What is protective relay?

 

Protective Relay

A protective relay is a device which initiates the circuit breaker to cut or off the faulty circuit from the healthy system. It acts just like a ‘ silence guard’, when any fault occurred in a system, the relay operates to close the trip circuit of the breaker and the breaker disconnects the faulty circuit.

Relay is the sensing unit of the electrical power system just like the brain of a human being which senses cold and hot things by touching it similarly, relay sense the abnormality in the system and gives trip command to the circuit breaker in order to disconnect the faulty section from healthy section.

A typical relay circuit is shown in fig. There are three parts in the relay circuit :-

1. First part is, the primary winding of a current transformer (C.T ) which is connected in series the line.
2. Second part , secondary winding of C.T is connected in series with relay operating coil.
3. Third part is the tripping circuit which may be either AC and DC source of supply, trip coil of the circuit breaker and relay stationary contacts are connected in series connection.

Now describe, how the relay disconnects the faulty circuit . 

Suppose when a short circuit occurs at a point F, on the line, the huge current  flows in the line. Hence, a heavy current  flow through the relay coil, causing the relay to operate by closing its contacts. Result complete the tripping circuit of the breaker and making the circuit breaker open.

Relays are very important for protection of the electrical system and any damage occurring to the costly equipment's of a substation of any industry. 

-eeeaf team