What is relay coil?

Answer 1

An electrical device, typically incorporating an electromagnet, which is activated by a current or signal in one circuit to open or close another circuit.

Based on Characteristic the protection relay can be categorized as-

1. Definite time relays
2. Inverse time relays with definite minimum time(IDMT)
3. Instantaneous relays.
4. IDMT with inst.
5. Stepped characteristic.
6. Programmed switches.
7. Voltage restraint over current relay.

Based on of logic the protection relay can be categorized as-

1. Differential.
2. Unbalance.
3. Neutral displacement.
4. Directional.
5. Restricted earth fault.
6. Over fluxing.
7. Distance schemes.
8. Bus bar protection.
9. Reverse power relays.
10. Loss of excitation.
11. Negative phase sequence relays etc.

Based on actuating parameter the protection relay can be categorized as-

1. Current relays.
2. Voltage relays.
3. Frequency relays.
4. Power relays etc.

Based on application the protection relay can be categorized as-

1. Primary relay.
2. Backup relay.

Primary relay or primary protection relay is the first line of power system protection whereas backup relay is operated only when primary relay fails to be operated during fault. Hence backup relay is slower in action than primary relay. Any relay may fail to be operated due to any of the following reasons,

1. The protective relay itself is defective.
2. DC Trip voltage supply to the relay is unavailable.
3. Trip lead from relay panel to circuit breaker is disconnected.
4. Trip coil in the circuit breaker is disconnected or defective.
5. Current or voltage signals from CT or PT respectively is unavailable.

As because backup relay operates only when primary relay fails, backup protection relay should not have anything common with primary protection relay.

Some examples of Mechanical Relay are-

1. Thermal
2. (a) OT trip (Oil Temperature Trip) (b) WT trip (Winding Temperature Trip) (C) Bearing temp trip etc.(a) Buchholz (b) OSR (c) PRV (d) Water level Controls etc.
3. Float type
4. Pressure switches.
5. Mechanical interlocks.
6. Pole discrepancy relay.

Answer 2

A protein that relays the signal transduction.