31 Types of Protections Relays in Substation
Channel: KashTech
Date: 18/08/2022

Types of Relays
Types of protection relays are mainly based on their characteristics, logic, on actuating parameters and operation mechanisms.

Based on operation mechanism protection relay can be categorized as an electromagnetic relay, static relay and mechanical relay. Actually, a relay is nothing but a combination of one or more open or closed contacts. These all or some specific contacts the relay change their state when actuating parameters are applied to the relay. That means open contacts become closed and closed contacts become open. In an electromagnetic relay, these closing and opening of relay contacts are done by the electromagnetic action of a solenoid.

In the mechanical relay, these closing and opening of relay contacts are done by mechanical displacement of different gear level system.

In static relay it is mainly done by semiconductor switches like thyristors. In digital relay on and off state can be referred as 1 and 0 state.

Based on Characteristic the protection relay can be categorized as:

Definite time relays
Inverse time relays with definite minimum time(IDMT)
Instantaneous relays.
IDMT with inst.
Stepped characteristic.
Programmed switches.
Voltage restraint over current relay.
Based on of logic the protection relay can be categorized as-

Differential.
Unbalance.
Neutral displacement.
Directional.
Restricted earth fault.
Over fluxing.
Distance schemes.
Bus bar protection.
Reverse power relays.
Loss of excitation.
Negative phase sequence relays etc.
Based on the actuating parameter the protection relay can be categorized as-

Current relays.
Voltage relays.
Frequency relays.
Power relays etc.
Based on application the protection relay can be categorized as-

Primary relay.
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 a 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,

The protective relay itself is defective.
DC Trip voltage supply to the relay is unavailable.
Trip lead from relay panel to the circuit breaker is disconnected.
The trip coil in the circuit breaker is disconnected or defective.
Current or voltage signals from Current Transformers (CTs) or Potential Transformers (PTs) respectively is unavailable.