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The relays that are usually employed for the protection of transmission lines include:
Over-current relay,
Directional relay,
Distance relay,
Under Voltage relay,
Under frequency relay
Thermal relay,
Differential relay,
Phase sequence relays,
pilot relays
It is an electrical device designed to initiate the isolation of a part of the electrical installation or to operate an alarm signal, in the event of abnormal condition or a fault. In simple words relay is an electrical device that gives the signal to isolation device (eg: Circuit Breaker) after sensing the fault and helps to isolate the fault system from the healthy electrical system
The Feedback in Control System in one in which the output is sampled and the proportional signal is fed back to the input for automatic correction of the error (any change in desired output) for further processing to get back the desired output.
