07b Busbar Differential

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Protection EngineeringAnd Research Laboratories

Session VII :

Busbar Protection

Dr. G. Pradeep Kumar

Training on Power System Element Protection,

9th

& 17th

March, 2007 at L&T Manappakam, Chennai.


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Contents

Introduction

Frame leakage protection

High impedance bus differential

Low impedance bus differential

Busbar configurations

Breaker failure protection


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Introduction


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Busbar Protection

Busbar faults are very rare.

Busbar protection not provided always

Without busbar protection

No dislocation of system due to accidental operation of

busbar protection.

Slow fault clearance.

Busbar faults are cleared by remote time delayed

protection on circuits feeding the faults:

Time delayed over current or

Time delayed distance protection


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Busbar Faults Are UsuallyPermanent

CAUSES :

Insulation failures

Circuit breaker failures

Falling debris

Isolators operated outside their ratings

Safety earths left connected

Current transformer failures

THEREFORE :

Circuit breakers should be tripped and locked out by busbar

protection


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Busbar Protection

BUSBARZONE

F2F1

WITH BUSBAR PROTECTION

Fast clearance by breakers at the busbars

Where busbars are sectionalised, protection can limit the

amount of system disruption for a busbar fault


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Busbar Protection

RELIABILITY Failure could cause widespread damage to the substation

STABILITY

False tripping can cause widespread interruption of supplies

to customers

DISCRIMINATION

Should trip the minimum number of breakers to clear the

fault

SPEED

To limit damage and possible power system instability


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Methods of Providing BusbarProtection

Frame to Earth (Leakage) Protection

Directional Comparison Protection

Differential Protection : High Impedance

Low Impedance


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Frame LeakageProtection


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Can only detect an earth fault

Involves measuring fault current from switchgear frame

to earth

Switchgear insulated by standing on concrete plinth

Only one earthing point allowed on switchgear

C.T. mounted on single earth conductor used to energiseinstantaneous relay

All cable glands must be insulated

Frame Earth Protection Scheme


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Current Distribution for ExternalFault

Outgoing feeder

Switchgear frameSwitchgear framebonding bar

Generator

SystemearthingresistorEarth bar

Frame-leakage currenttransformer

Earthing electroderesistance (< 1)

Frame insulationresistance to earth(> 10)

IF = I1+ I2

I1+ I2

I1I2

I1


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Frame Leakage Busbar Protection

Simple, economical.

Suitable for phase segregated indoor metal-clad

switchgear. Only E/F protection required.

Setting of instantaneous E/F relay (64)

= < 0.3 IF(min)

Disadvantages

Insulation of switchgear frame and between sections.

Insulation of cable glands to prevent spurious currents

during through faults.


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Frame Leakage Protection

Check Feature

To differentiate between a genuine busbar fault and a

fault in the secondary winding of a c.t.

The check feature provides a second line of defence.

The check relays pick up for both internal and external

faults.

Both check and discriminating relays must operate

before tripping can occur.


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Frame Leakage Protection

Check Feature

The various methods of obtaining the check feature are,

Neutral check provided by a relay energised from a single

c.t. in the power system neutral.

Residual check provided by a relay energized from a

residually connected c.t. on the busbar incomers.

Residual voltage check provided by a voltage relay

energized from a broken delta v.t. supply.

Check relays are normally self-reset in order to avoid

having to reset the relay after each external fault.


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Single Zone Frame - EarthProtection with Neutral Check

Switchgear frame

Neutral check relay

Trip all breakerscircuit

Frame-earthfault relay

G H J K

64

64

CH

+


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High Impedance BusDifferential


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G H

AB

CN

Differential relay

87A 87A 87A

Circulating CurrentDifferential Scheme


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High Impedance Protection

This is a versatile and reliable protection

system applied to many different busbar

configurations.

Simple system to apply and extend.

High sensitivity for phase and earth faults.

Extremely stable for external faults.


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High Impedance Protection

CT requirements:

Equal ratios

Class X

Requires stabilising resistors, RST

May require non-linear resistors (Metrosils)

If CT requirements are met, scheme

performance may be predicted by calculation

without heavy current conjunctive tests


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High Impedance BusbarProtection

RST

METROSIL

87


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Effective Setting

Since in each zone of protection there are several CTs inparallel with the relay and each other, the combined CT

magnetising currents will increase the primary operating

current (P.O.C).

P.O.C. = CT ratio (IR + INLR + nIM)where :-

IR = Relay setting current

IM = CT magnetising current (one CT at relay

setting voltage)n = Number of paralleled CTs

INLR= Non linear resistor current at relay setting

voltage


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Primary Operating Current(P.O.C)

The value of primary operating current should be

around 30% of minimum fault current available.

This ensures sufficient relay current during internal

fault conditions for high speed operation.


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Through Fault Stability

Busbar protection stability limit is based on maximum

through fault current.

Generally this value is derived from the rating of the

associated switchgear irrespective of existing fault

level, since it can be expected that system can growup to limit of rating.


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Check Feature

Provided by duplication of primary protection using second setof CTs on all circuits other than bus section and coupler units.

Check system forms one zone only, covering whole of busbar

systems and not discriminating between faults on various

sections.

Check zone

Zone A Zone B87A

87A

87A


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CT Wiring Supervision

Open circuit connections between CTs and relay circuit

result in unbalance currents which may operate the

protection.

Supervision is applied by a voltage relay across

differential relay circuit.

Supervision relay is time delayed, gives alarm and also

shorts out bus wires to protect differential relay circuit.

Typical effective setting is 25 primary amps or 10% of

lowest circuit rating, whichever is greater.


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M3

SP

M3

SP

M2

SPSP

SP

M4M3M21

Z

V

Z

V

Z

V

R

V

relaynsupervisiotheoperatecurrent tobalance-of-OutVsettingrelaynsupervisioIf

)Z||Z||Z||(RV

relaynsupervisiobymeasuredVoltage

CT1

Supervisionrelay

V

RST

RR

RZM2 ZM3 ZM4

I1

I1

I2 I3 I4

CT Wiring Supervision


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Differential Relay CircuitA

B

C

N

Zone bus wires

95X

95X

95X

Bus wire short

contacts

Supervision

relay

95

Stabilizingresistors

87 87 87

v v v

Non-linearresistors


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Current Transformer Wiring

Lead burdens between various sets of CTs must be kept

low. Usually bus wires are run in closed ring between

breaker control panels.

Typical route is :-

CTs to marshalling kiosk

Marshalling kiosk to isolator auxiliaries

Loop between marshalling kiosks

Normal conductor size is 2.5mm2


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Low Impedance BusDifferential


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Low Impedance Busbar Protection

Biased differential characteristics provides stability forthrough fault.

Modular scheme design allows relays to relate to each

circuit and function of the protection.

Optic inter module communication in numerical relays

High sensitivity for phase and earth faults. Protection for

each phase can be relatively independent.

Earlier schemes were less stable than high impedance

schemes. Modern schemes incorporate saturation

detectors and are extremely stable.


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Low Impedance Busbar Protection

Current transformers can be :

of different ratio

of relatively small output

shared with other protections

Current transformer secondary circuits are not

switched.

CT burden reduced in distributed architecture

Continuous supervision of CT circuits and constant

monitoring of vital circuits are included.


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Modular Low Impedance RelaySingle Bus Protection

F1 F2 F3 F4

Z2Z1

BS

FM1

FM2

FM3

FM4BSM

Z1ZCK Z2ZCK

ZCKZ1 Z2


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Modular Low Impedance RelayDouble Bus Protection

Z1

BCM1

Z3

Z2

Z4

BS

BC1 BC2F1 F2 F3 F4

BCM2

FM1

FM2

FM3

FM4BSM

Z1

Z3ZCK

Z2

Z4ZCK

Z1 Z2 Z4Z3 ZCK


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Bus Arrangements


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Effect of C.T. Location on BusbarProtection Performance

Circuitprotection

Busbar

protection

Overlapping C.T.s

Circuitprotection

Busbarprotection

Interlocked

over currentrelay

All C.T.s on line sideof circuit breaker

All C.T.s on Busbar sideof circuit breaker

Busbarprotection

Circuit

protection

Interlockedover currentrelay


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Typical Double BusbarArrangement

60MW

Generators

75MVA132/13.8kVTransformers

132kV


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Zones of Protection forDouble Bus Station

Zone G Zone H

Zone J

BC BC

BS

Typical Feeder Circuits


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Isolator Auxiliary Switches

R

M

A B C D

a b c d

r

Buswires

In order to maintain stability

on switching, auxiliary switches

should :

1) Close before the isolator

closes

2) Open after the isolator

opens

m


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Tripping Circuits

One trip relay is required for each feeder breaker

Two trip relays for each bus section or bus coupler

breakers.

The trip relays have to be lock-out type (hand reset)

Both main and check relays must be energized for the

tripping relays to trip all breakers associated with that

zone.


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Double Busbar with TransferFacilities

Main

Reserve / Transfer

By-passIsolator

By-passIsolator


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Triple Busbar

Main

TransferCB

Transfer

Reserve

TransferCB


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1 Breaker Scheme

Bus 1

Bus 2


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1 Breaker Bus Protection

87

87


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Mesh Busbar

T1

F1 F3

T4

T3

T2

F4 F2


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Mesh Busbar Protection

T1

F1 F3

T4

T3

T2

F4 F2

87

R1

87

R3

87R4

87R2


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Breaker Failure Protection


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Breaker Fail Protection

Detects failure of a circuit breaker to interrupt the fault

current even after the protection relay issues a trip

command

Where breaker fail protection is applied to a system,

back tripping of associated breakers is required in the

event of a breaker failure.

Often, breaker fail protection is arranged in conjunction

with busbar protection tripping circuits to initiate trippingof breakers on a busbar zone associated with the failed

breaker.


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Breaker Fail Protection

Inst.O/CEnable

Td

Breaker FailureTrip Initiation

+ -

PR Trip


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Thank you

07b Busbar Differential

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