Working Principle of a Protective Scheme

                       Fig. Basic connections of circuit breaker control for the opening operation

•The most common parameters which reflect the presence of a fault are:

–Voltage and currents at the terminals of the protected apparatus, or appropriate zone boundaries

–States – open or closed – of some contacts or switches

•A specific relay must use the inputs to determine that problem exists, and then initiate some action

•The fundamental problem is to distinguish between normal and abnormal  conditions in addition to the fact that ‘normal’ also means that the disturbance outside the zone of protection (security problem)

•The greatest significance in designing a secure relay system – dominates the design of all protection systems.

•From the previous figure, we can see that current flows from battery in the trip circuit. As the trip coil of circuit breaker is energized, the circuit breaker operating mechanism is actuated and it operates for the opening operation

•Thus the fault is sensed and the trip circuit is actuated by the relay and the faulty part is isolated

•Therefore, we can say that ‘a relay is an automatic device, which senses an abnormal condition of power system and closes its contacts

•These contacts in turn close and complete the circuit breaker trip coil circuit hence make the circuit breaker tripped for disconnecting the faulty portion of the power system from rest of the healthy part of the system .