Measurenents

ANATYSIS AND PRO1ECTION OF POWER SYSTE}T,S COIIRSE

BUSBAR PROTECTION

BY

G.A.

I{AIII,ET

1.

BUSBAR PROTECTION

1.0

INTRODUCTION

rn the early

of the electricity suppry ind.ustry, protective

equ.ipment
for plants connected. to a busbar instalration
was reried upon to crear
busbar faults' This resulted. in tine-delayed.
fault clearance
days

by time-grad.ed
protections such as distance relays or
overcurrent-tine relays.
with present-day widely meshed power systen
networks with

rine sections

varying in length and nurnerous intermed.iate

infeedsl fault elearance by
zone 2 or zone J of d'istance relay can
be difficult plus the inpossibility
of selective tripping of different br:s sections.
rn order to maintain
systen stability and ninimise da^mag:e due to
high fault levels tine-delayed

tripping for busbar faults is no longer acceptable.

rt is therefore
necessarJr to detect busbar faults selectivery
with a rrnit form of protection
system.

2r0

(i)

rt

must be conpletely

reliable, since the protectlon may only

called to operate once or twi.ce in the rife of the
switchgear
installation and failure to operate under fault eonditions

be

wourd

be unacceptable.

(ii)

rt

raust be absolute].y

stable r:nder all through fault conditi.ons

since failure to stabilise wourd cause unnecessarJr

(ii:.)

interruption of supply.
rt must be capable of comprete d.iscrimination

wid.espread

between sections

of
the busbars to ensr:re that the rninimum nr:nber
of circuit breakers
are tripped. to isolate the fau1t.
.

2.

(i")

It nust

possess high speed

maintaj.n system

fnd.oor

(ii)

outdoor type.

da.rnage and

stability.

There are two main types

(i)

of operation to minlmise

of busbar installation :-

or metalclad type.

Indoor or metalclad switchgear is rnainly used on medium voltage systems,

but with the introduction of sF5 gas as an insulation

med.ir:m,

it is

now

possible to have rnetalclad. busbar installations up to the highest system

voltages. rf the rnetarcrad type is fulry

phase segregated interphase

faults cannot occur and only earth fault protection is required. A1I other
types of busbars should be protected against both phase and earth faults
by varior:s types

of protection

schemes.

1.o

TYPES OF 3US3AR TA.OTECTION SCIfiME

1.1

SASIC CIRCULATING CURRE}IT SCHBIE

This is a sinple form of r.rnit protectlon which compares the current entering
and ]eaving the busbar as shown

in Fig. 1. If the curent transf

orrners were

perfect there would. be no cument through the relay circuj.t. In practice

there will be spill current through the relay circuit, which must not exceed
the relay cunent setting up to the rnarinr:n through fault current.

1.2

BTASFJD

DI$ENEMIAT. CIRCULATTNG CURRnIT

The basic schene using

SCFTN,IE

practical current transforrners cannot provide high

fault stability at the same tine.

A bias feature can be incorporated. The principle of operation is shown
in Fig. 2. The reray used has both bias and operating circuits. ?he
speed operation and guarantee through

former is energi-sed by the arithrnetric sr:m of all the circuit curents

whilst the latter energised by the vector sun. A set of rectifiers

ar:xi1iary sunnation cr:rrent transforners is required.

and

1.

,.1
The

DIRECTTONAT COMPAXISON

principle of operation is

in eurrent in all feeders

During an external

SCIII}E

shown

connected

in Fig. Ja. An internal fault results

to the bus flowing towards the bus.

fault, however, the cr:nent in the faulted

feed.er

will

flow outwards. Contacts from all the d.irectional relays are connected in
series to energise a muLti-contact trip reIay.
An

alternative

amangement

is to use an additional blocking relay.

The

d-irectional relay is to have a changeover output contact. Al-1 the break

contacts are paralleled and connected to the blocking relay. All the make
contacts are paralle1 and connected to the trip relay through a no::nally
crosed contact from the blocking reray. This is sholn in Fig. 1lb.
The d.irectional

relays are arranged to look into the bus. luring

norrnal

load eonditions at least one feed.er will ca^rry outgoing current so that
the blocking relay will nornally be energised. and there will not be contact
race to prevent tripping on an external fauIt.

relay can be provided for this

A tine deIay in the trip

purpose.

PHASE COMPARISON SCHEME

'.4
Fig. 4 shows a simplified single
3.

phase arrangernent using

high

speed.

relays

fault conditions the prinary fault currents are

in phaser but the cr:nent transforrner secondarJr cuxrents are out of phase.
A and

Under external

Soth relays A and B operate, no tripping

occrgs.

Ilnd.er

internal fault

conditionsr the primary currents are out of phase but the secondary cunents
are in phase. Relay A operates in the positive half cycle of the cunent
wavefbr"m

whilst relay 3 operates in the negative half cycle"

4.

1.5

FRII'IE LEAKAGE PROTECTTON

lhis is a sinple

and econonical

fom of

busba^r

protection which is ideal for

the protection of phase seg?egated. ind.oor metalclad. switchgear

where earth
fault protection only is reqr:-ired.. The main basic requirenent

is that

frarne

of the switchgear

mrrst be insulated from

the tn:e earth and

the

between

sections of the switchboard. This provision of insuration

between switchboard
sections is the main disadvantage of this forr of protection plus
the fact
that it is not possible to d.iscriminate between faults on tr*o
sets of busbars
rwrning through

).5.1

cornmon

switchgear fra.meso

Principle of Operation.

Refer to Fig. 5.

principle of operation of a frame leakage schene is based

on the fact
that any breakd'own of the witchgear lnsulation will raise the potential

T?re

the frame to earth

of

a eunent to flow in the connection between the

fra'me bonding bar and earth. A current transformer
connected. between the
and' cause

bonding bar and earth

operate a protective
CAG12

will therefore

re1ay.

measure thi-s earth

An instantarreous

fault current and.

cunent relay such as type

is sufficient for this application.

current transformer ratio used is not critical provid.ed the

fault setting can be obtained.
The

necessarJr

3.5.2
The switehgear nust be insurated. as a whore, usually
by stantling

concrete, ta-king care that the found.ation bolts

reinforcenent.
connections

that:-

No

d.o

it

on

not touch any steel

other earth connections of any Wpe including incid.ental

to structural steelwork should be present. This is to

ensure

5.

(i)

The

(ii)

No spurious

effective setting of the relay is not raised. by any path shwting

the principal earth connection and cr:*ent transformer,

tripplng will take place for an external earth fault

with current flowing into or out of the switchgear fra.&e.

insulation achieved should be greater than 10 ohns to ensr:re stability

under external fault cond.itions. This is illustrated
by considering Fig.
The

which shows the cuFent distribution for an external

earth fault. The fault
current splits between the switchgear frarne to earth insulation
resistance
and the resistance

of the earthing electrod.e. Since the latter has a value

is norrnarly less than 1 ohn the cu:rent 11 seen by the relay wirl
be
approximately 1CIrt of the total fault cunent. The relay
setting should be
greater than 1CI/o of the maxinun earth fault current to
achieve stability
for external- faults and should. be 1g1/o of the mininr:m earth fault
which

cr:rrent to

ensu.re

fast operation for busbar faults.

0n resistance earthed. systems r*here the earth

fault current is fairly

constant

there is no problern. However on soridly earthed. high vortage

systems the
difference between mininrrn and maxinun ea^rth faurt levers may

be considerable

and

this

may prevent

the use of fra.ne leakage protection

scheme unless

specially high insulation resistance is provided.

A11 cable glands nust be

insulated to prevent circuration of spurior:s cg:rent

produced by higb voltages induced in the cable
sheaths under through fault
conditions causing flashover between grand and srritchgear fr4ne.
on resistance earthed systems it is reconnended to use
a eonmon earthing
electrode for both the power source neutral and the switchgear frame.
rf
separate electrodes are used an internal fault cunent has to
flow thror:gh
both eleetrodes in series. rf either or both are of high resistance
or
inadequate current carrying capacity the fault cunent nay
be linited to
below the relay setting.

6.

If the electrode earthing of the switehgear fra^me is the offender the potential
of the fra.me may be raised to a d.angerous value as a1l the fault curent will
flow through the frane-to-earth insulation resistance.

Fig. 5 shows the preferred arrangenent with the earthing connection from the
switchgear frame rnade between the bottorn of the earthing resistor and the
earthing electrode.

t.r.t

Types

of

Fra.ne Leaka.ee Schelnes.

1.5,1.1 SinEle Susbar with Insulation Saniers on Both

Sides

of 3us Section Circult

The seheme is shown

Sreaker-

in Fig. 5. In this

case sepaxate zones

of protection

are fo::med with complete discrinination between them. Faults on the

on either sid.e

of the bus section brealer outside the bus section

in tripping of the bus section

brealcer and a1I

zones

zone

result

circuit breakers in the fJtea

Zono Faults on the bus section zone result in tripping of all circuit
brea.kers.

1.5.1.2 SinEIe Busbar with Insulation Sarrier

on
a

In this

agangement the bus section brealcer i.s insulated. on one side only

as shor.rn in Fig. 7. There will be a blind spot between the br:.s section
breaker a3d. the insulation bagier.

tr'aults on zone 1 or zone 2 will trip

the bus section breaker and all breakers connected. to the busbaxs in the

faulted Zoneo To cover the blind spot a sequential tripping circuit is

used whieh

is

arranged.

to trip circuit breakers in the non-faulted

if the protection in the faulted

zone

zone including the bus section breaker

remains operated. A time setting 1n the order

of 0.4

seconds

is

used. to

allow for the breakers in the faulted. zone to trip and the protection in

that

zone

to reset (for a genuine fault in that zone) before initiating

tripping of the other

zonse

7.

It is essential that an earthed. source of supply is

connected.

to the side of

the busbar not containing the bus section breaker.

1.5.1.1 Double Susbar.

rt is extremery diffieult to obtain d.iserimination between the two busbars

due to the practical difficurty in insuLating between them and
the fact that
the circuit breakers thenselves must also be includ.ed in the zones of
protection.
The schene shown

in Fig.

illustrates the various

zones arranged.

to trip

all circuit breakers connected to the faulty section of the nain busbars and
al1 breakers connected to the reserve busbars.
3.5.4

Check Feature.

The main

objection to the frame leaka6e protection

scherne

is the faet that

the discrininating relays in the various zones of protection will operate

whenever the cunent in the current transforrner is above their
effective

setting irrespective of whether it is due to a genuine busbar fault or

faults in the secondarSr wiring.

this diffieulty it is conmon practice to add a cheek featr:re

as a second line of d.efence. This takes the fo:m of another ind.epend.ently
operated' relay to detect earth faults. This relay is non-discrj.ni.natory
To overcorne

and operates

for both internal

and external

faul-ts. Both the eheck relay

diserininating relay m'st operate before tripping can occur.

The various nethod.s of obtaining a check feat're &Te !(i)
Neutral cheek provided by a relay energised. from a single cugent
transforner in the pouer systen neutral.
(ii)
Residual check proirid.ed by a relay energised fron residually
and

connected cr:rrent transforuers on the incoming

busbarso

circuits to

the

8.

(:.ii)

Residr:ar vortage check provided. by a voltage

reray energised from
an open delta voltage transforner supply.

cheek relays should' be

self reset to eliminate the need to manuarly reset

the relay after each external fault.
Fig. 9 shows a typical tripping and alarn circuit
for a frame leakage
protection schene with a eheck feature.
1.5

Tlris is a r:nit type protective scheme in which

cunents entering an. reaving
the busbar installation a-re compared continuously.
The object is to provide
fast operation at a 1ov fault setting on interraal faults
and. yet retain
stability up to the highest possible value of short circu:it
current on
through

faults'

cr:rrent transfonners on each of the busbar

circuits are
eonnected in paraller whlch will produee
a resultant cr:nent to operate a

relay for internal busbar faults onry. Theoreticarl_y

such a systen is
unaffected by through faults, but in practice
the associated. cu*ent
transfo::ners may not behave ideally when the
cu:rent exceed.s a certain vaIue.
Errors in transfo::station dr.rs to saturation of the
current transfo::mer cores
nay be sufflcient to cause rnaloperation if special preeautions
are not taken.
1

.5.1

consider Fig. 10. under external fault conditions

the current in relay R
should theoretically be zQToo However, if one
of the cr:nent transfo:mers
becones fu11y saturated' d'ue to high flux
in its core, its
secondanlr

becones zero and

this

can be represented by a short

e.n.f.

circuit acrosg its

magnetising impedance. This is the worst condition
for stability of the
relay and the high inped.ance principle is used to
ensure that the relay
circuit inped.anee is sufficientry high to prevent
its operation.

9.

Assuming

that CT rXf

becomes

ful1y saturated and ignoring the nagnetising

culrent in CT rYr r the secondarJr current Iy will split up betveen ttre relay

circuit

and the saturated current

transforrner. The relay circuit

inpedance

is adjusted so that the current flowing through the relay is less than its
cunent setting. The necessary inpedance can be calculated with a slight
safety nargin by assuming the cu:rent Iy flows th:ough the saturated. cunent
transformer only. This will develop a voltage Vj given by

vp = Iy
Ttre

relay circuit

(Rcr

* Ru)

irnped.ance

is then adjusted so that the

necessary voltage

to operate the relay is greater than the voltage Vp. This voltage
the setting voltage is given by

Yg =
fhus

where

rnR

Vg ca11ed

Ip = relay current setting.

R = relay circuit inp,edance.

Vg

i.er

iRR
.t.

> Iy (Rcr + RpE)

-1R

In order to obtain the required. value of R it is usually

an additional

relay coil

.'.

resistor called stabilising resistor

Rg1

necessary

to

use

in series with the

Rp.

Required value

of stabilising resistor :

Rst = R-RR
fault at tr' as shown in Fig. 11 the current transfolsners
will attenpt to transfom the fu1I fault cur'rent and. pass this through
the relay circuit. The voltage output Vp fron ttre current transformers
required to pass this cunent through the relay will be given by:For an internal

Vp =

fF.R

= fr'.a (na* + nr;)

fp

10.

relative values of relay setting fp and fault cunents

rp and ry the value of vp can be many KV. rt is not possibre for any

Depending on the

practical current transforners to develop such a high value an6 severe

saturation will oecu!. The saturated output consists of spikes of very
high voltage around the points of zero f1ux. To enable fast relay oper-

ation the curent transformer should have a knee-point voltage equal to at

least twice the relay voltage setting Vg.

1.5.2 Fau1t SettinEs.

Fig.

again. With a given relay current setting fp the overall

fault setting of the scheme is higher than Ip due to the magnetising cugent
Consider

11

taken by the curent transformers. This overall fault setting or the

effective setting Ig (referred. to cu:rent transformer secondary

amperes)-

is given by :

rs=rP+Zrm

vhere I,

is the magnetising cr:rrent

taken by one current transforroer

at the setting voltage Vg.

0r in terms of prinary cr:rrent the effective prinarJr current setting fp is
given by

rp

where

r =

= l:,1- + 2rm)
For a busbar installation with n circuits

transforner c'*ent

;"::

rp = r(rn+nr6).
In addition to the relay there may be a voltage limiting d.evice, a fault
setting resistor and supervision relay connected across the relay circuit.
These

will be discussed in later sections. Therefore, in generar,

effective prima:ry cur-rent setti.ng can be expressed as

fp = t (In + nI, + Iy + fSR + Iv)

the

11.

where r14

= currrent taken by voltage limiting

at Vg volts.

device

rsn = current taken by faurt setting resistor at

Vg volts.
Iv = cunent taken by supervi.sion relay at Vg volts.
The value of rp shoul-d' be in the order of
tq" of minimr:m fault cu*ent
available' This is to ensure sufficient fault cunent frowing
through
the relay under internal fault eonditions for high
speed. operation.
3.5.1

Throueh Fau1t

Stabilitv.

stabilj'ty lini't of a busbar protection scheme is based on the

naximun
through fault curent. rn general this takes the
value of the associated.

The

switchgear ratj-ng irrespective

of the existing or anticipated fauLt 1eve1s.

As shol'n previowly the stability limit is governed.
by the relay clrcuit
setting voltage. This must not be less than the stability
voltage of
the

systen, which is caleulated by assuming that the maxinr:rn through

fault
current flows in through one cilrrent transforrner and out
through a second.
onet the latter being assumed to be the most remote (in
terrns of secondarxr
Lead resistance) from the

further

asswaed

relay associated with the zone concerned. rt is

that the d.c. conponent of the offset prina:ry fault c'rrent

completely saturates the second. cr:nent transforraer whirst

the
continues to transforn perfeetly.

first

one

1.5.4 Fault SettinE Resistors.

to increase the effective primary fault setting by creating
a shunt resistance across the relay circuit as shown in Fig. ,rz,
They are
These are used

useful where a standard relay with a given setting

for arl the

busbar installations to achieve a given primary faurt
setting throughout.
i_s used.

12.

1.5.5

Cheek Feature

A second

line of

defence

is considered

good

practice in most

schemes

busbar protection, not

of

to give security against maroperation of the

prirnarSr protection d'ue to inherent
defects but to prevent incorrect tripping
as a resurt of damage to wiring and equipment
from
extra'eous sourees.

A check feature

is

provid.ed by duph_cation

of the primary protection using

a second set of curzent transforners on all circuits
other than bus section
and coupler units. The check systern is
ananged in a sinilar
manner to

the prinary protection but fozns one zone

only covering the whole of the
busbars and does not discriminate between
faults in the various

of the

sections

busbars.

under i'n-zone

faurt eonditi.ons, the high impedance relay circuit

constitutes
aJl excessive burden to the eu:rent transformers,
leading to the developnent
of a high voltage the waveform of which will be highly
distorted with a peak
value many tines the nominal saturation voltage.
AD approximate forrnula
based on experimental results conmonly used
for checking the magnitude
the

of

voltages vp deveroped by a current transformer

und.er internal
fault eonditions is gi.ven by :
peal<

Yp = 2rmvFEf

where

v6 =

eurrent transfor:mer
knee-point r.m.s.
voltage.

Vp =

maximurn ?orDrso

voltage

that would be produced

if the cunent transforser

did not saturate.

13.

This fornula does not hold for the open eircuit

condition and is inaccurate
for very high burd'en resi'stance values that approximate

ft only appl_ies for values of

to an open circuit,

V6 less than

v.F. ft should therefore be

used
2

as a guide to the possible peak voltage.

rf there are a number of current transformess in parallel

the peak voltage
vp as caleulated above for a single transforuer
wi]] be reduced by the shunt
conductance of the other current transformers.
The

insulation of the cu:=ent transformel

second.ary winding and

reray will

not be abre to withstand the very high voltages

that can be produced..
where neeessary the voltage is rinited
to less than ]kv peak by the use of
non-linear resistors caLled metrosirs eonnected
in paraller with the relay

circuit as shown in Fig.

The voltage/current

v = crp

1J.

characteristic of a netrosir is given by :

where the voltage

and current

are

peak values.

I(rms) = o.52I
C=

a constant depending on the

metrosi.l construction i.e. the

size and

number

of individ.ual

dises used in the metrosil

and.

whether connected. in series or

para11e1.

F = a constant in the range O.2 to

O.25.

14.

The values

of c

F are chosen so that the voltage across the metrosil is

linited to less than lkv peak at the maximr:n fault current. The value of c
and

nust al'so be sufficiently high to restriet the current tal<en by the netrosil
at the relay setting voltage Vg so as not to ad.versely affect the primary

fault setti.ng. Acceptable metrosil cuments are approxinately JgmA for use
with 1A current transforrners and. 1OOrnA for use with 5A cu:rent transfomers.
An approximate value

caLculated from

,fr

vs

of r(rms) at relay setting voltage vg(rms) can be

= c ((r(*");F
0.52
)

Suitable metrosils are chosen

(i)
(ii)

based.

on

relay setting voltage.

rated. cunent transformer second.arxr current

i.e. required prirnary-

cunent setting.

(:-ii)

rnaxirm:m

J.6.7
Open

eurrent transfo:rner

second.arxr

current under fault eonditions.

Circuited Current Transformers and Wiring

a cur=ent transformet secondary wind.ing or corueections between current

transfor:ners and the relay cireuit become open circuited, the resultalt

when

out-of-balanee current will flow through the paralleI combination of relay,

metrosilr fault setting resistor and cu:rent transfoxmer magnetising

inpedance. This nay cause the proteetion to operate for load. or through
fault conditions dependi.ng on the effective primary setting.
The

condition of an open circuit can be d.etected. by neasr.rring the voltage

across the relay circuit by a sensitive voltage-operated. relay as shown in

Fig. 14. This eelay is set to operate when the out-of-balance cument
equals about

2!

1U/o

of the least loaded feeder

amperes whichever

is the greater.

conneeted.

to the busbars or

'15.

rf accr:rate details of current transformer nagnetising

characteristics
available, the required setting ean be calculated..
Checks
should be

on

site to

ensuee

that the relay will not operate due to

are

done

normar unbalance

with the system and protection healthy.

operation of the supervision relay is arranged
to give an ala* that the
busbar protection is faulty and to short
circuit the buswires if this is
neeessarJr to prevent danage to the protective
relay and stabirising resistors.
when the busbar protection has a

faurt setting

fulr road of the

connected feeders it is very 1ike1y to
operate .ue to an open circuit
current transforrner' rn this case a check feature
is required to prevent
tripping. At the sarae tine it is inportant that
below

the buswires are short

circuited via the supervisi-on relay to prevent

therrnar damage to
protecti've relay and' stabilising resistors
which woul-d othe::wise

the

renain
continuously picked up und.er road conditions.
The supervision relay must have a tine
delay to prevent its operation due
to genui.ne busbar faults. A time delay of about
J seeonds is used.
1.5.8
3.5.9.1

inportant advantage of wing high impedance relay

in a circurating
current systen is the ability to predict the protective
An

scheme performance

in terus of prirnary fault setting

fault stability by calculation

without heary-cuEent eonjunctive tests. The
valid.ity of the eal-culation
is based on the assumpti-on that all the current
transforuers Lre of lowreactance type' A 1ow-reactance cunent transformer
is d.efined as one of
which a lcror+Iedge of the seconda:qr exciting
eu*ent, secondarxr winding
resi'stance and turns ratio is sufficient for
an assessnent of its performance.
and through

This covers current transformers with uniformly

distributed windings or
whose core leakage flux is negligiblq.

16.

1.6.8.2

l{1th hi-gh inpedance circulating current schemes, it is

of the utmost
importance that the l-ead. burdens between the various sets
of cu*ent
transformer be kept as low as possibre in order to obtain
the required

stability

sensi-ti-vity. rt is therefore ad.visable to run the buswires

in the forrn of a closed' ring between all the cireuit breaker control
cabinets.
and

This avoids the need for numerous radial loops between the
current transfo'mers
and the bus zone panel which would be required if the buswires
were formed in
the bus zone panel.
A closed

ring consisting of cores in multicore cables affords

increased

security against maloperation which may result from unbalancing

of the
protection due to inadvertent d.isconnection of bus
wires. rt also provid_es
easy extensicn of the protection when new circuits
are to be connected into
the protection zone.

of rwrning a nulti-core cable ring in the

arrangeinent is as fol1ows :
(i)
current transforrners to marshalling kiosk.
An example

(ii)

marshal-1ing

case

of a double busbar

kiosk to ar.rxiliary switches in the busbar serector

isolators.

(iii.)
The

loop between marshalling kiosks.

size of conductor

Howevet,

for the interconnecting pilots is 2.5rr2.

it is occasionally necessanxr to use para11e1 cores to reduce the
no::ma11y used

bu.rd.en.

j.6.9.1

rn a lot of

cases such as a d.ouble bus amangement where on-load

transfer of

a circuit is posslble, current transfonner outputs are switched. to

the
correct buswires by means of auxili-ary switches on the selecting
isolators.
These auxiliary srvitches should close before
the mai-n isolator closes and
should' open

after the main isolator

operati.on. This is

shorvn

in Fig.

opens

15.

to

ensure

stability during sr+itching

18.

The reserve busbar

is then included within the

feed.er

protection.

The

discrininating zone current transforrners on the bus coupler for the reserve
bars are no longer required and are used to replace the check zone current
transfor^mers on the

line circuit breaker.

additional cunent

Sometimes an

transforner may be provided on the bus coupler specifically for this pgrpose,

in which case the reserve bar discrirninating eurrent transformers

can be

short circuited. during bypass cond"itions.

1.5.8,) Current Transformer Locatj,on.

The three

alternative arrangements as shown in Fig. 18 are :

curent transfo:mers for feeder and busbar protection overlapping
the circuit breaker.

(i)

(u)
(

ii.i

(i)

all current transformers on rine side of circuit breaker.

alr cument transforiners on the busbar sid.e of circuit breaker.
rn this

arrangerrent

faults at F1 and F2 are cleared correctly by

the busbar and feeder protection respectively. Faults at

between the

forrners

F3

circuit breaker and feeder protection current trans-

will be creared by the busbar protection

by the remote end. of the feeder protection. No

and possibly also

'nneeessary

disruption to loads will result from this.

Faults at F4 will be seen by the feeder protection but also by the
busbar protection resurting in unnecessarJr tripping of the br:sbars

for what is essentially a feeder fault.

of this anrangement.
(ii)

This is the uost

This is the nain disad.vantage

cortrnon amangement where

all the current trassformers

are on the feeder side of the circuit breaker. However, there is

a blind spot at point F, where faults are seen by busbar protection

but not seen by the feed.er proteetion. With this arrangernent it is

therefore required to intertrip the renote circuit breaker when busbar
protection operates.

19.

fntertripping

can be achieved, by

and ean be instantaneous

unstabilising the feeder protection

or tirne delayed to

a1low clearance of

faults on the busbar sid.e of the circuit breaker before intertripping.

Alternatively an interlocked. overcurrent relay can be used. to intertrip
the renote circuit breaker. fhis relay in the form of a polyphase
induction disc is interlocked. wittr the br:sbar protection by means

of a shading
operates

(iii)

\,r,hen

wind.ing which

when the busbar protection

all the current transformexs are located on the busbar

of the circuit
and

is closed

brealcer a

sid.e

fault al T1 betveen the current trarrsformers

circuit breaker will continue to

be fed.

fron the busbars after

the circuit breaker has been tripped by the feed.er protection.

4n

interlocked overculrent relay whieh is interlocked with the feeder

protection is required to ensure that the busbars are only tripped

for this condition

and not

for faults on the feeder.

1.6.9 Typi.cal Susbar Protection Using High

I_grpedance

Circulating Cr:gent

Scheme

Figure 1! shows the current transfoxmer cireuits for a typical busbar

station layout comprising

breakers.

one bus section and two bus coupler

The busbar arrangment enables three zones

cireuit

of protection to

be obtained, and the cr:rrent transfoxme?s a"re connected

to provide over-

lapping at the bus section and bus coupler circuit breakers.

Discrininating Featureo
Three cunent transformers axe
and also on both sides
The

of the

fitted on all incoming and outgoing circuits

br:s section and bus coupler

circuit breakers.

star points of all the current transformers are connected. to a'buswire

which

is

earthed.

via a removable 1ink.

The other terminals

of the current

transformers in the same zone are connected to three more buswires, all

current transfo::mers in the

same phase bei.ng connected

thus giving a set of three busvires per zone and a

for all the three

zoneso

to the

eommon

sarne buswires,

neutral buswire

20.

One

triple pole re1ayl type

(device 8l) and stabilising resistor is

connected across the busr,rires to give phase and earth fault protection.
CAGJ4

rf a fault occrus outsid'e the protected. zone, the currents entering

an6

leaving the zone are equal and the current transfor:mers affected. will
circulate cu:=ent through the buswires. The schene is so designed. that
the voltage necessary to operate the relay is greater than the voltage
across the buswires under maxj.mum through fault cond"itlons, so that
the
rel-ay

will not operate

'nder such circumsta'ces.

rf a fault oceurs insid"e the protected. zone, the balance of seeond.arxr cu:rent
will be disturbed and the relay will operate.
fiscrininative tripping is obtained by the introd.uction of isoLator auxiliary
switches into the current transforrner circuits.

The auxiliaqy switch contaets

are silver plated, with two switches in paraller per phase, in order
to
mj-n:inise the possibility of high arr:riIiary switeh contact
resistance.
Check Feature.

This is simil-ar in operating principle to the d.iscrfuninating feature,

but
no current transfolsners are fitted. on the bus section and bus coupler

circuits,

and the cornplete busbar

installation is considered as one overaLl

zorlr The crrFent transfonners are fitted on all i.nconing and outgoing
circuits and' again all the current transfo::ners in each phase are paralleled
onto the buswires.
A triple pole relayr type cAci4 (device BJ) and stabilising resistor
is
connected across the buswires.
Continuous Buswire Supervision.

possibirity of faults in the cerent transfo:rners

wiring and interconnecting pi.lots a static relay type VTX

To guard against the

secondar;r

(device 95) is connected. across the busbar protection zone

busr.rires.

C1
Lta

The relay has a voltage

setting ad.justable

between

2 and 1d vo1ts, and an

inherent time delay of J seconds. Operation of the llr'IX relay

alarn

and.

sound.s an

takes the affected zone out of service by shorting the appropriate

buswires.
The

following faults are covered

Open

l+

Broken eurrent transfor.ner

Crossed

eircu-ited current transformers.

pilots.

current transfo:rner pilots.

Tripping Circuj-ts.

tripping circuits are arranged. so that the accidental operation of any

one of the circulating current relays (d.evice 8l) does not cause inad.vertent

The

tripping of a group of cireuit breaters. Both the rDiscrirainatingr

rCheckf high imped^ance voltage

and

differential relays must be energised before

the respective nain tripping relays (deviee )6) arc energised. one ma-in
tripping relay is required for each feed.er circuit breakerr altd two such
relays are required. for each bus section and each bus coupler circuit breaker.
This method presents several advantages

Spare contacts are always available on the arr:riliary

can be used when required

The zone

of protection

tripping relays,

and.

for intertripping duties.

rnay be

easily extended to cover additional circuit

breakets.

The

front

appearance

of the busbar protection panel is not

dependent on

the nwrber of circuit breakers in the zone.

Alarm Circuits.

Audible and visual alarrns are given under the following cond.itions

Busbar

Busbar protecti.on faulf,y.

fauIt.

tt Battery voltage

low.

22.

The r3usbar Proteetion Faultyr alarm

is

time delayed

to prevent

such an alarrn

being given und.er busbar fault condltions.

Figure 20 shows a WpicaL arrangement of the d.c. tri.ppingr alarrn and

indicating circr:-its.

1.6.10

Busbar protection Using Separate

Relavs &rg Each Circlrit Brea]<er.

Differential

J.6.10.1 laslg_Sgbgg.

Fig. 21 shows the basic arrangement. This

for

use where individ.ual

ation j.n U.K.

on a

is particularly suitable

relay roons are used on the 400kV busbar install-

Each room acconmodates both

per-circuit basis.

scheme

Each

the feeder and busbar protection

circuit breaker has its

own

individual

discrirninating and check high impedance relays, coincident operati.on of

which

trips the breaker.

battery. 0n this

Each breaker also has

basi.s buswire selection

its

onn individ.ual

is limited to the a.c. circuits

with no isolator switches in the d.c. trip circuits.

This will provide

greater secr:rity against mal-tripping and greater reliability

Another axrangement shown

in Fig.

trippihg

22 uses two

tripping routes for

circuit breaker. Direct tripping from the high

with each circuit breaker ls via a discrete

of tripping.

inpedance relays associated

busbax

protection trip relay.

The other route uses separate contacts on the individr:a1

check and dj.scrininating high-imped.ance relays

each

circult

breaker

to energise a second trip

relay associated with each circuit-breaker. fn this case the separate

contacts on all the high-impedance check relays ate connected in parallelt

the combination of which is in series with a single contact on the individual

high-impedance discriminating

relay associated. with each circuit breaker.

a7
-.)o

1.6.10.2 Basie Scheme with Ad.d.itional DiscriminatinE

and
a

Fig. 2) shows the

anangement

in a simplified forn.

increase the tripping reliability"

The additional relays

A sectionalised back tripping systern

with check and discrininating back tripping bus wj-res (each double pole
switched.) is incorporated which operates onto the ind.ivid.uaL busbar

protection trip relays. Operatj.on of the zone check and discrirninating

relays energise the back trip receive relay through the back trip buswires
and busbar selector

isolator aiuiliary switches.

TLre

receive relays in

turn pick up the busbar protection trip relay to trip the breaker.
I'urther reliability
back

of the tripping circuits is obtained by using

trip receive trip relays as

shor,m

in Fig. 24o This

provid.es

separate

virtual

dupli.cation of the fault detection and tripping functions apart fron curent
transformers and associated bus wiring"

1.5.10.1 Cheek Zone Sectionalisation

Provision of a single check zone covering all the br:sbars has the advantage

of relative simplicity

and economy.

At large busban stations having a

large mrmber of circuits it nay be necessary to depart from this

due

approach

to the difficulty i.n obtaining a suitable primary fault setting. fn

these cases sectionalised check zones are

greater security and flexibility

used. This provision leads to

during construction, naJ.ntenance

and

cornnissionlng. It requi.res additional cunent transforrners at the sectioning

points and separate high

impedance

relays per check

zonr

I+

bCTERNAL

trAU UT

rNTER-NAL EAUr-l

FIC,

BASTC CIRCULATING CURR.ENT STHLME

L5

BIAS

COILS

OPERATING COIL

EXTERNAL FAULT

INTERNAL FAULT

BIAS CURRENT CO|L 'o'

OPERATING CURRENT

BIAS CURRENT COIL.b,

FIGURE

LOW IMPEDANCE BIASED DIFFERENTIAL SCHEME

LXTLRNAL trhULT
NEGATIVE HALF

POSITIVE HETT CYCLE

CYCLE

t-

TRAN
SEEONDAR\ C\'RR.ENT

CTJRRENT

KELNY OPEBATI3I.I
FE-E.DE.R

FTeDER

INTE.RNAL

-(

FA.ULT

POqITIVE HALF CYCLE

<-<--

l
|

cuRRetr

TRANSFOF.t,tER

SEcON!ARy

A
FlG.4

CTTR.F.ENT

R.ELAY oPERATI0N

PHA:E coMPARi tON SCHEME

trEEXER

f-9

1r =I,+12
ar+Iz

swffc.HGEAg F('nmf

---T

-t

GErJ6(A'ro(

_l-

-J
gY9{6r,1 AGrH|rJG

BoIJDTNG 8AR.

RgsrsrAxcE
REr-AY

EACTH

I,+12

8AR

EAP:nrrrJG

tttict|aD

(sr3rAPeE

FrG.5. OPERATTNG

PRINCIPLE OF FRAME LEAKAGE PROTTCIIQN

lr.lsutAfioN

?*:t

Z.ONE Z

BePPR

Zot'tE 3

-t

-l
I

I
I

jl-

L/1

t_

-s/t-

l'
i----r-l
TRIP

t.

I
I

h'
t?rp'B'

FIG.6 FRAME LEAKAGE 5CHEMI \\IITH DOUBLE

INSULATION BARRIER

2ct

INSULATION BARRIER

ZONE

r---

- -1

0.2-1SEC.

-rTRtP

t,

FIGURE 7
FRAME - LEAKAGE SCHEME

INSULATION BARR IER

WITH SINGLE

fo

/,vSazaVaat B.sfPeR

Za*s./

ZavaZ

Z.va J
I

t'
I

'T

l
I
I

---

-d

./

.xF Y'
I

Zz

,11

(,

!. + )/L
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c(L\

l_
:

lG.8

f.<'fiyt - / aar4rr ^trfrZt

/6r< 4>a',Qz{'78

ta; @s

3t
IN

lr

ouT

Etod
I css'zt
I

r* olrr

rl

6+22-l

E:4
t t css-zz

TPtp

SUPPLY

SuPlvtSro^/

64Cl{-Z

AIRn

64zl_2

?alv

61a2-Z

74-

td

LAHP

OIJT

tu"zl

Arierr
Sv?6

FlG.g. TYPICAL TRIP

AN

ALARM

suPft.Y

tvl 6 rotJ

CIRCUI@

WITH DOUBLE INSULATION BARRIER ANb

CI{ECK

FEATURE

LTAl(AG

)4.

Q.et

Rr-v

Rsr
i

t
I

le

)*.

VR =

rv (R.r*Prr)

Vs>Ve

Ia R > .Ty (.x* R"r)

R > rv (Rr.x+(cr)

Ig

Psr =

R- Re

F'IG TO PRINCIPLE OF HIGH

IMPEDANCE

PROTECTION

3f

7,n

utl

I
I

FIG.1I INTE-RI{AL FAULT Ot.\ !\IGH MCI\EME

Vr=It'R
(R.. * R'*)
= r,' - !L
Is

E{(e.ct.\v<. Setti^q

Is=I(r2lm
-- Ietn I^

tJ'rL\

'^rit\

Cr r c,.l'i

tS

c-irtr"\tt

?r\'^o*., eftecL\,.re sett\^1

ip

or

: T'(r*+nI*)

tp =

T.(I**oI^+Im

i.* + lu)

c(-

lop = r** lae

FTG-12 USE Dtr FAULT SETTTNG RESISToR

35

t
I

ia
i
I

Ftc -13
t3

Met"osiI c\'ovoctertstic

l'

!:

V=

fiV*

,t/Z Vs
rr.l
o-s2

^FVs =

[s]F

16

- -T
_rl

{r4t',Crt

7*,

r-

)ll
)l

_l

t-

-j

ri

Jr-

cTr

6Ta-

1r

I,:L.+1,+I.
I.IEALTHY CD}.T}\T TA}J

vlt
i

CTr OPEN CIRCU lT , Lr FLo$rs THRDUGH MAGNET|STUG

IuFgUnUCE AN} g"EL$Y CIRIUIT IN PARALLLL
trIC.I4

SUFER.VISION AGAINST OPEN

a --

Voltac,.

m"o's.rrel

by

C\RCUITE} C.T..g

supe.rvisicn te,la1

y : l, (R ll zma ll zH3 il4.,o)

lC supervision

ra\a.1

seLl',*..

Crt-o{-bo.lo.,.ce. cr,rrrc-nt-

to

V.o

opc-vate

t\'

*9
l:s
L: )4'"
?-r'q7
4,
{
vsP

supetvls't6n Ta\ai

97

NORMAL

CIPER

ATIhIG

.(-oNAr-r,r qN

i\

.-- + \
f4ain s< reserve

zon-g

ultinate/y pr"lleled by
fsefvg b.rsbar se.\ectot

a*rihory

switches

ON-LoN} TRANSFER

FIC.15 BUSBAR SELECTOR A\-n.(ILIARY SWITCHES RECUIREMETIT

38

.<
4

{J
*

2
A.c

6ursx-=S

C/t/

FIG.15. CIRCUIT BREAKTR

BYPA5S.

/7

2
A.C.

tl/

EttswrRS

<//

FIG.IT.. CIRCUIT BREAIR, AND C.T.

BYPASS.

CreorrT

??o-tgiost

Co) oV&APPTNG c.-rs.

&lsgA

Pf.reer,o^t.

Crerrrf

e?org"",ot

(b) ArL c.ts ov t-r^/ sr0e

oF c'Perrff 6CsAEEl?
.

Sr.rs8t{?
{Xor6ctrroU
rrvrnLoc,1gFb

ove!uClN?

Qgtlv.

$WEir.D

orrtlerArrr
es.A\,
ClectrtT

(C). Al,t

c.Tj

oC G,lrsgAe

Eo'recrro{
gurBAe

P"lSeTorr,

FIG.IB IFFECT OF

C.T. LOCATION

srbE

oF crlevrT &f;eAglQ,

ON BUSBAR PRO]ECTION

PIRFORI4ANCE

+o

t1

IE
u3
6q
N;
aarJasr.r=<a(,e
:Y

1g
r-

a<(J
(r!

ff

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sg
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2
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uE
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;3i
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!
o
a
a

3s

fJ

vrc

-le
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o
N=

!
a=

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-3
tg
N;

:E
z=

RE
xB
(,

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C)o

3g

sV,

(
l

v,

I
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-lE u,

U
2tn

ra

(,
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5?
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i;3

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Qr

MlM2N
87CH-1

css-r'f2
ll

a7e-1

ll

al

css-R

o.c. BuswtREs
87CH-2

87M1

ssMf
93M2-1
95R-

-2

-t
1

95CH-l
30Mr

-'r

30[v''|-1

74-l
30R-l

95Mlx-1

EUZZER

93M2X-t
95R.X-1

95CnX-1

l3

30

Zonc indrcating rcl.y typ VAA

74

Alarm cancrllarion rclay rypc VAK

80

D.c. vohs rupcrvision rclay rypc VAX

87

95

95X Zooc bus wirec ahorting rctay rypc VAJHI3

l2

High impcdencc circularing currcnt relay rype CAG 34

8ur wircl supcrvision rclay rypc VTX 3l

FlG.20. DOUBLE BU5BAR scnEne.orve

CSS Conrrol schcror :wirch

tl
L2

lndicering lemp protcction in scrvic:

lndicating larnp protecrion out ol servicc

RerRv pEe.zorue .

o.c.crrcurrs.

+1_

h_-.
e*js-r
1i

z_--aT o-G)
s---olo-{F)

Eil

lrn

IY

tl @

@*r

;r @

@;r

6,

FIG.ZI

2o

BUSBAR. PROTECTION WITH SEPARATE RELAYS PEF.

CIRCUIT BITEAKtR

z--rre

\---o..F
io-rO
I

:IHE!I3

-:.\::i

ry-r
I

o-*is
a?

.?

f+-f

?
I

5s__ _ _

ol

Flc.2?
TRIPPI.NG ROUTE

gr@
.l

,1

tr
t

.J,

v3

OI

bl

,J
FlG

. 23

crRcU_!_a_F

fto- .1-

v-olo_,
I

-l:

@-*.].H
2t

ry

lr

i'

A,

FtG .24

O-- r-:r

CIRCUIT BREAKER .BAsIc sc}IEME WIT.H ADBITID\IAL

REunvs pgn znxE ANn.BhqK TRtpplNG rActL!.TlELJdtrH
SEPARATE TRIP R.ELAYS