busbar protection areva

Busbar Protection

p:/applics/Powerpoint Cabinet/Training Courses/APPS1

January 2004

Alan WixonSenior Applications Engineer

> Busbar Protection – January 20043 3

Without Busbar Protection (1)

There are fewer faults on busbars than on other parts of the power system. No dislocation of system due to accidental operation of busbar

protection. Slow fault clearance.

Busbar faults at F1 and F2 are cleared by remote time delayed protection on circuits feeding the faults:

Time Delayed Overcurrent orTime Delayed Distance Protection

F2F1


Without Busbar Protection (2)

Fast clearance by breakers at the busbars

Where busbars are sectionalised,

Protection can limit the amount of system disruption for a busbar fault

BUSBARZONE

F2F1


Busbar Faults Are Usually Permanent

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


Busbar Protection must be:

RELIABLE

Failure could cause widespread damage to the substation

STABLE

False tripping can cause widespread interruption of supplies to customers

DISCRIMINATING

Should trip the minimum number of breakers to clear the fault

FAST

To limit damage and possible power system instability


Methods of Providing Busbar Protection

Frame to Earth (Leakage) Protection

Differential Protection : High ImpedanceLow Impedance

Directional Comparison (Blocking Schemes) Protection


Frame Leakage Protection


Frame Earth Protection Scheme

Only an earth fault system

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 energise instantaneous relay

All cable glands must be insulated


Current Distribution for External FaultOutgoing feeder

Switchgear frameSwitchgear framebonding bar

Generator

System earthingresistorEarth bar

Frame-leakage current transformer

Earthing electroderesistance (< 1Ω)

Frame insulation resistance to earth (> 10Ω)

IF = I1 + I2

I1 + I2

I1 I2I1


High Impedance Protection


High Impedance Protection (1)

This is a versatile and reliable protection system applied to many different Busbar configurations.

If CT requirements are met, scheme performance may be predicted by calculation without heavy current conjunctive tests.


High Impedance Protection (2)

Simple system to apply and extend.

High sensitivity for phase and earth faults.

Extremely stable for external faults.

CT requirements: Equal ratiosClass ‘X’

May require stabilising resistors, RST.

May require non-linear resistors (Metrosils).

RST

METROSIL87


Effective Setting

Since in each zone of protection there are several CT’s in parallel 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 currentIM = CT magnetising current (one CT at relay

setting voltage)n = Number of paralleled CT’sINLR = Non linear resistor current at relay setting

voltage


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.


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 will develop up to limit of rating.


Check Feature

Usually provided by duplication of primary protection using second set of 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


Current Transformers


CT Wiring Supervision (1)

Open circuit connections between CT’s 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.


M3

SP

M3

SP

M2

SPSP

SP

M4M3M21

ZV

ZV

ZV

RV

relayn supervisio theoperate current to balance-of-OutV settingrelay n supervisio If

) // Z // Z // Z(R V relayn supervisioby measured Voltage

+++=Ι

=Ι=

CT Wiring Supervision (2)

I1

CT1

Supervisionrelay

VRST

RR

R ZM2 ZM3 ZM4

I1

I2 I3 I4


Differential Relay Circuit

ABCN

Zone bus wires

95X95X95X

Bus wire short contacts

Supervision relay

95

Stabilising resistors

87 87 87

v v v

Metrosilresistors


Current Transformer Wiring

Lead burdens between various sets of CT’s must be kept low. Usually buswires are run in closed ring between breaker control panels.

Typical route is :- CT’s to marshalling kiosk

Marshalling kiosk to isolator auxiliaries

Loop between marshalling kiosks

Conductor size :- Normally 2.5mm2


Effect of C.T. Location on Busbar Protection Performance

Busbarprotection

Circuitprotection

Circuitprotection

Busbarprotection

Overlapping C.T.s

Circuitprotection

Busbarprotection

Interlocked overcurrent relay

All C.T.s on line side of circuit breaker

All C.T.s on Busbar side of circuit breaker

Interlocked overcurrent relay


Busbar Arrangements


Typical Double Busbar Arrangement

60MWGenerators

75MVA132/13.8kVTransformers

132kV


Zones of Protection for Double Bus Station

Zone GZone H

Zone J

BC BC

BS

Typical Feeder Circuits


Isolator Auxiliary Switches

R

M

A B C D

a b c d

rm

Buswires

Auxiliary switches should :

1) Close before the isolator closes

2) Open after the isolator opens

In order to maintain stability on switching.


Tripping Circuits

One tripping relay (device 96) is required for each feeder breaker and 2 for each bus section or bus coupler breakers. Both main and check relays must be energised before tripping relays trip all breakers associated with zone.


Typical Trip Relay Arrangement

Double Busbar System

In Out

CSS - M1

CSS - M2

CSS - R

87M1 - 1

87M2 - 1

87R - 1

M1 RM2

a1

c1

b1

c2

D.C. Buswires

80T

96H2

96H1

96G

96F2

96F1

96E

96D2

96D1

-+87CH - 1


Double Busbar with Transfer Facilities

Main

Reserve / Transfer

By-passIsolator

By-passIsolator


Triple Busbar

Main

TransferCB

Transfer

Reserve

TransferCB


1½ Breaker Scheme


1½ Breaker Bus Protection

87

87


Mesh Busbar

T1

F1 F3

T4

T3

T2

F4 F2


Mesh Busbar Protection

T1

F1 F3

T4

T3

T2

F4 F2

87R1

87R3

87R4

87R2


Busbar Protection and Breaker Fail

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 tripping of breakers on a busbar zone associated with the failed breaker.


Low Impedance Protection


Low Impedance Busbar Protection

Fast

Modular scheme design allows relays to relate to each circuit and function of the protection. This enables the user to easily understand the principles of application.

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.

Duplicate measuring circuits are included.

Current transformers can be :

of different ratio

of relatively small output

shared with other protections

Current transformer secondary circuits are not switched.

Continuous supervision of CT circuits and constant monitoring of vital circuits are included.


Single Bus Protection

F1 F2 F3 F4

Z2Z1

BS

FM1

FM2

FM3

FM4BSM

Z1ZCK

Z2ZCK

ZCKZ1 Z2


Double Bus Protection

Z1

BCM1

Z3Z2Z4

BS

BC1 BC2F1 F2 F3 F4

BCM2

FM1

FM2

FM3

FM4BSM

Z1Z3ZCK

Z2Z4ZCK

Z1 Z2 Z4Z3 ZCK


Blocking Schemes


Busbar Blocking Protection

IF1

Incomer

O/C Relay

O/C Relay O/C Relay O/C RelayO/C Relay

BLOCK

IF2


Directional Comparison Busbar Protection

Bus zone protection and unit protection of feeders

Forward Forward Forward

F1 BS F2

F3 F4 F5 F6

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