Distance Protection and Dynamic Loading of Transmission Lines

5/21/2018 Distance Protection and Dynamic Loading of Transmission Lines

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AbstractDynamic loading of transmission lines is becoming

one of the tools that allows system operators to increase the

loading of the transmission line when required by the system or

the energy market. The paper discusses in detail the effect of

dynamic loading on different types of distance relays. Advanced

microprocessor based relays offer a variety of different

characteristics and functions that provide sufficient resistive

reach to cover the expected range of fault arc resistances, while at

the same time are not affected by encroachment of the load

impedance in the distance characteristic.

Index TermsProtective relaying, Dynamic loading,

Impedance characteristic.

I. INTRODUCTION

Distance relays have been successfully used for many

years as the most common type of protection of transmission

lines. The development of electromechanical and solid state

relays with mho characteristics can be considered as an

important factor in the wide spread acceptance of this type of

protection at different voltage levels all over the world.

Zone 1 of distance relays is used to provide primary high-

speed protection of a significant portion of the transmission

line. Zone 2 is used to cover the rest of the protected line and

provide some backup for the remote end bus. Zone 3 is the

backup protection for all the lines connected to the remote

end bus.

The implementation of distance relays requires

understanding of its operating principles, as well as the factors

that affect the performance of the device under different

abnormal conditions.

The setting of distance relays should ensure that the relay

is not going to operate when not required and will operate

when necessary.

The behavior of distance relays during several recent majorblackouts combined with the significant pressure on utilities

to increase the loading of their transmission systems are the

reasons to look at dynamic loading of transmission lines and

A. P. Apostolov is with ALSTOM T&D Energy Automation & Information, Los

Angeles, CA 90064 USA (e-mail: [email protected]).

D. Tholomier is with ALSTOM T&D Energy Automation & Information,

Levallois, France (e-mail: [email protected])

S. Richards is with ALSTOM T&D Energy Automation & Information,

Stafford, UK (e-mail: [email protected])

the effects that it has on the commonly used distance relays.

At the same time the characteristics of modern distanc

relays are analyzed in order to demonstrate that they can

provide better protection and at the same time are not affecte

by dynamic loading conditions.

II. DYNAMIC LOADING AND DISTANCE PROTECTION

REQUIREMENTS

The requirements for increase of the loading of man

transmission lines due to changing system or marke

conditions have to be considered when analyzing th

performance of distance relays, selecting protection device

with distance functions and calculating their settings.

Since the dynamic stability is a function of the loading o

the line and the duration of the fault, the operating time of th

distance relay will affect the level of loading of the protecte

line. As can be seen from Fig. 1 [3], shorter fault clearin

times allow increased power transfer.

Fig. 1. Typical power/time relationship for various fault types

The detection of a fault and a decision to trip is made by

modern distance relays in less than one cycle. However, th

operating time of the relay is not the only factor to b

considered while selecting a distance protection for

transmission line that requires dynamic loading.

The loading of transmission lines is typically limited b

their rating. The thermal rating is usually based on

conservative assumption of weather conditions. Since weathe

conditions are continuously changing, most of the time th

actual rating of the line can be significantly increased

especially if specialized monitoring equipment is being used

Distance Protection and Dynamic Loading of

Transmission LinesA. P. Apostolov, Senior Member, IEEE,D. Tholomier, S. H. Richards,Member, IEE


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Many utilities are experimenting with dynamic thermal line

rating. Reports [5] indicate that real-time rating allows 40 to

80 percent more power transfer compared to the static rating

that is usually used.

Figure 2 shows a comparison between the monitored line

loading and the static, emergency and dynamic rating [5] over

a period of time.

Fig. 2. Dynamic rating profile of a transmission line

The current interest in increase of the rating of

transmission lines however lacks enough attention to the

effect on the protection system.

The distance elements of protective relays have to beselected and configured in such a way that they will provide

sufficient resistive reach to ensure correct operation when a

fault is inside of the designed zone of protection. The

resistance of the arc has to be taken into consideration. It is

affected by many factors, such as the distance between the

phases and the extension of the arc by wind. The calculation

of the arc resistance will never be completely accurate, but

still there are formulas that can help in estimating the required

resistive coverage. For example, the protection engineer may

use the empirical formula derived by A. R. van C. Warrington

[4] to calculate the resistance of the arc:

Ra= 28710 L / I1.4

(1)

Where:

Ra = arc resistance (ohms)

L = length of arc (meters)

I = arc current (Amps)

Figure 3 below shows the protected transmission line in the

impedance plane with the area of arc resistance that has to be

covered by the protection element. Obviously, the

characteristic needs to have a shape and be wide enough to

provide this coverage.

At the same time the characteristic should have a shape

and be narrow enough so that the dynamically changing load

impedance does not enter inside the characteristic, that wil

result in undesired tripping of the protected line at the tim

when it is needed the most.

Fig. 3. Arc and Load impedance regions in the impedance plane

The effect of load on the operation of distance relays i

well known and studied for example [1, 2]. It may lead t

under or over-reaching of the distance characteristic. Th

apparent impedance seen by the relays under very heavy load

may lead to relay tripping. This is especially true in the cas

of long transmission lines or Zone 3 elements that have t

provide backup protection for lines outgoing from substationwith significant infeed. This is quite dangerous during wid

area disturbances and may result in quick deterioration of th

system and a blackout.

The analysis of recent blackouts in the Western and North

Eastern United States [6] clearly demonstrate this problem

with typical distance protection applications. Operation o

distance relays with Mho characteristics under increased loa

conditions resulted in tripping of transmission lines an

worsening of the overall system stability.

Utilities and regional industry coordinating bodies, such a

the WSCC , are analyzing their practices related especially t

the application of Zone 3 of distance protection relays. Load

encroachment has to be considered during the selection o

distance relays to be used and while calculating the setting

for each specific location.

From Figure 3 above it is clear that the distanc

characteristics for each zone of a multifunctional transmissio

line protection relay should lie between the fault + ar

impedance area and the load impedance area. The shaded par

of the load impedance region corresponds to the normal an

emergency rating of the line, while the white area is the loa

based on the dynamic rating.

X

R

ZLoad

Fault + arc impedanceregion

ZLine

Load impedance

region


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The analysis so far has been simplified, as in reality all

lines have two or three terminals, and if sources are present at

the remote terminals, they will infeed and contribute towards

any internal fault current. Figure 4 shows how for a fixed

amount of fault arc resistance, the apparent resistance as

measured by the distance relay at the local terminal appears to

magnify with increasing distance to the fault. This is because

the remote end current contribution increases proportionally

more, as the local current contribution decreases. For thisreason, it is common that the arc resistance reach of distance

zones might be four times that from the van Warrington

calculation.

The gray shaded region shows the possible fault

resistances measured when the load flow was forwards prior

to the fault (load export), and the solid lined region adds the

possible fault area when load import was the scenario. The

angular tilt of the resistance is an issue for the zone reactance

reaches of distance relays, and is not related to line

loadability. This is not addressed further in the paper,

reference [7] discusses in more detail.

X

R

ZLoad

Arc impedance with

Remote end infeed

ZLine

Load impedance

region

load export

load import

Fig. 4. Arc and Load impedance regions in the impedance plane

The electromechanical or solid state relays with Mho

characteristics have some problems with the above mentionedareas. They usually can not cover the arc impedance for faults

at the end of the protected zone, while at the same time are

subject to load encroachment, especially if the load is

dynamically changing above the static rating of the

transmission line.

Figure 5 shows a typical case of the Mho characteristics of

a transmission line protection relay with three forward looking

zones in the R X plane.

Zone 1 is not affected by the dynamic loading of the

protected line. Zone 2 may operate in the case of the highest

level of dynamic loading, while Zone 3 will operate during

dynamic or even emergency loading conditions.

Fig. 5. Arc and Load impedance regions and distance protection zones in th

impedance plane

Because of the significant problems with the application o

Zone 3 distance elements with Mho characteristic, som

utilities have disabled them in order to avoid potential lin

tripping during emergency system conditions. In other case

the reach settings are changed to reduce the probability fo

tripping under load conditions. However, this reduces theffectiveness of Zone 3 as a remote backup protection

element.

All of the above has been taken into consideration in th

design of modern microprocessor based transmission lin

protection relays with distance characteristics.

III. DISTANCE CHARACTERISTICS OF TRANSMISSION LINE

PROTECTION RELAYS

A. Lenticular Distance Characteristics

To avoid the operation of a Zone 3 distance element with

Mho characteristic one can select to use instead a lenticular

(lens-shaped) characteristic.From Figure 6 it is clear that the resistive coverage of thi

characteristic is restricted. The aspect ratio of the lens a/bi

adjustable. By selecting the configuration parameter a/b th

user can provide the maximum fault resistance coverage an

at the same time avoid the operation under maximum loa

transfer conditions. However, it is clear that the resistiv

coverage is not consistent along the length of the line an

varies with the location of the fault. Faults at the end of Zone

2 will probably be cleared by Zone 3 in the cases when ther

is arc resistance.

R

X

ZLoad

ZArc

ZLine

Zone 2

Zone 3


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This tripped zone indication can be confusing to system

operators and technicians, most of whom will not be distance

protection experts. There is thus the risk that the fault location

might be falsely presumed to be on a line downstream of the

actual faulted line.

Fig. 6. Zone 2 element with Lenticular characteristic

B. Distance Characteristics with Load Blinders

If we would still like to have a Mho distance characteristic

that provides sufficient arc resistance coverage but at thesame time eliminates the possibility for tripping under

maximum load condition, we can select to use a transmission

line protection relay that combines a Mho element with a load

blinder.

Figure 7 shows the characteristics of a distance relay with

load blinders for Zone 2 and Zone 3. The blinder restrains the

operation of the distance element for load impedance that

appears to the right of the blinder.

If the impedance seen by the relay is within the Mho

characteristic and to the left of the blinder, it is allowed to

operate and trip the breaker.

The setting of the resistive reach of the load blinder shouldtake into consideration the requirements for maximum arc

resistance coverage and at the same time elimination of the

possibility for operation of the distance element under

maximum load conditions. This means that the protection

engineer needs to know what is the maximum dynamic rating

of the protected transmission line.

Another option for combining Mho characteristics and load

blinders is by reducing the size of the Zone 3 element and

using at the same time forward offset in order to ensure

appropriate coverage of the outgoing lines at the remote end

substation. In this case a load blinder is required only for th

Zone 2 element. An advantage is that the non-blinded Zone

can better cope with the magnified fault resistance as wa

seen in Figure 4.

Fig. 7. Zone 2 and Zone 3 elements with Mho characteristics and load blinders

Fig. 8 Zone 2 with reverse offset Mho characteristic and load blinder an

forward offset Zone 3 Mho characteristic

A more advanced load blinder is designed to provide bette

resistive reach coverage. The blinder is basically formed from

an underimpedance circle, with radius set by the user and tw

blinder lines crossing through the origin of the impedanc

plane. It cuts the area of the impedance characteristic tha

may result in an operation under maximum dynamic load

X

R

ZLoad

ZArc

ZLine

Zone 2

Zone 3

ba

X

R

ZLoad

ZArc

ZLine

Zone 2

Zone 3

X

Zone 2

Zone 3

R

ZLoad


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conditions.

The radius of the circle should be less than the maximum

dynamic load impedance. The blinder angle should be set half

way between the worst case power factor angle, and the line

impedance angle.

In the case of a fault on the line it is no longer necessary to

avoid load. So, for that phase, the blinder can be bypassed,

allowing the full mho characteristic to measure. The resistive

reach during the fault condition is thus improved, as theblinder no-longer acts as a constraint.

Phase undervoltage detectors are the chosen elements to

govern switching of the blinders.

Figure 9 shows an example of such a load blinder

characteristic. Again it is possible to make use of a broader

Zone 2 and Zone 3 characteristic to cater for the fault

resistance magnifying effect in Figure 4.

Fig. 9. Advanced load blinder characteristic

C. Quadrilateral Characteristics

This form of impedance characteristic is shown in Figure

10.

Fig. 10 Zone 2 with quadrilateral characteristic and reverse offset Zone 3

quadrilateral characteristic

The characteristic is provided with forward reach and

resistive reach settings that are independently adjustable. I

therefore provides better resistive coverage than Mho typ

characteristic and is not affected by the load encroachment.

Quadrilateral impedance characteristics are highly flexibl

in terms of fault impedance coverage for both phase and

ground faults. For this reason, most digital and numerica

distance relays now offer this form of characteristic.

With this type characteristic, the resistive reach settings fo

each zone can be set independently of the impedance reachsettings.

The resistive reach setting represents the maximum

amount of additional fault resistance (in excess of the lin

impedance) for which a zone will trip.

Two constraints are imposed upon the settings, as follows:

The resistive reach must be greater than the maximum

expected phase-phase or phase-ground fault resistanc

(basically that of the fault arc)

It must be less than the apparent resistance measured

due to the heaviest dynamic load on the line

Figure 10 shows the Zone 2 and Zone 3 quadrilatera

characteristics of a transmission line protection relay. Zone is forward looking based on the reactive reach line, th

resistive reach blinders and a directional line.

It is clear from the figure that this characteristic provide

sufficient arc resistance coverage, and at the same time is no

affected by the maximum dynamic loading of the protected

line.

D. Polygon Characteristic

A polygon characteristic can be built from several blinder

and a directional element. An example of such characteristi

is shown on Figure 11.

Fig. 11 Polygon characteristic

RestrainRestrain Operate

X

R

ZLoad

ZArc

ZLine Zone 2

Zone 3

X

R

ZLoad

ZArc

ZLine

X1

R1PP


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This type of characteristic can provide (depending on the

settings) resistive coverage similar to the advanced load

blinder described earlier.

Setting the resistive reach and the slope angle allows the

definition of an optimal characteristic positioned between the

arc resistance and the load impedance areas.

IV. CONCLUSIONS

The requirements for increase of power transfer over

existing transmission lines based on system stability or energy

markets requirements is forcing utilities to find solutions that

will allow them to load the lines based on their dynamic

rating.

Successful pilot projects demonstrate that it is possible to

increase by more than 50 percent the loading of the lines.

On the other hand, experience with recent blackouts shows

that the dynamic changes of load may result in undesired

operation of distance elements due to the load impedance

entering the distance characteristic.

The different types of distance characteristics analyzed in

the paper demonstrate that by properly selecting and settingthe distance characteristics, the user can define an optimal

protection element that will provide sufficient arc resistance

coverage and at the same time eliminate the possibility for

tripping under maximum dynamic load conditions. It is

concluded that distance relays should not constrain the

loadability of transmission lines. The distance relay is

designed according to the power system needs not vice

versa.

Any loadability limit should be determined by the dynamic

rating of the transmission line.

V. REFERENCES[1] R. J. Marttila, "Performance of Distance Relay Mho Elements on MOV-

Protected Series-Compensated Transmission Lines," IEEE Trans. Power

Delivery, vol. 7, pp. 1167-1178, Apr. 1988.

[2] R. J. Marttila, "Effect of Transmission Line Loading on the Performance

Characteristics of Polyphase Distance Relay Elements," IEEE Trans.

Power Delivery, vol. 3, pp. 1466-1474, Oct. 1988.

[3] ALSTOM, Network Protection & Automation Guide, 2002

[4] A. R. van C. Warrington, "Protective Relays their Theory and Practice"

Chapman and Hall, 1962

[5] PIER, "Dynamic Circuit Thermal Line Rating," California Energy

Commission, Los Angeles, CA, Tech. Rep. TR-0200 (4230-46)-3, Oct.

1999.

[6] U.S.-Canada Power System Outage Task Force, Interim Report: Causes

of the August 14th Blackout in the United States and Canada, Nov. 2003

[Online]. Available: http://www.nerc.com/

[7] IEEE Std C37.113-1999 IEEE Guide for Protective Relay Applications to

Transmission. Lines.

VI. BIOGRAPHIES

Alexander Apostolovreceived MS degree in Electrical

Engineering, MS in Applied Mathematics and Ph.D.

from the Technical University in Sofia, Bulgaria. He has

worked for fourteen years in the Protection &Control

Section of Energoproject Research and Design Institute,

Sofia, Bulgaria.

From 1990-94 he was Lead Engineer in the Protection

Engineering Group, New York State Electric & Gas

where he worked on the protection of the six-phase line, application o

microprocessor relays, programmable logic and artificial intelligence in protection

1994-95 he was Manager of Relay Applications Engineering at Rochester

Integrated Systems Division. 1995-96 he was Principal Engineer at Tasnet.

He is presently Principal Engineer for AREVA (formerly ALSTOM) T&D EAI i

Los Angeles, CA. He is a Senior Member of IEEE and Member of the Powe

Systems Relaying Committee and Substations C0 Subcommittee. He is Vic

Chairman of the Relay Communications Subcommittee, serves on several IEEE PE

Working Groups and is Chairman of Working Group C3: New Technology Relate

to Power Systems Protection and Working Group C9: Guide for Abnorma

Frequency Load Shedding and Restoration.He is member of IEC TC57 and CIGRE WG 34.01.He is Chairman of the Technica

Publications Subcommittee of the UCA International Users Group. He holds thre

patents and has authored and presented more than 140 technical papers.

Damien Tholomier received a BEng in Electrical an

Automation Engineering in 1992 from the University o

Marseilles, France (Ecole Polytechnique Universitair

de Marseille). Damien joined ALSTOM T&D GmbH i

Stuttgart, Germany where he worked for 5 years in th

Protection & Control department as Power System

Application Engineer.

In 1997 Damien moved as Marketing Manager Hig

Voltage Protection Business Unit with Alstom T&D

Protection & Control in Lattes, France where he worke

on full scheme distance protection algorithms.

From 1999-2001 he was Sales & Service Director for Mediterranean Countrieand Africa.

Since 2002 he is Marketing Protection Relays Director for ALSTOM T&D EA

where he worked on new busbar relay (application of universal topology and CT

saturation detection algorithms).

Simon Richardsis the UK-based Marketing Directo

for Protection Products from AREVA. AREVA ha

recently acquired the Transmission and Distributio

businesses of ALSTOM. Protection Products are

part of AREVAs Energy Automation and Informatio

activity, and the author is based in Stafford, UK. H

has a B.Eng (Hons) in Electrical and Electroni

Engineering from the University of Bath, and is

Chartered Engineer, and MIEE. Previously a 25kV

electrification Distribution Engineer for the 500km

West Coast Main Line railway between London and Scotland, Simon also held number of protection applications engineering positions within ALSTOM prio

to his current role. The Marketing function provides technical support to Sale

and Service teams worldwide, and investigates opportunities for new produc

developments.

Distance Protection and Dynamic Loading of Transmission Lines

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