle starts, the sense unit senses the rms value of the line

voltage which is ~.'

The compensating voltage V;. which is constant is taken as input. The angle () is calculated

using the equation (2.7). The corresponding timing instance I, is calculated () using the

equation (2.4).

Then the next switching point li+1 is calculated using equation (2.5).

A set of timing instances is calculated for 20 ms time intervals. This is implemented at the

beginning of the next cycle.

The microcontroller outputs pulse HIGH to LOW (or LOW to HIGH) to drive the inverter.

After one cycle (20 ms) is elapsed the microcontroller uses the set of timings calculated in

previous cycle. An interrupt signal may stop the running of the microcontroller based

modulator.

The Table in figure 2.18 presents the value of () and ()+ 90° in each of the five steps for

which simulation was run.

34

Un compensated required phase angle required phase angle

Line Voltage of compensating for reference input

series voltage voltage

115V 0° 90°

172.5V 75.6° 165.6°

230V 104.5° 194.5°

287.5V 130.53° 220.53()

345V 180° 270°

Table 3. Calculated phase angle for different line voltage values.

2.4 Summary

The proposed voltage compensation system requires the components like transformer, PWM

modulator, Inverter and passive filter. The analogue implementation is also possible. It is not

difficult to design these modules. However, the microcontroller based sensing and control

system is also a good approach. The circuits are comparatively easier to implement. The Delta

PWM techniques offer a viable solution at comparatively lower cost and use less sophisticated

electronics.

"1-

35

CHAPTER 3

System Analysis and Results

3.1 Introduction

General description: The grid line power supply under investigation has been assumed to

have voltage supply which is not constant. It has sags and swells within a certain range.

This is a 230 volt supply system which varies +/- 50% or 115 V to 345V

From the practical vicw point of such supply two cases are very common.

First the voltage sags and swells can occur due the line resistance of the power supply

system. It happens when low voltage distribution system is extended to a long distance.

Voltage falls due to the line voltage drop, and the voltage rises due to the fact that the

generator or the step down transformer is designed to supply higher voltage to mitigate

the natural drop due to line resistance on normal load but the voltage rises during the time

when the system is under loaded.

The second case where such sags and swells occur is the generator or step down

transformer having very high regulation parameter or the generator supply has the

variation due to imperfect operation.

The total compensating system studied in this research can be divided into three major

parts,

I. The voltage supply will be compensated by a voltage fed in series by a transformer.

This ac voltage will be of fixed magnitude and will be fed or injected by changing its

phase relative to the line voltage. This technique is called phase addition or subtraction

method. In this way both the sag and the swell can be dealt with.

2. The compensating voltage will be generated by an Inverter. The source of extra energy

is the reactive power that is accumulated in a pair of large value capacitor. The amount of

energy accumulated is sufficient to compensate the voltage, hence the power required in

one cycle. A thyristor controlled rectifier is used to maintain a fixed voltage across the

capacitor. The inverter uses the dc of the capacitor and generates a train of pulses that

contains energy and has instantaneous rms value like a sinusoidal waveform.

36

3. The inverter is controlled by a modulator called Delta Pulse Width Modulator

(DPWM) that generates a pattern of rectangular waveform of varying pulse width using a

reference sinusoidal waveform at its input. Separate control logic maintains the phase

relation ship among the reference sinusoidal waveform, generated sinusoidal PWM signal

and the line sinusoidal waveform.

3.2 Results of Phase Addition

The variation in line voltages takes place due to two major reasons. These are discussed

here.

3.2.1 Higher Line Resistance

System that exhibits the variation of supply voltage due to higher line resistance IS

depicted in figure 3.1.

For simplicity the line resistance has been assumed to be R,. = 50, as the supply voltage

is set at higher value 300V to mitigate the over load that remains mostly in the supply

line. A 14 Ampere line load provides a proper voltage 230V on the load. When the load

increase by 50% the line current exceeds 20 A, and the voltage across the load drops to

200V. On the other hand the supply voltage rises to 275 volts when the line current falls

as low as 5 Amperes.

R-Line

5 Ohm

V-Line300V rv

-=-GND

R-Load

(15 to 55 Ohrrs)

Figure 3. J. The voltage on the load resistance varies from 275 V to 200 V as the load

resistance changes from 550 to 150 corresponding to 5A and 20 A load.

37

3.2.2 System with Variation in Supply voltage

System may also exhibit line voltage variation for the high regulation rating of the source

or step down transformer. In such cases the supply voltage drops although the line

resistance is very low. Atypical system with varying source voltage is depicted in figure

3.2.

R-Line

0.05 Ohm

V-Linerv

115V to 345V

-=-GND

R-Load

25 Ohms noninal

Figure 3.2 A system that supplies line voltage in range 115V to 345 V.

To compensate the variation of the voltage, phase addition or subtraction method has

been adopted. The compensating sinusoidal waveform is of fixed magnitude.

Figure 3.3 depicts the topology of the compensating system where V2 indicates the

voltage used to compensate the line voltage sags or swells. The output voltage is the

compensated voltage of rated value.

R-Line

V-Line

"-'115V to 345V

50hrrs

-=-GND

V6rv Series Corrpensation

115V (Oto 180 degree)

R-Load

25 Ohrrs noninal

Figure: 3.3 Line voltage VI is compensated by a fixed amplitude voltage V2

to obtain a rated output voltage in the system.

I I v.I 15VIU2AOl I U3Aro

U1Aro!3 i"--.

R11kM 70

Vf12k

~

15Vd~V5

1v ref

C1

.33uF

Swi1Ch-1

I ~ * 01.R5 '"10k Lme

7GND

V-

I~ 015

0.1 Ohm

Switch-2 '"

V1165\' I

3.18mFC2 L1

3.18mH

70V2

165'" I

1)(1

2,2uFC3

70

R-load10 Ohm

Figure 3.4: Circuit diagram for simulation and analysis using series filter

38

U1AOOI U2ACO'

3 i"---. l I I U3Aco

~A3

I .:.LV4

I R1

15Vdc ._

V ref V I 1_I10k

1k

"7

C1

0

.:lV5

.39uF

15Vdc +_

-'

10k

7

70

B

I

--

~~~07 165 Vdc -TL1

M R Line5mH

_ C4

26.3uF

1 Ohm r~7GNOl~

f R Load

Switch-2

! t 165 Vdc +_ BL2

10 Ohm

0826.3uH

1GNO 7

~v~".,..,.

- 9.Figure 3.5: Circuit diagram for simulation and analysis using parallel filter

39

40

3.3 Implementation

Figure 3.4 and figure 3.5 present the circuit diagram of the proposed voltage

compensation system using the series filter and parallel filter respectively.

3.3.1 Rectifier

The rectifier converts the line ac to a dc. The dc voltage is accumulated on a capacitor of

large value This reactive power is sufficient enough to supply energy needed to generate

the compensating voltage to meet the sag during one cycle. The second requirement that

the rectifier must meet is to maintain the dc voltage level all along.

3.3.2 Inverter and Delta modulation

The inverter consists of two power switches S I and S2, two dc voltage sources V I and

V2.The driving delta modulated pulse is applied to the controlling inputs of the switches.

They are switched on and off alternatively and complementarily. A square wave of peak

amplitude +165V to -165V is produced between the ground and the common point of the

switches. The voltage appears on the transformer winding TX I.

This inverter generates a rectangular wave pattern of varymg pulse width that is

equivalent to a sinusoidal waveform in respect to its instantaneous energy content. The

pattern is supplied to this inverter from a pattern generating Delta modulator. When the

transistors are switched by those pattern signals, it generates the rectangular wave

varying pulse width of exactly the same pattern but it contains required energy and

voltage level to be fed in the system.

The Delta PWM modulator consists of three operational amplifiers with associated

components. The first stage U I is a comparator that receives a reference sinusoidal

waveform at inverting terminal 2. The reference sinusoidal waveform extracted from the

line voltage, shifted by a certain angle 8. The reference voltage is of constant magnitudes.

A rising or falling triangular waveform is applied at the non-inverting terminal 3 of the

op-amp ..

The second op-amp U2 is a buffer that produces a properly shaped rectangular wave form

varying pulse width. The output is a delta modulated waveform which is used to drive the

41

main Inverter switches. The third stage U3 is an integrator which converts the rectangular

waveform into triangular waveform. The values of Rand C in the integrator decide the

carrier frequency. Here R2 is taken as IK and C, is of 0.33 uF. They provide a carrier

frequency about 6-7 kilo hertz.

301:1$

j-J-""!"7!.-=t==:

I -':'-1-1-

-++-I_L--t--f-L ..LI I

25m$201:1515ms

Time10"-15

Figure3.6: The reference sinusoidal waveform at the input of the Delta PWM and

the generated square waveform available at the output of the modulator.

The fi Itering of the pattern would yield a sinusoidal waveform that lags the reference

voltage (line voltage) by900 .This waveform is used to drive the set of power transistors

in the Invel1er.

The inverter uses the dc voltages on a pair of capacitors. In the figure the capacitors are

represented by a pair of batteries to exhibit the two important requirements that is the dc

voltages should be constant and the capacitors should supply the required energy. Power

transistors are switched to connect the pair of capacitors to the loa<i alternatively. The

capacitors are replaced by a pair of batteries for simulation purpose to ensure that it

supplies the whole energy.

42

3.3.3 The Filter

Two types of passive filters called secondary filters can be used in the system.

I. Series Filter. In figure 3.4 the series tilter consists of C2 and L 1 which is a 50 Hz. low

pass fi Iter.

2. Parallel Filter: In figure 3.5 a parallel tilter consisting of C4 and L2 works as stop

bandwidth filter at 50 Hz.

The design of the ripple filter connected in parallel to the pnmary winding of the

coupling transformer is performed according to the following assumption.

I. The filter has to filter out the high frequency component so that it does not enter the

main supply. For this purpose we either design a parallel filter, which has lower

impedance for higher frequencies, or design a series filter that will suppress the flow of

high frequency current to the power line.

2. Solution with the parallel band stop filter will force reactive currents to flow through

the filter unnecessarily, but with series filter it will reduce the flow of unnecessary

current.

3.3.4 The Coupling Transformer

The transformer used is a iron core low frequency transformer used to super impose the

compensating voltage in series between the source and the load. In OrCAD pspice the

simulation is carried out using the coupling factor unity. The inductances have been

selected suitable for low frequency operation. The transfer ratio has been assumed unity

for simplification. The VA rating of the transformer is to match the load current with the

compensating voltage i.e., 1J 5 volts p-p. The inverter is connected to the secondary

winding of the coupling transformer and the primary winding is connected in series with

the load.

The major function of this transformer is to isolate the PWM inverter from the line source

and to match the voltage and current rating of the PWM inverters with those of the power

distribution system. The coupling transformer that is used to couple the inverter with the

Power supply line plays an important role as it decides the quantity of injection of voltage

"c: ..

43

and current. The transformer has also a large influence on frequency selectivity of the

filter that is connected to secondary side of this transformer.

The total apparent power required by each coupling transformer is 1/3 the total apparent

power of the load. The turn ratio of the current transformer is specified according to the

inverter DC bus voltage and other factor related to the filter specification. The correct

value of the winding turn ratio must be specified according to the over all series power

filter performance.

3.4 Phase ControlThe reference voltage for the Delta PWM modulator can be generated using analogue

computation. The main part of the control scheme is the Delta Pulse Width Modulator.

The output pattern of the modulator presents the pulse width modulating wave equivalent

to sinusoidal waveform exactly in a phase which is needed for the phase addition.

Also the same controller can be designed and implemented by a microcontroller based

control system.

3.5 ResultsThe results of the study are being presented considering five specific cases. These are the

cases when the line exhibits 50%, 75%, 100%, 125% and 150% voltage variation of the

rated value 230V.

The compensating voltage, which is always I J 5V, is applied in series with proper phase

angle. The waveforms of the compensated output voltage have been presented in each of

the five cases.

3.5.1 Results for Series Filter

In figures (3.7 - 3.16), a complete compensation of voltage from 115V to 345 V are

depicted using typical cases of 115, 172, 230,287 and 345 volts.

I. Compensation of available Line voltage 115V (172.5V peak)

44

m' .............A • .......... -'.-::

11\ ......1,\,. · I \ 1/ \... . .......... I \I . \ .............. .........

•\ r \ I \ • I•

l. I \ I\ / . \ , . \ . /

•\ / I \ I

...

• . . OJ•

200V

OV

-200V20ms 40ms

" V(TX1:4)60ms

Time

BOrns lOOms l20ms

20V

OV

(a)

•

.

'\.

.,' / '\ I \ f \ I '\ I"

f f'

I ;\ .\ .\

\ ,/ \ I \ I \ / \ ,v

/

-20V2Oms 4Oms

" V(U1A:-)

200V

OV

-200V2Oms 4Oms

• V(D15:K)

60m3

(b)

60ms

(c)

Time

Time

80ms

BOrns

lOOms

lOOms

120ms

l20ms

Figure 3.7 Waveform s in the compensation when line voltage is 115V(rms)a) Available line voltage 115 V (172.5 peaks)b) Corresponding reference voltage of the modulator (() = 900

)

c) Output waveform of the inverter (() = 00)

4S

. . , ,f 1 I t r 1 I l j'.1.

1\ I \ IJ \ I \r ( . \......

,

•

I \ J \1, I ................. \ I \ J \. I. , , . \. ...J

,I""

...J '...J,• . .

200V

OV

-200V20ms

v V(TX1:3)40m3- V(TX1:4)

60ms

Time

BOrns lOOms 120ms

(a)

_.

1'\ f '\ j "' 7 '\ I II \ II \ I \ , \ i \

\ L \, , .\ I \ • I 1\ j\ L . \ ! \. / \ / \ l

. . ~

400V

OV

-400V20ms 40ms

o V (TX1:3)60ms

Time

BOrns lOOms 120ms

(b)

400V-r:=======r=======:r:=======:r==1

2oovttEEEIEEEtEEIEEEEjEIEEEEEE1EJj

1.5KHz1.0KHzO.5KHzOV1--Jc.L.-'~~--'----'-+-~--'--'----'--t--'----'--'---'---+-~-'OHz

o V(TX1:31Frequency

(c)Figure 3.8 Waveform s in the compensation when line voltage is 1ISV(rms)

a) The compensating series voltage of constant magnitude (series filtered)of the inverter for angle addition. (e = 0°)b) The compensated output voltage 230Vc) Spectrum of the compensated voltage.

~I

46

II. Compensation of available Line voltage I 72.5V (258.75V peaks)

l20mslOOmsBOrns

Time

60ms

: : ... . ......•.•. '.' ••••••••••• i, :; ~••••••••• ; ••••••••••••••••••• , ••••••••• j ••••••••• f.... .........•.................. + ,...........•...... t .

OV

........... L. ~ ~..

400V...........! i f 1..••.•

-400V20ms 40ms

o V(TX1:4)

.+ .

l20mslOOms

........i f +.......... .. + ~..

BOrns

Time

60ms

.......;...........•.........~ i f !

....... ~ + j .

OV

20V........... j .......•..•....•...• , .•••.••.

-20V20ms 40ms

o V(U1A:-)

200V

OV

-200V20ms 40ms

• V(D15:K)60ms 80ms lOOms l20ms

Time

Figure 3.9 Waveform s in the compensation when line voltage is 172.5V(rms)a) Available line voltage 172.5V (258.75V peaks)b) Corresponding reference voltage of the modulator (8 = 165°)c) Output waveform of the inverter (8 = 75.6°)

47

: .. •"-., , '1' ........ ........: f

'\ I ) :/ I ,\ I \ J' , { \ I.. . 1 ,

.\ :\ \:\ f \ . , \ .f l I \ J

\ ) V " / \ ;; \. /",

200V

OV

-200V20ms 40ms 60ms.v V(TX1:3) - V(TX1:4)

(a)Time

80ms lOOms 120ms

A Il. ....... A IT I'f~ I' Iv. I

I.... \ \ \.. ..\, I l, / ~ I ~ It

\ I \ I \ ! \ L \ L( :( ( '. •

\ \ \ .. \ :.•.......V- \r c,. "" : ""

• •,•

.. ;

..... ,

400V

OV

-400V20ms 40ms

o V(TX1:3)

400V

200V

OVOHzo V(TX1:3)

O.5KHz

60ms

(b)Time

80ms

1.0KHz

lOOms 120ms

1. 5KH z

Frequency

(c)

Figure 3.10 Waveform s in the compensation when line voltage is 172.5V (rms)a) The compensating series voltage of constant magnitude (series filtered)of the inverter for angle addition. «() = 75.6°).

b) The compensated output voltage novc) Spectrum of the compensated voltage.

\.

III. Compensation of available Line voltage BOY (345Y peaks)

48

I \ j \ / , ( \ ( \\ I \ / \ / \ 1/ \ ...... ... .....

\ I \ J \ / \ l \ I........ ....\ . / \ ( \ / \ •••• (....'d .

r T r r /"'

J \................ .. '\ ( \ ( \. ! \\ \ " \ \

( / ! ( I:\ / \ / \ p ~I " I'- '0 '- '-

•

400V

OV

-400V20ms 40ms

Q V(TX1:4)

20V

OV

-20V20ms 4Oms

o V(V3:+)

200V

OV

-200V20ms 40ms

• V(D15:K)

bOms

60ms

60ms

Time

Time

Time

80ms

80ms

80ms

lOOms

lOOms

lOOms

120ms

120ms

120ms

Figure 3.11 Waveform s in the compensation when line voltage is 230V(rms)a) Available line voltage 230V (475V peaks)b) Corresponding reference voltage of the modulator (() = 194.5°)c) Output waveform of the inverter (() = 104.5°)

49

"' i': i". •. 10 r

'\ I l I. 1 J 1 I J I.....\ ~

........

\ \ I........

\ I \ ,J 1 .....J f I ............. I........... r r r ........

I '\ ) , ) !\ ) \ }\,., v. 'tJ; v. 'V.

200V

OV

-200V20ms

v V(TX1:3)40ms- V(TX1:4)

60ms

Time

80ms lOOms 120ms

- _. ••••. ! t"".f / " Ir~....... ..... "[",;~ ~ \ .. \..\ J\ I \ .I. \ I \ I

\ I \, I \ I \ J ~ /---.---.; ••••• - ••• j .••• - •••.•

I r !' j\ III \

-..•. - ...., ~

400V

OV

-400V20ms 40ms

oV(TX1:3)60ms 80ms lOOms 120ms

Time4OOV,--------~-,-- __ -_-_-,--_- __ -_,_-__r-_,_,

20 OV t-IH--'---'----,--+--'--,-----,----,--+--'----'-----'----t---+--j

1.5KHz1.0KHzO.5KHzOV -j-Q'-'--'-_-'--'-_-'----j_-'-_'----'-_-'---+_-'----'-_-'----'_-+-_uOHz

o V(TX1: 3)

Frequency

Figure 3.12 Waveform s in the compensation when line voltage is 230V(rms)a) The compensating series voltage of constant magnitude (series filteredof the inverter for angle addition (e = 104.5°)b) The compensated output voltage 230Vc) Spectrum of the compensated voltage.

so

IV Compensation of available Line voltage 287V (431.25V peaks)

~'1' \ ~ ....

j \ P \ 7 \ ~/ / c ....

j \ / \ 1 \........

iT Ij " ..- ...j ...... ;....... I

\ / ~\ f ~.!\..

\ ( \ f \ f \f...\,... ..... r / • f

...\ 1..... \;

;

"""'; - = ~ .LJS, ...........

............ ;\/ '\

..........) \ I \ / '\ / Y"

/ \ \ ~ ), / \ I \J J

~ 'C;;.> ~ 'J 1= ;

~

500V

OV

-500V

20ms 40mso V(TX1:4)

20V

OV

-20V20ms 40ms

o V(V3:+)

60ms

60ms

Time

Time

80ms

80ms

lOOms

lOOms

120ms

120ms

200V

100V

OV-100V

-200V

20ms 40ms 60ms 80ms lOOms 120mso V(TX1:3) - V(TX1:4) 0 V(D15:K)

TimeFigure 3.13 Waveform s in the compensation when line voltage is 287V (rms)

a) Available line voltage 287V (431.25V peaks)b) Corresponding reference voltage of the modulator (B = 220.50)c) Output waveform of the inverter (Ii = 130.530)

.'-.

51

200V

100V

OV-100V-2oov,:k

20ms 40msv V (TX 1 :3) - v ITX 1 :4 I

60ms• V(TX1:3)

Time

80ms- V(TX1:4)

lOOms 12 Oms

--' - - -,

17 /' 17 ;\ Lf j ib\ \ \ \ \T , \ \ \ \

/ \ / / ~ I. // ri / / /, I :'\ "\ \. [. ,\.-.... ""','" : ••

400V

OV

-400V20ms 40ms

o V(TX1:3)60ms 80ms lOOms 120ms

.Time400V.--- __ -_-_-,--_-_-_-_---.-_-_- __ --,----,--,

20OV+-1I--'----'------,.---'---+---'--'----'--'---!--'----'---'----'---I--'-1

1.5KHz1.0KHzO.5KHzoV-f&ILLi--'-_'---'-_+-'--_--'----'--'--'-_-1--'-_--'-----'_-'-_-1----'-.JOHz

o V(TX1:3)Frequency

Figure 3.14 Waveform s in the compensation when line voltage is 287v(rms)a) The compensating series voltage of constant magnitude (series filtered)of the inverter for angle addition (B = 130.53°).b) The compensated output voltage 230vc) Spectrum of the compensated voltage.

••••

V. Compensation of available Line voltage 345V (517.5V peaks)

52

500V

OV

-500V

/ ... \ ....... , ....... / \..... ................

...... .......... ...

..........• \ I /

- ? '" "-I l;J, I \ ;( \ I \ I \

I

'\ .I.7 \ 'j \ j \ j \ ) \fii •.

•

20ms 40ms• V(TX1:4)

20V

OV

-20v20ms 40ms

o V(V3:+)

200v

Ov

-200v20ms 4Oms

o V(D15:K)

60ms

60ms

60ms

Time

Time

Time

80ms

80ms

80ms

lOOms

lOOms

lOOms

120ms

120ms

120ms

Figure 3.15 Waveform s in the compensation when line voltage is 345V(rms)a) Available line voltage 345V (517.5V peaks)b) Corresponding reference voltage of the modulator (() = 2700)c) Output waveform ofthe inverter (() = 1800 )

\•

53

200V

120mslOOms80ms

....

......"c.

OV100V

-100V

-200V20m3 40m3 60ms

o V(TX1:3) - V(TX1:4)Time

/< .. .j + .. j'!: ..

r...........

400V200V

OV-200V-400V

20ms 40ms" V(TX1:3)

60ms 80ms lOOms 120ms

Time

1.5KHz1.0KHzO.5KHz

,,,,,,TOV

OHz" V(TX1:3)

400V

200V

Frequency

Figure 3.16 Waveform s in the compensation when line voltage is 345v (rms)

a) ) The compensating series voltage of constant magnitude (series

filtered) of the inverter for angle addition for angle addition (B = 1800).

b) The compensated output voltage novc) Spectrum of the compensated voltage.

\

54

3.5.2 Results for l'arallelFilter

In figures 3.17 - 3.26 complete successful compensation of voltage from 115V -345Vhas been depicted using typical cases of 115V, I72V, 230V, 287V and 345V.

I. Compells:Jtioll of available Line voltage 115V (172.5V peak)

200V

OV

-200V20ms 40ms

o V(TX1:4)60ms

Time

BOrns lOOms l20ms

: . ! + ~ ..l20ms

..• .. ..........~ , .

lOOmsBOrns

: :..................., f ; ........f .

60ms

............~ f ,

40ms

, :........,.........•......... ;

OV

20V

-20V20ms

v V(V3:+)Time

200V

OV

-200V20ms 40ms

" V(Sl:4)60ms BOrns lOOms l20ms

Time

Figure 3.17 Waveform s in the compensation when line voltage is 115V (rms)a) Available line voltage 115 V (172.5 peaks)b) Corresponding reference voltage of the modulator (B = 90°)c) Output waveform of the inverter (B = 0°)

ss

. .

'\ '\ .I'\ ./ '\ / '\

j j '\ j \ 1 I \I / j 1 11

\ \ \, ;\ \

" I ' \ \ \ \ J

" J : ' J \ J ... ,." .. { \ T

, ; : , ,

200V

OV

-200V20ms

x V(Ll:2)40ms- V(TX1:4)

60ms 80ms lOOms l20ms

Time

• 77 '\ I '\ I '\ 7 '\ '\/ '\ / \ j \

.........) \ 7 \

II 1/ II Ii

\ .\ \\ \ \ / \ \ j

\ / \ J..

\ I.. \ r \ /

400V

OV

-400V20ms 40ms

A V(Ll:2)60rns 80rns lOOms l20ms

Time

1.33KHz1.00KHzO.50KHz

;,

.....

OVOHz

A V(L1:2)

400V

200V

Frequency

Figure 3.18 Waveform s in the compensation when line voltage is 11SV (rms)


Filtered) of the inverter for angle addition (B = 00)

b) The compensated output voltage 230V

c) Spectrum of the compensated voltage.

II. Compensation of available Line voltage 172.5V (258.75 peaks)

56

400V

OV

--_........ i--./' "- ff'\ /' "- /' "- \/ \ / \

......./ \

..... ,I

"-I \ I \ I \ , \ -j

"-./ "-./ "-

'.-t '( ;7 '(

-400V20ms 40ms

" V(TX1:4)60ms

Time

SOms lOOms l20ms

20V

OV

i, , , -

.. .....J-.. -b --. J-.."'"I', / \ / \ I \ I \

!' / / 1 ,I1\ 1\ 1\ \ 1\...\ I • / \ / \ / \ I

.,/ .,/....... ...-'"...

-20V20ms 40ms

6 V(V3:+)

200V

OV

-200V20ms 40ms

o V(Sl:4)

60ms

60ms

Time

Time

SOms

SOms

lOOms

lOOms

120ms

l20ms

Figure 3.19 Waveform s in the compensation when line voltage is 172.5V(rms)a) Available line voltage 172.5V (258.75V peaks)b) Corresponding reference voltage of the modulator (8 = 165°)c) Output waveform of the inverter (8 = 75.6°)

57

, ,, ,

.. .......\ 'I \ I.

.... \/. .......~ j S I:

I I I , I\ I \. \ \

", .... ..... . , r " 1 :\ j'\ f .........., ,

200V

OV

-200V20ms

Q V(Ll:2)40ms- V(TXl:4)

60ms

Time

80ms lOOms l20ms

/ \ / .\, / , I \ I!II \ \ \ \\ \ \ \ \

/ r l. I /L L 'r. ..Jl..... <I

\ \ J \ / \ I \J ...

.... - .... I• •

400V

OV

-400V20ms 40ms

D V(Ll:2)60ms BOrns lOOms 120ms

Time400V

1.33KHz1.00KHzO.50KHz

... .. ......

OVOHz

D V(L1:2)

200V

Frequency

Figure 3.20 Waveform s in the compensation when line voltage is I72.5V(rms)


filtered) of the inverter for angle addition (B = 75.7°).

b) The compensated output voltage 230Vc) Spectrum of the compensated voltage.

I

58

lII. Compensation of available line voltage 230V (345 peaks)

120mslOOmsBOrns

Time

60ms

,........... )

OV

400V

-400V20ms 40ms

• V(TX1:4)

120ms

......... ~ j .

. ; + i

lOOms

-r ....j •••••••••••...........!

80ms

Time

: : :...i ~ ;.

60ms

:: :......... ; + j •••••••••••••••••••... ! ! + .

...... j .•

OV

20V

-20V20ms 40ms

• V(V3:+)

200V

OV

-200V20ms 40ms

o V(Sl:4)60ms 80ms lOOms 120ms

Time

Figure 3.21 Waveform s in the compensation when line voltage is 230V (nns)


b) Corresponding reference voltage of the modulator (e = 194.5°)c) Output waveform ofthe inverter (e = 104.5°)

.• •

• ........... .

•

•

. .'"' • .~ .~ ~ ."C'\ . r , . /' '\ 1\ /" " .

.-\ }

.......\ , \ ! \. !

\ I. \ J \ f \ ! \ t...........f \.. f J '" j

.....f

'V ~ ~ ~ .

• •

200V

OV

-200V20ms

o V(L1:2)40ms- V(TX1:4)

60ms

Time

BOrns lOOms

59

120ms

/ .'11 / \ f"

f li. / '\

\ \ ~ \ \\ \ \ \ \

~ I. / I. I\ ./ ~ / ( r /\,/ \. ~ \. \, ;.

•\ Ii

••

400V

OV

-400V20ms 40ms

o V(L1:2)60ms

Time

BOrns lOOms 120ms

400V

200V

•:

•

OVOHzo V(L1:2)

O.50KHz

Frequency

1.00KHz 1.33KHz

Figure 3.22 Waveform s in the compensation when line voltage is 230V(rms)


filtered) of the inverter for angle addition (e = 104.5°).


• •

IV. Compensation of available line voltage 287.5V (431.25 peaks)

60

, ;"'j. \ I >;.... j \ j \ I \1 \ 1 \ 1 \ i \ i \) ) \ ) .,

\ J / .\ / \ J \ I\ I \ I \, I \ L \ I\ / ........... \ r' \ I \ I \ I

; ;

~ ~ ~ .c:-, ... .c:-, ...........

'"1 \ / Y I \ I \ I \I \ I \ I \ / \ I \J

":7 'i? "J '--'•

"J

;

500V

OV

-500V20ms 40ms

v V(TXl:4)

20V

OV

-20V20ms 40ms

• V(V3:+)

200V

OV

-200V20ms 40ms

o V(Sl:4)

GOms

GOms

GOms

Time

Time

BOrns

BOrns

80ms

lOOms

lOOms

lOOms

l20ms

l20ms

l20ms

TimeFigure 3.23 Waveform s in the compensation when line voltage is 287.5V(rms)



c) Output waveform ofthe inverter (e = 130.53°)

•61

-, T•

• IF I, • I' •, ....J 1\ ..~

......, r J , J...

• I

" ..•..1 1 l 1 , J , I\ ./ \ I \ J \ I \ l, f '\ I , I ,.. J \ JIl II.. ".

200V

OV

-200V20ms

o V{L1:2)40ms- V(TX1:4)

60ms 80ms lOOms 120ms

Time

•••• A' -' ..... -'I 'I \, T j '\ 1 ,\ \ \ " \ \\ \ \ \ \1 I I I \ I I

p / r I I. •.\.J \ l .\ j \ f" \• ';iii ......... .,.. >i;i ... ..•....

400V

OV

-400V20ms 40ms

o V(L1:2)60ms 80ms lOOms 120ms

Time400V~------~-_-~-_-_-_~-_--r--_-_-_~

200V-I--!4>---'----'---'---i----I----'-----'---i---i---1---i----'---'-l

1.33KHz1.00KHzO.50KHzOV +-..Ll--'-_---'- __ -'-_-'--_-+-_-'-_---'- __ '----_'----_-+-_--'-_---'-__ '-j

OHzo V(L1:2)

Frequency

Figure 3.24 Waveform s in the compensation when line voltage is 287.5V(rms)


filtered) of the inverter for angle addition (e = 130.53° ).

b) The compensated output voltage 230V

c) Spectrum of the compensated voltage.

\ •....

V. compensation of available line voltage 345V (517.5 peaks)

62

soov

ov

-soov

I \ I I ~1/ .... \ .i I \ > ......•.........

../

\ 1 \i/. \ I...

\ /' .r"- /

H

"- i!. "/ '\ / \ / \ . " \ I \T "I \

\ \ / '\i/ \ ;, ..\. ) \ / \ .7 \.., ;/ P , .

• •

20ms 4Omso V(TX1:4)

20V

OV

-20V20ms 40ms

o V (V3: +)

200V

OV

-200V20ms 40ms

• V(Sl:4)

60ms

(a)

60ms

(b)

60ms

(c)

Time

Time

Time

BOrns

BOrns

BOrns

lOOms

lOOms

lOOms

120ms

120ms

120ms

Figure 3. 25 Waveform s in the compensation when line voltage is 345V (rms)

a) Available line voltage 345V (517.5V peaks)

b) Corresponding reference voltage of the modulator (B = 2700)c) Output waveform of the inverter (B = 1800)

63

400V

200V

OV-200V

.. .. .. ..

-400V20ms

o V(L1:2)40ms- V (TXl: 4)

60ms

Time

80ms lOOms 120ms

•j \ ] \ ] \ 7 'ro / \.•j ~ } \ / \ I \ I \

\ \ \ \\ I \ J ~ I \ / \ /\ L \ L \ ( \ L \ /

, ,,

,• •

400V

OV

-400V20ms 40ms

D V(L1:2)

400V

200V

OVOHz

D V(L1:2)

(al

60ms 80ms

Time

O.50KHz

Frequency

lOOms

1.00KHz

120ms

1.33KHz

Figure 3.26 Waveform s in the compensation when line voltage is 345V(rms)


filtered) of the inverter for angle addition for angle addition (() = 1800).


\.

64

3.6 Filtering the harmonicsIn all cases either series 50 Hz band pass tilter can be used or parallel 50 Hz stop-band

filter can bc used. These filters have their own merits and demerits as mentioned.

All tive cases have been presented using the series filter.

The Fourier analysis reveals that Delta PWM pattern efficiently replicate the true

sinusoidal waveform. Using a very simple low pass (High pass during parallel filter) filter

perfectly shaped sinusoidal waveform has been found.

The smoothing filter consists of either a very low value parallel capacitor as used in case

of series filtering of the inverter output, or it may be a low value inductor connected in

series as has been used in the case of parallel filtering of the inverter output.

3.7 SummaryThe result shows good compensated waveform at the output with proper magnitude. The

control system which may be an analogue sensing or a micro-controller based control

which can produce required phase for compensating voltage. The Delta PWM modulator

has the ability of self generating carrier frequency which is in the range of 6-7 KHz. This

is easier to remove by designing a narrow band-pass parallel filter or a narrow band series

stop filter.

In this thesis work the used series filter was a 50 Hz. resonant filter. In the other case the

filter used has been parallel resonant band-stop filter.

65

Chapter 4

Summary and Conclusions

4.1 IntroductionThis chapter concludes the work of the thesis AC Voltage Regulation Using Delta PWM

Inverter Fed Series Compensator. The simulation work and analysis has been carried out

successfully.

4.2 ConclusionsThe objective of this research work has been to investigate a voltage compensation

scheme for sags caused either by the overloading or improper line parameters like copper

resistances, and line swells caused by under loading or improper regulation in generator

or power distribution system.

A Delta PWM Inverter feeding a series transformer with load has been used as voltage

compensating device with the capability of handling power required by the system to be

compensated. Complete compensation was investigated for various conditions other than

normal voltage of the system.

Energy Storage needed for the Inverter was simulated using capacitors that would store

reactive energy from the supply in actual case. A rectifier has been connected with the

line to provide the dc energy to the capacitor. Since the switching speed of power

transistors of the inverter is high the amount of energy required to be stored has been very

low.

The circuit analysis was carried out using spice simulating software. Parameters were

selected to design the circuit in such a way that it would reflect the practical behavior.

Phase addition is implemented via a transformer, the primary of which has been

connected to the inverter output and the filter. The filtered waveform appears on the

secondary windings of the transformer. The secondary winding of the transformer is

placed in series between the load and the source. The voltage of the transformer

66

secondary has been used as compensating voltage in series with line between the main

source voltage and the load.

The magnitude of compensating voltage has been chosen 115 volts. It is 50% of the rated

voltage. The compensation is achieved by placing the compensating voltage at an angle

between 0 and 180 degrees corresponding to line voltage between 115V and 345 V.

Although it could be leading or lagging and the result would be the same, phase addition

with leading angle have advantage of improvement of power factor especially for

inductive load. The phase addition is an uninterrupted process that continues even if the

supply voltage is normal.

Use of either series or parallel filter at the transformer output yielded good results.

A single-phase compensation scheme has been carried out. Same procedure can be

adopted in 3-phase compensation. The three phase implementation of this thesis work

could be done by modifying the control unit and using 3-phase inverter along with a rated

3-phase transformer with secondary windings mutually isolated and placed in series with

the corresponding load.

4.3 Recommendations for future worksThe proposed scheme can be adopted for compensating the line voltage for non linear

loads. In such case this inverter will be required to inject the harmonics to the main line

to feed the non-linear load.

A series active filter and a Delta PWM inverter fed transformer can be used as source of

harmonics that would be needed by the non-linear loads.

In this thesis battery has been used as Energy Storage device. Switching speed indicates

that less Energy Storage will also serve the purpose. Hence storage of energy in a

capacitor or in an inductor can also be investigated for voltage compensating purposes.

It is worth mention here that fixed magnitudel variable phase voltage compensation

instantly improve the power factor of the line. For inductive or resistive load

compensating voltages should be placed in leading mode to achieve the power factor

improvement.

67

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•

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voltage regulation using deltapwminverter fedseriescompensator

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