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 the

whole 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 bus faults

• 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 metal

clad 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 protection

systems.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 two independent

measurements of fault quantities .

• Security of both stability and

operation is obtained by providing

three independent discriminating

channels,the trip outlets of which are

connected 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 distribution

systems,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 fault system,and ,in principle ,involves simply measuring the fault current flowing from the switchgear frame to earth.To this end a current transformer is mounted on the earthing conductor and is used to energize 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 direct

application 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 is

obtained 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,

GENERATOR TRANSFORMER, 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 bus

bar 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. secondaries

against 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 will

be initiated

6. It shall include D.C. supply

supervision.

7. Include adequate number of high

speed tripping relays.