Busbar Protection AREVA

Busbar Bus bar Protectio rotection n

 Al  A l an Wix Wi x o n Senior Se nior A ppli pplications cations Engin ee eer  r 

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

January Janu ary 2004 2004

Busbar Bus bar Protectio rotection n

 Al  A l an Wix Wi x o n Senior Se nior A ppli pplications cations Engin ee eer  r 

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

January Janu ary 2004 2004

With ithout out Bus Busbar bar Prot Prote ecti ction on (1)

F1

F2

There There are are fewer faults faults on bus bars than on other parts of the pow er system. 

No dislocation of s ystem due to accidental accidental operation of busbar protection.



Slow f ault c learance. learance. Bus bar faults at F1 and F2 are cleared cleared by r emote tim e delayed delayed protection on c ircuits feeding feeding the faults: faults: Time Delayed Overcurrent or  Time Delayed Delayed Distance Protect ion

With ithout out Bus Busbar bar Prot Prote ecti ction on (2)

BUSBAR ZONE F1

F2

 Fast Fast cleara c learance nce by breake b reakers rs at the th e busbars busb ars

Where bus bars are section alised,  Where Protection can limit the amount amount of system disrupti on for a busbar fault fault

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

Busbar Protection must be: 

RELIABLE 



STABLE 



False trippi ng can cause widespread interruption of supplies to custo mers

DISCRIMINATING 



Failur e could cause widespread damage to th e substation

Should trip the minimum n umber of breakers to clear the fault

FAST 

To limit damage and poss ible power system instabilit y

Methods of Providing Busbar Protection

 Frame to Earth (Leakage) Protection

 Differential Protection :

High Impedance Low Impedance

 Directional Comparison (Blocking Schemes)

Protection

Frame Leakage Protection

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



 All cabl e gland s must be insulated

Current Distribution for External Fault Outgoing feeder  IF

Switchgear frame bonding bar 

Switchgear frame Frame-leakage current transformer 

= I1 + I2

Generator 

System earthing resistor 

Earth bar 

I1

I1

Frame insulation resistance to earth (> 10Ω)

I1

I2

+ I2

Earthing electrode resistance (< 1Ω)

High Impedance Protection

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.

High Impedance Protection (2) Simple system to apply and extend. High sensitivity f or phase and earth faults . Extremely stable for external faults .

RST 87

CT requirements: Equal ratios METROSIL Class ‘X’ May require stabilising resistor s, RST. May require non-linear resistor s (Metrosils).

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 ( IR + INLR + n IM) where :IR IM

n

INLR

= = = =

Relay setting current CT magnetising current (one CT at relay setting vol tage) Number of paralleled CT’s 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 suffi cient relay current during int ernal fault conditions for high speed operation.

Through Fault Stability

Busbar protection stability li mit is based on maximum t hrough fault cur rent.

Generally this value is derived from the rating of the associated sw itchgear irrespective of exist ing fault level, since it can be expected that system will develop up to limit of rating.

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.

Zone A 87A

87A

Check zone 87A

Zone B

Current Transformers

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 s hort 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.

CT Wiring Supervision (2) I1

CT1

I2

RST R

V Super  vision relay

I4

ZM4

ZM3

ZM2

I3

RR I1

Voltage measured  by supervision relay V

= Ι1

(R // Z M2 // Z M3 // Z M4 )

If  supervision relay setting

=

VSP

Out - of  - balance current to operate the supervision relay Ι =

VSP R 

+

VSP Z M2

+

VSP Z M3

+

VSP Z M3

Differential Relay Circuit  A B C N

Zone bus wires

95X 95X

Bus wire short contacts

95X 95

Metrosil resistors

v

v 87

Stabilising resistors

v 87

Supervision relay

87

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 :

Normally 2.5mm2

Effect of C.T. Location on Busbar Protection Performance

Interlocked overcurrent relay

Circuit protection

Circuit protection Busbar  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 

Busbar Arrangements

Typical Double Busbar Arrangement 60MW Generators 75MVA 132/13.8kV Transformers

132kV

Zones of Protection for Double Bus Station Zone H

Zone G BS

BC

BC

Zone J

Typical Feeder Circuits

Isolator Auxiliary Switches

R M

 Auxiliary switches should :  A

B

C

a b

Buswires

r  m

D

c d

1)

Close before the isolator closes

2)

Open after the isolator opens

In order to maintain stability on switching.

Tripping Circuits

One trippi ng 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

87M1 - 1

M1 M2R

87CH - 1 96D1

CSS - M1 87M2 - 1

96D2

a1 96E

CSS - M2 87R - 1

c1

CSS - R

96F1

96F2

b1 96G

c2 96H1 96H2

D.C. Buswires 80T

Double Busbar with Transfer Facilities

Main

Reserve / Transfer 

By-pass Isolator 

By-pass Isolator 

Triple Busbar 

Main

Reserve

Transfer 

Transfer  CB

Transfer  CB

1½ Breaker Scheme

1½ Breaker Bus Protection

87

87

Mesh Busbar  F1

F3

T1

T3

T4

T2

F4

F2

Mesh Busbar Protection F1

F3

87 R1

87 R3

T1

T3

T4

T2

87 R4

87 R2

F4

F2

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.

Low Impedance Protection

Low Impedance Busbar Protection Fast Modular scheme design allows r elays t o relate to each c ircui t and func tion of the protection. This enables the user to easily un derstand the principles of application. High sensit ivity for phase and earth faults. Protection for each phase can be relatively independent. Earli er schemes were less stable than hig h impedance schemes. Modern schemes incor porate saturation detectors and are extremely stable. Duplicate measuri ng cir cuit s 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.

Single Bus Protection

Z2

Z1 F1

BS

F2

F M1

F M2

BSM

F3

F4

F M3

F M4

Z2 ZCK

Z1 ZCK

Z1

ZCK

Z2

Double Bus Protection

BS

Z1

Z2 Z4

Z3

BC1

F2

F1

BCM 1

FM 2

FM 1

BSM

BC2

F4

F3

FM 4

FM 3

BCM 2

Z1 Z3 ZCK

Z2 Z4 ZCK

Z1

Z3

ZCK

Z2

Z4

Blocking Schemes

Busbar Blocking Protection Incomer  O/C Relay

BLOCK

IF2

O/C Relay

IF1

O/C Relay

O/C Relay

O/C Relay

Directional Comparison Busbar Protection  Bus zone protection and unit pr otection of feeders Forward DOC F1

OC

Forward DOC BS

OC F3

Forward

F4

DOC F2

OC F5

OC F6