an enhanced performance of distance relay algorithm to prevent (tesis)

7/25/2019 An Enhanced Performance of Distance Relay Algorithm to Prevent (Tesis)

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AN ENHANCED PERFORMANCE OF DISTANCE RELAY ALGORITHM TO PREVENT

UNDESIRABLE ZONE 3 OPERATION DURING LOAD ENCROACHMENT

MR.SATHAPORN SITTIWONG

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF MASTER OF SCIENCE

IN ELECTRICAL POWER ENGINEERING

SIRINDHORN INTERNATIONAL THAI-GERMAN GRADUATE SCHOOL OF ENGINEERING

(TGGS)

GRADUATE COLLEGE

KING MONGKUT'S UNIVERSITY OF TECHNOLOGY NORTH BANGKOK

ACADEMIC YEAR 2007

COPYRIGHT OF KING MONGKUT'S UNIVERSITY OF TECHNOLOGY NORTH BANGKOK


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Name : Mr.Sathaporn Sittiwong

Thesis Title : An Enhanced Performance of Distance Relay Algorithm to

Prevent Undesirable Zone 3 Operation During Load Encroachment

Major Field : Electrical Power Engineering

King Mongkuts University of Technology North Bangkok

Thesis Advisor : Assistant Professor Dr.Teratam Bunyagul

Academic Year : 2007

Abstract

Distance relays are used to detect and protect faults in transmission lines. The

distance relay provide three zones of protection. Zone 3 is necessary as a backup for

Zone 1 and Zone 2. However Zone 3 has a disadvantage that during a heavy load

condition it may operates because the high load impedance moves into the zone. If

Zone 3 misoperates, it causes disconnection to transmission line consequently the

adjacent lines may be over rated because the change of power flows. This may lead to

cascading outage and power system instability. The thesis propose the use of steady

state signal and transient state signal to distinguish between faults and load

encroachment into Zone 3. The models are built and simulated in PSCAD/EMTDC

software. Then transient components are detected to distinguish from high load

moving into Zone 3. The thesis propose the use of steady state signal and transient

state signal to distinguish between faults and load encroachment into Zone 3. The

models are built and simulated in PSCAD/EMTDC software.

(Total 83 pages)

Keywords : Distance Relay, PSCAD/EMTDC, Load Encroachment

______________________________________________________________ Advisor


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: :

3

:

: . : 2550

3 3 1 2 3 1 2 3 PSCAD/EMTDC

(83)

: , PSCAD/EMTDC,

_____________________________________________


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ACKNOWLEDGEMENTS

I would like to express my sincere gratitude for Assistant Professor Dr.Teratam

Bunyagul, my advisor, for providing excellent guidance and introduce me to the area

of power system protection including the subjects in this thesis. It is because of his

constant assistance that this thesis has been successful. Under her guidance, I have

learned much more than scientific knowledge.

I would also like to thank my teachers, friends, the staff of the Electrical

Engineering Department, Faculty of Engineering and the staff of Sirindhorn

International Thai-German Graduate School of Engineering, King Mongkut's

University of Technology North Bangkok for all their support

Finally, I am extremely grateful to my family for always loving and supporting

me. This thesis would have not been finished without their encouragement and

inspiration throughout the duration of my study.

Sathaporn Sittiwong


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TABLE OF CONTENTS

Page

Abstract (in English) ii

Abstract (in Thai) iii

Acknowledgements iv

List of Tables vii

List of Figures viii

Chapter 1 Introduction 1

1.1 Distance relay in power systems 1

1.2

Basic Requirement of Protection Systems 1

1.3

Fundamental of Distance Protection Systems 2

1.4 Protection Zones for Distance Relays 3

1.5 Performance of Zone Protection 4

1.6

Zone Protection and Disturbance 4

1.7 Problems of Distance Protection 8

1.8 Purpose of The Study 8

1.9

Scope of The Study 8

1.10 Methods 9

1.11 Utilization of The Study 9

1.12 The Structure of Thesis 9

Chapter 2 Distance Relays and Zone Protection 11

2.1 Fundamentals of Distance Relaying 11

2.2

Operation of Distance Relays 12

2.3

Zone Application of Distance Relaying 132.4 General Characteristics of Distance Relays 15

2.5

Application of Distance Relays 21

2.6 Types of Disturbances 23

2.7 Load Power Effect to Distance Relays 26

2.8

Load Encroachment 29

Chapter 3 Literature Survey 31

3.1 Previous Works on Disturbance for Zone 3 of Distance

Protection 31


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TABLE OF CONTENTS (CONTINUED)

Page

Chapter 4 Algorithm and Signal Analysis 39

4.1 Fundamental Considerations 39

4.2 Digital Distance Measurement 39

4.3 Fourier Analysis 41

4.4 Fourier Analysis Based Algorithm 42

4.5 Principle of Algorithm 43

Chapter 5 Software Model in PSCAD/EMTDC 45

5.1 Power System Parameters 45

5.2 Protection System and Evaluation Setting Values 47

Chapter 6 Simulation Results 53

6.1 The Sequence of Simulation 53

6.2 Three-Phase Fault Condition 54

6.3 Line-to-Line Fault Condition 66

6.4 Single Line-to-Ground Fault Condition 70

Chapter 7 Simulation Results 77

7.1 Conclusion 77

7.2 Discrimination 78

7.3 Speed 79

References 81

Biography 83


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LIST OF TABLES

Table page

5-1 Data of transmission lines and generator source 46

6-1 Setting of threshold value 76


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LIST OF FIGURES

Figure page

1-1 Distance relays characteristic 3

1-2 The three protection zones for the transmission line 3

2-1 The distance relay connected to line and instrument transformers 12

2-2 Principle of the distance relay 12

2-3 Beam type distance relay 13

2-4 Step time zones of distance relay protection 14

2-5 Step time zones of distance relay protection 15

2-6 Distance relay characteristics on R-X diagram 15

2-7 Distance relay of mho characteristic type 16

2-8 Increased arc resistance coverage 17

2-9 Minimum load impedance permitted with lenticular, offset mho and

impedance relays 18

2-10 Quadrilateral characteristic 19

2-11 Application of out-of-step tripping relay characteristic 20

2-12 Distance relays on various lengths of adjacent line section 21

2-13 Effect of infeed on impedance measured by distance relays 22

2-14 The apparent impedance as the complex power vary and the power

factor vary 27

2-15 An apparent impedance as the active and the reactive power vary 29

2-16 Load encroachment on mho distance element characteristics 30

3-1 The impedance changes when a power swing with oscillation frequency

equal to 0.1 Hz 31

3-2 Power swing blocking characteristic 32

3-3 Block diagram of the proposed fault detector 35

4-1 Distance protection measuring principle PD = protection device,

FR = fault resistance 40

4-2 Operation of transmission system 43

4-3 Fault 1 occur at 200 km both breaker 5 and 6 43


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LIST OF FIGURES(CONTINUED)

Figure page

4-4 Fault 2 occur at front of breaker 3 44

5-1 The single line diagram of circuit typical 230 kV substation 45

5-2 PI section model for a transmission line 46

5-3 Distance relay zone protection 47

5-4 Input signal are into Fast Fourier transform Block 48

5-5 Input signal are into sequence component Block 48

5-6 Calculation the line-to-line impedance block 49

5-7 Calculation the line to ground impedance block 49

5-8 Mho characteristics of distance relay 50

5-9 Model of circuit typical 154kV substation 50

6-1 Model when fault occur on power system 53

6-2 Active power when fault between 2 s to 2.1 s 54

6-1 Active power when fault at 5.4 s 55

6-2 Change rates of active powerbetween 2 s to 2.1 s 55

6-3 Change rates of active powerat 5.4 s 56

6-6 Trajectory of impedance distance relay 1 56

6-7 State of distance relay 1at fault(1) and fault(2) 57


6-9 State of distance relay 2 58


6-11 State of distance relay 3 606-12 Trajectory of impedance distance relay 4 60




6-16 Change rates of active power between 2 s to 2.1 s 62

6-17 The power swing and fault(2) at 5.4 s 63

6-18 State operation of distance relay when fault(1) and fault(2) 63



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LIST OF FIGURES(CONTINUED)

Figure page

6-20 The power swing of impedance and fault(2) at 5.4 s 65


6-22 The power swing impedance and fault(2) at 5.4 s 66



6-25 Change rates of active power at 5.4 s 68

6-26 The trajectory of impedance when fault(2) at 5.4 s 686-27 State of distance relay 1 69







6-34 R-X diagram of distance relay 1 72




6-38 The power swing impedance and fault(2) at 5.4 s 75



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CHAPTER 1

INTRODUCTION

This chapter introduces fundamental operations of a distance relay and theories

of transmission line protection. It also describes the purpose and the background of

the research. The study focuses on discrimination of the distance relays, misoperation

of distance relays and the effect of disturbance to distance relays.

1.1 Distance Relay in Power Systems

Distance relays are protective devices in electrical power system. They detect

faults in transmission systems and isolate abnormal or fault conditions by sending trip

signals to associated circuit breaker.

In the event of an electrical fault, the normal balance between generation and

load requirement is suddenly disturbed. Most of the energy previously supplied to the

load is diverted into the fault path. In order to retain system stability and to limit the

damage at the fault location, rapid isolation of the faulted section is necessary.

1.2 Basic Requirement of Protection Systems

Maintaining continuity of electrical supply to customers is a major work of the

electricity generating authority. Protective relays are responsible for discriminating

between normal and faulted conditions in the system. When fault happens, a local

relay should have a high speed operation. The distance relay should detect the faultwithin 20 to 40 milliseconds [1]. Allowing the time of about 40 milliseconds for

circuit-breaker to operate a total fault clearance time of 80 millisecond or less is

desirable [1]. Measurement of distance from the current and voltage waveforms

during the fault period may contain transient errors which affect the accuracy of the

distance relay. We must be careful when considering the output requirement of the

current and voltage transformers.


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Mal-operation of protective devices can give rise to widespread disturbance in

the electricity supply system. This emphasizes the need for continued improvements

in speed of operation, discrimination and reliability [1].

1.3 Fundamental of Distance Protection Systems

Distance protection systems are typically used for the protection of transmission

lines. The protection is managed in overlapping zones. No part of the system is left

unprotected. A comparison of local signals (voltage and current) enables the relay to

decide which zone contains the fault [1]. In this way, a distance assessment is made

from the relay location to the fault location.

Distance relaying applies the principle of ratio comparison between the voltage

and current which equates to the impedance. The relay, located at the beginning of a

line calculates the apparent impedance of the line using the measurements of the

current and voltage transformers at the same location as the relay. Impedance of

transmission line increases with the length of the line.Each type of faults produces a

different impedance value. Due to this reason the settings of a distance relay must be

selected to distinguish the faults between the phase and the ground. Furthermore, a

fault resistance may create a problem for distance measurement because the value of

the fault resistance may affect apparent impedance. This causes inaccuracy of the

apparent impedance calculation.

The distance relay is capable of rapidly detecting faults on the transmission

line. It operates when the impedance of the line is within the impedance characteristic

of the distance relay (impedance plane circle).The provided operating characteristics

of distance relay may have different shapes, depending on application and suitability.The characteristic consideration must be adapted to the changes of the loading

levels, different values of fault resistance, effects of power swings, and reversals of

fault direction. Most of the relay characteristics that we use are quadrilateral and mho.

Quadrilateral relay is suitable for protecting ground fault. Mho relay is very effective

with detecting phase fault. The different operating characteristic shapes are shown in

Figure 1-1 [2].


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FIGURE 1-1 Distance relays characteristic

1.4 Protection Zones for Distance Relays

Most of the transmission line protections are arranged into three protection

zones. Each zone uses different protect sections. Figure 1-2 shows the arrangement of

the three protection zones. These protection zones are used to define the relay reach

and their operation times.

FIGURE 1-2 The three protection zones for the transmission line

Each protection zone is set to cover the pre-defined length of the transmission

line. Typical selection of the zones in the transmission line protection is to cover 80 to

90% of the line in Zone 1, 120150% in Zone 2, and 200250% in Zone 3 [1]. The

relays operation times associated with each zones are different: in Zone 1 the relay

operates instantaneously, Zone 2 is delayed to allow zone 1 relays to operate first, and

Zone 3 times allows the corresponding relays closer to the fault to operate first in

either the zone 1 or zone 2.

This time-step approach for different protection zones allows the relays closest

to the fault to operate first. If they fail to operate, the relays located at the remoteterminals that see the same fault as in Zone 2 will still disconnect the failed


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component. If Zone 2 relay fails to operate, the relays located further away from the

faulted line will operate next with the zone 3 settings.

The advantage of this approach is a redundant coverage of each line section.

They are also covered with multiple relay zones of the relay located on the adjacent

lines, ensuring that the faulted component will be eventually removed even if the

relay closest to the fault fails.

1.5 Performance of Zone Protection

When protective relays correctly tripped a faulted line, its current flow will be

transferred to other lines which may become overloaded. These overloaded lines may

trip Zone 3 backup relay, eventually leading to a widespread blackout.

If the overloaded lines can be identified as the flow transferring from the faulted

line and thus preventing the relays from tripping, cascading trips can be prevented and

actions such as under-voltage load-shedding and manual load-shedding can be taken

to save the system and avoid whole system blackout [2].

Some expert systems are presented to prevent cascading trips but these expert

systems emphasize only the optimal fault clearance and how to avoid incorrect

operation of backup relay. They are not, however, intended to avoid the Zone 3

distance relay trip caused by the aforementioned overloading of other lines.

Zone 3 distance relay is used to provide remote backup protection in case the

primary protection fails. Operation of the distance relay depends on the settings of the

characteristics distance relays. If the relays only take local measurements (such as the

voltage and current) as inputs without taking the impact of the whole network to make

decisions to trip, the system will be deteriorated. In the worst scenario even resultingin cascading trips due to flow transferring overloadafter the clearance of a faulted line

[2].

1.6 Zone Protection and Disturbance

Power utility networks are becoming larger and transmission lines are now

operating closer to their limit than ever before. Power systems are subjected to a

variety of small or large disturbances during its operating conditions. Changes in the

regulations and the opening of the power markets result in rapid changes in the way


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that the power grid is operated. As a result, power system are more vulnerable to

disturbances which increases the possibility that an interconnected network may lose

stability thereby leading to regional or even total blackout.

The background of blackout, the power swing and the load encroachment will

be briefly outlined in this section.

1.6.1 The background of blackout

The major blackouts in the United States of America, such as the two blackouts

in 1996 in the Western region on 2ndJuly 1996 and again on 10thAugust, and 2003 in

the Midwest and the Northeast on 14 th August, are the results of heavy load and a

number of multiple outages that had occurred within a short period of time.

For example, the Northeast Blackout on 9thNovember 1965 resulted in the loss

of over 20,000 MW of load. The initiation of the disturbance was the faulty setting of

a relay thus resulting in the tripping of one of the five heavily loaded 230-kV

transmission lines. The flow of power on the disconnected line was thus shifted to the

remaining four lines causing them to become overloaded and to trip successively in a

total of 2.5 seconds. The cascading tripping of additional lines then began which

resulted in the regional blackout.

On 2ndJuly 1996 the West Coast Blackout resulted in the loss of 11,850 MW of

load and affected 2 million people in the West. The outage began when a 345-kV

transmission line in Idaho sagged into a tree and tripped out. A protective relay on a

parallel line also detected the fault and incorrectly tripped a second line. Other relays

tripped two of the four generating units at Jim Bridger plant. About 23 seconds later,

the Mill Creek to Antelope 230-kV line was tripped by Zone 3 relay. Remedial action

relays separated the system into five pre-engineered electric islands which collapsedeventually due to stability problems [2].

On 10thAugust 1996 the second West Coast Blackout resulted in the loss of

over 28,000 MW of load. Faults caused by treesput three 500-kV line sections out of

service: (1) at 3:48 p.m., the Keeler-Allston 500-kV line had sagged into a tree and

tripped, which caused the loss of 1300 MW of loading. The transmission line outages

overloaded parallel lower-voltage lines in Portland area. (2) About 5 minutes later a

relay failure tripped a 115-kV line and (3) at the same time a 230-kV line had sagged

into a tree and also tripped. At about the same time generators at the McNairy


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hydroelectric plants started tripping because of the faulty relays. Increasing

oscillations soon caused synchronous instability and the ensuing cascading tripping of

transmission lines broke the interconnection into four electric islands [2].

On 14th August 2003, the latest blackout in the Midwest and the Northeast

regions resulted in the loss of 70,000 MW of load. The blackout happened

consecutively in different locations. At 1:31 p.m. the Eastlake 5 generation unit was

tripped and at 2:02 p.m. the Stuart-Atlanta 345-kV line was tripped due to contact

with a tree. From 3:05 to 3:57 p.m. a 345-kV line was tripped due to contact with

trees. Later, from 3:39 to 4:08 p.m., a 138-kV line was tripped due to overloading.

Then from 4:05 to 4:10 p.m. many lines which operated on Zone 3 impedance relays

eventually led to cascading trips and blackout [2].

These types of events most likely occur following sequential outages on a

stressed system when the system is operated marginally in compliance with planning

criteria. For example, some generators and/or lines for maintenance, line trips due to

fault which overload other lines. If another disturbance occurs, for example, another

line gets in contact with a tree and then trips, it will bring the system into a more

serious state, which may result in a blackout [2].

1.6.2 The power swing

The power swing is the variation in three phase power flow which occurs when

the system generator rotor angles are advancing or retarding relative to each other in

response to changes in load magnitude such as line switching, loss of generation,

faults, and other system disturbances. In most cases, the power swings are stable if the

generators do not slip poles and the system reaches a new state of equilibrium.

On the other hand, if a system is transiently unstable and the power swingresults in the generator experiencing pole-slipping which eventually leads to a loss of

synchronism between groups of generators, it is called an out-of-step (OOS)

condition.

The power system should be maintained to survive larger types of disturbances

such as faults, loss of a large generator, line switching and heavily loaded. Power

system disturbances could cause loss of synchronism between a generator, low

network voltages and consequent voltage or angular instabilities or severe loss of

interconnected power systems of neighboring utilities. Depending on the severity of


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the disturbance, the system may remain stable or return to a new stability state

experiencing what is referred to as a stable power swing.

Large power swings, make stable or unstable happen and unwanted relay

operations at these events, which it can aggravate further the power system

disturbance and possibly lead to cascading outages and power blackouts. When power

swings happen the distance relay must not operate due to the reason that the apparent

impedance (falls below)decreases withinthe relays operating characteristic.

Operations of distance relays due to a power swing result in undesired tripping

of transmission lines or other power system elements, thereby weakening the system

and possibly leading to cascading outages and the shutdown of major portions of the

power system.

Therefore, distance relays should not trip during power system disturbance

conditions from stable or unstable power swings because the power system may

return to a stable operating condition after some time.

Power swing has been proven to influence the distance relays. In some

situations of power swing and out-of-step conditions, the distance relay cannot

distinguish the power swing from the three-phase line fault.

1.6.3 The load encroachment

The load encroachment is a problem particularly in Zone 3 distance relay

caused by increasing load. This Zone 3 is given a delay time longer than that

associated with Zone 2 to achieve time coordination, and the time delay is typically in

the range of 1-2 s.

Because Zone 3 is set to detect faults down adjacent lines out of the remote

station, in-feed at the remote station causes the relay to under-reach. Likewise, out-feed causes an over-reach effect.Additionally, distance relays may misoperate during

events such as transient and voltage instabilities. This undesirable Zone 3 tripping has

often contributed to cascading outages.

According to a report from the latest 2003 blackout in the US, a lot of Zone 3

distance relays operated under the overload and power swing situations, which further

stressed the system thereby causing the cascading blackout in the end. Power swing,

either stable or unstable, may have impacts on distance relays judgment. This kind of

relay mal-operation may further weakened the system.


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1.7 Problems of Distance Protection

Many problems affect the application of distance protection in a power system.

These problems are outlined as follows :

1.7.1 The transient of current and voltage signals at the occurrence of a fault.

1.7.2

The distance relay mal-operates during a power swing condition due to the

reason that the apparent impedance in system may come inside the zone.

1.7.3 Discrimination between a healthy load and a system fault condition which

the distance relay must be able to identify these condition to prevent mal-operation.

1.7.4 The fault resistance may create problems for distance measurements

because the value of the fault resistance makes to distance relay produce error

apparent impedance which may be difficult to forecast.

1.7.5 The current in-feed from other transmission lines causes a voltage drop on

the fault resistance. Affect to distance relays to measure incorrect apparent

impedance. This may contribute to incorrect calculation of the apparent impedance.

1.8 Purpose of The Study

1.8.1

To study PSCAD/EMTDC software especially for distance relay

modeling.

1.8.2 To study and simulate a distance relay model for Zone 3 protection.

1.8.3

To study the effect of power swing on zone protections.

1.8.4 To identify faults occurring during power swings.

1.9 Scope of The Study

The Thesis studies the prevention methods of undesirable Zone 3 protectionoperation during load encroachment and power swing. The aims are to design an

algorithm, model and simulate the transmission systems using PSCAD/EMTDC

simulation software; and to design the algorithm using MATLAB software.

The distance relay is designed to identify between a power swing and a fault

conditions in order to prevent cascading trips under the power swing condition, which

might result in a widespread blackout.


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1.10 Methods

1.10.1 To study zone protection of transmission line and distance relay setting.

1.10.2

To learn the PSCAD/EMTDC simulation software.

1.10.3 To study, model and simulate transmission systems using the

PSCAD/EMTDC software.

1.10.4 To designs an algorithm that distinguishes between a fault and a power

swing.

1.10.5

To test the algorithm with PSCAD/EMTDC simulation.

1.11

Utilization of The Study

1.11.1

The algorithm can be used to detect and distinguish a fault occurrence

during a power swing.

1.11.2 To prevent the maloperation of distance relay due to power swing.

1.12 The Structure of Thesis

The research presents the study of the discrimination of distance relays

protection at fault occurrence during a power swing. The distance relay is modeled

and simulated using PSCAD/EMTDC and MATLAB softwares. (The simulation

results of the distance relay operating sequence and time are analysed.) The model of

distance relays is confirmed by simulation results of the operating sequence and

operating time.

The methodology, data and analysis of the research is divided into six chapters :

Chapter 2: The basics of the distance relay protection.

Chapter 3: Literature survey.Chapter 4: The basic algorithm of the mathematic used for the research.

Chapter 5: The transmission line and the distance relays parameters are

evaluated and modeled using PSCAD/EMTDC.

Chapter 6: The simulation results of PSCAD/EMTDC.

Chapter 7: The conclusion of the research and recommendation for future work.


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CHAPTER 2

DISTANCE RELAYS AND ZONE PROTECTION

This chapter discusses fundamental, operation, zone application, characteristics

and the load power effect of the distance relays as well as the disturbance in power

system and the load encroachment. The causes of the problems in distance relay and

the disturbances which affect the zone operation of the relay are also presented.

2.1 Fundamentals of Distance Relaying

Distance relay detects the fault impedance obtained from the measured ratio of

voltage to current faults. The apparent impedance is measured between the relay

location and the point of fault occurrence. If the measured fault impedance is smaller

than the impedance set for the distance relay to indicate an internal fault happen, then

send a trip signal to the circuit breaker. At the fault occurrence on a transmission line

the impedance will move into zone protection impedance because the fault impedance

is smaller than the load impedance.

The distance protection scheme is normally a multi-zone arrangement in which

the first zone of protection provides instantaneous tripping and the next zones

incorporate time delayed tripping. Three protective zones are usually included, giving

the stepped time-distance characteristic. The aim of the scheme is to provide correct

high speed tripping of circuit-breakers, with adequate discrimination provided

between internal and external faults [1].

This kind of relay is known as a distance relay and is designed to operate only

for faults that occur between the relay location and the selected reach point thus

giving discrimination for faults that may occur in different line sections [3].

The reach point of a relay is the point along the line impedance locus that is

intersected by the boundary characteristic of the relay. Since this is dependent on the

ratio of the voltage to current and the phase angle between them, it may be plotted on


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an R/X diagram. The locus of power system impedances as seen by the relay during

faults, power swings and load variations may also be plotted on the same diagram [3].

2.2 Operation of Distance Relays

The distance relay is connected with a power line through instrument

transformers, as shown in Figure 2-1. Assuming that the fault occurs on the line at a

distance of 1LnZ W from the relay, the voltage RV on the relay will be the R LI nZ

drop from the distance relay to the fault since the voltage at the fault is 0 V [4].

FIGURE 2-1 The distance relay connected to line and instrument transformers

Therefore, the impedance of the distance relay is :

R R 1LR 1L

R R

V I nZZ nZ

I I= = = Eq. 2-1

FIGURE 2-2 Principle of the distance relay

In Figure 2-2, When a fault occurs between the distance relay and LZ , the

impedance from the relay is ( )L LZ Z- D W . The restraint force will then be


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( )R L LI Z Z- D and it will be less than the operating force for the same current

magnitude. As a result, the beam will incline down at the current end, closing the

contacts. If the fault is beyond LZ distance, say at ( )L LZ Z+ D W from the relay, then

the restraint force RV will be greater than the operating force RI . The beam will then

incline down at the voltage end, and the contacts will not close [4].

In Figure 2-3 the voltage provides a magnetic force or pull on one end of the

beam. If, for this fault, the current or the operating force RI on the other end of the

beam is adjusted to be equal to the voltage or the restraint force RV , the beam will be

balanced.

FIGURE 2-3 Beam type distance relay

2.3 Zone Application of Distance Relaying

Throughout all transmission lines there will be three protection zones which are

used to protect a line section and to provide backup for the remote section. The

common practice has been to use separate distance units for the several protection

zones. This is in contrast with distance relays that use a single distance-measuring unit

initially set for Zone 1 reach. If the fault persists, the reach is extended by switching

to Zone 2 after T2time delay, then after T3to Zone 3.

Separate units provide the comfort of redundancy because for faults in the Zone

1 primary reach area, all three distance units will operate. Thus, Zones 2 and 3 are

used back up in case Zone 1 unit fails.

These zones and typical settings are illustrated in Figure 2-4 which shows the

zones at several locations. Each of the three zones uses instantaneous operating

distance relays. Zone 1 is set for 80 to 90% of the line impedance. The resulting 10-


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20% safety margin ensures that there is no risk in the Zone 1 protection over-reaching

the protected line due to errors in the current and voltage transformers, inaccuracies in

line impedance data provided for setting purposes and errors of relay setting and

measurement.

FIGURE 2-4 Step time zones of distance relay protection

Zone 2 is adjusted for 100% of the line, plus approximately 50% of the shortest

adjacent line off the remote bus which operates through a timer T2. Zone 3 is set for

100% of both lines, plus approximately 25% of the adjacent line off the remote bus.

Wherever possible, Zones 2 and 3 provide backups for all the adjacent lines at

operating times of T2and T3[2]. These original settings define the protection zones

only if there are no infeed effects.

Figure 2-5 shows the operating circles for the three zone at bus G, breaker 1

(solid line) and at bus H, breaker 2 (broken line) plotted on the RX diagram. Therelays operate when the ratio of fault voltage to current falls within the circles.

Load can be represented on these R-X diagrams as an impedance phasor,

generally lying near the R axis (depending on the power factor of the load current on

the line). The phasor lies to the right (first quadrant of the R-X diagram) when

flowing into the protected line from the bus and to the left (third quadrant of the R-X

diagram) when flowing out of the line to the bus. The operating circles must be set

such that they do not operate on any system swings from which the system can


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recover. Such swings occur after a system disturbance, such as faults, sudden loss of

generation or load, or from switching operations.

R

X

Load areafrom H to G

Load areafrom G to H

G

H

R

FIGURE 2-5 Step time zones of distance relay protection

2.4 General Characteristics of Distance Relays

The distance relay characteristics plotted on the R-X diagram are shown in

Figure 2-5. The operating zones are defined such that the apparent impedance falls

inside the circles for the relay characteristics labeled a, b, and c. In another words,

whenever the ratio of V/I falls inside the circle, the distance unit operates [4].

FIGURE 2-6 Distance relay characteristics on R-X diagram

Figure 2-6a has the non-directional impedance characteristic. When it is used for

fault protection, a separate directional unit is added to limit the tripping to line faults

[13]. The mho characteristic of Figure 2-6c is a circumference that passes through theorigin.


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In the event that high values of earth fault resistances are expected, the

reactance protection scheme of Figure 2-6b may be used. In this scheme, it is

important to ensure that the reach setting of the fault detector dose not result in

operation during maximum circuit loading condition [1].

2.4.1

Mho characteristics

The mho characteristic in distance protection scheme, as shown in Figure 2-6c,

is widely used for Zones 1 and 2, with Zone 3 providing back-up protection for fault

immediately at the relay [1].

FIGURE 2-7 Distance relay of mho characteristic type

Figure 2-7 shows the impedance element that will operate only for faults in the

forward direction along line AB. Advantages are fixed reach as a function of the

protected line impedance and so independence of system operating and fault levels

over a very wide range [2].

2.4.2 Effect of fault resistance with mho relay

The impedance reach varies with the fault angle. The fault angle will bedependent upon the relative values of R and X at the system operating frequency.

Under an arcing fault condition, or an earth fault involving additional resistance, such

as tower footing resistance or fault through vegetation, the value of the resistive

component of fault impedance will increase. This increase results in a change to the

impedance angle. Thus relay having a characteristic angle equivalent to the line angle

will under-reach under resistive fault conditions [3].

In Figure 2-8 the relay characteristics angle (RCA) is set at less than the line

angle so that it is possible to accept a small amount of fault resistance without causing


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under-reach. When the line AB corresponds to the length of the line to be protected.

With set to be less than , the actual amount of line protected, AB, would be equal

to the relay setting value AQ multiplied by cosine (q-j). Therefore the required relay

setting AQ is given by

( )AB

AQcos

=q - j

Eq. 2-2

FIGURE 2-8 Increased arc resistance coverage

When the earth resistance happen, it should be realized that this does not need to

be considered with regard to the relay settings other than the effect that reduced fault

current may have on the value of arc resistance seen. The earthing resistance is in the

source behind the relay and only modifies the source angle and source to line

impedance ratio for earth faults. It should therefore be taken into account only when

assessing relay performance in terms of system impedance ratio.

2.4.3 Application of lenticular characteristic

Figure 2-9 shows the risk that the offset mho relay may operate under

maximum load transfer conditions if Zone 3 of the relay has a large reach setting. A

large Zone 3 reach may be required to provide remote back-up protection for faults on

the adjacent feeder [3].


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ZD3

R

X

Offset Lenticular

Characteristic

LOAD

AREA

ZD2ZD1

a

b

0

Offset Mho

Characteristic

Impedance

Characteristic

FIGURE 2-9 Minimum load impedance permitted with lenticular, offset mho

and impedance relays

To avoid maximum load, a shaped type of characteristic may be used, where the

resistive coverage is restricted. With a lenticular characteristic, the aspect ratio of the

lens ( )a b is adjustable, enabling it to be set to provide the maximum fault resistancecoverage consistent with non-operation under maximum load transfer conditions.

Figure 2-9 shows how the lenticular characteristic can tolerate much higher

degrees of line loading than offset mho and plain impedance characteristics [3].

Reduction in load impedance from D3Z to D1Z corresponds to an equivalent increase

in load current.

2.4.4 Quadrilateral characteristic

The quadrilateral impedance characteristic is shown in Figure 2-10. The

characteristic is provided with forward reach and resistive reach settings that are

independently adjustable. It therefore provides better resistive coverage than any

mho-type characteristic for short lines. This is especially true for earth fault

impedance measurement where the arc resistances and fault resistance to earth

contribute to the highest values of fault resistance.

To avoid excessive errors in the zone reach accuracy, it is common to impose a

maximum resistive reach in terms of the zone impedance reach. Recommendations in

this respect can usually be found in the appropriate relay manuals [3].


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FIGURE 2-10 Quadrilateral characteristic

Quadrilateral elements with plain reactance reach lines can introduce reach error

problems for resistive earth faults where the angle of total fault current differs from

the angle of the current measured by the relay. This will be the case where the local

and remote source voltage vectors are phase shifted with respect to each other due to

pre-fault power flow. This phase difference can be overcome by using a phase current

for polarization of the reactance reach line. Polygonal impedance characteristics are

highly flexible in terms of fault impedance coverage for both phase and earth faults.

2.4.5 Protection against Power Swings-use of the ohm characteristic.

During severe power swing conditions in which a system is unlikely to recover,

the system might return to stability if the swinging sources are separated. Where such

scenarios are identified, power swing or out-of-step tripping protections can be

deployed to strategically split a power system at a preferred location. Ideally, the split

should be made so that the plant capacity and the connected loads on either side of the

split are matched.

Normally this type of disturbance cannot be correctly identified by an ordinary

distance protection. As previously mentioned, it is often necessary to prevent distance

protection schemes from operating during stable or unstable power swings in order to

avoid cascade tripping.


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To initiate system separation for a prospective unstable power swing, an out of

step tripping scheme employing ohm impedance measuring elements can be

deployed.

FIGURE 2-11 Application of out-of-step tripping relay characteristic

Ohm impedance characteristics are applied along the forward and reverse

resistance axes of the R/X diagram and their operating boundaries are set to be

parallel to the protected line impedance vector, as shown in Figure 2-11.

As the impedance changes during a power swing, the point representing the

impedance moves along the swing locus, entering the three zones in turn and causing

the ohm units to operate in sequence.

When the impedance enters the third zone the trip sequence is completed and

the circuit breaker trip coil can be energized at a favorable angle between system

sources for arc interruption with little risk of restriking.

Only an unstable power swing condition can cause the impedance vector to

move successively through the three zones. Therefore, other types of system

disturbance such as power system fault conditions will not result in relay element

operation.

Discrimination of the protection zones can be achieved using distance relays

provided that fault distance is a simple function of impedance. While this is true in

principle for transmission circuits, the impedances actually measured by a distance

relay also depend on the following factors:


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2.4.5.1 The magnitudes of current and voltage (the relay may not see all

the current that produces the fault voltage).

2.4.5.2

The fault impedance loop being measured.

2.4.5.3 The type of fault.

2.4.5.4

The fault resistance.

2.4.5.5 The symmetry of line impedance.

2.4.5.6 The circuit configuration (single, double or multi-terminal

circuit).

2.5

Application of Distance Relays

2.5.1

Distance relays with various lengths.

When several remote lines have different lengths as shown in Figure 2-12 the

settings of Zones 2 and 3 are compromised. Since line HV is short compared to lines

HS and HR, setting Zone 2 at G for 50% of line HV provides a maximum of 5.5%

coverage for line HR and 8.4% for line HS. This coverage is further reduced by the

infeed effect.

FIGURE 2- 12 Distance relays on various lengths of adjacent line section

Additional coverage could be obtained by increasing the G Zone 2 setting and

the corresponding T2 setting to coordinate with the T2 times on lines HV, VW, and

WX. The result would be long end-zone clearing for line G. If pilot relaying is used

for primary protection, increased backup with longer T2 times could be employed.

Setting Zone 3 to cover line HR would provide coverage through severalsections HV, VW, WX, and XY requiring a longer T3 setting. Again, the infeed effect


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22

from lines HS and HV probably would not provide T3 coverage for line HR. This fact

re-emphasizes the need for local backup in modern power systems.

2.5.2

The infeed effect on distance relay

On interconnected power systems, the effect of fault current infeed at the remote

busbars will cause the impedance presented to the relay to be much greater than the

actual impedance to the fault. This effect needs tobe taken into account when setting

Zone 3. In some systems, variations in the remote busbar infeed can prevent the

application of remote back-up Zone 3 protection but on radial distribution systems

with single end infeed, no difficulties should arise [3].

Undesired operation of Zone 3 distance relays applied for remote backup

protection during major system disturbances has caused the magnitude of the scope of

such disturbances to be expanded. Large ohmic settings are typically applied to the

Zone 3 relaying in order to obtain the desired backup protection. Power swings and

low voltage conditions that often exist during system disturbances resulted in the

impedance seen by the Zone 3 relay to be within its operating characteristic for a

sufficient length of time for it to initiate a trip command. Such experiences have

resulted in utilities restricting the use or reach applied to Zone 3 relaying [2].

FIGURE 2-13 Effect of infeed on impedance measured by distance relays

When there is a source of fault current within the operating zone of the distance

relay, its reach will be reduced and variable. This infeed effect can be seen from

Figure 2-13 where there are other lines and sources feeding current to a fault at F from

bus H. The relays at bus G are set beyond this fault point to F . With a solid 0-V fault

at F, the voltage for the relay at G is the drop along the lines from the fault to the

relay, or


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( )G G L G H HV I Z I I Z= + + Eq. 2-3

Since relay G receives only current IG, the impedance appears to be

GG apparent

G

HL H H

G

HL

VZ

I

IZ Z Z

I

ZZ

K

=

= + +

= +

Eq. 2-4

and

( )G

G H

IKI I= +

Eq. 2-5

Where K is the current distribution factor (phasor). This apparent impedance are

compared to the actual impedance to fault F of [4].

Gapparent L HZ Z Z= + Eq. 2-6

If HI is 0 (no infeed), Z apparent equals to Z actual. As the infeed increases in

proportion to GI , Z apparent increases by the factor( )H G HI I Z . Since this impedance,

as measured by the distance relay, is larger than the actual impedance, the reach of the

relay decreases. That is, the relay protects less of the line as infeed increases.

Since the reach can never be less than LZ as shown in Figure 2-13, Zones 2 and

3 provide protection for the line. However, remote backup for the adjacent line may

be limited since infeed is very common and can be quite large in modern power

systems [4].

Note that the infeed effect varies with system configurations and that the

apparent impedance may be a maximum under either maximum or minimum system

conditions [4].

2.6

Types of Disturbances

The disturbances happen when the power systems are heavy loaded and a

number of multiple outages occur within a short period of time, causing power

oscillations between near utility systems, low voltages of network, and consequent

voltage instability or angular instability.


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All of these disturbancescause loss of generation and loads. Power system faults

such as line switching, generator disconnection, and the loss of load result in sudden

changes of electrical power. These system disturbances affect the oscillations in

machine rotor angles and can result in severe power flow swings.

Distance relay for transmission line protection is designed to isolate the faults

that occurred within the desired zone only. It is not supposed to trip the line during a

power swing caused by the disturbances outside the protected line. Even for the out-

of-step conditions, the preferred operation is to separate the system with an out-of-

step tripping (OST) protection at pre-selected network locations and blocking other

distance relays by out-of-step blocking (OSB) protection.

2.6.1

Power swing

This situation is a variation in three phase power flow which occurs when the

generator rotor angles are advancing or retarding relative to each other in response to

changes in load magnitude and direction, line switching, loss of generation, faults, and

other system disturbances.

Operation of distance relays during a power swing may cause undesired tripping

of transmission lines, thereby weakening the system and possibly leading to cascading

outage and the shutdown of major portions. Distance or other relays should not trip

during abnormal system conditions such as stable or unstable power swings (OOS),

and sometime after stable power swing may be the power system to return to a stable

operating condition.

Distance relay elements possible to operate during stable or transient power

swings should be restrain from operating to prevent system separation from this event.

Power Swing Block (PSB) function is available to prevent unwanted distancerelay element from operating during power swings. The main purpose of the PSB

function is to identify the faults and power swings and to block distance or other relay

elements from operating during a power swing. Faults that occur during a power

swing must be detected and cleared with a high degree of selectivity and

dependability.

The effect of system disturbances results from large separation of generator

rotor angles, large swings of power flows, large swing of voltages and currents, and

last mean loss of synchronism between groups of generators or between neighbouring


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25

utility systems. Large power swings, stable or unstable, can cause unwanted relay

operations which can aggravate further the power-system disturbance and possibly

lead to cascading outages and power blackouts.

The power swing block and out of step will use the difference in the rate of

change of the positive-sequence impedance to detect a power swing or an out of step

condition and then send a to block (or not to block) signal before the apparent

impedance move into the protective relay operating characteristics.

Power swing detectors prevent distance relays from mal-operation under power

swing and out-of-step conditions. In another words, the rate of change of the

impedance will be observed from a trajectory slow during power swings.

It takes a finite time for the generator rotors to change position with respect to

each other because of their large inertias. On the contrary, the rate of change of the

impedance phasor is very fast during a system fault.

In theory, the impedance rate of change is normally measured using two

impedance measurement elements together with a timing device. If the measured

impedance stays between the settings of the two impedance measurement elements

for a predetermined time, the relay sees and decision be a power swing condition and

thus issues a blocking signal to block the distance relay element operation. After a

predetermined time the relay will trip if the power swing element is not reset.

It is not recommended to apply power swing blocking for unstable power

swings without some form of OST being applied at some predetermined location.

2.6.2 Out-of-Step Detection

Out-of-step or unstable power swing are conditions that result from system

instability such as short-circuits fault, line switching and generator tripping [5]. Whensystem instability occurs in a power system they may lead to outage in the

transmission system and also create stress on the electrical equipment.

At a damped oscillation the generators will be able to return to a normal state

condition which is known as stable power swing. In some cases the swing is so large

that the generators lose synchronism and run out of step. This power swing and out of

step situations cause the current and the voltage of the power system network not to

be constant and affect also the swing in amplitude and phase.


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26

An impedance calculation error based on voltages and currents will also

oscillate in amplitude and phase with the power swing frequency. Every time a power

swing or out of step happen the apparent impedance can become small such that it

will enter the zone detect fault of operation distance relay or as well the instantaneous

zone of a distance relay and lead to a mal-operation of the distance relay element.

A trip due to out of step and power swing situation can be desirable on

transmission lines to separate the unstable interconnect. An additional logic in the

protection relay should distinguish between a stable power swing where the system

get back to stable system and an unstable out of step situation particularly issued only

for the out of step situation, must be sent logic trip to circuit breaker tripping in

system [6].

The main operation of Out-of-Step Trip (OST) is provided in distance relays to

detect a power swing and block the operation of a distance relay due to the power

swing. To maintain power system stability and service continuity the most common

method used to detect a power swing is to measure the rate of change of impedance as

it travels into the protection zones of the relay.

2.7 Load Power Effect to Distance Relays

The load which is serviced by the transmission line and affect to the distance

relay as impedance. When the load increases will affect to the impedance in distance

relay measurement to decreases also. In some situations the load is large enough that

it overlaps the relay characteristic. This is referred to generally as load encroachment.

If it occurs, the distance relay will detect the reduced load impedance. It can move

within the characteristic circle of distance relay. The distance relay not identifybetween fault and actually load. The decrease impedance affect to see as indicating a

fault condition on the line, and will trip the circuit breaker. This of course is no fault

condition is in fact present on the line. It is a false trip, which is undesirable the false

trip occurs at a very inconvenient time in the operation of the power system, when the

demand for power is very high [7].

The load encroachment caused by load power is a problem to distance relay

using Zone 3. An impedance depends on the magnitude and the power factor of the

load impedance.


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The apparent impedance as seen by a distance relay is given by

( )

2 2

2 2 2a

V P jQ V S Z

P Q S

+= =+

Eq. 2-7

aZ = The apparent impedance seen by a distance relay.

V = The line voltage.

S = The complex power.

,P Q = The active and reactive powers

If the magnitude of complex power fixed is follow in Equation 2-8. The

apparent impedance is illustrated in Figure 2-14 where the radius of circle is increased

because of the larger of the magnitude of the complex power and the m1 direction of

apparent impedance as the power factor decreases [8].

2

a

VZ

S= Eq. 2-8

a1

a2

r1

r2

m1

m1 m2

R

X

bZone 3

FIGURE 2-14 The apparent impedance as the complex power vary ( )1 2,a a and

the power factor vary ( )1m

In Figure 2-14, when the power factor is fixed, the apparent impedance is

illustrated follows and the m2 direction of apparent impedance as the magnitude of

the complex power becomes larger. This scenario is possible as the distance relay

mal-operates due to the reason that the load power becomes greater [8].


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The apparent impedance of the real parts and imaginary part are written as

2

2 2

V P

RP Q= +

Eq. 2-9

2

2 2

V QX

P Q=

+ Eq. 2-10

The R and X can written as

4

2 2

2 2

VR X

P Q+ =

+ Eq. 2-11

If Equation 2-11 written depend on R and X from Equation 2-9 and Equation

2-10 can be written as

2

2 2 R V

R XP

+ = Eq. 2-12

22 4

2

22 4

V VR X

P P

- + =

Eq. 2-13

2

2 2 X VR X

Q+ = Eq. 2-14

22 4

2

22 4

V VR X

Q Q

+ - =

Eq. 2-15

When P and Q fixed, Equation 2-12 and Equation 2-14 is a circle with

Center of P :

2

,0

2

V

P

, Radius of P:

2

2

V

P

Center of Q:

2

0,2

V

Q

, Radius of Q:

2

2

V

Q

In Figure 2-15 the apparent impedance is the direction when the active power

increases. In part direction when reactive power increases. Therefore, the distance

relay may lead to mal-operation because to encroach Zone 3.


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X

R

Zone3

a

m1

b

m2

0

2

V

Q

2

V

P

FIGURE 2-15 An apparent impedance as the active and the reactive power vary

2.8 Load Encroachment

The maximum load that, sometime greater than loadability limit in system. Until

distance relay can not distinguish from a fault and last will lead to trip of transmission

line.The impedance of heavy loads can actually be less than the impedance of some

faults. However, the protection must be made selective enough to discriminate

between load and fault conditions. Unbalance aids selectivity for all faults except

three-phase faults [9].

When power flows out, the load impedance is in the wedge-shaped load-

impedance area to the right of the X-axis. When power flows in, the load impedance

is in the left-hand load impedance area.

There is overlap (shaded solid) between the mho circle and the load areas.

Should the load impedance lie in the shaded area, the impedance relay will detect the

under-impedance condition and trip the heavily-loaded line. Such protection

unnecessarily limits the load carrying capability of the line.

For better load rejection, the mho circle can be squeezed into a lenticular or an

elliptical shape. Unfortunately, this also reduces the fault coverage.

Alternatively, we could use additional comparators to make blinders parallel to

the transmission line characteristic to limit the impedance-plane coverage, and


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exclude load from the tripping characteristic. Or, we could build quadrilateral

characteristics, which box-out load.

FIGURE 2-16 Load encroachment on mho distance element characteristics

All traditional solutions have the same common approach: shape the operating

characteristic of the relay to avoid load. The traditional solutions have two major

disadvantages:

2.8.1 Reducing the size of the relay characteristic desensitizes the relay to faults

with resistance. Avoiding a small area of load encroachment often requires sacrificing

much larger areas of fault coverage.

2.8.2 From a user's point of view, the more complex shapes become hard to

define, and the relays are harder to set.

However, long lines, infeed, and load encroachment may cause difficulties to

obtain that result in a secure manner. It is the opinion of the authors that uncontrolled

disconnection of power lines should be avoided during voltage instability [10].


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CHAPTER 3

LITERATURE SURVEY

3.1 PreviousWorks on Disturbance for Zone 3 of Distance Protection

This chapter present previous works on disturbance affect distance protection.

Which simulation on software to test influence of disturbance on distance protection.

3.1.1 Evaluation and Performance Comparison of Power Swing Detection

Algorithms[11]

Research comparison of power swing detection algorithms. The research

studied operation of the power swing detectors, analyzed for different conditions and

the operation of different algorithms. The method utilize compare such as the

decreased impedance method, the Vcosj algorithm, the superimposed method and

the decreased resistance method. Result of research found that the decreasing

resistance algorithm has the best behavior, because this method is able to detect slow

and fast power swings and can detect three-phase fault during power swing better

other method.

FIGURE 3-1 The impedance changes when a power swing with oscillation

frequency equal to 0.1 Hz


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Testing simulating different types of conditions on the power system and

include power swing without and with fault, and different types of faults without

power swing. Varying the load angle of generator with different frequencies and the

maximum power angle creates different power swings. Such disturbances on the

system will cause the power to swing with different oscillation frequencies.

Simulation result for the power swing curve of the proposed model is given in

Figure 3-1. These figures show the impedance changes when a power swing with

oscillation frequency equal to 0.1 Hz has occurred.

3.1.1.1 The Decreased Impedance Method

The principle of this method is based on the impedance locus changes slower

during a power swing than when a fault occurs. Figure 3-2 shows power swing

blocking characteristics, which consists of two concentric mho characteristics. The

outer characteristic is an offset mho characteristic and set concentric with the inner

mho characteristic. The criteria for operation is the time t taken for the impedance

locus to pass through the area between the two mho characteristics. If there is a fault

condition, the impedance locus will move instantaneously from the load position to

the fault position inside the power swing blocking characteristic, and no blocking will

occur. During a power swing however, the locus moves much slower, at a speed

determined by the inertia of the system, and if the time taken to travel between the

outer characteristic and the inner characteristic exceeds setting t, the power swing

blocking unit will operate.

FIGURE 3-2 Power swing blocking characteristic


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3.1.1.1.1 The simulation in various frequency of slip

Test the method can the outputs of distance relay and Power Swing Detector

(PSD) for a power swing with fs = 0.1 Hz and max = 180 degrees. As shown the

PSD has operated at t=1.6 s and the distance relay has operated at t=2s. Operation of

PSD will block operation of the distance relay. When power swing occurs the

impedance enters the distance relay characteristic, and cause relay misoperation if

steps are not taken to prevent this.

Then to test in case at fs = 5 Hz and max = 90 degrees. It shows that the PSD

has not operated but the distance relay has operated. More tests show that this method

cant distinguish power swings with slop frequency of 5 Hz or more than that.

3.1.1.1.2

The simulation when earth fault with high resistance

When test an earth fault, the outputs as shown, the distance relay has not

operated but the PSD has operated. The time the impedance locus will take to pass the

power swing blocking characteristic at this type of fault has been longer than setting

time ( tD ) and therefore the PSD has operated.

a) The conclusion disadvantage of the method as

follows.

b)

This method is not able to distinguish fast power

swings.

c) This method may operate for earth faults with high

fault resistance.

d) If during a power swing a three-phase fault occurs, it

is possible that the blocking relay will not reset.

3.1.1.2

The Vcos Algorithm.When power swings occur different electrical quantities vary as a function of

the angle at a greater or lesser speed. When a fault occurs, however, these quantities

change suddenly. Those, which vary considerably regardless of the location in a wide

range around phase opposition, are the voltage component Vcos.

This algorithm evaluates the change in Vcos as a function of time. The

criterion upon which the response by the power swing-blocking relay depends is the

steady reduction in the absolute value of Vcos. This value is measured once every

half-cycle.


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As shown, after the fault, the value of Vcos decreases as before fault.

Therefore if during a power swing a three-phase fault occurs, it is possible that the

blocking relay will not reset and that the distance relay will remain blocked.

3.1.1.3 The Superimposed Method.

A power swing produces continuous superimposed signals as a result of the

change in current and voltage signals between power frequency cycles. This method

uses the superimposed current signals for detecting the power swing.

When test simulated changes in superimposed currents for a three phase fault

that has occurred during a power swing with fs = 0.1 Hz and max = 180 degrees. As

shown, after the fault, the value of superimposed currents change as continually.

Therefore if during a power swing a three phase fault occurs, the blocking relay will

not reset and the distance relay will remain blocked. As shown, after power swing

superimposed currents increased for two cycles and this increase can be used as the

criteria for detection of faults. But when faults with high resistance occur, this

increase is low and cannot detect these faults.

3.1.1.4 The Decreased Resistance Method.

When power swing occurs, the resistance component of the measured

impedance will change continuously. But when a fault occurs, the resistance

component of the measured impedance will not change except at the initial instant of

short circuit. This phenomenon is proposed to be used to distinguish between the

system fault and power swing.

Results of simulation show that is able to detect slow and fast power swings.

This method can even detect three-phase fault during power swing.

Result to compare the behavior of different algorithms, the response of each oneof the algorithms was obtained for different power swings with presence of fault and

without it. The test results of different algorithms during power swings with different

power angles and different slip frequency. As shown, the Vcos method and the

superimposed method have operated for all conditions but the decreased impedance

method is unable to detect fast power swings.

The test results of different algorithms for different three-phase faults during

power swings with different conditions. It is shown that the decreasing resistance

algorithm is always able to detect faults during power swings at different conditions


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and for more cases other methods are not reset and the distance relay has remained

blocked.

The performance studies that the superimposed method is better than other

methods. This method is able to detect different power swings and it can detect faults

during power swing. In addition it is stable for high resistance faults.

This paper describes the evaluation and performance comparison of different

power swing detectors. The purpose several conventional algorithms were evaluated.

It is found that the decreasing resistance algorithm has the best behavior, because this

method is able to detect slow and fast power swings and can even detect three-phase

fault during power swing.

3.1.2

An Enhanced Zone 3 Algorithm of a Distance Relay Using Transient

Components and State Diagram[12]

This research presents a novel zone 3 scheme based on combining the steady-

state components and the transient components. The simulation results show the novel

zone 3 distance relay elements using the proposed method operate correctly for the

various events.

The frequency components can also be used to detect the transient state during

faults. The fundamental frequency component of the voltage or current is very large

and the other frequency components are much smaller in pre-fault. However, during a

fault, the other frequency components become prominent. In this paper are used to

extract the TCs by digital algorithms based on fast Fourier transform (FFT).

FIGURE 3-3 Block diagram of the proposed fault detector

Figure 3-3 shows a block diagram of the fault detector using the steady-state

and transient state characteristics. Signal S are generated by the fault detector using

steady-state. At the same time signal T are generated by the fault detector using


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36

transient state characteristics. In this paper, the signal S is generated when the

apparent impedance as seen by a relay is within zone 3, whereas the signal T is

generated when the sum value of the transient components (amplitudes) exceeds the

preset threshold value.

Conclusion of a novel scheme for transmission line fault detection using zone 3

and the transient components that are combined by using the state diagram. Two fault

detection signals and are generated based on the steady-state and transient state

characteristics, respectively. A comprehensive set of simulation results has shown that

a distance relay employing the proposed method has no problems of the

maloperations that are caused by the heavy loading or the voltage instability

associated with conventional distance relay. Furthermore, as an adjunct to the

aforementioned attributes, faults can also be detected during encroachment of zone 3

due to heavy loading.

3.1.3

Use of Wavelets for out of step Blocking Function of Distance Relays

[13]

Present wavelet analysis to detect power swings. Analysis power swing during a

symmetrical fault occurrence. Different power swing conditions and fault instants are

simulated with PSCAD/EMTDC software to test the methodology.

Out of step blocking is function in distance relays to detect a power swing and

block the operation of a distance relay due to the power swing. The method to detect a

power swing is to measure the rate of change of impedance as it travels into the

protection zones of the relay. However, if there is a fault during a power swing, this

function must unblock and agree to let the relay trip.

However, if a symmetrical fault occurs during a power swing when theimpedance has already passed the blinders, it is impossible to detect with this

methodology. This could result in the relay not being able to clear a fault.

A distance relay can also trip on load. In relaying parlance, this behavior is

known as load encroachment. Usually, loads have a large power factor. Hence, the

relay characteristic is modified to avoid this region. A line-tripping contingency can

also lead to load encroachment. It can be detected by computing the relay impedance

from a load flow analysis.


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37

3.1.4Adaptive Blinder for Distance Relay Based on Sensitivity Factors[14]

After clearance of fault, some transmission lines may be overloaded, which may

cause cascading trips even total blackout. This paper presents an adaptive distance

relaying based on linear sensitivity

In this research study one part of algorithm to enhance discriminate overload

and fault, and block zone 3 trip caused by overload. The result can take work together

with blinder scheme of distance relays.

3.1.5

A Novel Scheme to Identify Symmetrical Faults Occurring During Power

Swings[15]

In this paper present fast unblocking scheme for distance protection to identify

symmetrical fault occurring during power swings. When the power swing occurs

affect to a change appear in the reactive voltage phase angle between machine. The

method use the change rate of change of active power and reactive power. When a

fault occurs, the rate of change will level off to zero. Convention algorithm for power

swing detection such as the decreasing impedance, the rate of change angle voltage,

the superimposed current method.

The conclusion of disadvantage of the purpose. First, the relay may not respond

to genuine faults occurring during the power swing period since it is blocked from

operation. Second, the time delay has to be set with a knowledge of the likely speed of

movement of the impedance during the power swing.


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CHAPTER 4

ALGORITHM AND SIGNAL ANALYSIS

4.1 Fundamental Considerations [16]

Continuously vary signal usually sinusoidal current and voltage from C.T. and

P.T. are applied to the input of the converter. These, however, may not just consist of

the fundamental, but may include superimposed high frequency interference,

harmonics, subharmonics and also a DC component. The general equation for the

input signal is thus.

( ) ( ) ( )ap

t T1 1 1 0 k 1 k

k 2

v t V cos t V e V cos k t-

=

= w - a + + w - a Eq. 4-1

Where

1V = Amplitude of the fundamental at rated frequency

2 pV ...V = Amplitude of the harmonics k times higher than the rated frequency

0V = Initial value of the DC component which decays at a time constant aT

4.2 Digital Distance Measurement [16]

By far the greater part of all the publication on the subject of selective digital

protection are concerned with the determination of the distance between the relay

location and fault on highvoltage line. This complex of problems is so dominant that

even the equation for the discrete power were mere by-products during the

development of the algorithm for calculating reactance and resistance.

As is the case with analog protection device, the operation principle of most

digital distance protection systems is based on the determination of whether the values

of resistance and reactance (or resistance and inductance) measured at the relay

location lie inside the operating characteristic, thus indicating a fault in the zone of

protection.

The majority of digital distance relay perform their task by first calculating the

resistances

R and the reactance s 1 sX L= w from the given input variable for the


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40

current ( )i t and voltage ( )u t and then determining whether the values obtained lie

within the operating characteristic in the R X plane.

FIGURE 4-1 Distance protection measuring principle PD = protection device,

FR =fault resistance

There many factors which influence the accuracy of the values determined for

the resistance and reactance of a fault on the values determined for the resistance and

reactance of a fault on an overhead line. Amongst these are:

4.2.1

Distortion of the current and voltage curves by decaying DC component

and high frequency oscillation

4.2.2 Fault resistance ( )FR

4.2.3 C.T and P.T. errors, primarily saturation phenomena caused by decaying

DC components.

4.2.4 Inductive coupling and insulation breakdown the conductors of the

faulted line, respectively between the faulted and healthy circuit of a double circuit

line.

The derivations of the algorithm for calculating sR and sX generally only take

account of the above factors, because it is assumed that the mutual impedance

between conductors can be compensated with the aid of the zero-sequence

component, respectively with the aid of the current of healthy lines. It is also assumed

that the amplitude and phase errors of C.T. and P.T. and the error due to the fault

resistance FR are relatively small compared with the reactance sX and may therefore

be neglected. As is explained later, these assumption are not permissible in all cases.


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41

4.3 Fourier Analysis [17]

4.3.1 Trigonometric form

Any periodic function ( )f t can normally be represented by a Fourier series of

discrete harmonics

( ) 0 n 0 n 0n 1 n 1

af t a cos n t b sin n t

2

= =

= + w + w Eq. 4-2

Where 0w is the angular fundamental frequency 02 f 2 T= p = p

T is the time period of the fundamental component

0nw is the nth harmonic angular frequency

1t is arbitrary

Given the known function ( )f t (which in most practical situation is a function

is a function that varies with time), the coefficients 0 1 1 n na , a , b ,...a , b can be

determined from expressions of the from of Equation 3-2 to 3-4

( )1

1

t T

0

t

2a f t dt

T

+

= Eq. 4-3

( )1

1

t T

n 0

t

2a f t cos n t dt

T

+

= w Eq. 4-4

( )1

1

t T

n 0

t

2b f t sin n t dt

T

+

= w Eq. 4-5

Alternative, by combining corresponding sine and cosine terms of the same

frequency, Equation 4-2 can be written as

( ) ( )n 0 nn 0

f t A cos n t

=

= w + q Eq. 4-6

Where

0n 0

aA , 0

2= q = Eq. 4-7

And

( )

2 2 1 n

n n n n

bA a b , tan n 1,2,...

a

-= + q = = Eq. 4-8


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4.3.2 Complex form

In some applications it is more convenient to use the complex form of Fourier

series give in Equation 4-9. It will be apparent that the latter equation is directly

equivalent to the basic Fourier series expression given in Equation 4-2.

( ) 0jn tnn

f t F e

w

=-

= Eq. 4-9

Where

( )n n n

0

F a jb 2,n 1, 2 ...

a 2, n 0

= - =

= =

By substituting Equation 4-2 to 4-4 into the above equation, nF reduces to

( )

( )

01

1

1

1

jn tt T

nt

t T

t

f t e dt, n 1, 2...1F

T

1f t dt, n 0

T

- w+

+

= =

= =

Eq. 4-10

4.4 Fourier Analysis Based Algorithm [17]

Function of time ( )f t can be represented by s Fourier series and each

coefficient of the series can be found according to the formula given in Equation 4-1

to 4-3.

Voltage and current waveforms are of course function of time and they can be

consequently expanded using the Fourier series. If we take, for example, a voltage

waveform ( )v t , then

( ) 0 n 0 n 0n 1 n 1

av t a cos n t b sin n t2

= == + w + w Eq. 4-11

And from Equation 4-4 and 4-5

( )1

1

t T

n 0

t

2a v t cos n t dt,n 0,1,...

T

+

= w = Eq. 4-12

( )1

1

t T

n 0

t

2b v t sin n t dt, n 1,2,...

T

+

= w = Eq. 4-13


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43

Where 0w is the angular frequency of the fundamental component and T is its

period.

Equation 4-12 and 4-13 show that the fundamental component of a voltage and

current waveform can be extracted from the corresponding faulted waveform simply

by setting n 1=

4.5 Principle of Algorithm

Figure 4-5 to add block diagram of sT and Theshold part. This blocks will

detect fault. The detecting fault will operate when change rate power increase morn

than threshold value setting.

FIGURE 4-2 Algorithm of detection fault

In Figure 4-3, the algorithm suppose fault 1 occur between circuit breaker 5 and

6 at 2 second. Then fault clear at 2.1 second. After that the power swing occurring and

effect to circuit breaker 1, 2, 3 and 4. This occurrence result from the power swing

blocking (PSB) function to operate because the impedance move to zone protection of

distance relay. Then fault 2 occurring in front of circuit breaker 3.

FIGURE 4-3 Fault 1 occur at 200 km both breaker 5 and 6


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As fault 2 occurrence during power swing, the relay cannot to operate because

relay perform blocking from power swing blocking(PSB). This is maloperation of the

relay and effect to system instability.

FIGURE 4-4 Fault 2 occur at front of breaker 3

In thesis to present solve this problem by unblock scheme to detect fault

occurring between power swing. When power swing occur cause distance relay no

operate. Because distance relay block operate by power swing block function.


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CHAPTER 5

an enhanced performance of distance relay algorithm to prevent (tesis)

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