109713451 busbar protection

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LESSON 34

BUSBAR PROTECTION



OUTLINE OF THE LESSON

• INTRODUCTION

• BUSBAR FAULTS

• PROTECTION REQUIREMENTS• TYPES OF PROTECTION SYSTEMS

–FRAME EARTH PROTECTION

–DIFFERENTIAL

PROTECTION



INTRODUCTION

• The protection scheme for a

power system should cover thewhole system against all

probable type s of fault.



• Unrestricted forms of line protection

such as overcurrent and distance

systems,meet this requirement,

although faults in the Busbar zone

are cleared only after some time

delay.



• If unit protection is applied to

feeder and plant the busbars are

not inherently protectected



• Busbars have been left without

specific protection for one or more of

the following reasons:

– The busbars and switchgear have high

degree of reliability ,to the point of

being regarded as intrinsically safe.



• It was feared that accidental

operation of busbar protection might

cause wide spread dislocation of the

power system ,which ,if not quickly

cleared ,would cause more loss than

would the very infrequent actual busfaults



• It was hoped that system protection

or back-up protection would provide

sufficient bus protection



BUSBAR FAULTS

• Majority of bus faults involve one

phase and earth, but faults arise

from many causes and a significant

number are inter-phase clear of earth.



•

With fully phase -segregated metalclad gear ,only earth faults are

possible ,and a protective scheme

need have earth fault sensitivity only.



• For outdoor busbars , protection

schemes ability to respond to inter-

phase faults clear of earth is an

advantage



BUSBAR FAULT STATISTICS

• For a particular system in which total

faults equal to 129 , the distribution is :

– PHASE TO EARTH 87

– TWOPHASE TO EARTH 15

– THREE PHASES TO EARTH 19

– THREE PHASE 6

– UNKNOWN 2



PROTECTION EQUIREMENTS

• Although not basically different from

other circuit protections,the key

position of busbars intensifies the

emphasis put on the essential

requirements of speed and stability



SPEED

• High speed operation is required for

limiting the damage due to fault.

• Removal of bus faults in less time

than could be achieved by back –up

protection ,with the object of maintaining system stability.



• Most of the modern busbar

protection are based on low

impedance biased or high impedance

unbiased differential protectionsystems.The operating time is of the

order of one cycle .



STABILITY

• The stability of bus protection is of

paramount importance

• In order to maintain the high order of

integrity needed for bus protection,it

is an almost invariable practice to

make tripping depend on twoindependent measurements of fault

quantities .



• Security of both stability and

operation is obtained by providing

three independent discriminating

channels,the trip outlets of which areconnected in series and in pairs as

shown below:



TWO OUT OF THREE PRINCIPLE

FIG: Two out of three principle



• Three channels x,y and z from three

independent trip circuits ,x+ y,y+z,and

z+x. This means that any kind of

failure can occur in any one channel

without prejudging either stability or

ability to operate for the scheme as a

whole.



TYPES OF PROTECTION

SCHEMES

• System protection used to cover

bus bars• Frame –earth protection

• Differential protection



SYSTEM PROTECTION

• A system protection that includes

overcurrent or distance systems will

inherently give protection cover to the

busbars.



• Overcurrent protection will only be

applied to relatively simple distributionsystems,or as a back-up protection set

to give considerable time

delay.Distance protection will provide

cover with its second zone.

• In both cases,therefore ,the busbar protection so obtained is slow



FRAME-EARTH ROTECTION

•

This is purely an earth faultsystem,and ,in principle ,involves

simply measuring the fault current

flowing from the switchgear frame toearth.To this end a current

transformer is mounted on the

earthing conductor and is used toenergize a simple instantaneous

relay.



FRAME – EARTH PROTECTION



DIFFERENTIAL PROTECTION

• BASIC PRINCIPLE

–Basic Principle

–Summation scheme



BASIC PRINCIPLE

• The merz-price principle is applicable

to multi-terminal zone such as a

busbar.The principle is the directapplication of kirchhoff’s first

law.Usually the circulating current

arrangement is used.



BASIC CIRCULATING CURRENT

SCHEME

Fig Basic Circulating Current Scheme



SUMMATION SCHEME

• By using current transformers of

different ratios, for example

300/1,400/1and 500/1in each three

phase group,connected in parallel

and interconnected between groups

,by two bus wires only a response isobtained to all types of fault.



• An alternate arrangement to that

above is obtained by using current

transformers of equal ratio but cross

-connecting two of them and taking a

centre tap from the third.



RECOMMENDATIONS ON BUSBAR

PROTECTION

(Reference: Manual on PROTECTION

of GENERATOR, GENERATORTRANSFORMER, and 220 KV and

400KV NETWORKS, Publication No.

274(revised) CBIP New Delhi).



1. Busbar protection must be

provided in all new 400 kV and 200

kV substations as well as

generating station switchyards.

2. It shall be of 3-phase type and

operate selectively for each busbar section.



3. It shall operate on differential

principle and provide independent

zones of protection for each bus.

4. It shall be stable for through fault

conditions up to maximum 40KA fault

level.



5. It shall incorporate continuous

supervision for C.T. secondariesagainst any possible open circuits. In

case of detection of any open circuit of

C.T. secondaries, after a time delay, the

affected zone of protection shall be

rendered inoperative and an alarm willbe initiated



6. It shall include D.C. supply

supervision.

7. Include adequate number of high

speed tripping relays.

109713451 busbar protection

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