Breaker failure relays are called back-up protection devices are applied as back-up protection relays for the main relays. As example Breaker failure relay is used as a back up device for differential relay of a power Transformer and also for distance relay of a Transmission line .
Why Breaker Failure ?
In the fault condition of any equipment it is expected that the circuit breaker of this equipment will be tripped by main protection relays of such equipment if there is a failure in the tripping circuit of the circuit breaker or in the circuit breaker it self .
In this case the circuit breaker will not tripped and so, to isolate this fault ,the bus bar at which this equipment is connected must by completely isolated by tripping all the equipment connected to it by a delayed time .then , the only connected equipment to this bus bar is the faulted one. In this case , the faulted equipment has no current passing through it so , it is can be disconnected manly by isolates.
How Breaker Failure Operate
There are three conditions must be satisfied For operation of breaker failure relay which are
- Tripping signal from any main protection relay of the faulted equipment is exist.
- Circuit breaker of the faulted equipment is still connect (n.o. auxiliary contact from circuit breaker is used).
- There is current passing through this equipment even if it is value is very small.
Although one of the second and third condition is sufficient for healthy operation of breaker failure relay, it is preferable to use both for more security.
Figure (55) shows wiring diagram of breaker failure circuit. Tripping circuit of breaker failure relay is similar to tripping circuit of bus bar protection relay. In general breaker failure relay may use the same tripping circuit of bus bar protection relay.
