Seminer on Bus Bar Protection

Presented by:Madhusmita Baruah

Chandan Jyoti Baishya

Introduction Bus zone protection : The bus bar in electric power station and substations are

one of the most vital elements . Experts have different opinion regarding bus zone protection.

A section of them said that local bus protection should not be provided and bus faults should be cleared back up relays at neighbouring stations. According to them local bus protection would certainly increase the risk of inadvertment tripping. The other group of specialist believe that although bus bar faults are rare the cannot be totally neglected,as the damage resulting from one uncleared fault can be so dangerous that it may result upto the complete loss of station which means prolonged interruption of supply.

Although bus zone faults are rare, but bus zone protection is highly desirable in large and important stations such as grid substations. However where local bus protection is provided care should be taken by providing two independent protective circuits both of which must be satisfied before tripping can occur.

Bus bar protection needs special attention because of the following reasons:Fault level at busbar is very high.Fault on busbar would result in

widespread supply interruption.System stability is adversely affected

by fault in bus zone.

The desirable features of bus bar protection are given below:

High speed operation (less than 3 cycles) Discrimination between fault in the

protected section and fault elsewhere. Stability for external faults. Freedom from unwanted operation . No operation due to C.T. saturation or

power swings.

Various bus bar arrangement

Single bus bar arrangement. Double bus bar arrangement. One and half breaker arrangement. Ring main arrangement.

Single bus bar arrangement

This is the simplest arrangement consisting of a single set of bus bars for the full length of the switchyard. And to this set of busbars all the generators ,transformers and feeders are connected as illustrated by figure.

Double bus double breaker arrangement

Double bus double breaker arrangement:

In a very important power stations two circuit breakers are employed for each circuit. In such arrangement bus couplers are not required.

Switch over from one bus to the other can be done when ever desired without interruption.

This arrangement is very costly and maintenance cost is also high.

One and a half breaker arrangement

One and a half breaker arrangement:

This is an improvement over double bus double breaker arrangement.

This arrangement needs 3 CB s for 2 CKTs.

Any CB can be switched off for the purpose of maintenance without the provision of bypass.

Ring main arrangement

Ring main arrangement

This is an extension of the sectionalized bus bar where the ends of the bus bar are returned upon themselves to form a ring.

The arrangement provides greater flexibility as each feeder is supplied by two paths, so that the failure of the section doesn’t cause any interruption of the supply.

Bus zone faults

According to statistical information majority of faults are Single phase in nature.

The causes of the bus zone faults are -failure of support insulator due to ageing resulting

in earth fault. -flash over across support insulator caused by

prolonged and excessive over voltages. - Human error - foreign objects accidentally falling over busbars. - failure of circuit breaker under through fault

condition.

Types of protection

Back up protection Frame earth protection Differential protection Directional comparision Phase comparision protection.

1. Back Up Protection In principle it is the simplest of all bus bar protection

methods.

Figure -a. back up protection.

In this type of protection no separate bus protection is provided but distance protection is provided for the feeders connected to the bus, it is possible to cover the bus bar by time graded over current and earth fault protection of the distance relays of nearby zones.

Refering to figure-a. The bus A is covered in the 2nd step of distance protection of B. Thus when fault occurs on bus A the distance protection B will operate. The operating time of the 2nd step can be of the order of 0.4 sec. in such systems protection is slow and there can be unwanted disconnections of all incoming circuits. Ref. to fig. a. the local over current protection at station A provides the bus zone protection to zone A.

The remote over current protection or impedence protection at station B provides a back up protection to Buszone A. In case protection A fails protection B provides a back up protection.

2. Frame leakage protection

Fig. b Frame leakage scheme

Fig b1 Fig b2

Frame leakage protection It is one of the most simple form of protection and

applicable to small sized metal clad switch gear. this method consists of insulating the bus supporting

structure and its switch gear from ground, interconnecting all the frame work circuit breaker tanks etc. and providing a single ground connection through a CT that feeds an over current relay as shown in fig.b1. An impedence is also connected in the earth connection to limit the short circuit current during line to earth fault. It is nescessary to isolate the switch gear frame work from lead cable seaths ,cable boxes, and conduit fittings so that when a leakage to the framework occurs the only path to the leakage current should be through the connection from the frame work to earth.

3. Differential protection of busbar

Fig. c Differential protection

Differential protection :For the main bus-bars in the power stations due their importance in the operating conditions it is required that the disconnection be without any delay in case of faults.Differential current protection without time delay is imperative to use.

The protection is based on simple circulating current principle that under normal operating conditions or under external fault conditions ,the sum of the currents entering into the busbar will be equal to the sum of the currents leaving the bus bar. In case the sum of these currents is not zero ,it must be due to a short circuit either a ground fault or phase to phase fault.

Hence this protection scheme is applicable to both types of faults i.e. phase to phase faults as well ground fault. Fig c. shows the application of differential circulating current principle to a bus with four circuits. The CT are inserted in each phase of the incoming and outgoing feeders of the busbar and the secondaries are connected in parallel. And the relay operating coil is connected across the pilot wires in such a way that the summation current of secondaries flows through it. All the CTs must have same ratio.

Flow of current in the relay is the indication of fault within the protected zone. And will initiate opening of the breakers of each generator and feeder.

Bus bar section can also be protected by means of Voltage differential protection.

4. Directional comparision busbar protection.

Fig:e Directional blockage schemeFig:d Series trip scheme

Directional protection During an external fault the faulty feeder

current flows away from the busbar whereas the current fed to the fault by healthy feeders flow towards busbar. On the other hand during a busbar fault the direction of current through all the feeders feeding to the fault will be towards the bus. Directional relaying schemes utilizing this fact are as follows.

1. The series trip scheme

2. The directional blocking scheme.

1.Series trip scheme (fig d.):

Here all the contacts of directional relays connected to different feeders are connected in series with the trip coil. When ever a bus bar fault occurs, all the directional relays will close their contacts, thereby energizing the trip coil which will trip all the circuit breakers. On the other hand during an external fault the faulty feeder directional relay will not operate. Thereby it will prevent trip coil energization . Malfunctioning of the scheme due to the presence of too much of series contact maybe there because much importance hasn’t been given upon this fact.

2. Directional blocking scheme (fig.e):

As long as the blocking relay remain energized the trip coil cannot be energized. Hence the blocking relay contacts connected in the trip circuit will be open. Here all the relay contacts are connected in parallel and connected with the trip circuit and all the breaker contacts are paralled and connected to the blocking relay so that it could block tripping operation.

As these scheme is very rarely used not much informations are available about this scheme.

5. Phase comparison protection

Phase comparison technique is the most widely used technique for all practical directional , distance, differential and carrier relays. In phase comparator the operation of the relay takes place when the phase relation between two inputs suppose s1 and s2 varies within certain specified limits. Both inputs must exist for an output to occur . Ideally operation is independent of their amplitudes and depends only only on their phase relationships.