TYPES OF PROTECTION
When fault occurs in any part of system, it must be cleared quickly to avoid damage or interference with rest of the system. Protection scheme is divided into two classes- primary protection and back up protection.
It’s used for protection of component parts of power system. In fig 1 each line has an over current relay that protects the line. If a fault occurs on any line, it will be cleared by its relay and circuit breaker. This forms the primary or the main protection and serves as the first line of defense. Sometimes faults are not cleared by primary relaying because of trouble within the relay, wiring system, or breaker. Under such conditions back up protection is used.
Back up protection
It’s the second line of defense in case of failure of primary protection. It’s designed to operate with sufficient time delay so that primary relaying is given enough time to function if it is able to. In fig 1, A provides back up protection for each of the four lines. If a line fault is not cleared by its relay and breaker, relay a will operate after a definite time delay and clear the entire group of lines. When back up protection functions a larger part is disconnected than when primary relaying functions correctly. Therefore greater emphasis is given on better maintenance of primary relaying.