There are three types of back-up relays:
Centrally coordinated back-up
Remote back-up:- When back-up relays are located at a neighbouring station, they back-up the entire primary protection scheme which includes the relay, circuit breaker, P.T., C.T. and other elements, in case of a failure of the primary protective scheme. It is the cheapest and simplest form of protection and it is widely used for transmission lines. It is most desirable because of the fact that it will not fail due to the factors causing the failure of the primary protection.
Relay back-up:- This is kind of a local back-up in which an additional relay is provided for back-up protection. It trips the same circuit breaker if the primary relay fails and this operation takes place without delay. Though such a back-up is costly, it can be recommended where a remote back-up is not possible. For back-up relays, principles of operation that are different from those of the primary protection are desirable. They should be supplied from separate current and potential transformers.
Breaker back-up:- This is also a kind of local back-up, which is necessary for a bus bar system where a number of circuit breakers are connected to it. When a protective relay operates in response to a fault but the circuit breaker fails to trip, the fault is treated as a bus bar fault. In such a situation, it becomes necessary that all other circuit breakers on that bus bar should trip. After a time delay, the main relay closes the contact of a back-up relay which trips all other circuit breakers on the bus if the proper breaker does not trip within a specified time after its trip coil is energised.
Centrally coordinated back-up:- This is also a kind of local back-up. In this method primary protection is at various stations. There is a central control room and back-up protection for all the stations is at the central control room. Central control room continuously inspects the load flow and frequency in the system. If any element of any part of the system fails, load flow gets affected which is sensed by the control room. The control source consists of a digital computer which decides the proper switching action.
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