Voltage Dips and Mitigation System using Dynamic Voltage Restorer

By

MD NAJIB B MD YUSOF

FINAL REPORT

Submitted to the Electrical & Electronics Engineering Programme

in Partial Fulfillment of the Requirements for the Degree

Bachelor of Engineering (Hons) (Electrical & Electronics Engineering)

Universiti Teknologi PETRONAS

Bandar Seri Iskandar

31750 Tronoh

Perak Darul Ridzuan

© Copyright 2011

. by

Md Najib b Md Yusof, 2011

CERTIFICATION OF APPROVAL

VOLTAGE DIPS AND MITIGATION SYSTEM USING DYNAMIC VOLTAGE RESTORER

Approved by,

by

Md Najib b Md Yusof

A dissertation submitted to the Electrical & Electronics Engineering Programme

Universiti Teknologi PETRONAS

in partial fulfilment of the requirement for the

Bachelor of Engineering (Hons) (Electrical & Electronics Engineering)

Assoc Prof Dr K.S. Rama Rao

Project Supervisor

UNIVERSITI TEKNOLOGI PETRONAS

TRONOH, PERAK

May 2011

ii

CERTIFICATION OF ORIGINALITY

This is to certifY that I am responsible for the work submitted in this project, that the

original work is my own except as specified in the references and acknowledgements,

and that the original work contained herein have not been undertaken or done by

unspecified sources or persons.

, MD NAJIB B MD YUSOF

iii

ABSTRACT

Quality of the output power delivered has become a major concern for heavy

duty industry for present era. The poor power quality is due to some disturbances

such as voltage sags/dips, surge, flicker, harmonics, imbalance and interruptions. It

becomes more important especially with the sophisticated devices whose

performance is very useful to the quality of power supply. Power quality problem in

industries, factories, university, residential areas etc cause failure to the equipment

and malfunction which can make the operating system stop and at the end gives a big

impact to the plant production. These issues become worse if there is no mitigation

system to counter back the problem. A voltage dip is loss of rms voltage which is a

major power quality problem. In specific term of duration and retained voltage, it can

also be representing as the lowest rms remaining voltage at the lowest point during

the dips at the industry. So, it is the responsibility of the user and customers to

identify the root cause of the problem and create a mitigation system to overcome the

problem. To overcome this abnormal problem custom power devices are connected

closer to the end of the load. One of the devices is the Dynamic Voltage Restorer

(DVR) which is the most efficient custom power device used in power distribution

networks. This paper presents modeling, simulation, and analysis of a Dynamic

Voltage Restorer (DVR) using PSCAD (Power System Computer Aided Design)

software. The main component of a DVR is voltage source converter (VSI) which is

operated by pulse width modulation (PWM) technique and a PI controller. There are

many different type of fault has been created and shown reliable result after mitigate

usingDVR.

iv

ACKNOWLEDGEMENT

I would like to thank Universiti Teknologi PETRONAS for giving me to use their

resources and work in such excellent environment.

First and foremost I take this golden opportunity to express my gratitude to my guide

Assoc Prof Dr K.S Rama Rao and Mr Bahr Eldin for their guidance during my

project work. This project would not be success without their help and the valuable

time that they given to me even though they are quite busy with their work.

I would like to extend my gratitude to my friends and technician who always

supported and encouraged me to complete my work.

v

TABLE OF CONTENTS

CERTIFICATION iii

ABSTRACT IV

ACKNOWLEDGEMENT . v

TABLE OF CONTENTS vi

CHAPTER I: INTRODUCTION . 1

1.1 Background of Study •

1.2 Problem Statement 2

1.3 Objectives and Scope of Study 2

CHAPTER2: LITERATURE REVIEW . 3

2.1 Power Quality problem 3

2.2 Classification of Voltage dips. 4

2.3 The causes 6

2.4 The effect 6

2.5 IEEE standard • 7

2.6 Custom Power Devices 8

2.7 Dynamic Voltage Restorer 9

2.7.1 Introduction . 9

2.8 Basic Structure of DVR 10

2.8.1 Energy Storage Unit 11

2.8.2 DC charging • 11

2.8.3 PWM Voltage Source Inverter 11

2.8.4 PWM Current Source Inverter 14

2.8.5 Passive filter • 15

vi

CHAPTER3:

CHAPTER4:

CHAPTERS:

REFERENCES

2.8.6 Voltage injection transformer. 16

2.9 DVR operating 17

2.9.1 Protection Mode 17

2.9.2 Standby Mode • 18

2.9.1 lrifection/Boost Mode • 18

METHODOLOGY 19

3.1 Project Identification • 19

3.2 Modeling. 21

3.3 Equation related to DVR 24

3.3.1 Mathematical model for voltage

injection by DVR system 25

3.4 Tools and Equipment Required 27

3.4.1 Power System Computer Aided Design

(PSCAD)

3.4.2 What is PSCAD

3.5 DVR structure.

3.6 Energy Storage

3.7 Pulse Width Modulation

3.8 PI controller . 3.8.1 Tuning the PI controller

RESULTS AND DISCUSSION

4.1 Result •

CONCLUSION AND RECOMMENDATION

5.1 Conclusion

5.2 Recommendations

vii

27

28

29

30

30

34

34

36

38

46

47

48

LIST OF TABLES

Table I IEEE definitions for the voltage sags and swells .................................. 6

Table 2 Comparison between UPS and DVR ................................................. 8

Table 3 Advantage and Disadvantage of Voltage Source Inverter ...................... .14

Table 4 Advantage and Disadvantage of Current Source Inverter ...................... .15

viii

LIST OF FIGURES

Figure 1: Single line diagram of power distribution system ............................... .3

Figure 2: Classification of Voltage Variations .............................................. .4

Figure 3: Sample voltage dips waveform ..................................................... 5

Figure 4: Location ofDVR ...................................................................... 9

Figure 5: Basic operation ofDVR ........................................................... .IO

Figure 6: Schematic of Dynamic Voltage Restorer ....................................... .I 0

Figure 7: The power converter structure of various PWM-VSI drives ................. .l2

Figure 8: Simplified Circuit diagram of CSI.. ............................................. .14

Figure 9: Different filter placement .......................................................... 16

Figure I 0: Connection methods for the primary side of the injection transformer .... 16

Figure II: Protection Mode .................................................................. .l8

Figure 12: Standby Mode .................................................................... .18

Figure 13: Flow diagram of project work ................................................................... 20

Figure 14: Single ine diagram ................................................................ 21

Figure 15: Timed fault configuration ........................................................ 22

Figure 16: Configuration data of the voltage transformer ................................. 23

Figure 17: Calculation for voltage sags (dips) ............................................. 25

Figure 18: Calculation for DVR injection transformer .................................... 26

Figure 19: PSCAD Software Version 4.2.1 ................................................. 27

Figure 20: DVR structure ..................................................................... 29

Figure 21: Setting mode for breaker operation ............................................. 29

Figure 22: Pulse Width Modulation generation ........................................... 30

Figure 23: Sinusoidal Pulse Width Modulation block diagram .......................... 32

Figure 24: PWM graph ........................................................................ 33

Figure 25: PI controller block diagram ...................................................... 34

Figure 26: Defining the PI controller parameter ........................................... .35

Figure 27: Complete power system network with DVR ................................. .36

Figure 28: Pulse Width Modulation ........................................................ .37

Figure 29: Switching signals for inverter leg ............................................. .37

ix

Figure 30: Three phase and load voltage at normal condition ........................... 38

Figure 31: 3 phase voltages with single phase fault without DVR .................... .39

Figure 32: Voltage at load side without DVR ............................................. 39

Figure 33: 3 phases voltage at double phase fault without DVR ....................... .40

Figure 34: Voltage at load side without DVR ............................................. .40

Figure 35: 3 phases voltage at three phase fault without DVR .......................... .41

Figure 36: Voltage at load side without DVR ............................................. .41

Figure 37: 3 phase injected voltage with DVR ............................................ .42

Figure 38: Voltage at load side with DVR for single phase fault... ............................ 43

Figure 39: Voltage at load side with DVR for double phase fault ...................... .44

Figure 40: Voltage at load side with DVR for three phase fault ............................... .45

X

Var

PSCAD

PI

EMTDC

UPS

P.U

DVR

PWM

D-Statcom

PLC

VSI

FACTs

CSI

svc Hz

IEC

IGBT

IEEE

KV

MW

ms

GTO

LIST OF ABBREVIATIONS

Volt- Amperes Reactive

Power System Computer Aided Design

Proportional Integral

Electromagnetic Transient Direct Current Software

Uninterrupted Power Supply

per unit

Dynamic Voltage Restorer

Pulse Width Modulation

Distribution Static Compensator

Programmable Logic Controller

Voltage Source Inverter

Flexible ac Technology

Current Source Inverter

Static Var Compensator

Hertz

International Electro technical Commission

Insulated Gate Bipolar Transistor

Institute of Electrical and Electronics Engineers

kilovolt

Megawatt

mili second

Gate Tum-off thyristors

xi

1.1 Background of study

CHAPTER I

INTRODUCTION

Nowadays, most of industries, universities, and utilities use modem

equipment based on the electronic devices such as PLC (Programmable Logic

Controller), laptop, washing machine, printers etc. These devices are very sensitive to

the disturbances and become less efficient to the power quality problems such as

voltage sags, swells, flicker and harmonic.

With increasing usage of power electronics in industrial oil and gas plants,

manufacturing plants etc the issue of power quality has contributed significant

adverse effects on the plant performance. The proliferation of non-linear loads (i.e.

computers, variable speed drives, cranes, elevators, furnaces, motors, discharging

lighting, UPS (Uninterrupted Power Supply) etc ) in industries have produced

substantial voltage disturbance in Electrical Distribution System. Voltage dips can be

considered as one of the major disturbances.

Voltage dips can create unexpected and unwanted operating problems with all

types of equipments, ranging from nuisance tripping, miss-operation and failure of

electrical plant equipment (i.e. transformers, conductors, motor, generators etc.)

Plant's electrical substations experience voltage dips due to the distribution circuit

from operation of multiple large electric equipment at the plant facilities.

There are many ways to overcome the problem of voltage dips. One of the

electronic equipment that can be used to regulate the voltage is by using Dynamic

Voltage Restorer (DVR) which can provide the reliable solution to overcome the

issue of distribution power quality. The DVR mainly use for very sensitive load that

effect the efficiency of the output voltage.

I

1.2 Problem statement

Voltage dips can cause additional voltage stress to the electronic components,

electrical machines, cables, transformers, switchgears, circuit breakers, bus bars, light

sources etc also voltage dips cause reduction in equipment life span for example in

electric machines. At terminals of induction motor it causes changes in torque and

slip and consequently affects the production process. In worst case it may lead to

vibration, reducing mechanical strength and shortening the motor service life. These

uncorrected problems make process control being harder. Process variations slow

production rates, decrease product quality, increase energy consumption and the

worst thing make the plant upset. By exposing plant transformers and motors to

voltage dips may shorten their lives and increase the chances of catastrophic failure.

1.3 Objective

a. To study problems and scenarios related to the voltage dips in power quality

systems.

b. To learn about Dynamic Voltage Restorer (DVR) system that can reduce the

voltage dips problem.

c. To develop a simulation model of a distribution network incorporating the

DVR.

1.4 Scope of Study

a. Power System Computer Aided Design (PSCAD) V4 software is used for

modeling and simulations in the project. By creating large load running in a

weak power network system to create voltage dips and model the mitigation

system DVR to reduce voltage dips.

b. Understanding the Power System Computer Aided Design (PSCAD)

software.

2

CHAPTER2

LITERATURE REVIEW

2.1 Power quality problems

Step-down transformer 132/115 KV

Step-down transformer 115/11 KV

-~--1 21t.O[olm] 1

~-~ ~-w~r~---H· 2ll.O ]otrn] I

-wv----1 20.0 [Ohm]

Medium scale industries

The consumers

Figure 1: Single line diagram of power distribution system

In ideal case, power distribution system should provide a better performance

without any interrupted voltage to the customer activities. However, because of the

nonlinear loads it will affect the quality of power supplies. So due to this problem a

smooth voltage waveform cannot be achieved thus it will end up with the power

quality problem. Power Quality problem can be classified in much definition such as

voltage dips (sags), swells, flicker, harmonic distortion, transient and interruption.

3

2.2 Classification of Voltage dips

Voltage dip is change of nns voltage in short interruption at load bus. A

voltage dip means that the required energy is not being delivered to the load and this

can have serious consequences depending on the load that involved. When heavy

load are started, such as electric machine, the high starting current may drop the

voltage at certain level for a short tenn. The magnitude of the voltage usually is

below than the 10% nominal supply voltage (16]. The classification of nns voltage

variations is shown in Figure 2 as plot of voltage against duration of disturbances.

Figure 3 (a) and (b) is shown the voltage dip wavefonn.

Voltage vs duration of

Voltage (nns) I%

Voltage swells

455.5V 110%

415.0V 100% Voltage changes and fluctuations

373.5V 90%

Voltage dips

Short interruption Long interruptions

I ~ec I min 1 ho~r duration of disturbances

Figure 2: Classification of Voltage Variations

4

10.0 • ()Aput ~!l!tl4!e i

8.0 - .. - ., . - - ' ....... I -.. - . - .. i --6.0 -- . --

4.0

20 -I~Hi!HHI·IHtltiHitfjljtJljiiHI 11111111 0.0 fltlt lttt'ltttlftHtttt

IIIII Ill II -20 +I-IHl-H

1111

.. J !··

-4.0 --

-6.0 -ao -- - !

-10.0 '-- .J ................ J. .. .. . .. ' -···-·········

0.00 0.10 0.20 0.30 0.40 0.00 0.60 0.70 0.60 0.90

(a)

0\ 5'\

0.. 4 1---1--'W----If---+--+--+--+--+-t--++--+---1--+--11---i

w 1\ ~ 3 " ~ ~ ' ~ ./BORDERLINE OF IRRITATION !z 2 1-1--' 'kc---+-+--+-f_!lloooot,_+--+'-f-'--1'1-+-'-t'-+--+-+---l w ......... ""'-- "-BORDERLINE OF VISIBILITY OF FLICKER

~ 1 r1~ ''-ri'Ff~~4~=i~~t~~~4+~t;~/1 .... ..., II _.,~~

0 1 2 5 10 20 30 1 2 5 10 20 30 1 2 5 10 20

DIPS PER HOUR DIPS PER MINUTE DIPS PER SECOND

I l I I I FREQUENCY OF ~IPS I I I I I I 3D 12 6 3 2 1 30 12 6 3 2 1 .3 .2 .1 .05

MINUTES SECONDS TIME BETWEEN DIPS

(b)

Figure 3: Sample voltage dips wavefonn

5

2.3 The causes

Voltage dip is sudden reduction between 10 % - 90 % from nominal voltage

supply [16). It happened because high current, short circuit, heavy loads and earth

faults in the grid or electric equipment. When voltage drops down below than 90 %,

the time taken for a voltage dip to occur is from millisecond up to I minute [3]. If

number of motors start at the same time or the same repeatedly start and stop, the

frequency of voltage changes may produce dips in electrical equipment that is

perceived by human eyes. The voltage dips also may cause by distribution or

transmission network fault, cable fault during the construction, lighting strike that trip

the breaker and the higher rise building without proper grounding [4]. Other causes

due to voltage dips are:

~ Storm activity -lightning, wind.

~ Object coming in contact with power line and tripping breaker - tree

branches, animals etc.

~ Utility fault.

~ Construction activity.

~ Accidents- motor vehicle, other.

~ Equipment failure.

~ Overloading.

~ Poor grounding.

~ Non-linear loads.

2.4 The effect

Voltage dip is due to high starting current that may cause the electric equipment rms

voltage drop. Due to time taken by voltage dip it may affect the process of transaction

of data and the communication system such as email and box mail system. Moreover

it may cause spurious tripping of relays and electronic equipment. In addition,

induction motor that operates at maximum torque may also damage the bearings etc.

Severe voltage dips may not allow other loads to be started due to reduction in the

supply voltage. Also major effects can occurs such as motor may not maintained the

6

required speed even for a good variable speed drive, motor contactors drop out with

around 65% of the nominal voltage and also many variable speed drives trip out when

a voltage dip is more than 15%.[4].

2.5 IEEE Standard

IEEE 446-1995, "IEEE recommended practice for emergency and standby

power system for industrial and commercial application range of sensibility loads".

The standard discusses about the effect of voltage sags on sensitive equipment such

as motor starting, etc. It shows principles and examples on how systems shall be

designed to avoid voltage sags and other power quality problems when backup

system operates.

IEEE 493-1990, "Recommended practice for design of reliable and

commercial power system"

The standard proposes different technique to predict voltage sag

characteristic, magnitude, duration and frequency. There are few important things of

interest for voltage sags. The differences can be summarized as follows:

• Calculating voltage sag magnitude by calculating voltage drop at critical load

which includes the network impedance, fault impedance and location of fault.

• By studying protection equipment and fault clearing time it is possible to

estimate the duration of voltage sag.

• Further it has been found that the voltage dips/sag with magnitude 70 % of the

nominal value is more common than complete outages [5]. Sags/ dips and

surges can be identified by the voltage magnitude and the time duration it can

sustain. IEEE 519-1992, IEEE 1159-1995 as shown in Table 1(6].

Table 1: IEEE definitions for the voltage sags and swells

Disturbance Voltage Duration

Voltage Sag 0.1- 0.9 p.u 0.5- 30 cycle

Voltage Swell 1.1-1.8 p.u 0.5- 30 cycle

7

2.6 Custom Power Devices

Nowadays, a lot of devices have been developed to mitigate voltage dips such

as Uninterrupted Power Supply (UPS), Distribution Static Compensator (D-Statcom)

and Dynamic Voltage Restorer (DVR). Among these devices UPS is well known and

most of the industries use UPS configuration for Power Distribution System

application. Previously DVR and D-Statcom are less popular devices that being used

even though they are highly efficient and in term of cost more effective. Because of

the rapid development of power electronic industry and low cost devices, DVR

becomes most popular among the industries.

DVR is used to eliminate two different types of abnormality that affect power

quality. There are two types of load situation which are linear loads and non-linear

loads. To describe that the load is linear is when the entire dependent and

independent variables changes linear with each other. Resistor is the best example to

describe as a linear device. For non-linear devices the entire dependent and

independent variables does not change with each other. Capacitors and inductors are

the best examples to describe as non-linear devices.

Among the custom power devices, UPS and DVR can be considered as the

devices that can boost up the voltage waveform of the power distribution line. The

comparison between UPS and DVR is shown in Table 2.

Table 2: Comparison between UPS and DVR

UPS( Uninterrupted Power Supply) DVR (Dynamic Voltage Restorer)

Supplying the full voltage to the load Voltage will inject during the voltage

whether there is no distortion or not. dips occur.

High power rating Low power rating

Low efficiency High efficiency

High cost Low cost

8

2.7 Dynamic Voltage Restorer

2. 7.1 Introduction

Many power quality problems such sags, swells, harmonics etc, voltage dips

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

custom power devices has been introduced recently. One of the devices is Dynamic

Voltage Restorer which consists of power circuit and control circuit. The DVR

provides three phase independent controlled voltage source. It controls the voltage

vectors which have magnitude and angle. These voltages are injected into the ac

distribution system at the reference voltage. DVR is the most efficient and an

effective device that has been used for power system network. DVR is a series

connection to the powers system network that injects voltage into the system that

regulates the load side voltage. Normally it is connected into a power system network

between the supply and the critical load at a common point of coupling. Other than

voltage dips, DVR also can be used for other application such as harmonic

compensation, transient in voltage and current and etc. Figure 4 and 5 show the

location ofDVR and basic operation ofDVR respectively.

A B

100.0[MV~ 0.05 [chmP.4B06 P1[

,-------1#~~-..!!.JB 11 (\ ~ B 0.05 [chmP.4805 ~I U I •

20.0[chm[ [

-H' ~_j 20.11[ohml

100.0!M'I>'l ---1#---~--'-jA tOO.O[MV~ A --~-, 0001 ~ 0.005P'[ 1 0.1 [chm[ 0.1925P'[ J'WI"--'i.,;y A A_l"------fN--~---"-1., A y., 1"--e --t--r--AII---------+-1

U U O.OG1[ahm[ 0.005[HJ U 0.1 [ohm] 0.1928[1-!]

----11'1----~----"--c 1t511<1 ---N>~-~-L-_18.7.:..,11<1'-' 0.1[chm[ 0.1928P'[ 11.001 [ohm[

Figure 4: Location ofDVR

9

Voltage wavefonns ,r'rlrirlnnn ,,, r, n .... "r ,.-. _..r,nnr'-'d!iUU\illl, + \} \1 \.! - () J J \! '"H' IJ \J \1!,;

r'\ Voltage (2-:; l.~-.AAJ sensitive

~ Load

DVR

Figure 5: Basic operation ofDVR

2.8 Basic structure ofDVR

Figure 6 shows general configuration of the DVR which consists of:

i. Booster Transformer

ii. A harmonic filter

iii. Storage device

iv. A voltage source converter

v. DC charging circuit

bl'ql E'(} al\CE!'

Vm:a __.. LOAD I

l Vs ..AA.A.. VL l

'\.. Supply T ' T

-- Filter

control vsc system

Figure 6: Schematic of Dynamic Voltage Restorer

10

2.8.1 Energy Storage Unit

The energy storage unit is responsible to store DC voltage. It can use a super

capacitor or bank of batteries that being to storage energy. Its supply real

power to the system that is required when DVR is used for compensation.

2.8.2 DC charging

The DC charging circuit has two tasks:

• To charge back the energy that has been used after the voltage dips

compensation.

• To maintain the de voltage at the nominal de voltage.

2.8.3 PWM Voltage Source Inverter (VSI)

With the cost being the driving factor, general purpose PWM-VSI devices

with low kilowatt ratings (fractional to several tens of kilowatts) are generally

designed with minimum component count and complexity. In ac motor drive

applications, a single/three phase diode bridge circuit rectifies the AC input voltage

and a de link capacitor filters the rectified voltage forming a stiff de voltage source.

The PWM-VSI converts the de voltage to three-phase ac voltages with controllable

magnitude and frequency via a PWM method. Below about hundred volts MOSFETs

and at higher voltages IGBT power semiconductor switching devices are utilized in

the VSI, and the switching frequency can be as high as many tens of kilohertz. The

power circuit may also contain a de link capacitor pre-charging circuit, a small EMI

filter, and transient voltage suppression devices [14].

With the ac utility line power quality regulations becoming more stringent, at power

ratings higher than several tens of kilowatts the input rectifier device performance

becomes important. An inductor (de reactor) is inserted in series with the rectifier in

order to reduce the harmonic content on both the ac line and the de link capacitor. In

higher power non-regenerative applications 12-pulse rectifiers together with input

transformers with star primary and star-delta secondary windings are frequently

utilized to reduce the input current harmonic distortion. In the megawatt power range,

11

the PWM-VSI may be formed from parallel inverters, inverters with parallel devices,

or more complex inverter device structures such as the three levels VSI.[15]

c

(b)

(c)

c .-1-+----i-+-·· Va. ·VO

'""

(d)

Figure 7: The power converter structure of various PWM-VSI drives

In ac motor drive applications, shown in Figure 7 (a) a single/three-phase

diode bridge circuit rectifies the ac input voltage and a de link capacitor filters the

rectified voltage forming a stiff de voltage source.

As shown in Figure 7 (b) an inductor (de reactor) is inserted in series with the

rectifier in order to reduce the harmonic content on both the ac line and the de link

capacitor. Alternatively, a three phase reactor may be inserted in series with the ac

line to suppress the harmonics. Also a dynamic brake formed by a resistor and a gate

tum-off switch 20 is connected across the de bus capacitor in order to limit the de bus

voltage under transients and during regeneration (Figure 7(c)). In regenerative

applications with emphasis on energy efficiency back-to-back connected thyristor

12

bridges or more commonly a PWM-VSI can be utilized (Figure? (d)).

However, with the PWM-VSI a relatively large and therefore expensive three

phase reactor is required. Therefore, this solution is the most expensive approach. Up

to near a megawatt the switching device choice remains to be IGBT due to its cost

and performance advantages.

The power semiconductor devices can be IGBT's, GTO's or some of the

recently developed high voltage IGBT's and other new devices. Due to the switching

losses and the thermal issues associated with them, the switching frequency of the

PWM-VSI decreases from tens of kilohertz at lower tens of kilowatts level to less

than a kilohertz at the megawatt level. At increasing power levels, typically snubber

circuits are added to reduce the commutation losses and the peak stresses on the

devices [ 15].

13

2.8.4 PWM Current Source Inverter (CSI)

An inverter of the current source type makes it possible to convert the energy

supplied by a de source into the mains without using an auxiliary de to de converter.

The current source inverter's efficiency depends on the input de voltage of the

inverter, so the rating of the de source voltage influences the overall efficiency of the

system.

Figure 8 shows the simplified circuit of the CSI. The limited literature on the

CSI for Var compensation concentrates on the steady-state characteristic analysis and

main circuit design. These references employ a PWM switching pattern for the CSI

which is pre-calculated for harmonic elimination, where the modulation index is fixed

and the CSI is controlled by its firing angle [12]. The advantages and disadvantages

of voltage source inverter and current source inverter is shown in Table 3 and 4.

V'l'li, V:r~

AC System

Figure 8: Simplified Circuit diagram of CSI

Table 3: Advantage and Disadvantage of Voltage Source Inverter

Advantages Disadvantages

Simple and robust High switching frequency

Easy to control in the feed-forward voltage Large switching losses.

or feedback current mode

Wide ranges of frequency, magnitude of Conducted and radiated electromagnetic

fundamental output voltage are interference.

attainable.

PWM allows direct control of magnitude Hazardous over-voltage in long cables.

and phase of output space vector.

Accelerated deterioration of insulation in

bearings and supplied motors.

14

Table 4: Advantage and Disadvantage of Current Source Inverter

Advantages Disadvantages

Can transfer power in both direction Inductance slows down response to

current magnitude control commands,

can cause overvoltage if current path is

broken.

In square-wave mode is more efficient The output current is a square-wave

than a PWM VSI waveform.

Power circuit is simpler and more robust High level of low-order harmonics.

than the VSI

Lack of free-wheeling diodes, large de- When supplying a motor, can cause

link inductance, current control in the voltage spikes in the stator leakage

rectifier results in inherent protection inductance.

from over-currents.

PWM CSis are large and costly, and they

lost their efficiency advantage over VSI.

2.8.5 Passive filter

To eliminate the unwanted harmonic waveforms that are generate from VSI

process the filters can be placed on the high voltage side or on low voltage side of the

injection/booster transformer, thus higher order harmonics are eliminated from

passing through the voltage transformer. But there can be a phase shift and voltage

drop in the inverted output. This can be reduced by placing the filter on the load side.

But in the case of the higher order harmonic current passing through to the secondary

side of the transformer, a higher rating of the transformer is necessary. Figure 9

shows the different filter placement in the system.

15

. .q <j _J"' ~l~--.+ -<· -i'1 +~

.. rl' . Fi~ atlhe orimary side

Figure 9: Different filter placement

2.8.6 Voltage Injection Transformers

The booster transformer is specially designed to limit the coupling noise and

also to limit the transient energy from the primary and secondary side. Its main tasks

are:

• The DVR is connected to the power system network to H-V windings and

transforms and injects is specified voltage from supply source.

• It is also to isolate the load form the system.

The high voltage of the injection transformer is connected in series with

powers system network while the low voltage side is connected to the DVR power

circuit. The three-single phase transformers and the three-phase DVR can be

connected either star or delta open as shown in figure 10[10].

;;; >----~----r---_,__J

~====~-_j

Delta/open configuration

::;; >----'

£ Star/open configuration

Figure 10: Connection methods for the primary side of the injection transformer

16

The transformer winding ratio depends on the voltage required by the

secondary of the transformer. Normally the supply voltage must be equal to DVR

needed at the full voltage dips compensation. The higher the ratio of transformer

winding the higher current will flow at primary side which will affect the

performance of the electronic devices.

The winding configuration of the transformer depends on the power system. If

the transformer uses delta/star ( /1 - Y ) connection with the grounded neutral, during

the fault, earth fault happens on the high voltage side, no zero sequence current flows

to the secondary and only positive and negative sequence components will flow. So

the DVR can only compensate with the positive and negative sequence components

and for the open configuration it will be used to compensate the positive and negative

sequence. Similarly for other winding configuration such as delta/ delta connection of

the transformer, during the fault, earth fault occur, the positive, negative and zero

sequence components flow through the secondary side [8].

2.9 DVR operating system

The basic function of DVR is to control the voltage, V DVR generated by the

voltage source converter to the booster transformer. If there is a different voltage that

is caused by power system network, the DVR will compensate by injecting to the

distribution line from voltage source converter through the injection/booster

transformer.

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

mode and injection! booster mode.

2.9.1 Protection mode

If the load current exceeds the limit of the winding transformer current, the

DVR is isolated from the system by using bypass switches whereby S2 and S3 will

open and S 1 will close to supply another path to the current to flow as shown in

Figure 11.

17

S'

Sl

lll'"P.l.~f>'

!<'\\UC'III!.'

n ~I!iSJTIU WAD

Figure 11: Protection Mode

2.9.2 Standby Mode (VnVR = 0)

In the standby mode the injection/booster transformer low winding will not

inject voltage to the system. In this case the voltage source converter is not switched

on and the full load current will pass through the primary as shown in Figure 12.

j~:QTJRJ'.'F 1;'Emnt\"E L(l.-'.D w BCH!STER TR~NSFORI">IF£

l 1• IIINJ:!Ntl yyy

.! 1- l

lt\"P.-\.~;; }.'WITt.'HFS (('~)N~ 'IRU a.

.....

I r!LTER I l!:\IT

Figure 12: Standby Mode

2.9.3 Injection/ Boost Mode: (V DVR > 0)

In this case, the DVR injects a voltage trough the booster transformer due to

the disturbances on the power system network

18

CHAPTER3

METHODOLOGY

3.1 Procedure Identification

The project is carried out according to the flow chart as shown in Figure 13. It

is started with study of the voltage dips and the mitigation system using dynamic

voltage restorer. Then data is collected on DVR part by gathering the information

from internet resources, technical papers and also books. During this phase, voltage

dip fundamental also must be studied in order to achieve expected result at the end of

the research. Current technology also needs to be taken into account so that a clear

understanding on the operation ofDVR can be obtained.

In addition, the control system also needs to be studied to get better

understanding on DVR and its application. Preparing weak power system network

with load need also to be studied in order to apply and enhance the knowledge of this

type of power quality problem. Next, simulate the weak power network system and

analyze the voltage dip in order to get voltage dip graph. In this stage, collecting data

of varies type of fault is needed.

After all the data have been taken, the next phase is designing the structure of

DVR and mitigates the poor voltage using DVR. After that, the controller part needs

to apply to the system so that the voltage can be control automatically. After doing all

the simulation and achieve the data for every parameter needed, analysis the result

will conducted.

19

Study power quality problem in term of voltage dip

Study on Dynamic Voltage Restorer to mitigate the voltage dips problem

! Recognize and design part of Dynamic Voltage Restorer

l Preparing weak power system network with load

! Simulate the system and analyze the voltage dip

l Collect the data with several type of fuult

! Mitigate the voltage dips using Dynamic Voltage Restorer

! Design the controller part to control the Dynamic Voltage

Restorer

• Analyze the result

Conclusion

Figure 13: Flow diagram of project work

20

~vv'

-g .Q ..c .-=: :;: § !;))

"' ::a ~w 1000 !MVAI

~ ib = ·;;;

l"v'N'---'Y~Y !J. 63 ~Vv ~ 132!k'll 115!kVJ 115!kVJ ~Vv

<I> .;;

~ 0

-fii .:!!

ell ~ = -= e .. "C §l, = ::s li:

"! ..,

20~1 w.--H

20.0 !ohrq [

Wr--20.0 !ohrq

Vfa 0.1!ohml 019~6~ .-H

A

Vfb 0.1Johrt1 0.1926~

'" 0.1Johrq 0.1926!HI

115!kl'l IC · 1 lie 1B.71kVI • I I ' "' _,,,- ~

8 .> --<)

Vrms

c FAULTS

':"

~ _::g

""' <I>

:§ <I> bh -.s '"" "' ~ .... f! = ~

Single line diagram shown in Figure 14 of the system is composed by a

132kV, 50 Hz generation system, feeding two transmission lines through a 3-

winding transformer connected in star/delta/delta, 132/115/115 kV. Such transmission

line feed two distribution networks through two transformers in delta/star, 115/ 18.7

kV and 115/llkV. The DVR is simulated to be in operation only when the fault

occurs. The duration of fault considered to be is 300 ms. The Figure 15 show the

timed fault configuration and Figure 16 (a), (b) and (c) show the transformer voltage

configuration.

Transformer Name

3 Ph8se Transformer MVA

Base opermon frequency

-11Type

-#2Type

-#3Type

Delta lagS or leads v

[tfaultn] Timed Fault Logic L8::J

Time to Apply Fault

Duration of F au1t

Jo.3(s]

Jo.3(s]

OK Cancel Help ...

Figure 15: Timed fault configuration

Jr1 _, LretoUnovoftage(RMS)

J1 00.0 (MVA] """"ng 2 Uno to Line vollege (RMS)

Jso.O (Hz] """"ng 3 Lreto Uno volloge (RMS)

Jv iJ

Pos. seq.lefi<age reactance (#1-*2)

Pos. seq.leakage react&nce (#1-13)

Pos. seq. --ce (#2-I'J)

Joello iJ Joelo iJ JLogs iJ J0.2(pu]

[O:liPu-1 --10.1 (pu]

~--il'~

Jon(puJ Copper lOSses Jo.o(puJ Tap changer on winding

Concel Help ...

(a)

22

Cancel

J132(kV]

J115(kV]

Help ...

-

xfmr Jplw] J Phase J Winding I ransformer ~

Transformer Name IT1 3 Phase Transformer 'ttfiiA 1100.0 [MVA)

Base operation frequency I so O(HZJ

~11Type ro;; 3 ~12Type IV .:J Dele Lags or L~ Y I Lags ::J Posllve seQI8'lCe leek1lge reectn:e I o.2 (pu)

Ideal Transformer Model ~ ~ No Iced loss8s 0.0 (pu)

Copper losses 0.0 (pu)

TlltJ clw1ger on wirdng !N;;;" ~ Grll)hlcs Display 13 phase view ~ Display Detells? 3

Cancel ..... (b)

xfmr Jplw] J Phase 7 Wmdmg I ransformcr rsi ----r,rrf•.ll j' -,t "

Transformer Name rn 3 ~Transformer 'ttfiiA I 100.0(MVA)

Base operation~ l soo IHZI ~11Type l0e1a -:::J ~12Type fY .:J Dele Lags or Leeds Y I Leads 3 Poslive ~ leekage reectence 1 o.1 (pu)

Jdeel Transformer Model ~ 3 Noloedmses 1 o.o (pu)

--Copper losses l oo[puJ

TlltJ changer on~ jt;;; 3 OrepNcs Display 13 phase view ..:J Display Detels? 3

OK Cancel .. .. (c)

~ 1l.ile lo lile YIUJe (RMS) lm[kVJ --~ 21.i'le lo l.ile wtage (RillS) 118 7!kVl --

C.-eel I ~ ...

. . I xfmr lplwj l PhdSc I W1nd1np, I rdn~formcr &'I

Mllq 11.ile to Lne volage (RillS) 115 [kV)

Mllq 21.ile to l.ile volage (RillS) j 11[kV) -

OK c.lC8I ~·~

Figure 16: Configuration data of the voltage transformer

23

3.3 Equation related to DVR

Referring to Figure 17, for normal operation (no fault) current will flow

through the balanced load A and B. Both 2 load is balance load. When there is fault

occur for example at load A, a high current will flow into the feeder A. So based on

the Kirchhoff's Law current will flow into feeder B. A voltage drop occurs at feeder

B.

For example:

Load A impedance = Zload A

Load B impedance = Zload B

Feeder A reactance = X1

Feeder B reactance = X2

Current flow from power supply = I

Current flow from feeder A = h

Current flow from feeder B = I2

Therefore, I = I 1 + I 2

In normal condition (without any fault)

Vz I = =--=-­Xz+ ZLOADB

(1)

When fault occurs at feeder A, because of the short circuit occur, the high

current will flowing through feeder B as well as through the source. At this moment

voltage at feeder B will decease due to the voltage drop across source reactance X,.

So from this condition voltage at feeder B will decrease from nominal voltage.

During fault, the source current,

(2)

24

FaDit A

V•8p.u..

V• I. p.IL

v~ o~s p.u. r Sag

Figure 17: Calculation for voltage sag (dip).

3.3.1 Mathematical model for voltage injection by DVR system.

Consider the schematic diagram shown in Figure 18. The impedance

represented Zth where the real part of the impedance Rth and the imaginary part in

reactance X th.

(1)

(2)

Where

V1 = The desired voltage magnitude

Zth = Load impedance

h = Load current

Vth =System voltage during the fault

Thus, when the voltage drop occurs at feeder B, DVR will inject a voltage through

the transformer so that the magnitude of the voltage is same as the load voltage.

(3)

25

The load current h is given by,

h= (4)

Where

PL = Real power

QL = Reactive power

When V L is a voltage reference,

VvvR < VvvR < a = VL < 0 + Zth h < (fJ - fJ) - Vth < a (5)

a,p and a are angles of VvvR and fJ is load angle.

The complex power injection of the DVR can be describe as

DVR

Voltage source converter

Figure 18: Calculation for DVR injection transformer

26

(6)

(7)

3.4 Tools and Equipment Required

The simulation and analysis uses the Power System Computer Aided Design

software.

3.4.1 Power System Computer Aided Design (PSCAD)

In this project PSCADIEMTDC software as shown in Figure 19 is used to

model, simulate and analyze voltage dips and mitigation using DVR. The student

version is used to familiarize with the software. This student edition is limited but

provides examples and tutorial is important to understand the software functionality.

To have more knowledge about this software, liaise with Simulation Research and

Support Engineer, Manitoba HVOC Research Center, Canada or access to the

website PSCAD such as www.pscad.com

AboutPSCAD ~

PSCAD v4 2 .1

Copyrigtt (C) 2006

Manloba HVOC Research Certre

a.ild Version 42.1 (Aug 11 2006)

Operefilg System \o"Mdows XP (Service Pack 2)

License Host locatlost

License I 1

Expiratial Date <Never>

OK

Figure 19: PSCAD Software Version 4.2.1

27

3.4.2 What is PSCAD?

PSCAD (Power System computer aided design) is software used to perform

the modeling analysis of such controller for a wide range of operating conditions.

PSCAD also enables the user to construct the circuit schematically, run simulation,

analyze the result and imaging the data in completely integrated in graphical

environment. Online plotting function, control meters are also included so that the

user can alter the system parameters during a simulation run and view the result

directly. PSCAD comes completely with a library of pre-programmed and tested

models, ranging from simple passive elements and control functions. It is easy to the

user to do any type of modeling such as FACTS devices. transmission line, breakers,

transformers etc. If a particular model does not exist, PSCAD provides the flexibility

of simple custom model either by assembling those graphically using existing models

or by utilizing an intuitively designed from Design Editor.

The following are some common models found in system studied using

PSCAD:

I. Resistors, inductors, capacitors

11. Mutually coupled windings, such as transformers.

III. Frequency dependent transmission lines and cables(including the most

accurate time domain line model)

IV. Current and voltage source.

V. Switches and breakers.

VI. Protection and relaying

VII. Diodes, thyristors and GTO's

VJJI. Analog and digital control functions

JX. AC and OC machines, exciters, governors, stabilizers and inertial model

X. Meters and measuring functions.

XI. Generic OC and AC controls

XII. HVDC,SVC and other FACTS controllers

XIII. Wind source, turbines and governor.

28

3.5 DVR structure

The power circuit of DVR consists of Energy Storage (DC supply), PWM

(Pulse Width Modulation) inverter and injection transfonner and is shown in Figure

20. Figure 21 is shown the setting mode for breaker operation.

Gn 2

g1

0 II

a:

Gb ' 2

g4

Timed Bleaker

()

~~ BRKA Gn G b

2 "' 2 g3 gS

c If',

B d1C

.. If',

A \t1c r If', BRKA

~c

-* ' G I Gt 2 1 ° 1 2

g6 g2

Figure 20: DVR structure

, Pbredkn] Timed Hredker I ogic ~)

I of Breaker Operltiol.s r.liel State

r 1 - Close

r. 2 r;" Open

Time of Fi'st Breaker Operation I 0.3 [s)

Time of 2nd Breaker Operation I O.S(s)

OK - Help ... J

Figure 21: Setting mode for breaker operation

29

I ..I I .. ~ I .. ~ ~~

- .. -..., ..., ..., •

3.6 Energy Storage

The de (direct current) voltage is used to provide the real power during the

voltage dips compensation.

3. 7 Pulse Width Modulation (PWM)

Pulse Width Modulation is one of the techniques of controlling the voltage

within de-de converter. With this technique the converter output voltage involves a

pulse modulated wave and the voltage is controlled by varying the duration of the

output voltage pulses. The control voltage will be achieved by comparing the

triangular wavefonn with a switching frequency. The reference control voltage output

obtained from the sinusoidal wave is compared with a high frequency triangular wave

of 5000 Hz with 50 % duty cycle. The high value of the switching frequency is

required for voltage dips compensation. Figure 22 to 24 represents the generation of

SPWM (Sinusoidal Pulse Width Modulation) signal for the VSI (Voltage Source

Inverter).

(a)

I ,;igndlgcn I '>•gndl Lcncr<>lor lw Inter pold .. IRJ

Fr~y Of Signal

Initial Phase of Slgnel

OK Cencet J

(b)

30

jsooo.o IH%1

~~--

'Triangle 3 j SOI%1

~ 1·1

- Help .•• _j

g1

[compar] Two Input Comparator ~,

OUpj when A>8 fht OCCIIS 11 OUpj when no translictl OCCIIS ro-()UpJ when A<-8 trst OCCli'S I ' 1 _ _ _ _

Help ... J (c)

Figure 22: PWM generation

a) The comparator generate an output equal to I when the magnitude of the

triangular waveform higher than sine wave/ control waveform.

b) The comparator generates an output equal to 0 when the magnitude of the

triangular waveform lowers or equal than sine wave/ control waveform.

T

Figure 23: Sinusoidal Pulse Width Modulation block diagram

31

<> g5

~:r -1 - ~ r I I

.. ill l l ~ li ...

~1

1 ~ ~ r1n~:r:r1 r: .. l ! l . . ·1 l. d ~~~~

Figure 24: Sinusoidal Pulse Width Modulation signal

32

3.8 PI controller

For the control system part, a Proportional Integration (PI) controller was used

to regulate the error between the supply voltage (measured) to the reference voltage

as shown in Figure 25. The reason PI controller being chosen is because the function

of the proportional itself will respond quickly toward the changes of error occurs. The

integration function is used to remove the offsets of the reference of steady state with

the input. Before DVR start to inject the voltage to the system, the timed

consideration is allowed for the synchronization process. The synchronization process

was made according to the possible system frequency deviation. The system is not

much deviated from 50 Hz frequency. Hence, it will help the load voltage without

phase jumps.

Vrg;s----

litt-~ _, p

PI comoler pererneters : Cortr ... -

rtegral gat! Pr~ lei gat! -100 -10

--0

5.17284 --0

0.493627

Figure 25: PI controller block diagram

33

Delta

3.8.1 Tuning the PI controller

Figure 26 show the interface to key in the parameter for the PI controller.

(pi _c tlr] PI Controller \w lnterpoldtion ~

Proportional Gail

lrtegral Tme Const.ert

Maxint.ln Linit

MlniTun Linl

r.ial ~ of lrtejJ'ator

lrtegration h4ethod

lOp IGi rw.o--1-10.0

lo.o

~lletpotatioll ~y

r. r

OK - ~-·· J

(a)

~, , [var] Variable Input Shder ~ 1 [vdr] Var1able Input Shder ~

Tile (for c1sp1ey n panel and meter) I ~I!!J'ai gail Tile (tor c1sp1ey n panel and meter) I Proportional gail

~ l PI cortroler pera Gr~ I PI cortroler pera

Display tile on icon? r;;- _:j Display tile on icon? I No .:] MaxilunVu roo- MaxilunVu f10 --hhlunVu lo.o hhlunVu ro.o --ra~vu 15.17283950317 rlial Vak.Je 1 0.493827160494

l.tis (eg: kA, kV, pu) r l.tis (eg: kA, kV, pu) I oa colediln (TtuNI c:uVa> leriulus 3 o.a colediln (TtuNI coma) leriulus 3

OK ~-·· OK ~ ... - .

(b)

Figure 26: Defining the PI controller parameter

34

The PSCAD simulation diagram ofthe proposed distribution network together

with the DVR and with control circuit is presented in Figure 27.

1 0 0 ~· • }

lkma •F

lv.£ " · 1 ooe 8 ~~-

? •

I --· 18'1531

I . ,. 1- .... I I

- :.. OlfiiJifZI

(a)

0.1. '"""' (• +~ CD 0

(b)

""-{ ~ 1.0 ,J~f

so.rf~

Figure 27: Complete power system network with DVR

35

CHAPTER4

RESULTS AND DISCUSSIONS

The simulation is carried out and the results are analyzed for different voltage

dip and fault condition.

Different voltage dip conditions

(i) No fault

(ii) Single line to ground fault without DVR

(iii) Double line to ground fault without DVR

(iv) Three phase to ground fault without DVR

(v) Injection voltage with DVR

(vi) Single line to ground fault with DVR

(vii) Double line to ground fault with DVR

(viii) Three phase to ground fault with DVR

Besides that, the graph of the output voltage VSC will shows as one of the

DVR part to generate 3-phase signals from de to ac voltage. Also the injection

transfonner graph will shows to see the changes before and after voltage dip occurs.

36

PSCAD simulation diagram of the proposed PWM is presented in Figure 28.

The signal (here the red, yellow and blue) is compared with the triangular waveform

(green) to generate ac waveform. Figure 29 shows the ac signal. Each of the signal is

120 degree a part each others.

1.50 • Vlri Vred Vyf!loN -~

1.00

0.50

0.00

-0.50

-1.00

-1.50 r---~--~--~----r---~---r--~----~--~---.

O.IXX> 0.010 O.ao!O O.<XJl 0.040 0.000 O.IB> 0.070 0.(8) O.ca> 0.100

Figure 28: Pulse Width Modulation

500 • fa

«X)

:m 200 100

~ 0

>- -100

-200

-D)

-400

-500

O.IXX> 0.010 O.ao!O O.<XJl 0.040 O.CB:l O.IB> 0.070 0.(8) O.ca> 0.100

Figure 29: Switching signals for inverter leg

37

4.1 Result

Case l : No fault

This section shows the simulation and results when no fault occurred at the system.

Voltage dip created at t = 0 s

Figure 30 shows the injected voltage and load voltage during normal condition.

:llOO • VII> ·- •lt!!l

I.J&I\hn~ :~

till ·~

1Bl

1.«1

~ O.IJJ >-

Q.Bl

0.<10

Q.2l

0.00

0.00 0.10 Q.2l 0.«1 O.!il

Figure 30: Three phase and load voltage at normal condition

38

Case 2: Single phase to ground fault without DVR

This section shows the simulation and results when single phase fault occurred at the

system.

Voltage dip created at t = 0.3 - 0.6 s

Figure 31 shows the red voltage phase drop during the voltage dip. Figure 32 shows

the load voltage drop during the voltage dip.

•'180

Figure 31: 3 phase voltages with single phase fault without DVR

ttl •YI!!I

tm

QID

... 0.110

Qt)

O.tl 100

Figure 32: Voltage at load side without DVR

39

Case 3: Double phase to ground fault without DVR

This section shows the simulations and results when double phase fault occurred at

the system.

Voltage dip created at t = 0.3 - 0.6 s

Figure 33 shows the blue and green voltage phase drop from 9.0 p.u to 3.0 p.u during

the voltage dip. Figure 34 shows the load voltage drop from 0.9 p.u to 0.6 p.u during

the voltage dip.

15.0 • VSI

10.0

5.0

~ 00 >.

-6.0

-10.0

-15.0

0.3) O.«l O.!iO 0.8> 0.10

Figure 33: 3 phases voltage at double phase fault without DVR

"""vu. cn.n 1.3> • Y!!!!

100

0.80

08) >.

O.«l I 0.3)

0.00

0.00 0.10 0.30 O.«l O.!iO 0.8> 0.10 0.8> 0.9J

Figure 34: Voltage at load side without DVR

40

Case 4 : Three phase to ground fault without DVR

This section shows the simulations and results when three phase fault occurred at the

system.

Voltage dip created at t = 0.3 - 0.6 s

Figure 35 shows all voltage phase drop from 9.0 p.u to 3.0 p.u during the voltage dip.

Figure 36 shows the load voltage drop from 0.9 p.u to 0.15 p.u during the voltage dip.

10.0 • V&l -~

ao 6.0

40 2.0

~ 0.0 >- -2.0

-4.0

~0

.a.o -10.0

02) 0.~ 0.«> 050 0.00 0.70

Figure 35: 3 phases voltage at three phase fault without DVR

VnT6 vu. ~ 1.20 • .Ym:l

1.00

O.!Kl ~. f 0.00

I >-

0.40 \

0.20 \_ 0.00

0.00 0.10 0.~ 0.40 0.50 0.00 0.70 O.!Kl O.!K>

Figure 36: Voltage at load side without DVR

41

Case 5 : Injected voltage with DVR

This section shows the simulations and results when fault occurred at the system.

Voltage dip created at t = 0.3- 0.6 s

Figure 37 shows the injection voltage from DVR system during the fault occurred.

•.w. • liSt

040 QlO

Figure 37: 3 phase injected voltage with DVR

42

Case 6 : Single phase to ground fault witb DVR

This section shows the simulations and results improvement of load voltage when

single phase to ground fault occurred at the system.

Voltage dip created at t = 0.3- 0.6 s

Figure 38 shows the improvement ofload voltage when DVR connected to the

system.

tal

noo --­nm Ul Oll till

Figure 38: Voltage at load side with DVR for single phase fault

43

Case 7 : Double phase to ground fault with DVR

This section shows the simulations and results improvement of load voltage when

double phase to ground fault occurred at the system.

Voltage dip created at t = 0.3- 0.6 s

Figure 39 shows the improvement of load voltage from 0.15 p.u up to 0.9 p.u when

DVR connected to the system.

1.2) •VmB

1.00

, .. --... ---IIWNW'NtNW,Wl'fiN~WJ#:~\ ~ o.oo r- ·- , v -

{

o.ro I

O.«l f

021

0.00

Figure 39: Voltage at load side with DVR for double phase fault

44

Case 8 : Three phase to ground fault with DVR

This section shows the simulations and results improvement of load voltage when

three phase to ground fault occurred at the system.

Voltage dip created at t = 0.3 - 0.6 s

Figure 40 shows the improvement of load voltage from 0.6 p.u up to 0.9 p.u when

DVR connected to the system.

Vmsvu:QcdB

121 •Vms

1.00

'1/>fWNA\~Wt/MW##I'#N.J'v./,.---O.lll ,...,.-

1/

O.En I t - j

0.40 I I I

021

0.00

0.00 0.10 O.al O.ll 0.40 O.fn O.En 0.10 O.lll 0.00

Figure 40: Voltage at load side with DVR three phase fault

45

CHAPTERS

CONCLUSION AND RECOMMENDATION

5.1 Conclusion

Voltage dips are most common problems faced by the consumer. Many

industries face the problem of voltage dips problem leading to loss in production cost.

The common solution for this problem is to use full UPS system but it is more costly.

This project describes a solution to the voltage dip problem and the

compensation technique using custom power electronic device DVR which was

designed by using PSCAD software. The design and also the application of DVR for

voltage dip and the simulation results for different fault were presented. A PWM

based control scheme was implemented to generated 3-phases signal and this PWM

scheme only require voltage measurement. To control the voltage, PI controller has

been developed to control the voltage automatically. It uses a closed loop control

system to detect the phase difference and magnitude error between the voltage during

and before the voltage dip. In addition to mitigation of voltage dip, the simulation

result indicates that DVR provides additional benefit of output voltage improvement.

The DVR injects appropriate voltage component to correct rapidly any voltage dip to

keep the load voltage balanced and constant at the nominal value. The efficiency and

effectiveness in voltage dip compensation is demonstrated by the simulation results

ensuring high quality in power supply.

46

5.2 Recommendation

In order to verify the simulation results, understanding the behavior of the

Dynamic Voltage Restorer is important to mitigate the voltage dips by selecting the

right configuration of DVR. This project mainly concentrated on about mitigation of

the output voltage whereas the other power quality issues are not taken into account.

So in future work, voltage and current harmonics also need to be considered as one of

the power quality issues and DVR is to be designed to mitigate the voltage and

current harmonics.

47

REFERENCES

[1] David Chapman, "Voltage dips", Copper Development Association, pg 1-6,

2001.

[2] C.Benachaiba and B.Ferdi, "Power Quality Improvement using DVR", American

Journal of Applied Sciences (6), vol3, p.p 396-400, 2009.

[3] C. DiPerna, P.Verde, A.Sannino, M. H.J. Bollen, "Static series compensator for

voltage dips mitigation with zero sequence injection capability", Bologna Power

Tech Conference, p.p 23-26, June 2003.

[4] K.W.E.Cheng, Y.L.Ho, S.L.Ho, K.P.Wong, NCCheung and T.K.Cheung,"

Investigation of Voltage Dips Restorer Using Square Wave Inverter" the 30'h

Annual Conference of the IEEE Industrial Electronics Society, Busan, Korea, p.p

2-6, November 2004.

[5] C. S. Chang, Zhemin Yu, "Distributed Mitigation of Voltage Sag by Optimal

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49