distance relay in power systems

Power SystemProtection

S.A.Soman

Zones ofProtection

Zone 1ProtectionZone 1 Setting

Infeed andOutfeedEffects

Zone 2 andZone 3 Setting

OverlapProblem forZone 2

Problem ofLoadEncroachmentSolution for LoadEncroachment

Summary

Power System Protection

S.A.Soman

Department of Electrical EngineeringIIT Bombay

Setting of Distance Relays


S.A.Soman

Zones ofProtection






Summary


1 Zones of Protection

2 Zone 1 ProtectionZone 1 Setting

3 Infeed and Outfeed Effects

4 Zone 2 and Zone 3 Setting

5 Overlap Problem for Zone 2

6 Problem of Load EncroachmentSolution for Load Encroachment

7 Summary


S.A.Soman

Zones ofProtection






Summary


Zones of Protection

A distance relay has to perform the dual task of primaryand back up protection.

The primary protection should be fast and without anyintentional time delay.

Back up protection should operate if and only ifcorresponding primary relay fails.

Distance relays are provided with multiple zones ofprotection to meet the stringent selectivity andsensitivity requirements.


S.A.Soman

Zones ofProtection






Summary



S.A.Soman

Zones ofProtection






Summary


Zone 1 of Protection

Zone 1 provides the fastest protection.

There is no intentional time delay.

Operating time can be of the order of 1 cycle.

It is set to cover 80% of the line length.

It is because of the difficulty in distinguishing betweenfaults which are close to bus B.


S.A.Soman

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7 Summary


S.A.Soman

Zones ofProtection






Summary


Zone 1 does not cover the entire length because of thefollowing

Limited accuracy of CT and PT because ofPartial or complete saturation of CT during fault.It results in errors in measurement of apparentimpedance.This makes it difficult to determine fault location at theboundary accurately.

Working of numerical algorithms is prone to errorsbecause of

Model limitation of transmission line.Transients accompanied with the fault.


S.A.Soman

Zones ofProtection






Summary


With only local measurements and a small time window,it is difficult to determine fault impedance accurately.The impedance seen by the relay for fault F will dependupon the current contribution from the remote end.Thus,

ZR = xZl + ZF + ZFIBF

IAF

There are infeed and outfeed effects associated withworking of distance relays.


S.A.Soman

Zones ofProtection






Summary

Infeed and Outfeed Effects

Consider relay R1 for a fault F.~IBF = ~IAB + ~IED


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Zones ofProtection






Summary


Hence,VR1 = ~IABZ1 + xZl2

~IBC

VR1

~IAB

= Z1 + xZl2 + xZl2

~IED

~IAB

If there is an equivalent generator source at E, then itfeeds the fault current. Thus ~IAB and ~IED are in phase.This is known as infeed effect. Thus relay R1 perceivesfault to farther away from its actual location.

If there is an equivalent load at bus E, relay perceivesfault to be at a point closer than its actual location.


S.A.Soman

Zones ofProtection






Summary


Setting of Zone 2Zone 2 protection is delayed by coordination timeinterval.Zone 2 is set to 120% of primary line impedance.It also provides back up protection to a part of theadjacent line.If ZP + 1.5ZB > 1.2ZP , where ZP and ZB are the positivesequence impedance of primary and the shortest backup line, then zone 2 is set to reach 50% of shortestback up line.Else if the shortest back up line is too short then Z2 isset to 1.2ZP .

Setting of Zone 3It is set to cover the longest remote line.It is set up to operate with a time delay of 2CTI.


S.A.Soman

Zones ofProtection






Summary



S.A.Soman

Zones ofProtection






Summary


Overlap Problem for Zone 2

If the reach of Z2 of relay R1 is extended too much, thenit can overlap with Z2 of relay R3.

Both R1 and R3 will compete to clear the fault.

In order to avoid this situation Z2 of relay R1 has to befurther slowed down by CTI.

This will further delay the fault clearing time in 20%region of primary line.

It is not possible to avoid this overlap problem when theshortest line to be backed up is too short.

In such situations, pilot relaying can be used to improvespeed.


S.A.Soman

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S.A.Soman

Zones ofProtection






Summary


Load Encroachment Problem

The apparent impedance seen by the relay R for the steadystate positive sequence model of a transmission line isgiven by,

ZR =|Vi |2

Pij − jQij=

|Vi |2

P2ij + Q2

ij

(Pij + jQij)


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Zones ofProtection






Summary


The following conclusions can be derived from the equation:

Quadrant of ZR in the R-X plane correspond to thequadrant of apparent power Sij in Pij − Qij plane.

The apparent impedance seen by the relay isproportional to square of the magnitude of bus voltage.

The apparent impedance seen by the relay is inverselyproportional to the apparent power flowing on the line.

During peak load conditions, these effects will reducethe apparent impedance seen by the relay to a smallvalue to fall within Z2 or Z3 characteristic.

Then the relay will pick up and trip the circuit after itstime dial setting requirements are met and the relay issaid to trip on load encroachment.


S.A.Soman

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7 Summary


S.A.Soman

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Summary


Solution for Load Encroachment

Loads have large powerfactor and hence Zapp

with large RX ratio.

But faults are more orless reactive in natureand X

R ratio is high.

In order to preventtripping on loadencroachment, relaycharacteristics aremodified by excluding anarea in R-X planecorresponding to highpower factor.


S.A.Soman

Zones ofProtection






Summary


Summary

Setting of distance relays for zone 1 protection

Zone 2 and zone 3 setting

Infeed and outfeed effects

Overlaping problem

Load encroachment problem and its solution

Documents

distance relay in power systems