Power System Protection Engineering... · Power System Protection S.A.Soman Introduction Islanding with Generation Load Balance Out of Step Blocking Relays Out of Step Tripping Relay

Power SystemProtection

S.A.Soman

Introduction

Islanding withGenerationLoad Balance

Out of StepBlockingRelays

Out of StepTripping Relay

Setting of Outof StepTripping Relay

ElectricalCenter

ThreeSteppedDistanceProtection

Summary

Power System Protection

S.A.Soman

Department of Electrical EngineeringIIT Bombay

Power Swing Detection, Blocking and Out of StepRelaying


S.A.Soman

Introduction





ElectricalCenter


Summary

Power Swing Detection, Blocking and Out ofStep Relaying

1 Introduction

2 Islanding with Generation Load Balance

3 Out of Step Blocking Relays

4 Out of Step Tripping Relay

5 Setting of Out of Step Tripping Relay

6 Electrical Center

7 Three Stepped Distance Protection

8 Summary


S.A.Soman

Introduction





ElectricalCenter


Summary


Power Swings can be classified as

Stable: When the power swing is consequence ofstable disturbance, unwanted tripping lead to instability.Unstable: When the power swing is consequence ofunstable disturbance, interconnected operation of thesystem will not be possible.

Then the system has to be split into multiple islands,each having synchronism of generators.In each island, generator load balance has to beascertained.If an island has excess generation, it should be shelved.If an island has excess load, load shedding is required.The boundary of islands has to be selected in such away that loss of service is minimized.


S.A.Soman

Introduction





ElectricalCenter


Summary


During an unstable disturbance, let the electrical centerbe on line AB.Relays R1 and R2 will trip and thereby islanding thesystem.There will be loss of load in island B and loss ofgeneration in island A.Thus during unstable swings, distance relays should beblocked from operation.


S.A.Soman

Introduction





ElectricalCenter


Summary


Out of Step Blocking Relays

It is similar to the Mho unit.

It is a circle concentric to mho tripping characteristicswith larger radius

This ascertains that any power swing which enterstripping will first enter the out of step characteristic.

If the transit time is larger than the preset interval, it willoperate auxilliary relays to block tripping.

An out of step blocking scheme with an impedance alsohave the same principle.


S.A.Soman

Introduction





ElectricalCenter


Summary



S.A.Soman

Introduction





ElectricalCenter


Summary



S.A.Soman

Introduction





ElectricalCenter


Summary


Out of Step Tripping Relay

This scheme consists of two modified reactance unitswhose characteristics are set parallel to the systemimpedance.

The unstable swing will cross these two characteristics.

The two crossing indicate that swing has crossed theimpedance characteristics and hence it is a loss ofsynchronism condition.

This will lead to an out of step trip decision.

This scheme will pick up for swing movement in theopposite direction and for the swings behind the relay.

In order to avoid tripping on swings far away from theline, this relay is supervised by an overcurrent relay.


S.A.Soman

Introduction





ElectricalCenter


Summary



S.A.Soman

Introduction





ElectricalCenter


Summary


Setting of Out of Step Tripping Relay

The primary step is to freeze the location of linesegments.

This setting is done in such a way that swing locus willremain between XX 1 for atleast 0.005sec.

This is the operating time of the auxiliary relays whichdetermines a loss of synchronism.

The characteristics of these two units should not be setso far apart to pick up on load conditions.


S.A.Soman

Introduction





ElectricalCenter


Summary



S.A.Soman

Introduction





ElectricalCenter


Summary

Power Swings and Distance Relaying

Thus it can be seen thatThe trajectory of the impedance measured by relayduring the power swing is straight line.The angle subtended by a point in the locus on S and Rend points is the angle δ.The swing intersects the line AB, when δ = 180.This point of intersection of swing impedance trajectoryon the impedance line is known as electrical center ofthe swing.At the electrical center, angle between two sources is180◦.The existence of the electrical center is an indication ofsystem instability. i.e, the two generators are now out ofstep.If the post fault system is stable, the power swingretraces its path at δmax .


S.A.Soman

Introduction





ElectricalCenter


Summary



S.A.Soman

Introduction





ElectricalCenter


Summary


If ESER

= k 6= 1 then the power swing locus is an arc of thecircle.


S.A.Soman

Introduction





ElectricalCenter


Summary


It can be shown that,

ES

ES − ER=

k(cosδ + jsinδ)

k(cosδ + jsinδ)− 1=

k [(k − cosδ)− jsinδ]

(k − cosδ)2 + sin2δ

Then,

Zseen = −ZS +k [(k − cosδ)− jsinδ]

(k − cosδ)2 + sin2δZT

The location of electrical center depends upon |ES ||ER |

ratio.

Appearance of electrical center on a transmission lineis a transient phenomenon.

The electrical center vanishes after some time.


S.A.Soman

Introduction





ElectricalCenter


Summary


At the electrical center voltage is zero and the relays atboth ends will trip the line.

Thus, existence of electrical center indicates systeminstability and nuisance tripping of the distance relay.


S.A.Soman

Introduction





ElectricalCenter


Summary



S.A.Soman

Introduction





ElectricalCenter


Summary



S.A.Soman

Introduction





ElectricalCenter


Summary


Let δZ1, δZ2 and δZ3 are the rotor angles when swing justenters the 3 zones, then,

If δmax < δZ3, then the swing will not enter the relaycharacteristics.

If δZ3 ≤ δmax ≤ δZ2, then the swing will enter in zone 3.

If δZ2 ≤ δmax ≤ δZ1, then the swing will enter in bothzone 2 and 3.

If δmax ≥ δZ1, then the swing will enter in all the 3 zonesand the relay will trip on zone 1.


S.A.Soman

Introduction





ElectricalCenter


Summary

Setting of Distance Relays

Summary

Introduction to power swings

Distance relay perspective of power swings

Swing locus seen by distance relay

Electrical center

Three stepped distance protection

Possibility of distance relay tripping on power swings

Documents

Power System Protection Engineering... · Power System Protection S.A.Soman Introduction Islanding with Generation Load Balance Out of Step Blocking Relays Out of Step Tripping Relay