Busbar Protection AREVA

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Bus

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Bus

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Janu

January 200ary 20044

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There are are fewer fare fewer faults ults on buson bus bars than on other parts bars than on other parts of the powof the pow er system.er system.



 _{No dislocation of s ystem due to a}_{No dislocation of s}_{ystem due to accidental operation of busbar}_{ccidental operation of busbar}

protection. protection.



 _{Slow f ault c}_{Slow f}_{ault c lea}_learance._rance.

Bus

Bus bar faults bar faults at F1 at F1 and F2 and F2 are cleaare cleared by rred by r emote timemote tim e delayee delayedd protection on c

protection on c ircuits ircuits feefeeding ding the fathe faults:ults: Time Delayed Overcurrent or

Time Delayed Overcurrent or Time De

Time Delayed Distance Protectlayed Distance Protect ionion F F₂₂ F

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 _Fa_{Fast c}_{st cleara}_{learance by b}_{nce by breake}_{reakers at th}_{rs at the busb}_{e busbars}_ars

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 _Whe_{Where bus}_{re busbars are section}_{bars are section alised,}_alised,

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Protection can limit rotection can limit the athe amount mount of of systemsystem disrupti

disrupti on for a busbar faon for a busbar faultult

BUSBAR BUSBAR ZONE ZONE F F₂₂ F F₁₁

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Busbar Faults Are Usually Permanent

CAUSES :

 _{Insulation failures}

 _{Circuit breaker failures}  _{Falling debris}

 _{Isolators operated out side their ratings}  _{Safety earths left connected}

 _{Current transfor mer failures}

THEREFORE :

Circuit breakers should be tripp ed and lo cked out by bus bar protection

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Busbar Protection must be:

 _RELIABLE

 Failur e could cause widespread damage to th e

substation  _STABLE

 False trippi ng can cause widespread interruption of

supplies to custo mers  _{DISCRIMINATING}

 Should trip the minimum number of breakers to clear

the fault  _FAST

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Methods of Providing Busbar Protection



_{Frame to Earth (Leakage) Protection}

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_{Differential Protection : High Impedance}

Low Impedance

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_{Directional Comparison (Blocking Schemes)}

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Frame Earth Protection Scheme

 _{Only an earth fault s yst em}

 _{Involves measuring fault curr ent f rom s wi tchgear frame to earth}

 _{Switchg ear insul ated by standing on c oncrete plinth}

 _{Only one earthin g poi nt allowed on sw itchg ear}

 _{C.T. mount ed on singl e earth cond uctor used to energise}

instantaneous relay

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Current Distribution for External Fault

Outgoing feeder Switchgear frame Switchgear frame bonding bar Generator System earthing resistor Earth bar Frame-leakage current transformer Earthing electrode resistance (< 1Ω) Frame insulation resistance to earth (> 10Ω) I_F = I₁ + I₂ I₁ + I₂ I₁ _I₂ I₁

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High Impedance Protection (1)

This is a versatile and reliable prot ection system

applied to many different Busbar configurations.

If CT requirements are met, scheme performance

may be predicted by calculation without heavy

current conjunctive tests.

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High Impedance Protection (2)

Simple system to apply and extend.

High sensitivity f or

phase and earth faults . Extremely stable for external faults.

CT requirements: Equal ratios Class ‘X’

May require stabilising resistor s, RST.

May require non-linear resistor s (Metrosils).

R_ST

METROSIL 87

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Effective Setting

Since in each zone of pro tecti on th ere are several CT’s in p arallel with the relay and each o ther, the combin ed CT magnetising c urrents w ill increase the primary

operating current (P.O.C).

P.O.C. = CT ratio (I_R + I_NLR + _nI_M) where

:-I_R = Relay setting current

I_M = CT magnetising current (one CT at relay setting voltage)

n = Number of paralleled CT’s

I_NLR = 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 suffi cient relay current during int ernal

fault conditions for high speed operation.

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Through Fault Stability

Busbar protection stability li mit is based on

maximum through fault cur rent.

Generally this value is derived from the rating of

the associated sw itchgear irrespective of existing

fault level, since it can be expected that system

will develop up to limit of rating.

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Check Feature

Usually provided by duplication of pri mary protection using second set of CTs on all cir cuits other than bus section and coup ler unit s. Check system forms one zone only, covering whole of b usbar systems and not discrimin ating between faults on various sections.

Check zone Zone A Zone B 87A 87A 87A

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CT Wiring Supervision (1)

Open circu it connection s between CT’s and relay circu it result in unbalance currents which may operate the

protection.

Supervision is applied by a voltage relay acros s dif ferential relay circuit .

Supervision relay is time delayed, gives alarm and also short s out bu s w ires to protect differential relay cir cuit.

Typical effective setting is 25 prim ary amps or 10% of low est circ uit rating, whi chever is greater.

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M3 SP M3 SP M2 SP SP SP M4 M3 M2 1

Z

V

Z

V

Z

V

R

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// Z

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measured

Voltage

+ + + = Ι = Ι =

CT Wiring Supervision (2)

I₁ CT1 Super vision relay V R_ST R_R R _Z M2 ZM3 ZM4 I₁ I₂ I₃ I₄

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Differential Relay Circuit

A B C N

Zone bus wires

95X 95X 95X

Bus wire short contacts

Supervision relay 95 Stabilising resistors 87 87 87 v v v Metrosil resistors

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Current Transformer Wiring

Lead bur dens between various sets of CT’s must be kept low. Usually busw ires are run in clos ed ring between breaker contro l p anels.

Typical rou te is

:- CT’s to m arshalling k iosk

 Marshallin g kios k to isol ator auxili aries  Loop between marshallin g kio sks

Conducto r size

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Effect of C.T. Location on

Busbar Protection Performance

Busbar protection Circuit protection Circuit protection Busbar protection Overlapping C.T.s Circuit protection Busbar protection 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

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Typical Double Busbar Arrangement

60MW Generators 75MVA 132/13.8kV Transformers 132kV

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Zones of Protection for Double Bus Station

Zone G Zone H

Zone J

BC BC

BS

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Isolator Auxiliary Switches

R M A B C D a b c d r m Buswires

Auxiliary switches should : 1) Close before the isolator

closes

2) Open after the isolator opens

In order to maintain stability on switching.

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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.

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Typical Trip Relay Arrangement

Double Busbar System

In Out CSS - M1 CSS - M2 CSS - R 87M1 - 1 87M2 - 1 87R - 1 M1M2R a1 c1 b1 c2 D.C. Buswires 80T 96H2 96H1 96G 96F2 96F1 96E 96D2 96D1

-+

87CH - 1

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Double Busbar with Transfer Facilities

Main Reserve / Transfer By-pass Isolator By-pass Isolator

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Triple Busbar

Main Transfer CB Transfer Reserve Transfer CB

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1½ Breaker Bus Protection

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 87R3 87 R4 87R2

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Busbar Protection and Breaker Fail

Where breaker fail pro tection is appli ed to a syst em, back trip ping o f associated breakers is requir ed in the event of a breaker failu re.

Often, breaker fail protection is arranged in conjunc tion with busbar protection trip ping circui ts to initi ate tripping o f

breakers on a busbar zone associated with the failed breaker.

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Low Impedance Busbar Protection

Fast

Modular scheme design allows r elays t o relate to each circuit and

function of the protection. This enables the user to easily un derstand 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 incor porate saturation detectors and are extremely stable.

Duplicate measuring cir cuits are inclu ded. Current transformers can be :

of dif ferent ratio

of relatively small outp ut

shared w ith other pr otections

Current transform er secondary cir cuits are not sw itched.

Continuous sup ervision of CT circuits and constant monitoring of vital circuits are included.

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Single Bus Protection

F1 F2 F3 F4 Z2 Z1 BS F M1 M2F BSM M3F M4F Z1 ZCK Z2 ZCK ZCK Z1 Z2

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Double Bus Protection

Z1 BCM 1 Z3 Z2 Z4 BS BC1 BC2 F1 F2 F3 F4 BCM 2 FM 1 FM 2 FM3 FM 4 BSM Z1 Z3 ZCK Z2 Z4 ZCK Z1 Z3 ZCK _Z2 Z4

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Busbar Blocking Protection

I_F1

Incomer

O/C Relay

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

BLOCK

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Directional Comparison Busbar Protection



_{Bus zone protection and unit pr otection of feeders}

Forward Forward Forward

F1 BS F2

F3 F4 F5 F6

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