thesis in one file

HARDWARE ENACTMENT FOR SAGACIOUS

COMPENSATION OF VOLTAGE SAG BY

SINGLE PHASE DYNAMIC VOLTAGE

RESTORER USING DSPACE RTI 1104

Dissertation submitted to

Visvesvaraya National Institute of Technology, Nagpur

in partial fulfillment of the requirement for the award of

degree of

MASTER OF TECHNOLOGY

in

INTEGRATED POWER SYSTEMS

By

MR. B. HANUMANTHU NAIK

Under the Guidance of

MRS. R. J. SATPUTALEY

Assistant Professor

DEPARTMENT OF ELECTRICAL ENGINEERING

VISVESVARAYA NATIONAL INSTITUTE OF TECHNOLOGY

NAGPUR 440010 (INDIA)

NOV 2013

HARDWARE ENACTMENT FOR SAGACIOUS

COMPENSATION OF VOLTAGE SAG BY

SINGLE PHASE DYNAMIC VOLTAGE

RESTORER USING DSPACE RTI 1104

Dissertation submitted to

Visvesvaraya National institute of Technology, Nagpur

In partial fulfillment of the requirement for the award of

degree of

M.Tech

in

Integrated Power Systems

by

B. HANUMANTHU NAIK

Under the Guidance of

Mrs. R.J.Satputaley

Assistant Professor

DEPARTMENT OF ELECTRICAL ENGINEERING

VISVESVARAYA NATIONAL INSTITUTE OF TECHNOLOGY

NAGPUR 440010 (INDIA)

2012 - 2013 © Visvesvaraya National Institute of Technology (VNIT) 2013

Visvesvaraya National Institute of Technology

Department of Electrical Engineering

Certificate

This is to certify that a project report on “HARDWARE

ENACTMENT FOR SAGACIOUS COMPENSATION

OF VOLTAGE SAG BY SINGLE PHASE DYNAMIC

VOLTAGE RESTORER USING DSPACE RTI 1104”,

being submitted by Mr. B. Hanumanthu Naik , in partial

fulfillment of the requirements for the award of degree of

MASTER OF TECHNOLOGY IN INTEGRATED

POWER SYSTEMS , is a record of the student’s own work

carried by him under my supervision and guidance .

Date: (Mrs. R.J Satputaley)

Assistant Professor

Countersigned by

(Dr.M. V. AWARE)

Professor & Head of theDepartment


V.N.I.T Nagpur

Visvesvaraya National Institute of Technology


DECLARATION

I, hereby, declare that the thesis titled “Hardware Enactment for

Sagacious Compensation of Voltage Sag by Single Phase Dynamic

Voltage Restorer using dSPACE RTI 1104”, submitted herein for the

award of degree of Master of Technology has been carried out by me in the

Department of Electrical Engineering of Visvesvaraya National Institute of

Technology, Nagpur. The work is original and has been not submitted earlier as a

whole or in part for the award of any degree/diploma at this or any other

Institution/University.

BB .. HH aann uu mm aann tthhuu NN aa ii kk

DD aatt ee ::

ACKNOWLEDGEMENT

Acknowledgement is a very small bouquet for the appreciation,

recognition actuated by gratitude towards those valuable help, guidance and

criticism that led my project to the utter success.

I take up this precious opportunity to express my sound gratitude

towards my Guide Assistant Prof. Mrs. R. J. SATPUTALEY for her

treasured guidance throughout the project work.

I thank to Dr. M. V. AWARE, Professor and Head of the Electrical

Engineering Department, and extend my thanks to Dr.V.B.BORGHATE ,

Associate Professor & Dr. M.A.CHAUDHARI, Associate Professor who

were kind enough to provide me all the help so that I could work voraciously,

baring the time limit.

I also want to thank all my classmates especially (Ashok, Srinivas,

Suresh, Sandy, Rambabu, Swamy, Aneesha, Srikant, Shankar, Rachananjali

&Geethanjali) and the staff of college who directly or indirectly helped and

contributed to successful completion of my project.

And Finally I thank my parents (Mr.B.Sama Naik & Mrs.B.Mary Bi)

for their love, support and encouragement. And last but not least I am very

thankful to my Almighty- God and powerful one, who loves me abundantly.

Date: (B. Hanumanthu Naik)

CONTENTS

Page No.

ABSTRACT i

LIST OF FIGURES ii-v

LIST OF TABLES vi

CHAPTER 1 INTRODUCTION 1-07

1.1 Background and motivation 1

1.1.1 Power quality problems 1

1.1.2 New trends in power quality 3

1.1.3 Power electronic controllers for voltage dip

Mitigation 3

1.2 The Dynamic Voltage Restorer 5

1.3 Aims and objectives 6

1.4 Outline of Project 7

CHAPTER 2 MAJOR POWER QUALITY PROBLEMS 8-26

2.1 Introduction 8

2.2 What is Power Quality? 9

2.3 Causes of Poor Power Quality 9

2.4 Impact of Poor Power Quality 10

2.5 Symptoms of Power Quality Problems 10

2.6 Why Power Quality is so important? 11

2.7 Different Power Quality Problems 11

2.7.1 Voltage sag 12

2.7.1.1 Introduction 12

2.7.1.2 Voltage Sag Definition 13

2.7.1.3 General Causes and Effects of Voltage

Sags 14

2.7.1.4 Voltage Sags due to Faults 14

2.7.1.5 Voltage Sags due to Motor Starting 15

2.7.1.5.1 Motor-starting methods 16

2.7.1.6 Voltage Sags due to Transformer

Energizing 17

2.7.1.7 Estimating Voltage Sag Performance 18

2.7.1.8 Estimating the costs for the voltage sag

Events 18

2.7.2 Voltage Swell 19

2.7.3 Voltage Fluctuations 20

2.7.4 Transients 20

2.7.4.1 Impulsive Transients 21

2.7.4.2 Oscillatory Transients 21

2.7.5 Harmonics 23

2.7.6 Interruption 24

2.8 Power Quality issues mitigation techniques 25

CHAPTER 3 DYNAMIC VOLTAGE RESTORER (DVR) 27-35

3.1 Dynamic Voltage Restorer (DVR) 27

3.2 Principle of DVR Operation 27

3.3 Basic Configuration of DVR 28

3.3.1 Injection/ Booster transformer 29

3.3.2 Harmonic Filter 29

3.3.3 Voltage Source Converter 29

3.3.4 DC Charging Circuit 30

3.3.5 Control and protection 30

3.4 Operating modes of DVR 31

3.4.1 Protection mode/Bypass mode 32

3.4.2 Standby Mode 32

3.4.3 Active/Injection/Boost Mode 32

3.5 Voltage injection methods of DVR 33

3.5.1 Pre-sag/dip compensation method 33

3.5.2 In-phase compensation method 34

3.5.3 In-phase advanced compensation method 34

3.5.4 Voltage tolerance method with minimum energy

injection 35

CHAPTER 4 DESIGN AND SPECIFICATIONS OF DYNAMIC

VOLTAGE RESTORER 36-53

4.1 Design parameters 36

4.1.1 Voltage and current rating 36

4.2 Design of the DVR elements 37

4.2.1 Design of voltage source converter 38

4.2.1.1 Inverters 38

4.2.1.2 Pulse width modulation (PWM) 40

4.2.1.3 Total harmonic distortion (THD) 44

4.2.1.4 dSPACE RTI 1104 45

4.2.2 Harmonic filter 49

4.2.3 Injection/booster transformer 50

4.2.4 Storage devices 52

4.2.5 Detection and control block 52

CHAPTER 5 DISCUSSION ON RESULTS 54-75

5.1 Matlab simulink model and simulation results 54

5.2 Hardware enactment 58

5.2.1 Main specifications 59

5.3 Problems faced 59

5.3.1 Basic conditions 60

5.4 Modes of operation 60

5.4.1 Open loop control 60

5.4.2 Closed loop operation 68

CHAPTER 6 CONCLUSIONS & FUTURE SCOPE 76

REFERENCES

APPENDIX A DATASHEETS

Abstract

i

ABSTRACT

The battle of theoretical and practical accomplishment of DVR lies in the fact

that it has versatile usage for mitigation of voltage sag occurred in power system

loads. As the power quality issues are the drastic scenario of power system world the

importance have been amplified after the prologue of sophisticated devices, whose

concert is so responsive during the Power quality issues like voltage sag/dip, swell,

harmonic contents etc. Application of custom power devices is essential to knob out

this form of issues. So, Dynamic Voltage Restorer (DVR) is one of the technologies

which have the ability to ensure consumers for improved power quality. DVR can

provide the most commercial solution to mitigate voltage sag by injecting voltage as

well as real power into the system. The mitigation capability of these devices is

mainly influenced by the maximum load; power factor and maximum voltage dip to

be compensated. The aim and beauty of the DVR lies in the fact that it can be used to

increase the stability and overall power quality in distribution network. This

documentary mainly speaks about the mitigation of voltage sag by single phase DVR

with the use of dSPACE RTI d1104. Hardware implemented results have been also

presented to verify the performance of Single phase DVR.

ii

LIST OF FIGURES

Fig.1.1: CBEMA Curve 02

Fig.1.2: Double conversion Uninterruptible Power Supply (UPS) with

an energy storage 04

Fig.1.3: Series connected dynamic voltage restorer (DVR) with an energy

Storage 04

Fig.1.4: Solid state transfer switch (SSTS) to switch between two

supply lines 05

Fig.1.5: Operation principle of a DVR. The DVR can by series voltage

injection compensate for a voltage dip at the supply side and

restore the load voltage for a sensitive load 06

Fig .2.1: Power Quality Problems at distribution side 12

Fig.2.2: Peak-Peak & RMS representation of the Voltage sag 13

Fig.2.3: Different faults cause for different sags 14

Fig. 2.4: Voltage sag due to a cleared line-ground fault 15

Fig. 2.5: Voltage sag due to motor starting 16

Fig. 2.6: Voltage sag due to transformer energizing 17

Fig.2.7: Example of a Voltage Swell 19

Fig.2.8: Voltage Flicker Caused by Furnace Operation 20

Fig. 2.9: Lighting stroke current impulsive transient 21

Fig.2.10: Oscillatory Transients 21

Fig.2.11: Second and Third Harmonics in voltage waveform 24

Fig.2.12: Interruption 25

iii

Fig. 3.1: Location of DVR 27

Fig. 3.2: Principle of DVR with a response time of less than one

Millisecond 28

Fig. 3.3: Schematic diagram of DVR 29

Fig. 3.4: Equivalent circuit diagram of DVR 30

Fig. 3.5: Protection Mode (creating another path for current) 32

Fig. 3.6: Standby Mode 32

Fig. 3.7: Pre-sag compensation method 33

Fig. 3.8: In-phase compensation method 34

Fig. 3.9: Voltage tolerance method with minimum energy injection 35

Fig .4.1: Single-phase simplified model for the DVR 36

Fig .4.2: Block diagram of Dynamic Voltage Restorer 38

Fig .4.3: Single Phase Full wave Bridge Inverter 39

Fig .4.4: Hardware implementation of Inverter circuit 40

Fig .4.5: Single Pulse Width Modulation 41

Fig .4.6: Multiple Pulse Width Modulation 42

Fig .4.7: Sinusoidal Pulse Width Modulation Technique 43

Fig .4.8: dSPACE RTI 1104 connector panels 46

Fig.4.10: (a) Power supply required to operate Gate driver, (b) Gate driver

circuit of IC 4425 (c) Hardware implementation of Gate driver

circuit, (d) Pulses to Gate driver from dSPACE Control Panel

to interface 49

iv

Fig .4.11: Design of LC Filter and its hardware implementation 50

Fig .4.12: Transformers placement after polarity Test 51

Fig .4.13: Hardware implementation of Injection Transformer 52

Fig .4.14: Equivalent design of energy storage device and its hardware

implementation 52

Fig.4.15: Complete Hardware set up of DVR 53

Fig.5.1 Creation of voltage sag using large loads 54

Fig.5.2 Output waveform supply voltage, supply current and load voltage 55

Fig.5.3 Creation of 70% sag in the load voltage 55

Fig.5.4 Mitigation of sag using DVR 56

Fig.5.5 Closed-loop control using the PI controller 56

Fig.5.6 Output voltage of the single phase inverter using with and

without filter 57

Fig.5.7 Mitigation of voltage sag using DVR 57

Fig.5.8 (a): Basic Model of Open Loop Control for pulse generation 62

Fig.5.8 (b): Hardware pulses from Dspace 63

Fig.5.8 (c): Hardware pulses after gate driver circuit 63

Fig 5.9 Source Voltage 64

Fig 5.10 Load Voltage during PreSag 64

Fig 5.11 Load Voltage when sag dip occurs 65

Fig 5.12 Load Voltage when sag occurs 65

Fig 5.13 Injected Voltages by DVR 66

v

Fig 5.14 Injected Parameters by DVR 66

Fig 5.15 Load Voltage after mitigation by DVR 67

Fig 5.16 THD Analysis of Load Voltage 67

Fig 5.17 Basic Model of Open Loop Control for pulse generation 69


Fig 5.19 Load Voltage 70

Fig 5.20 Load Voltage when sag Dip occurs 71




Fig 5.24 Load Voltage after mitigation 73

Fig 5.25 Injected DVR Voltages VDVR 73

Fig 5.26 THD Analysis of Load Voltage 74

Fig 5.27 Complete Hardware set up for sag mitigation using DVR 75

vi

LIST OF TABLES

Table No: Table Name Page

2.1 Types of Voltage Swell 19

4.1 Switching States 39

5.1 List of Hardware components 58

5.2 System parameters 59

5.3 Overall specifications of DVR in

open loop condition 62

5.4 Overall specifications of DVR in

closed loop condition 69

Introduction

Page 1

CHAPTER 1

INTRODUCTION

The research documented in this thesis relates to the design and control of

power converters for Custom Power System (CUPS). More precisely, focus is put on

a special class of Custom Power System device called a Dynamic Voltage Restorer.

Further information regarding the background and motivation for the current research

on the dynamic voltage restorer is provided below. Also, the aim of the thesis is

defined including an outline of the thesis.

1.1 BACKGROUND AND MOTIVATION:

Power quality has been a topic of great interest for decades and several issues

have triggered interest in monitoring and improving the power quality.

1.1.1 POWER QUALITY PROBLEMS:

A North American analysis of power quality included data collection from

1057 site-months at 112 locations from 1990 to 1994[1], [2]. The measured data were

analyzed and classified according to the standards of ANSI C84.1-1989 and the

Computer and Business Equipment Measurement Association (CEBEMA) curve,

illustrated in Fig.1.1. More than 1, 60,000 power disturbances were recorded over the

four monitoring periods, which showed the unavailability of commercial power with

an estimated mean value of 6.17 hour per site, per year. The power disturbances were

categorized into four major events; low RMS events, high RMS events, transients and

interruptions.

The transient events took a major part with around 60%, while the interruption

took minor part with around 1%. The transients mainly came from capacitive

switching operations, when the utility applies a large bank of capacitors on a high

voltage power line to help to regulate voltage and to compensate for a poor power

factor.More than 26% of the power disturbances came from the low RMS events,

while 13% of that came from the high RMS events. Some of the high RMS events

were suspected to come from the incorrect setting of the transformers. The majority of

the low RMS events (90%) lasted less than one minute, while 4% lasted more than

thirty minutes.

Introduction

Page 2

Fig.1.1: CBEMA Curve

In a power quality investigation in Denmark from November 1996 to May

1998 around 200 delivery points was measured located in the distribution networks.

Medium and large industrial companies were excluded from the survey and more than

60% of the measuring proceeded in urban site. In total 700 data sets, each one of them

representing one week of measuring, were analyzed by the new standards, EN 50160,

IEC 1000-3-2 and the CBEMA curve. All interruptions longer than 3 minutes were

removed from the data.

The data were categorized into the events of flicker, total harmonic distortion

(THD), dip and swell, unbalance, DC voltage, and transients. Among these events, the

dips appeared as a top issue. Around 30 % of the measurements were under 85 % of

the nominal voltage, which must be avoided according to the European Standard EN

50160.

Voltage dips can cause tripping of sensitive loads and the cost associated with

short duration voltage dips can in some cases justify the insertion of power electronic

equipment to compensate for the poor power quality [3].

Introduction

Page 3

1.1.2 NEW TRENDS IN POWER QUALITY:

Some of the issues, which have renewed and triggered the interest in power

quality, can be stated as:

Higher demand on supreme power quality. IT-technology, automated

production plants and commercial activities require a good and reliable power

supply [3].

De-regulating and commercializing of the of electric energy markets has made

power quality a parameter of interest to achieve a higher price per kilowatt, to

increase the profit and share of the market [4].

Decentralization of the production of electricity with integration of alternative

energy sources and small generation plants has increased certain power quality

problems like surplus of power, voltage variations and flickers [5].

The improvements in the power electronics area and data processing capability

have made improvement in power quality possible by means of relative cost-

effective power electronic controllers [6].

These trends have triggered interest in different types of power electronic

controllers to mitigate power quality problems.

1.1.3 POWER ELECTRONIC CONTROLLERS FOR VOLTAGE DIP

MITIGATION:

There are two general approaches to mitigate power quality problems. One

approach is to ensure that the process equipment is less sensitive to disturbances,

allowing it to ride-through the disturbances [7]. The other approach is to install a

custom power device to suppress or counteract the disturbances.

Many CUPS devices are commercially available in the market today such as,

active power filters (APF), battery energy storage systems (BESS), distribution static

synchronous compensators (DSTATCOM), distribution series capacitors (DSC),

dynamic voltage restorer (DVR), power factor controller (PFC), surge arresters (SA),

super conducting magnetic energy storage systems (SMES), static electronic tap

changers (SETC), solid-state transfer switches (SSTS), solid-state circuit

breaker(SSCB), static var compensator (SVC), thyristor switched capacitors (TSC)

and uninterruptible power supplies (UPS).

Introduction

Page 4

Focusing on the compensation of voltage dips the number of devices can be

narrowed down, and in [8] three types of devices have been compared, they are:

UPS: Uninterruptible Power Supply. This could be a static converter with

double conversion to mitigate most type of power quality disturbances.

The topology is illustrated in Fig.1.2.

DVR: Dynamic Voltage Restorer is a series-connected device, which

corrects the voltage dip and restore the load voltage in case of a voltage

dip. The topology is illustrated in Fig.1.3.

SSTS: Solid State Transfer Switch to change from a faulted feeder to a

healthy feeder. The topology is illustrated in Fig.1.4.

Some of the advantages and disadvantages with the three solutions are summarized in

Table 1.1.

Fig.1.2: Double conversion Uninterruptible Power Supply (UPS) with energy storage

Fig.1.3: Series connected dynamic voltage restorer (DVR) with an energy storage.

Introduction

Page 5

Fig.1.4: Solid state transfers switch (SSTS) to switch between two supply lines.

In [8] and economic comparison of the three solutions have been investigated

regarding the expected savings, cost of solution per kVA, annual operating cost, total

annual cost and a benefit/cost ratio. The SSTS has the highest benefit/cost ratio if a

secondary independent feeder is present and if not the DVR is considered to be the

most cost effective solution.

1.2 THE DYNAMIC VOLTAGE RESTORER:

The dynamic voltage restorer is a series connected device, which by voltage

injection can control the load voltage. In the case of a voltage dip the DVR injects the

missing voltage and it avoids any tripping the load. Fig.1.5 illustrates the operation

principle of a DVR. Table.1.1 discusses the comparison of DVR with other custom

power devices.

Table1.1: Comparison of three solutions to protect sensitive loads from voltage

deviations.

Device Advantages Disadvantages

UPS -Can compensate for interruptions. -High cost per KW.

-High losses.

DVR -Low losses, injects only the missing

part of the supply voltage.

-Cost effective.

-Difficult to protect.

-Cannot compensate for

interruptions.

SSTS -Low stand by losses.

-Low system cost if the second

feeder is present.

-Can compensate for interruptions

and voltage dips.

-High benefit/cost ratio if a second

feeder independent feeder is present.

-Needs a second undisturbed feeder.

-Difficult to ensure an undisturbed

feeder.

-Slow response.

Introduction

Page 6

Fig.1.5: Operation principle of a DVR. The DVR can by series voltage injection

compensate for a voltage dip at the supply side and restore the load voltage for a

sensitive load

The DVR is still very rarely inserted in the grid and only relative few devices

have been inserted around the world. Commercial projects/products regarding the

DVR have been reported in [9], [10], [11], [12], [13] and [14]. Most of the described

projects include limited information about potential problems and a detailed

description of the design and control aspects.

Even though the DVR is commercially available today, the DVR is not a

matured technology and several areas regarding the design and control of this type of

devices are at the basic research level. The design of a DVR has been treated in [15],

[16] and [17] with focus on the sizing of the voltage, power and current rating. The

control strategies to limit the energy storage have been addressed and some control

issues regarding series compensation of unbalanced supply voltages have been

treated. Additionally, the DVR is a series connected device and one of the drawbacks

with series connected devices is the difficulties to protect the device during short

circuits and avoid interference with the existing protection equipment. Which have

been treated in [18] and [19].

1.3 AIMS AND OBJECTIVES:

Mitigating the problem related to Voltage Sag by Using Dynamic Voltage

Restorer (DVR).

To inject appropriate voltage component to correct rapidly any change in

supply voltage to keep the load voltage balanced and constant.

Introduction

Page 7

Modeling, analysis and simulation of dynamic voltage restorer (DVR) using

MATLAB/SIMULINK.

Hardware implementation of DVR using dSPACE for mitigating voltage sag.

1.4 OUTLINE OF PROJECT:

This thesis studies about “Hardware Enactment For Sagacious

Compensation Of Voltage Sag By Single Phase Dynamic Voltage Restorer Using

dSPACE RTI 1104”.

The present report is structured in six chapters.

The abstract of thesis may contain a short background, and the objectives of the

current thesis.

The First chapter represents an overview of project. Firstly, Introduction about

the power system and it problems followed by brief introduction of the DVR.

Secondly, Aim and Objective of the project followed by outline of the thesis.

The Second chapter discuss about Major Power quality Problems which gives

an overview of power quality issues, which are relevant for the design and control of a

dynamic voltage restorer with focus on voltage dips and interruptions.

The Third chapter describes about the Dynamic Voltage Restorer (DVR) which

gives an overview of the introduction to DVR, the location of a DVR and the main

system, operation principle of DVR, basic configuration of DVR and converter

topologies suited for a DVR.

The Fourth chapter deals with the Design and Specifications of Dynamic

Voltage Restorer which consists of main design process and parameters for the DVR.

The Fifth chapter includes Discussion on Results with the modeling process

and parameters included in the models using MATLAB/SIMULINK. It also includes

a verification of the system model with a comparison between simulations and

experimental setup using dSPACE RTI 1104.

The Sixth chapter represents the conclusion and future scope of the thesis.

Major Power Quality Problems

Page 8

CHAPTER 2

MAJOR POWER QUALITY PROBLEMS

2.1 INTRODUCTION:

Since last 25 years there has been an increase in the use of solid state

electronic technology. This new, highly efficient, electronic technology provides

product quality with increased productivity. Today, we are able to produce products at

costs less than in the years passed, with the introduction of automation by using the

solid state electronic technology .This new technology requires clear electric power

quality. The conventional speed control systems are being replaced by modern power

electronic systems, bringing a verity of advantages to the users. Classic examples are

DC & AC drives, UPS, soft stators, etc. Since the thrusters converter technology is

rapidly gaining in the modern industrial plants, the power supply systems are

contaminated as the ideal sinusoidal current and voltage waveforms are getting

distorted. This is in turn is affecting the performance of the equipment in the electrical

network.

Adequate to superior power quality is essential for the smooth functioning of

critical industrial processes. As industries expand, utilities become more

interconnected and usage of electronically controlled equipment increases, power

quality is jeopardized. Most large industrial and commercial sites are served by

overhead lines with feeders that are subject to unpredictable and sporadic events,

e.g. lightning and contact with tree limbs. Most distribution circuits have resoling

devices that clear temporary faults through a timed series of trip and close operations.

This minimizes the possibility of long-term outages but leads to a number of

minor power disturbances. These typically occur several times a month. Many electric

utilities have increased the voltage at which they distribute power. This allows a

single circuit to serve more customers or deliver higher loads, and reduces energy

losses in the system. But it often means the overhead distribution circuit is longer,

with more exposure to disturbances. The disturbances travel farther because of lower

system impedances associated with higher voltage circuits. Sophisticated new systems

are providing vastly increased efficiency and control in critical processes. But with


Page 9

their high sensitivity even to brief variations in electric power quality, today's

computer-driven devices fail when power is disturbed for even a few milliseconds.

2.2 WHAT IS POWER QUALITY?

Electric Power Quality is a term which has captured increasing attention in

power engineering in the recent years. Even though this subject has always been of

interest to power engineers, it has assumed considerable interest in the 1990's. Electric

power quality means different things for different people. To most electric power

engineers, the term refers to a certain sufficiently high grade of electric service but

beyond that there is no universal agreement. The measure of power quality depends

upon the needs of the equipment that is being supplied. What is good power quality

for an electric motor may not be good enough for a personal computer. Usually the

term power quality refers to maintaining a sinusoidal waveform of bus voltages at

rated voltage and frequency.

‘Power Quality [20]’ broadly refers to the delivery of a sufficiently high grade

of electric service.

As per IEEE standards Power Quality [21] is defined as “It is the concept of

powering and grounding sensitive equipment in a manner that is suitable for

operation of system (equipment)”.

As per IEC “Electromagnetic compatibility is the ability of equipment or

system to function satisfactorily in its environment without introducing intolerable

disturbances to others equipment’s”.

Many problems can result from poor power quality, especially in today's

complex microelectronics environment. Electrical disturbances on mechanical

equipment went unnoticeable in the past but can upset today’s high-tech equipment

operations severely. Because approximately 80% of all power-quality problems

originate from the customer's side of the meter, facility owners, managers, designers

and other high-tech equipment users need to understand and avoid power

disturbances.

2.3 CAUSES OF POOR POWER QUALITY:

The causes of poor quality can be attributed to:

• Variations in voltage, magnitude and frequency.


Page 10

• Variations in magnitude can be due to sudden rise or fall of load, outages,

repetitive varying loading pattern in rolling mills, power electronic converters,

lightning etc.

• Variations in frequency can rise of out of system dynamics or harmonics

injection. Consequently the voltage or current waveforms of a power system

cease to be purely sinusoidal in nature but consist of harmonics and other

noises.

2.4 IMPACT OF POOR POWER QUALITY:

The effect of these aforesaid poor power quality problems has serious

implication on the utilities and customers. Utility side impacts higher losses in

transformers, cables etc. In conductors the neutral wires can burn due to the presence

of third harmonics generated by non-linear loads. The power factor correction

capacitors may puncture due to resonant conditions at resonant frequencies near lower

order harmonics. The energy-meters, which are calibrated to operate under pure

sinusoidal conditions, may give erroneous readings. The solid-state protective relays

can mal operate due to poor power quality. There can be increased losses in cables,

transformers and conductors. The customer side of the power network also experience

adverse effects of poor power quality. The automatic processes employing adjustable

speed drives may shut down because of nuisance tripping due to even short voltage

sags. The induction synchronous motors can have increased copper and core loses,

pulsating torques and overheating with derating effect. The non-sinusoidal power

supply thus reduces torque and efficiency of the motors. The computers and

telecommunication equipment encounter loss of data and mal-operation due to poor

power supply quality. The domestic electronic gadgets such as digital clocks, VCRs

and TVs are also affected by voltage distortions.

2.5 SYMPTOMS OF POWER QUALITY PROBLEMS:

Electronic controlled systems that stop unexpectedly.

Many systems reboots required.

Abnormal failure rate of electronic systems.

Transformers overheating.

Motors failing.

PF capacitors failing.


Page 11

Test results unreliable.

May cause financial losses to the network operators and the equipment

manufacturers too.

2.6 WHY POWER QUALITY IS SO IMPORTANT?

Power quality is an increasingly important issue for all industries. Problems with

powering and grounding can cause data and processing errors that affect production

and service quality.

Lost production: Each time production is interrupted, your business loses the

margin on the product that is not manufactured and sold.

Damaged product: Interruptions can damage a partially complete product,

cause the items to be rerun or scrapped.

Maintenance: Reacting to a voltage disruption can involve restoring production,

diagnosing and correcting the problem, clean up and repair, disposing of

damaged products and, in some cases, environment cost.

2.7 DIFFERENT POWER QUALITY PROBLEMS:

Transients

Impulsive

Oscillatory

Flicker

Long Duration Voltage Variation

Over voltage

Under voltage

Sustained interruption

Voltage Unbalance

Noise

Short Duration Voltage Variation

Sag

Swell

Interruptions

Frequency Variation

Wave Distortion


Page 12

Most common problems at distribution side:

Fig.2.1 resembles most of power quality problem [22] commonly occurring at

distribution system

Fig.2.1: Power Quality Problems at distribution side

2.7.1 VOLTAGE SAG:

2.7.1.1 INTRODUCTION:

Voltage sags are huge problems for many industries, and it is probably the

most pressing power quality problem today. Voltage sags may cause tripping and

large torque peaks in electrical machines. Tripping is caused by under voltage

protection or over current protection. These two protections operate independently.

Large torque peaks may cause damage to the shaft or equipment connected to the

shaft. Some common reason for voltage sags are lightning strikes in power lines,

equipment failures, accidental contact power lines, and electrical machine starts.

Despite being a short duration between 10msec to 1sec event during which a

reduction in the RMS voltage magnitude takes place, a small reduction in the system

voltage can cause serious consequences.

POWER QUALITY

FLICKER

VOLTAGE SAG

VOLTAGE SWELL

HARMONICS

TRANSIENTS


Page 13

2.7.1.2 VOLTAGE SAG DEFINITION:

The IEEE defines voltage sag as a decrease to between 0.1 and 0.9 p.u.in rms

voltage or current at the power frequency for durations of 0.5 cycle to 1 min. The

amplitude of voltage sag is the value of the remaining voltage during the sag.

The IEC terminology for voltage sag is dip. The IEC defines voltage dip as: A

sudden reduction of the voltage at a point in the electrical system, followed by voltage

recovery after a short period of time, from half a cycle to a few seconds. The

amplitude of a voltage dip is defined as the difference between the voltage during the

voltage dip and the nominal voltage of the system expressed as a percentage of the

nominal voltage.

Fig.2.2 shows an rms and peak-peak representation of voltage sag, the sag

starts when the voltage decreases to lower than the threshold voltage Vth (0.9 pu) at

time T1. The sag continues till T2 at which the voltage recovers to a value over the

threshold value, hence the duration of the voltage sag is (T2-T1) and the magnitude of

the voltage sag is sag to Vsag.

Fig.2.2: Peak-Peak & RMS representation of the Voltage sag


Page 14

2.7.1.3 GENERAL CAUSES AND EFFECTS OF VOLTAGE SAGS:

Fig.2.3: Different faults cause for different sags

There are various causes of voltage sags in a power system. Fig.2.3 illustrates

different voltage sags which can be caused by faults (more than 70% are weather

related such as lightning) on the transmission or distribution system or by switching

of loads with large amounts of initial starting or inrush current such as motors,

transformers, and large dc power supply.

The general causes and effects of voltage sags are:

Voltage Sags due to Faults

Voltage Sags due to Motor Starting

Voltage Sags due to Transformer Energizing

2.7.1.4 VOLTAGE SAGS DUE TO FAULTS:

Voltage sags due to faults can be critical to the operation of a power plant, and

hence, are of major concern. Depending on the nature of the fault such as symmetrical

or unsymmetrical, the magnitudes of voltage sags can be equal in each phase or

unequal respectively.

The magnitude and phase angle of the voltage sag is determined by the type

and the position of the disturbance and the lasting time of the voltage sag depends on

the protect way. Both balanced and unbalanced voltage sag could appear in this case.

For a fault in the transmission system, customers do not experience interruption, since

transmission systems are looped or networked. Fig.2.4 shows voltage sags on all three

phases due to a cleared line-ground fault.


Page 15

Fig.2.4: Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system are:

i. Distance to the fault

ii. Fault impedance

iii. Type of fault

iv. Pre-sag voltage level

v. System configuration

a. System impedance

b. Transformer connections

The type of protective device used determines sag duration.

2.7.1.5 VOLTAGE SAGS DUE TO MOTOR STARTING:

Since induction motors are balanced 3 phase loads, voltage sags due to their

starting are symmetrical. When the induction-machine starts, the starting current

could be 5~10 times of the normal value in steady state, and the power factor will be

low. Each phase draws approximately the same in-rush current. This large current

will, by flowing through system impedances, cause voltage sag which may dim lights,

cause contactors to drop out, and disrupt sensitive equipment. The situation is made

worse by an extremely poor starting displacement factor usually in the range of 15 to

30 %. The time required for the motor to accelerate to rated speed increases with the

magnitude of the sag, and excessive sag may prevent the motor from starting

successfully.

The magnitude of voltage sag depends on:

i. Characteristics of the induction motor

ii. Strength of the system at the point where motor is connected.

Time (cycles)

RM

S V

olt

ag

e (V

)


Page 16

Fig.2.5: Voltage sag due to motor starting

Fig.2.5 represents the shape of the voltage sag on the three phases (A, B, and C) due

to voltage sags. The magnitude of three-phase fundamental voltage decreases at the

beginning, and then it recovers since the current return to the normal value.

2.7.1.5.1 MOTOR-STARTING METHODS:

Energizing the motor in a single step (full-voltage starting) provides low cost

and allows the most rapid acceleration. It is the preferred method unless the resulting

voltage sag or mechanical stress is excessive.

Autotransformer starters have two autotransformers connected in open delta.

Taps provide a motor voltage of 80, 65, or 50 percent of system voltage during start

up. Line current and starting torque vary with the square of the voltage applied to the

motor, so the 50 per cent tap will deliver only 25 per cent of the full-voltage starting

current and torque. The lowest tap which will supply the required starting torque is

selected.

Resistance and reactance starters initially insert impedance in series with the

motor. After a time delay, this impedance is shorted out. Starting resistors may be

shorted out over several steps; starting reactors are shorted out in a single step. Line

current and starting torque vary directly with the voltage applied to the motor, so for a

given starting voltage, these starters draw more current from the line than with

autotransformer starters, but provide higher starting torque. Reactors are typically

provided with 50, 45, and 37.5 percent taps.

Part-winding starters are attractive for use with dual-rated motors (220/440 V

or 230/460 V). The stator of a dual-rated motor consists of two windings connected in

parallel at the lower voltage rating, or in series at the higher voltage rating. When

operated with a part-winding starter at the lower voltage rating, only one winding is

RM

S V

olt

age

(V)

Time (cycles)


Page 17

energized initially, limiting starting current and starting torque to 50 per cent of the

values seen when both windings are energized simultaneously.

Delta-wye starters connect the stator in wye for starting and then, after a time

delay, reconnect the windings in delta. The wye connection reduces the starting

voltage to 57 per cent of the system line-line voltage; starting current and starting

torque is reduced to 33 per cent of their values for full-voltage start.

2.7.1.6 VOLTAGE SAGS DUE TO TRANSFORMER ENERGIZING:

The causes for voltage sags due to transformer energizing are:

i. Normal system operation, which includes manual energizing of a transformer.

ii. Reclosing actions

The voltage sags are unsymmetrical in nature, often depicted as a sudden drop

in system voltage followed by a slow recovery. The main reason for transformer

energizing is the over-fluxing of the transformer core which leads to saturation.

Sometimes, for long duration voltage sags, more transformers are driven into

saturation. This is called Sympathetic Interaction. Fig.2.6 shows the voltage sag due

to transformer energizing

A large impulse current will be generated when the transformer injects to the

power system with no-load. The relationship among transient magnetic flux ϕ, static

maximum main magnetic flux and input initial phase angle α is derived in equation

Seen from the equation, the maximum value of transient main magnetic flux

could be twice as the value in static when the initial phase angle input as zero.

Considering the residual magnetism, it will reach to 2.3 times. Since the iron core is

saturated when the transformer runs normally, the excitation current could reach to

several ten times of regular value and it is so enough to cause voltage sag.

Fig.2.6: Voltage sag due to transformer energizing

RM

S V

olt

age

(V)

Time (cycles)


Page 18

2.7.1.7 ESTIMATING VOLTAGE SAG PERFORMANCE:

It is important to understand the expected voltage sag performance of the

supply system so that facilities can be designed and equipment specifications

developed to assure the optimum operation of production facilities. The following is a

general procedure for working with industrial customers to assure compatibility

between the supply system characteristics and the facility operation:

1. Determine the number and characteristics of voltage sags that result from

transmission system faults.

2. Determine the number and characteristics of voltage sags that result from

distribution system faults (for facilities that are supplied from distribution

systems).

3. Determine the equipment sensitivity to voltage sags. This will determine the

actual performance of the production process based on voltage sag

performance calculated in steps 1 and 2.

4. Evaluate the economics of different solutions that could improve the

performance, either on the supply system (fewer voltage sags) or within the

customer facility (better immunity).

2.7.1.8 ESTIMATING THE COSTS FOR THE VOLTAGE SAG EVENTS:

The costs associated with sag events can vary significantly from nearly zero to

several million dollars per event. The cost will vary not only among different industry

types and individual facilities but also with market conditions. Higher costs are

typically experienced if the end product is in short supply and there is limited ability

to make up for the lost production. Not all costs are easily quantified or truly reflect

the urgency of avoiding the consequences of a voltage sag event.

The cost of a voltage sag disturbance can be captured primarily through three

major categories:

Product-related losses, such as loss of product and materials, lost production

capacity, disposal charges, and increased inventory requirements.

Labour-related losses, such as idled employees, overtime, clean up, and

repair.

Auxiliary costs such as damaged equipment, lost opportunity cost, and

penalties due to shipping delays.


Page 19

2.7.2 VOLTAGE SWELL

Fig.2.7: Example of a Voltage Swell

An increase in RMS voltage or current at the power frequency for duration of

0.5cycles to 1 minute, called voltage swells. Swells have serious impact on equipment

function; however, they are not as common as sags. Fig 2.8 illustrates an example for

voltage sag.

Table.2.1: Types of Voltage Swell

Causes

Voltage swells are usually associated with system fault conditions - just like

voltage sags but are much less common. Due to a single line-to-ground (SLG) fault on

the system, the result is a temporary voltage rise on the un-faulted phases, which last

for the duration of the fault. Voltage swells can also be caused by the de-energization

of a very large load. The abrupt interruption of current can generate a large voltage,

per the formula Moreover, the energization of a large capacitor bank can also cause a

voltage swell, though it more often causes an oscillatory transient.

Effects:

The effects of a voltage swell are often more destructive. It may cause

breakdown of components on the power supplies of the equipment, though the effect

may be a gradual, accumulative effect. It can cause control problems and hardware

failure in the equipment, due to overheating that could eventually result to shut down.

Also, electronics and other sensitive equipment are prone to damage due to voltage

swell.

0 0.05 0.1 0.15-1.5

-1

-0.5

0

0.5

1

1.5

Time (sec)

mag

nitu

de

Voltage swell Magnitude Duration

Instantaneous 1.1 to1.8 0.5 cycle to 30 cycle

Momentary 1.1to 1.4 30 cycle to 3 sec.

Temporary 1.1 to1.2 3 sec. to 1 min.


Page 20

2.7.3 VOLTAGE FLUCTUATIONS:

A variation in input voltage(shown in Fig.2.8), either magnitude or frequency,

sufficient in duration to allow visual observation of a change in electric light source

intensity left unchecked, high and low-voltage conditions can result in equipment

damage, data lose erroneous readings on monitoring systems. Overloaded power

circuits are typically the cause behind under voltage conditions. Heavily loaded

motors such as air conditioners can result in intermittent low voltages. Less common

but more damaging, facilities with rapidly varying loads can cause over voltage

conditions.

Fig.2.8: Voltage Flicker Caused by Furnace Operation

Method of Characterization

• Frequency ,Magnitude

• 90 to 110 V

• 8 to 10 Hz

Causes

• Non-linear load

• Welding and arcing devices

Vulnerable equipment:

Computers; fax machines; variable frequency drives; CNC machines;

extruders; motors

Effects:

Data errors; memory loss; equipment shutdown; flickering lights; motors

Stalling/stopping; reduced motor life

2.7.4 TRANSIENTS:

The main difficulty with transients is in detection, since they manifest only as

a short duration change in voltage. The switching on and off of the electric motors


Page 21

those power air conditioners, power tools, furnace ignitions, electrostatic copiers, arc

welders and elevators causes low energy swells. Lighting usually causes larger swells.

Electrical noise is another, milder transient power irregularity that often manifests as a

computer glitch rather than an equipment failure. Essentially, electrical noise is

created when one piece of equipment interacts negative with another, or with building

grounding or wiring. Loose connections or the equipment itself can be responsible for

noise. Known noise-generating equipment includes everything from computers, radios

and fluorescent lights to fax machines, welders and light sockets

2.7.4.1 IMPULSIVE TRANSIENTS: It is unidirectional.

Fig.2.9: Lighting stroke current impulsive transient

2.7.4.2 OSCILLATORY TRANSIENTS: It is bidirectional.

Fig.2.10: Oscillatory Transients

Where do Transient voltages come from?

Transients can be generated internally, or can come into a facility from

external sources. The least common of the two are externally generated transients.

They've been described as "electronic rust".

External sources:

• Lightning

• Switching on or off of facility loads,

• Opening and closing of disconnects on energized lines,

• Switching of capacitor banks,


Page 22

• Re-closure operations and tap changing on transformers

• Poor or loose connections in the distribution system can also generate

• Transients.

Internal Sources:

• The vast majority of transients are produced within your own facility.

• The main culprits are device switching, static discharge, and arcing.

• Each time you turn on, turn off, load, or unload an inductive device, you

Produce a transient.

Arcing can generate transients from a number of sources. Faulty contacts in

breakers, switches, and contactors can produce an arc when voltage jumps the gap.

When this gap is "jumped" the voltage rises suddenly and the most common effect is

an oscillatory-ring-type transient.

What are the effects of transient activity?

Electronic Equipment:

• Electronic devices may operate erratically. Equipment could lock up or

produced garbled results.

• Electronic devices may operate at decreased efficiencies. Damage is not

readily seen and can result in early failure of affected device.

• Integrated circuits may fail immediately or fail prematurely.

Motors:

Motors will run at higher temperatures when transient voltages are present.

Transients can interrupt the normal timing of the motor and result in "micro-jogging".

This type of disruption produces motor vibration, noise, and excessive heat. Motor

winding insulation is degraded and eventually fails. Motors can become degraded by

transient activity to the point that they produce transients continually which

accelerates the failure of other equipment that is commonly connected in the facility's

electrical distribution system. Transients produce hysteresis losses in motors that

increase the amount of current necessary to operate the motor. Transients can cause

early failures of electronic motor drives and controls.

Lighting:

Transient activity causes early failure of all types of lights. Fluorescent

systems suffer early failure of ballasts, reduced operating efficiencies, and early bulb


Page 23

failures. One of the most common indicators of transient activity is the premature

appearance of black "rings" at the ends of the tubes. Transients that are of sufficient

magnitude will cause a sputtering of the anodes--when these sputters deposit on the

insides of the tube, the result is the black "ends" commonly seen. Incandescent lights

fail because of premature filament failures. The same hysteresis losses produced in

motors are reproduced in transformers. Do you want to see a graphic illustration of

the results of transient activity on fluorescent tubes? Look at the ends of your

tubes.....see those dark rings? Effective transient suppression will eliminate those

rings and make your bulbs last 4 to 6 times longer.

Electrical Distribution Equipment:

The facility's electrical distribution system is also affected by transient

activity. Transient degrade the contacting surfaces of switches, disconnects, and

circuit breakers. Intense transient activity can produce "nuisance tripping" of breakers

by heating the breaker and "fooling" it into reacting to a non-existent current demand.

Electrical transformers are forced to operate inefficiently because of the hysteresis

losses produced by transients and can run hotter than normal.

2.7.5 HARMONICS:

Harmonics are sinusoidal voltages or currents having frequencies that are

whole multiples of the frequency at which the supply system is designed to operate.

Any signal component having a frequency which is not an integer multiple of the

fundamental frequency is designated as an inter-harmonic component or referred to

more simply as an inter-harmonic.

Harmonics and inter-harmonics are basically the result of modern

developments in electricity utilization and the use of electronic power conditioning

modules. Using switching power supplies to control loads and to reduce power

consumption results in unwanted frequencies superimposed on the supply voltage.

The presence of voltage at other frequencies is, as far as possible, to be avoided

Sources of Harmonics:

Single phase load

• Switched mode power supplies (SMPS)

• Electronic fluorescent lighting ballasts

• Small uninterruptible power supplies (UPS) units


Page 24

Fig.2.11: Second and Third Harmonics in voltage waveform.

Three phase loads

• Variable speed drives

• Large UPS units

Effects of Harmonics:

Problems caused by harmonic currents

• Overloading of neutrals

• Overheating of transformers

• Nuisance tripping of circuit breakers

• Skin effect

Problems caused by harmonic voltages

• Voltage distortion

• Induction motors

• Zero-crossing noise

Problems caused when harmonic currents reach the supply

• Neutral conductor over-heating

• Effects on transformers

• Nuisance tripping of circuit breakers

• Over stressing of power factor correction capacitor banks

2.7.6 INTERRUPTION:

An interruption is a complete power loss, which can last a second or several

hours. There are momentary and sustained interruptions. A momentary interruption is

a voltage loss (<10% of nominal) for a time period between 0.5 cycles and 3 seconds).


Page 25

Fig.2.12: Interruption

A temporary interruption is a voltage loss (<10% of nominal) for at time

period between 3 seconds and 1 min. A sustained interruption is the complete loss of

voltage for a time period greater than 1 min.

Momentary interruptions are more common and can happen 6 to 12 times a

year. Most utility companies such as PG&E have devices to keep these interruptions

temporary, clearing them within a few seconds. The most noticeable result of a

momentary interruption is a blinking digital clock. Sustained interruptions typically

last between 30 minutes to several hours. They occur less often, once or twice a year.

A UPS/battery backup helps protect you against a complete loss of power.

Effects of Interruptions

• Stoppage of sensitive equipment (i.e. computers, PLC, ASD)

• Unnecessary tripping of protective devices

• Loss of data

• Malfunction of data processing equipment.

2.8 POWER QUALITY ISSUES MITIGATION TECHNIQUES:

Solutions to improve the reliability and performance of a process or facility

can be applied at many different levels. The different technologies available should be

evaluated based on the specific requirements of the process to determine the optimum

solution for improving the overall voltage sag performance. The solutions can be

discussed at the following different levels of application:

1. Protection for small loads [e.g., less than 5 kilovolt amperes (KVA)]. This

usually involves protection for equipment controls or small, individual

machines. Many times, these are single-phase loads that need to be protected.


Page 26

2. Protection for individual equipment or groups of equipment up to about

300KVA. This usually represents applying power conditioning technologies

within the facility for protection of critical equipment that can be grouped

together conveniently. Since usually not all the loads in a facility need

protection, this can be a very economical method of dealing with the critical

loads, especially if the need for protection of these loads is addressed at the

facility design stage.

3. Protection at the medium-voltage level or on the supply system. If the whole

facility needs protection or improved power quality, solutions at the medium-

voltage level can be considered.

The size ranges in these categories are quite arbitrary, and many of the

technologies can be applied over a wider range of sizes. The following sections

describe the major technologies available and the levels where they can be applied.

1) Uninterruptable Power Supply (UPS)

A continuous power supply will be used during the period of voltage sag/swell

in high efficient as 92%~97%. An expensive expense and restricted capacity is the

disadvantage of UPS.

2) Constant Voltage Transformer (CVT)

It’s generally used under 20KVA power system. A balanced voltage will be

supplied even the voltage drops to the 70% of normal value and its efficiency is

between 70%~75%.

3) Static Transfer Switch (STS)

It’s set in a dual-power system. When one of the power supplies has problem,

the STS will switch to another one as the power supply for the load.

4) Transformer Tap-Change (TC)

It can reduce the effect of voltage sag/swell during a certain extent, which is

determined by the adjusting range of the tap changer.

5) Motor-Generator (MG)

The inertia of the motor could be used to keep the normal voltage when the

voltage sag/swell happens.

6) Dynamic Voltage Regulator (DVR)

It’s the cheapest device according to the other ones. It has a high efficient,

since the DVR only works when the voltage sag/swell happens.

Dynamic Voltage Restorer (DVR)

Page 27

CHAPTER 3

DYNAMIC VOLTAGE RESTORER (DVR)

3.1 DYNAMIC VOLTAGE RESTORER (DVR):

Among the power quality problems (sags, swells, harmonics…) voltage sags

are the most severe disturbances. In order to overcome these problems the concept of

custom power devices is introduced recently. One of those devices is the Dynamic

Voltage Restorer (DVR) [23], which is the most efficient and effective modern

custom power device used in power distribution networks. DVR is a recently

proposed series connected solid state device that injects voltage into the system in

order to regulate the load side voltage. It is normally installed in a distribution system

between the supply and the critical load feeder at the point of common coupling

(PCC). Other than voltage sags and swells compensation, DVR can also added other

features like: line voltage harmonics compensation, reduction of transients in voltage

and fault current limitations.

3.2 PRINCIPLE OF DVR OPERATION:

A DVR is a solid state power electronics switching device consisting of either

MOSFET or IGBT, a capacitor bank as an energy storage device and injection

transformers. It is connected in series between a distribution system and a load that

shown in Fig.3.1. The basic idea of the DVR is to inject a controlled voltage

generated by a forced commuted converter in a series to the bus voltage by means of

an injecting transformer. A DC capacitor bank which acts as an energy storage device,

provides a regulated dc voltage source. A DC to Ac inverter regulates this voltage by

sinusoidal PWM technique.

Fig.3.1: Location of DVR


Page 28

SENSITIVE

LOAD

DVR

CONVERTER

ENERGY

STORAGE

SUPPLY

Voltage Sag Injected Voltage Restored Voltage

Fig.3.2: Principle of DVR with a response time of less than one millisecond

During normal operating condition, the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses.

However, when voltage sag occurs in the distribution system, the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load

by injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load.

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system. The response time of DVR is very short and is limited by the

power electronics devices and the voltage sag detection time. The expected response

time is about 25 milliseconds, and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers.

3.3 BASIC CONFIGURATION OF DVR:

The general configuration of the DVR consists of:

i. An Injection/ Booster transformer

ii. A Harmonic filter

iii. Storage Devices

iv. A Voltage Source Converter (VSC)

v. DC charging circuit

vi. A Control and Protection system


Page 29

DVR

ENERGY

STORAGE

SENSITIVE

LOAD

SUPPLY

LARGE

LOAD

CBPCC

FILTER

CONTROL

UNIT

Fig.3.3: Schematic diagram of DVR

3.3.1 INJECTION/ BOOSTER TRANSFORMER:

The Injection / Booster transformer is a specially designed transformer that

attempts to limit the coupling of noise and transient energy from the primary side to

the secondary side. Its main tasks are:

It connects the DVR to the distribution network which transforms and couples

the injected compensating voltages generated by the voltage source converters

to the incoming supply voltage.

In addition, the Injection / Booster transformer serves the purpose of isolating

the load from the system (VSC and control mechanism).

3.3.2 HARMONIC FILTER:

The main task of harmonic filter is to keep the harmonic voltage content

generated by the VSC to the permissible level.

3.3.3 VOLTAGE SOURCE CONVERTER:

A VSC is a power electronic system consists of a storage device and switching

devices, which can generate a sinusoidal voltage at any required frequency,

magnitude, and phase angle. In the DVR application, the VSC is used to temporarily

replace the supply voltage or to generate the part of the supply voltage which is

missing.


Page 30

There are four main types of switching devices: Metal Oxide Semiconductor

Field Effect Transistors (MOSFET), Gate Turn-Off thyristors (GTO), Insulated Gate

Bipolar Transistors (IGBT), and Integrated Gate Commutated Thyristors (IGCT).

Each type has its own benefits and drawbacks. The IGCT is a recent compact device

with enhanced performance and reliability that allows building VSC with very large

power ratings. Because of the highly sophisticated converter design with IGCTs, the

DVR can compensate dips which are beyond the capability of the past DVRs using

conventional devices.

The purpose of storage devices is to supply the necessary energy to the VSC

via a dc link for the generation of injected voltages. The different kinds of energy

storage devices are Superconductive magnetic energy storage (SMES), batteries and

capacitance.

3.3.4 DC CHARGING CIRCUIT:

The dc charging circuit has two main tasks.

The first task is to charge the energy source after a sag compensation event.

The second task is to maintain dc link voltage at the nominal dc link voltage.

3.3.5 CONTROL AND PROTECTION:

The control mechanism of the general configuration typically consists of

hardware with programmable logic. All protective functions of the DVR should be

implemented in the software. The basic proportional control scheme is implemented

which is discussed below. Firstly the error signal is detected by the comparison of the

sag voltage with the supply voltage and this error signal is proportionally used to the

generation of the pulses which are given to the switches of the voltage source inverter.

Equations related to DVR

Zline ZDVR

Load

Vsource

Vinj

Vload

Iload

Fig. 3.4: Equivalent circuit diagram of DVR


Page 31

The system impedance depends on the fault level of the load bus. When

the system voltage ( ) drops, the DVR injects a series voltage through the

injection transformer so that the desired load voltage magnitude can be maintained.

The series injected voltage of the DVR can be written as

VDVR = VL + ZTH IL - VTH

Where VL: The desired load voltage magnitude

ZTH: The load impedance.

IL: The load current

VTH: The system voltage during fault condition

The load current IL is given by,

L LL

P jQI

V

When VL is considered as a reference equation can be rewritten as,

0 0 ( )DVR L TH THV V Z V

∝, β, δ are angles of VDVR, ZTH, VTH respectively and θ is Load power angle

1tan L

LP

The complex power injection of the DVR can be written as,

*

DVR DVR LS V I

It requires the injection of only reactive power and the DVR itself is capable of

generating the reactive power.

3.4 OPERATING MODES OF DVR:

The basic function of the DVR is to inject a dynamically controlled voltage VDVR

generated by a forced commutated converter in series to the bus voltage by means of a

booster transformer. The momentary amplitudes of the three injected phase voltages

are controlled such as to eliminate any detrimental effects of a bus fault to the load

voltage VL. This means that any differential voltages caused by transient disturbances

in the ac feeder will be compensated by an equivalent voltage generated by the

converter and injected on the medium voltage level through the booster transformer.

The DVR has three modes of operation which are: protection mode, standby

mode, injection/boost mode.


Page 32

3.4.1 PROTECTION MODE/BYPASS MODE:

If the over current on the load side exceeds a permissible limit due to short circuit

on the load or large inrush current, the DVR will be isolated from the systems by

using the bypass switches (S2 and S3 will open) and supplying another path for current

(S1 will be closed) as shown in Fig.3.5.

The DVR is disconnected in case of short circuit occurring inside the facility to

protect its sensitive components from excessive short circuit currents.

Fig.3.5: Protection Mode (creating another path for current)

3.4.2 STANDBY MODE: (VDVR= 0)

In the standby mode the inverter is not active in the circuit to keep the losses to a

minimum. The booster transformer’s low voltage winding is shorted through the

converter. No switching of semiconductors occurs in this mode of operation and the

full load current will pass through the primary.

Fig.3.6: Standby Mode

3.4.3 ACTIVE/INJECTION/BOOST MODE: (VDVR >0)

In the Injection/Boost mode the DVR is injecting a compensating voltage through

the booster transformer due to the detection of a disturbance in the supply voltage.


Page 33

3.5 VOLTAGE INJECTION METHODS OF DVR:

Voltage injection or compensation methods by means of a DVR depend upon the

limiting factors such as; DVR power ratings, various conditions of load, and different

types of voltage sags. Some loads are sensitive towards phase angel jump and some

are sensitive towards change in magnitude and others are tolerant to these. Therefore

the control strategies depend upon the type of load characteristics.

There are four different methods of DVR voltage injection which are

i. Pre-sag compensation method

ii. In-phase compensation method

iii. In-phase advanced compensation method

iv. Voltage tolerance method with minimum energy injection

3.5.1 PRE-SAG/DIP COMPENSATION METHOD:

The pre-sag method tracks the supply voltage continuously and if it detects

any disturbances in supply voltage it will inject the difference voltage between the sag

or voltage at PCC and pre-fault condition, so that the load voltage can be restored

back to the pre-fault condition. Compensation of voltage sags in the both phase angle

and amplitude sensitive loads would be achieved by pre-sag compensation method.

DVR prefault sagV V V

θDVRθS

θL

VDVR

IL

VL

VS

Fig.3.7: Pre-sag compensation method


Page 34

3.5.2 IN-PHASE COMPENSATION METHOD:

θS

θL VSag VDVR

IL

Vprefault

VL

Fig.3.8: In-phase compensation method

The phase angles of the pre-sag and load voltage are different but the most

important criteria for power quality that is the constant magnitude of load voltage are

satisfied. This is the most straight forward method. In this method the injected voltage

is in phase with the supply side voltage irrespective of the load current and pre-fault

L prefaultV V

One of the advantages of this method is that the amplitude of DVR injection

voltage is minimum for certain voltage sag in comparison with other strategies.

Practical application of this method is in non-sensitive loads to phase angle jump

3.5.3 IN-PHASE ADVANCED COMPENSATION METHOD:

In this method the real power spent by the DVR is decreased by minimizing

the power angle between the sag voltage and load current. In case of pre-sag and in

phase compensation method the active power is injected into the system during

disturbances. The active power supply is limited stored energy in the DC links and

this part is one of the most expensive parts of DVR. The minimization of injected

energy is achieved by making the active power component zero by having the

injection voltage phasor perpendicular to the load current phasor.

In this method the values of load current and voltage are fixed in the system so

we can change only the phase of the sag voltage. IPAC method uses only reactive

power and unfortunately, not al1 the sags can be mitigated without real power, as a

consequence, this method is only suitable for a limited range of sags.


Page 35

3.5.4 VOLTAGE TOLERANCE METHOD WITH MINIMUM ENERGY

INJECTION:

Fig.3.9: Voltage tolerance method with minimum energy injection

A small drop in voltage and small jump in phase angle can be tolerated by the

load itself. If the voltage magnitude lies between 90%-110% of nominal voltage and

5%-10% of nominal state that will not disturb the operation characteristics of loads.

Both magnitude and phase are the control parameter for this method which can be

achieved by small energy injection.

Design And Specifications of DVR

Page 36

CHAPTER 4

DESIGN AND SPECIFICATIONS OF DYNAMIC

VOLTAGE RESTORER

This chapter deals with some general design procedures for the Dynamic

Voltage Restorer which are presented and followed by a more specific design. The

DVR prototype is constructed in conjunction with the M.Tech project. At the end of

the chapter the main specifications for the hardware setup is presented.

4.1 DESIGN PARAMETERS:

A simplified model for the DVR is illustrated as shown in Fig. 4.1

Vsupply

Vconv

+

-

Zsupply RDVR XDVR

+-VDVR

+

-

VLOADZLOAD

+-

Fig.4.1: Single-phase simplified model for the DVR.

Where Vsupply is the supply voltage, Zsupply is the source impedance, VDVR is the DVR

voltage, VLOAD is the load voltage and ZLOAD is the load impedance.

4.1.1 VOLTAGE AND CURRENT RATING:

The Simplified model which is illustrated in Fig.4.1 is a single phase DVR

which consists of a controllable voltage source and a fixed resistance. This is

equivalent to the losses in the DVR and a fixed reactance and is equivalent to the

reactive elements in the DVR. The main design parameters for the DVR are the

voltage injection capability, the current handling capability and the size of the energy

storage. The voltage injection capability can be expressed as:

%

,

100%DVRDVR

Supply rated

Vv

V

The voltage injection capability should be chosen as low as necessary in order

to reduce equipment cost and standby losses. Losses will tend to increase if the

voltage rating of the DVR is increased if it is assumed that current rating of the DVR

current is fixed. The DVR resistance is mathematically expressed as:


Page 37

,DVR

DVR DVR R

DVR

VR v

I

The losses in the DVR can be grouped as losses in the transformer followed by

filter and the converter losses. The voltage injection is mostly decided based on the

requirements of compensating symmetrical and non-symmetrical voltage dips

followed by deciding the size & current rating of the DVR. As the DVR is a series

connected device, the design of the current handling capability of the DVR depends

upon:

In-rush phenomenon: as for the down-stream loads, such as starting of large

motors and magnetization of transformers.

Non-linear loads: Leads to increase in higher peak currents.

P (U) and Q (U) characteristics: as for the down-stream loads; If the DVR is

utilized to compensate severe voltage dips, the load currents may increase.

Future load extension: Currents may vary if else load is extended further.

Standby losses: Increase in the current rating of DVR tends to decrease in

standby losses of the system.

The main parameter to decide the current rating of the VSC is based upon the

peak currents and the RMS currents of the grid. The main feature of IGBT switch is

that it has an overload current limit, which helps to influence the system during short

circuit and heavy loading conditions. The current handling capability of a DVR can be

defined as:

%

,

100%DVRDVR

load rated

Ii

I

4.2 DESIGN OF THE DVR ELEMENTS:

The most attractive mode of the thesis has come to an end by introducing this

section which includes the design of the DVR. There are many constraints which are

the most essential part to be considered while designing. The general configuration of

the DVR consists of:

i. Voltage Source Converter

ii. Harmonic Filter

iii. Injection/Booster Transformer


Page 38

iv. Storage Devices

v. Detection and Control Block

4.2.1 DESIGN OF VOLTAGE SOURCE CONVERTER:

Voltage Source Converter has been used over here for the development of

DVR, which is equipped with fully controllable switches. The three most common

switch types considered in the required power range are MOSFET’s, IGBT’s and

IGCT’s. The MOSFET switch has been chosen for VSC converters, because it is easy

to trigger and best suited for the actual power range. The IGBT is not necessarily the

best switch, because the current limiting behaviour is not particularly as per the

requirement of DVR.

DVR

ENERGY

STORAGE

SENSITIVE

LOAD

SUPPLY

LARGE

LOAD

CBPCC

FILTER

CONTROL

UNIT

Fig.4.2: Block diagram of Dynamic Voltage Restorer

4.2.1.1 INVERTERS:

A Power electronic device that converts DC power to AC power at the desired

output voltage and frequency is called an Inverter. Phase controlled converters when

operated in the inverter mode are called line commutated inverters. But line

commutated inverters requires an existing AC supply at the output terminals which is

used for their commutation purpose. This means that line commutated inverters

cannot function as isolated AC voltage sources or as variable frequency generators

with DC power at the input. Therefore voltages and frequencies on the AC side of the

line commutated inverters cannot be altered. On the other hand, forced commutated

inverters provide an independent AC output voltage with an adjustable voltage and

adjustable frequency and therefore it has wider applications.

Based on the operations, Inverters can be broadly classified into two types:

1. Voltage Source Inverters (VSI)

2. Current Source Inverters (CSI)


Page 39

1. Voltage Source Inverter: This type of inverter produces an AC output

voltage which can be independently controlled and isolated from load side. Due to

this feature, the VSI have many industrial applications such as adjustable speed drives

(ASD) and also in Power system especially in the field of FACTS (Flexible AC

Transmission).

2. Current Source Inverter: This type of inverter produces an AC output

current which can be independently controlled and isolated from load side. These are

widely used in medium voltage industrial applications, where high quality output is

required.

Single Phase Full Bridge Inverter: It consists of two arms with a two power

semiconductor switches on each arm with anti-parallel freewheeling diodes embedded

with it (for discharging the reverse current). In case of resistive /inductive loads, the

reverse load current flow through these diodes. These diodes provide an alternative

path to inductive current which continues to flow during the turn OFF condition.

Fig.4.3: Single Phase Full wave Bridge Inverter

Table 4.1: Switching States

S1 S2 S3 S4 VA VB VAB

ON OFF OFF ON 2

SV -

2

SV VS

OFF ON ON OFF +2

SV +

2

SV - VS

ON OFF ON OFF 2

SV -

2

SV 0

OFF ON OFF ON -2

SV +

2

SV 0


Page 40

The switches S1 & S2 or S3 & S4 should operate in a pair to produce an

output. These arms of the bridge are switched such that the output voltage is shifted

from one to another and hence the change in polarity occurs in output voltage. If the

shift angle is zero, the output voltage is also zero & if the shift angle is π, then

maximum voltage develops at the output. The schematic diagram of the inverter

shown in the Fig.4.3 and hardware is implemented as shown in Fig.4.4.

Fig.4.4: Hardware implementation of Inverter circuit.

4.2.1.2 PULSE WIDTH MODULATION (PWM):

The Pulse Width Modulation (PWM) is a technique which is characterized by

the generation of constant amplitude pulse by modulating the pulse duration by

modulating the duty cycle. Analog PWM control requires the generation of both

reference and carrier signals that are fed into the comparator and based on some

logical output, the final output is generated. The reference signal is the desired signal

output may sinusoidal or square wave, while the carrier signal is either a sawtooth or

triangular wave at a frequency significantly greater than the reference.

There are various types of PWM techniques and so we get different output and

the choice of the inverter depends on cost, noise and efficiency.


Page 41

Basic PWM Techniques

There are three basic PWM techniques:

1. Single Pulse Width Modulation

2. Multiple Pulse Width Modulation

3. Sinusoidal Pulse Width Modulation

1. Single Pulse Width Modulation:

In this modulation there is an only one output pulse per half cycle. The output

is changed by varying the width of the pulses. The gating signals are generated by

comparing a rectangular reference with a triangular reference. The frequency of the

two signals is nearly equal.

Fig.4.5: Single Pulse Width Modulation

The rms ac output voltage 0

22ON

S S

tV V V

T

Where

=duty cycle= ONt

T

Modulation Index (MI) = r

c

V

V

Where Vr = Reverence signal voltage

Vc = Carrier signal voltage

By varying the control signal amplitude Vr from 0 to Vc the pulse width ton can be

modified from 0 secs to T/2 secs and the rms output voltage Vo from 0 to Vs.

2. Multiple Pulse Width Modulation:

In this modulation there is multiple numbers of output pulses per half cycle

and all pulses are of equal width. The gating signals are generated by comparing a

rectangular reference with a triangular reference. The frequency of the reference

signal sets the output frequency (fo) and carrier frequency (fc). The number of pulses

per half cycle is determined by p: 02

Cfpf


Page 42

Fig.4.6: Multiple Pulse Width Modulation


0

1

p

mS S

m

pV V V

Where =duty ratio= ONt

T

The variation of modulation index (MI) from 0 to 1 varies the pulse from 0 to π/p and

the output voltage from 0 to Vs.

3. Sinusoidal Pulse Width Modulation:

In this modulation technique, there are multiple number of output pulse per

half cycle and pulses are of different width. The width of each pulse is varying in

proportion to the amplitude of a sine wave evaluated at the centre of the same pulse.

The gating signals are generated by comparing a sinusoidal reference with a high

frequency triangular signal.


0

1

p

mS S

m

pV V V

Where p=number of pulses and δ= pulse width

Sinusoidal pulse-width modulation technique (SPWM) is one of the most

popular modulation techniques applied in power switching converters. This technique

has many inherent advantages including simplicity of implementation, relatively low

harmonics on the output, low switching losses, etc. The fundamental idea of the

SPWM technique is to compare a high frequency signal known as the carrier that is a

triangular signal with frequency fs to a low frequency signal known as the modulating

signal that is usually a sinusoidal signal with frequency fm. The sinusoidal


Page 43

modulating signal has the same frequency as that of the desired output of the power

switching converter.

SPWM technique is used to control the fundamental component of the line-to-

line converter voltage. Three-phase converter voltages are obtained by comparing the

same triangular voltage with three sinusoidal control voltages as shown in Fig.4.7.

The frequency of the triangular voltage (fs-carrier frequency) determines the converter

switching frequency and the frequency of the control voltages determine the

fundamental frequency of the converter voltage (f1-modulating frequency). Hence,

modulating frequency is equal to supply frequency in DVR.

Fig.4.7: Sinusoidal Pulse Width Modulation Technique

Amplitude modulation ratio ma is defined as: controla

tri

Vm

V

Where Vcontrol is the peak amplitude of the control voltage

Vtri is the peak amplitude of the triangular voltage.


Page 44

The magnitude of the triangular voltage is kept constant and the amplitude of

the control voltage is allowed to vary. Linear range of SPWM is defined for 0≤ma≤1

and over modulation is defined for ma<1.

Frequency modulation ratio mf is defined as: 1

sf

fm

f

mf must be chosen as an odd integer to form an odd and half wave symmetric

converter line-to-neutral voltage (VA0). Therefore, even harmonics are eliminated

from the VA0 waveform. Moreover, mf is chosen as a multiple of 3 in order to

eliminate the harmonics at mf and odd multiples of mf in the converter line-to-line

voltages. Harmonics in the converter voltages appear as sidebands, centered on the

switching frequency and its multiples. This is true for all values of ma in the linear

range. The frequencies of converter output voltage harmonics can be expressed as:

1( )h ff jm k f

The fundamental component of the converter line-to-neutral voltage varies

linearly with the amplitude modulation ratio ma irrespective of the frequency

modulation ratio mf as shown in. Fundamental component of the converter line-to-line

voltage is also expressed as:

0ˆ ; 1

2

dA a a

VV m m

1

30.612 ; 1

2 2LL a d a d aV m V m V m

4.2.1.3 TOTAL HARMONIC DISTORTION (THD):

The total harmonic distortion, or THD, of a signal is a measurement of the

harmonic distortion and is defined as the ratio of the sum of the powers of all

harmonic components to the power of the fundamental frequency.

When the input is a pure sine wave, the measurement is most commonly the

ratio of sum of the powers of all higher harmonic frequencies to the power at the first

harmonic or fundamental frequency.

2 3 4 2

1 1

n

n

PP P P P

THDP P

This can be written as

1

1

totalP PTHD

P


Page 45

Measurements based on amplitudes (e.g. voltage or current) must be converted

to powers to make addition of harmonics distortion meaningful. For a voltage signal,

for example, the ratio of the squares of the RMS voltages is equivalent to the power

ratio

2 2 2 2

2 3 4

2

1

V V V VTHD

V

Where Vn is the RMS voltage of nth harmonic and n=1 is the fundamental

frequency. THD is also commonly defined as an amplitude ratio rather than a power

ratio. Resulting in a definition of THD which is the square root of that given above

2 2 2 2

2 3 4

1

nV V V VTHD

V

Measurements for calculating the THD are made at the output of a device under

specific conditions. The THD is usually expressed in percent as distortion factor or in

dB as distortion attenuation.

4.2.1.4 DSPACE RTI 1104:

By using the above SPWM technique pulses has been generated in

MATLAB/Simulink which has been fed to the switches of the inverter, but in order to

get the real time pulses we have used an interface for simulated pulses and inverter

known as dSPACE RTI 1104.

The dSPACE system is based on the DS1104 R&D Controller Board which

comprises both hardware and software. The DS1104 R&D Controller Board is a

standard board that can be plugged into a PCI slot of a PC. The DS1104 is specifically

designed for the development of high-speed multivariable digital controllers and real-

time simulations in various fields. It is a complete real-time control system based on a

603 PowerPC floating-point processor running at 250 MHz. For advanced I/O

purposes, the board includes a slave-DSP sub-system based on the TMS320F240 DSP

microcontroller.

For purposes of rapid control prototyping (RCP), specific interface connectors

and connector panels provide easy access to all input and output signals of the board.

Thus, the DS1104 R&D Controller Board is the ideal hardware for the dSPACE

Prototype development system for cost-sensitive RCP applications.

Using an adapter cable one can link external signals from the 100-pin I/O

connector on the board to Sub-D connectors. So it can make a high-density


Page 46

connection between the board and the devices of specified application via Sub-D

connectors.

Specific interface connector panels provide easy access to all the input and

output signals of the DS1104 R&D Controller Board:

The CP1104 Connector Panel provides easy-to-use connections between the

DS1104 R&D Controller Board and devices to be connected to it. Devices can

be individually connected, disconnected or interchanged without soldering via

BNC connectors and Sub-D connectors. This simplifies system construction,

testing and troubleshooting.

In addition to the CP1104, the CLP1104 Connector/LED Combi Panel

provides an array of LEDs indicating the states of the digital signals.

Fig.4.8: dSPACE RTI 1104 connector panels

Connector Pins: The CP1 to CP16 connectors are female BNC connectors.

Their shells are connected to GND. Among this CP1 to CP8 are ADC connector pins

and CP9 to CP16 are DAC connector pins. Digital I/O Connector Slot CP17 & Slave

I/O PWM Connector CP18 which is used for interfacing signals from outer system.

As dSPACE can bear a maximum voltage of +/-10V it is to be cautioned that a

voltage divider must be used to maintain a constant voltage and to ensure that it

should not exceed the limit. The hardware sensing model used for this DVR has been

shown in Fig.4.9.


Page 47

230 V

1Ø AC

SUPPLY

100K

2K

Vou

t

Fig.4.9: Voltage Sensor

S outout

out

V RV

R R

The voltage rating of the outputs being gathered from dSPACE is of 5V which

is difficult to drive gate pulse required by the MOSFET switches of the inverter. A

minimum of +12V is required to TURN ON MOSFET switch and -12V is required to

TURN OFF MOSFET, So to amplify 5V of dSPACE gate signal, Gate driver circuit

is required.

1-Ф AC

Supply

470µ

F

47µ

F

470µF 47µF15V DC

5VDC

7815

7805

(a)


Page 48

5V

2 4

7 5

2 36

6N13

7

15 V

VC

C

VE

GN

D

N/C

N/C

N/C

IN A

IN B

GN

D

N/C V

S

470Ω

470Ω

47

0Ω

50Ω

50Ω

1KΩ

1KΩ 0.01µF

4.7V

TO SWITCH (S1)

1A 1Y 2A

2Y

3A 3Y

GN

D

VC

C

6A

6Y 5A

5Y

4A 4Y74LS07

GA

TE

DR

IVE

R

TO SWITCH

(S2)

TO SWITCH

(S3)

TO SWITCH

(S4)

GA

TE

DR

IVE

R

GA

TE

DR

IVE

R

GA

TE

DR

IVE

R

44

25

FROM dSPACE

BU

FF

ER

IC

OP

TO

CO

UP

LE

RD

RIV

ER

IC

(b)


Page 49

(c) (d)

Fig.4.10: (a) Power supply required to operate Gate driver, (b) Gate driver circuit of

IC 4425 (c) Hardware implementation of Gate driver circuit, (d) Pulses to Gate driver

from dSPACE Control Panel to interface

4.2.2 HARMONIC FILTER:

1. Inductor Design:

Magnetic cores used in power electronic applications like transformers

and inductors usually fall in four broad categories. The first is bulk metal, like

electrical steels which are processed from furnace into ingots and then hot and cold

rolled. Second is powdered core materials where are manufactured from various types

of iron powders mixed with special binding agents and then die-pressed into toroids,

EE cores and slugs. The third is ferrite materials which are ceramics of iron oxide,

alloyed with oxides or carbonate of Mn, Zn, Ni, Mg, or Co. The most recent category

is of metallic glasses where the bulk metal is rapidly quenched from molten state to

obtain a ‘glassy’ state without a regular arrangement of metallic atoms in the material.

One of the design objectives is to derive most general procedures for inductor

construction. Theoretically, it should be possible to accurately design the inductor

using just the property of permeability of the core material. But practically, the design

procedure for Ferrite, Amorphous and Powdered material is different, mainly because

vendors follow different conventions and specify the material properties in many

ways. Amorphous and powder cores also have nonlinear permeability, ie the

permeability varies with the applied field, temperature, air gap etc. Hence the design

procedure for different materials is heavily affected by the available data from


Page 50

vendors, and it is not possible to define a single generalized accurate design process

for all materials.

2. Capacitor Selection:

Metallised Polypropylene capacitors are AC capacitors that are

especially designed for high frequency current operation. These capacitors are

constructed from polypropylene films on which an extremely thin metal layer is

vacuum deposited. The metal layer typically consists of aluminium or zinc of

thickness in range of 0.02mm to 0.05mm. Several such layers are wound together in a

tubular fashion to get higher capacitance.

Metallised film capacitors are characterized by small size, wide operating

frequency range, low losses, and low to medium pulse handling capabilities, low

parasitic impedances and self-healing. In regular film-foil capacitors, if the electrode

foils of opposite potential are exposed to each other because of wearing away of the

dielectric, the foils will short and the capacitor will be destroyed. But in case of

metallised polypropylene capacitors, because of the extremely thin metal layer, the

contact points at the fault area are vaporised by the high energy density, and the

insulation between foils is maintained. Due to the above reasons, these capacitors are

perfectly suited for grid connected filter operation. The designing parameters of

inductor as 10 mH and capacitor as 105 µF of LC filter have been analysed from the

reference paper [24].

FR

OM

IN

VE

RT

ER

TO

IN

JE

CT

IO

N T

RA

NS

FO

RM

ER

10 mH

105

µF

LC FILTER

Fig.4.11: Design of LC Filter and its hardware implementation.

4.2.3 Injection/Booster Transformer:

The transformers used throughout this project should be run over by a polarity

test which is the most accurate point while making a coupling with source voltage. It

is based upon the process of aiding and apposing flux direction which plays a crucial


Page 51

role while placing the device in the circuit. By using injection transformer (after

running through polarity test) it has been shown in the Fig.4.12 clearly regarding the

connection of sensitive loads with the high loads.

The filtered inverter output is injected to the line with the help of an injection

transformer. The Injection / Booster transformer is a specially designed transformer

that attempts to limit the coupling of noise and transient energy from the primary side

to the secondary side. The basic function of this transformer is to connect the DVR to

the distribution network couples the injected compensating voltages generated by the

voltage source inverters to the incoming supply voltage and it has to maintain low

impedance on the load side to avoid voltage drop across the load. The design of this

transformer is very crucial because, it faces saturation, overrating, overheating, cost

and performance. The injected voltage may consist of fundamental, desired

harmonics, switching harmonics and dc voltage components. If the transformer is not

designed properly, the injected voltage may saturate the transformer and result in

improper operation of the DVR and a practical hardware has been designed as shown

in Fig.4.13

CONVERTER

ENERGY

STORAGE

LARGE

LOAD

FILTER

CONTROL

UNIT

1:1

230/230V1:1

115/115V

SENSITIVE

LOAD

SUPPLY

CB

PCC

DV

R

Fig.4.12: Transformers placement after polarity Test


Page 52

Fig.4.13: Hardware implementation of Injection Transformer.

4.2.4 Storage Devices:

DC energy storage device provides the real power requirement of the DVR

during compensation as shown in Fig.4.14 with the use of rectifier circuit. Various

storage technologies have been proposed including Flywheel energy storage, Super-

conducting magnetic energy storage (SMES) and Super capacitors these have the

advantage of fast response. An alternative is the use of lead-acid battery batteries were

until now considered of limited suitability for DVR applications since it takes

considerable time to remove energy from them. Finally, conventional capacitors also

can be used. But rectifier has been used here to convert 230 V AC to (0-230) V DC

which maintain variable DC supply rather than constant DC.

Fig.4.14: Equivalent design of energy storage device and its hardware implementation

4.2.5 Detection and Control Block:

The basic proportional control scheme is implemented which is discussed

below. Firstly the error signal is detected by the comparison of the sag voltage with

the supply voltage and this error signal is proportionally used to the generation of the

pulses which are given to the switches of the voltage source inverter.


Page 53

Fig.4.15: Complete Hardware set up of DVR

Discussion on Results

Page 54

CHAPTER 5

DISCUSSION ON RESULTS

5.1 MATLAB SIMULINK MODEL AND SIMULATION RESULTS

Now the scenario of real time DVR system comes into picture. The real word

proposed DVR topology and control algorithm has been used here for emergency

control during the voltage sag. The heavy load has been considered as the cause of

disturbance in the simulations. The test system is modeled first in

MATLAB/SIMULINK [25] software and enacted as hardware. Fig.5.1 shows the

intentional creation of voltage sag by switching ON large loads suddenly. Fig.5.2

shows the initial conditions, supply voltage of 230Vrms and load voltage of 200Vrms

when there is no occurrence of sag (Pre Sag condition). Switching ON of heavy load

of 1.3KW across a sensitive load has created voltage sag of 130Vrms which is

resembled in Fig.5.3 and Fig.5.4 shows the SIMULINK for mitigating 70% voltage

sag by introducing a DVR in series with the sensitive load. The DVR has been

modeled by its components in the MATLAB/SIMULINK software to make more

effect of real simulation results. A single phase inverter has been used so that each

phase could be controlled separately.

Fig.5.1: Creation of voltage sag using large loads


Page 55

Fig.5.5 shows the closed loop control using PI Controller to compare the sag

and produce the error signal to trigger as pulses. After the pulses are fed to inverter

which generates the output voltage as shown in Fig 5.6. After injecting these voltages

into the load in series with load voltage the occurred sag will be mitigated. The

Fig.5.7 shows the mitigation of voltage sag after the voltage parameter has been

injected by DVR

Fig.5.2: Output waveform supply voltage, supply current and load voltage

Fig.5.3: Creation of 70% sag in the load voltage


Page 56

Fig.5.4: Mitigation of sag using DVR

Fig.5.5: Closed-loop control using the PI controller


Page 57

Fig.5.6: Output voltage of the single phase inverter using with and without filter.

Fig.5.7: Mitigation of voltage sag using DVR


Page 58

5.2 HARDWARE ENACTMENT

The Hardware enactment had been so crucially built up. Each and every device

has been used with utmost care. Many practical problems have been observed while

developing this project throughout the tenure of this project. Even the day before

starting this thesis to script, small difficulties have been overcome to process out an

attractive and exact desired output of this project.

The main focus to describe the above thought is that all the switches used over

here, IC’s etc will never work as of ideal case. So cautiousness and constant patience

is more important while building up a hardware project.

Table.5.1: List of Hardware components

Sr

No

Component Rating Purpose

1 Buffer 74LS07 Driver Circuit

2 Regulator 7805, 7815 Driver Circuit

3 Driver IC MIC 4425 Driver Circuit

4 Opto Coupler 6N137 Driver Circuit

5 Capacitors 470 µF, 47 µF, 0.01 µF Driver Circuit

6 Resistors 50 Ω, 100 Ω, 470 Ω Driver Circuit

7 Bridge Rectifier MIC W08M Driver Circuit

8 Zener Diode 4.7 V Driver Circuit

9 Connectors 2 Pin, 5pin Driver Circuit

10 MOSFET IRF460 Inverter

11 Heat Sinks For Every Switch Inverter

12 Connectors 2 Pin, 4 Pin Inverter

13 Resistors 1 KΩ Inverter

14 Resistors 100 KΩ, 2 KΩ Voltage Divider

15 Capacitors 0.01 µF Voltage Divider

16 Connectors 4 Pin Voltage Divider

17 DC Capacitor 330 µF, 450 V DC-Link

18 Resistor 10 KΩ (10W) DC-Link

19 Bridge Rectifier MIC KBPC2510 DC-Link

20 Switch SPST DC-Link


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21 Dimmerstat (0-230)V, 8A, 1Φ Power Supply

22 dSpace RTI 1104 Trigger Pulse Interface

23 Transformers 230/230V, 1 KVA (1:1) Isolation

24 Step Down Transformer 230/115V, 2 KVA Isolation

25 Transformer 110/110V 1 KVA (1:1) Injection Transformer

26 Capacitor 105 µF, 250V Harmonic Filter

27 Inductor 10 mH Harmonic Filter

28 Rheostat 1 Ω, 5A Source Impedance

29 MCB 10 A, 240 V High Load Switch

30 Bulb 40 W, 230 V Sensitive Load

31 Bulb 1000 W, 230V High Load

5.2.1 MAIN SPECIFICATIONS:

Table.5.2: System parameters

Sr No Specifications Rating

1 Nominal Grid Voltage 110 Vrms

2 Nominal Load Voltage 110 Vrms

3 Switching/Sampling freq 1 KHz

5.3 PROBLEMS FACED:

While measuring triggering pulses of Driver circuit it is mandatory to use

differential probes to avoid many mis-matches and problems by measuring

with normal probes.

Specifications of Switches need be analyzed perfectly while choosing

MOSFET switches.

Gate resistors used for gate terminal of MOSFET Switches should be placed

near Switch and avoid placing near driver IC which may get overheated and

leads to damage to driver IC.

Transformers should be necessarily used for isolation purpose to avoid

problems while interfacing devices.


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Polarity test need to be checked out thoroughly before using any transformers

to clear out aiding or opposing techniques.

After measuring the output of inverter, analyze THD and identify the

harmonics, based on which designing of filters is processed.

5.3.1 BASIC CONDITIONS:

DVR is basically connected in series with the load.

The gate pulses provided for the inverter has been bypassed through dSPACE

system.

A Resistive load (Bulbs) has been used as shown in with a RMS load voltage

of 110 Volts for open loop.

Intentionally sag has been induced by switching a high resistive load (higher

ratings Bulbs) suddenly, which is connected along with actual sensitive load

(lower rating Bulb).

Differential probe has been used here with a multiplying factor of ‘20X’ for

every measurement throughout the tenure of the project.

5.4 MODES OF OPERATION:

1) Open Loop Control

2) Closed Loop Control

5.4.1 OPEN LOOP CONTROL:

The working of DVR can be segregated into two basic operational modes. One

of the modes is without controlling process known as open loop and with the help of

controlling process as the closed loop control.

Open loop control deals with the process of operating DVR model manually

whenever voltage sag occurs across the load. This process speaks about the

comparison of voltage sag with normal voltage to generate pulses and further feeding

it to inverter for generating an injected voltage. But this voltage can be injected only

when DVR is turned ON manually. So the mitigation of Sag [26] can be delayed by

processing it manually.


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The basic model of open loop control for pulse generation required in

hardware for DVR [27] using MATLAB Simulink has been shown in

Fig.5.8(a).

MATLAB simulink pulses have been interfaced with dSAPCE which are

shown in Fig.5.8 (b).

By using gate driver circuit pulses are amplified from 3Vp-p to 30Vp-p which is

shown in Fig.5.8 (c).

The Fig.5.9 speaks about the load working normally for sensitive load of Bulb

40 W with a Source voltage =16.0 V p-p (i.e. 16×20 = 320 Vp-p i.e

113.3Vrms).

Now suddenly when a high load of 1000W is switched ON a voltage sag has

been occurred across the load as shown in Fig.5.10 working under pre sag

condition with a Load voltage =15.6 V p-p (i.e. 15.6×20 = 312 Vp-p i.e

110.3Vrms) and it clearly shows the marks for various modes of operation.

The load voltage has been working with voltage dip developing voltage sag.

Point A resembles the occurrence of sag by switching ON high resistive load

which dips the sensitive load voltage and is clearly visualized in Fig.5.11

(12.4×20 = 248 Vp-p i.e 87.6 Vrms).

It means that sag has been occurred for 64 Vp-p as shown in Fig.5.12 (i.e

1.60×2×20 = 64 Vp-p i.e. 22.7 Vrms). So the contribution of DVR comes into

picture.

Area B resembles the time when DVR has been switched ON manually as

shown in Fig.5.10.

Now the injected voltage of 64 V p-p by DVR has been observed which is

represented in Fig.5.13 (i.e VDVR = 3.2×20 = 64 Vp-p i.e 22.6 Vrms).

Fig.5.14 shows the way of injected voltage by DVR which increases the

voltage from 248 Vp-p to 312 Vp-p.

After the injection of voltage as shown in Fig.5.15, the measured Load

voltage=15.6 Vp-p (i.e. 15.6×20 = 312Vp-p i.e 110.3 Vrms).

The overall THD analysis of Load voltage sag has been observed as shown in

Fig.5.16 which has been reduced to nearly 5.7%, maintaining a THD within

limit.


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Point C (Fig.5.10) again shows about the time when DVR has been turned

OFF to have a clear understanding regarding the operation of DVR and the sag

continues.

The overall specification which has been measured for complete operation of

DVR in open loop condition is tabulated below.

Table.5.3: Overall specifications of DVR in open loop condition

Parameter Peak to Peak (×20) RMS

Vs = 16.0 V p-p 320 V 113.3 V

VL= 15.6 V p-p 312 V (Pre Sag) 110.3 V

VL= 12.4 V p-p 248V (During Sag) 87.6 V

Sag = 1.6×2 = 3.2V p-p 64V 22.7 V

Injection = 3.2V 64V 22.7 V

Fig.5.8 (a): Basic Model of Open Loop Control for pulse generation


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Fig.5.8 (b): Hardware pulses from dSPACE

(Channel 1: Carrier signal Channel 2: Reference Signal

Channel 3: Pulse for switches S1 & S2 Channel 4: Pulse for switches S3 & S4)

Fig.5.8 (c): Hardware pulses after gate driver circuit

(Channel 1: Pulse for switches S1 & S2 Channel 2: Pulse for switches S3 & S4)


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Fig.5.9: Source Voltage:

(Channel 1: Voltage across Source Vs Channel 2: Voltage across Load VL

Channel 3: Injected DVR Voltage Vdvr Channel 4: Voltage across inverter Vinv)

Fig.5.10: Load Voltage during PreSag :




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Fig.5.11: Load Voltage when sag dip occurs:



Fig.5.12: Load Voltage when sag occurs :




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Fig.5.13: Injected Voltages by DVR:

Fig.5.14: Injected Parameters by DVR


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Fig.5.15: Load Voltage after mitigation by DVR

Fig.5.16: THD Analysis of Load Voltage

VL


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5.4.2 CLOSED LOOP OPERATION

Closed loop control deals with the process of operating DVR model

automatically whenever voltage sag occurs across the load. This process speaks about

the comparison of voltage sag with normal voltage to generate pulses and further

feeding it to inverter for generating an injection voltage. This process automatically

injects the voltage in series with the load to mitigate the voltage sag. Here it ensures

that there is no delay in processing it due to controlling methods.

The basic model of closed loop control for pulse generation required in

hardware for DVR [28], [29] using MATLAB Simulink [30] has been shown

in Fig.5.17.

The Fig.5.18 speaks about the Load voltage working normally for sensitive

load of Bulb 40 W with a Load voltage =17.6 Vp-p (i.e. 17.6×20 = 352 Vp-p

i.e 124.5 Vrms).

Now suddenly when a high load of 1000W is switched ON a voltage sag has

been occurred across the load as shown in Fig.5.19 working under pre sag

condition with a Load voltage =17.6 V p-p (i.e. 17.6×20 = 352 Vp-p i.e

124.5Vrms). The load voltage has been working with a voltage dip developing

voltage sag.

Point A resembles the occurrence of sag by switching ON high resistive load

which dips the sensitive load voltage and is clearly visualized in Fig.5.20

(12×20 = 240 Vp-p i.e 84.9 Vrms).

It means that sag has been occurred for 5.6 Vp-p as shown in Fig.5.21 &

Fig.5.22 (i.e 2.8×2×20 = 112 Vp-p i.e. 39.5 Vrms). So the contribution of

DVR comes into picture.

Area B resembles the time when DVR has been switched ON automatically as

shown in Fig.5.23.

Now the injected voltage of 5 V p-p by DVR has been observed which is

represented in Fig.5.25 (i.e VDVR = 5×20 = 100 Vp-p i.e 35.4 Vrms).

After the injection of voltage as shown in Fig.5.24, the measured Load

voltage=17.6 Vp-p (i.e. 17.6×20 = 352Vp-p i.e 124.5 Vrms).

The overall THD analysis of Load voltage sag has been observed as shown in

Fig.5.26 which has been reduced to nearly 5.45%, maintaining a THD within

limit.


Page 69

Point C (Fig.5.23) again shows about the time when DVR has been turned

OFF automatically to have a clear understanding regarding the operation of

DVR and the sag continues.

The overall specification which has been measured for complete operation of

DVR in closed loop condition is tabulated below.

Table.5.4: Overall specifications of DVR in closed loop condition

Parameter Peak to Peak (×20) RMS

Vs = 17.6 V p-p 352 V 124.5 V

VL= 17.6 V p-p 352 V (Pre Sag) 124.5 V

VL= 12.0 V p-p 240 V (During Sag) 84.9 V

Sag = 2.8×2 = 5.6 V p-p 112 V 39.5 V

Injection = 5V 100 V 35.4 V

Fig.5.17: Basic Model of Open Loop Control for pulse generation


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Fig.5.18: Source voltage

(Channel 1: Voltage across Source Vs; Channel 2: Voltage across Load VL)

Fig.5.19: Load voltage



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Fig.5.20: Load voltage when sag Dip occurs


Fig.5.21: Load voltage when sag occurs



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Fig.5.22: Load voltage when sag occurs


Fig.5.23: Source voltage


Channel 3: Injected DVR Voltage Vdvr )

A

B

A C


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Fig.5.24: Load voltage after mitigation



Fig.5.25: Injected DVR Voltages VDVR




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Fig.5.26: THD Analysis of Load Voltage

The thought of the author conveys many issues experienced during the tenure of

this project. The main origin of any project starts with the software analysis rather

than indulging directly into hardware enactment. So the complete production of this

project started with the bean of MATLAB SIMULINK which has initialized and

enhanced the desire of developing DVR. So basically this episode prologue with

simulation results of DVR which has involved an epic device known as dSPACE

which showered all the eradication of minute problems while generating triggering

pulses required for the heart of the DVR, the inverter. As the simulation results

excited the next episode of Hardware analysis with the production of driver circuit

which has migrated the analysis towards the production of injection voltage required

to mitigate the voltage sag which is the basic aim of this project. Finally it has

sprinkled a spark of merriment in heart, the day when DVR has worked through and

implemented its duty in a desired manner and that’s the epilogue of this project.

It is the case when power system network is connected to DVR and complete set

up has been shown in Fig.5.27 for mere understanding. It has been noticed that

controlling scheme is the important fact of the complete system for future scope.


Page 75

Fig.5.27: Complete Hardware set up for sag mitigation using DVR

Conclusion & Future Scope

Page 76

CHAPTER 6

CONCLUSION & FUTURE SCOPE

CONCLUSION:

The documentary of this thesis has been pleasantly scripted with keen

curiosity and paramount heed. Each and every theme has been reviewed by many

skilled members to avoid any perplexity or gaffe throughout the libretto. The

simulation results have healed up the wounds before initializing this project which

excited the view of power quality issues to instigate a hardware project.

The triggering of this project escorted to edifice up DVR model in a software

analysis which has allowed creating a power quality issue of voltage sag. This voltage

sag wag intentionally created by switching on high loads and it has been analyzed.

The harvest of simulation was completely satisfied by analyzing the mitigation of sag

intellectually. It has then migrated towards the building up DVR model in hardware

era.

Hardware scenario has been geared up with drafting up driver circuit which

had induced the driving pulses with the help of dSPACE by comparing normal

voltage with voltage sag and fed to inverter. This inverter has yielded with harmonic

distortions. This distorted harvest has been filtered out by LC filter. The furnished

capitulate has been tried to infuse in series with the load and it has been keenly

pragmatic that DVR has involved in mitigating sag effectively. The efficient

mannered mitigation was superiorly observed for closed loop.

FUTURE SCOPE:

As the DVR modeled over here is of single phase, it can have a glowing

developed scope in future for three phase DVR by using different level inverters.

Many modulation techniques and few control methods can be adopted in the area of

harmonic eradication.

I would like to cease it by saying that DVR is not only reserved to mitigate sag

but it can have wider applications when equipped with many practical working

devices for controlling different types of power quality disturbances, by using

advanced controllers.

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thesis in one file

Documents

master of technology

guidance of

associate professor

award of degree

dspace rti

hardware enactment

project work

integrated power systems