Why we use relay and what are the types of relay

Definition of  Relay

A relay is automatic device which senses an abnormal condition of electrical circuit and closes its contacts. These contacts in turns close and complete the circuit breaker trip coil circuit hence make the circuit breaker tripped for disconnecting the faulty portion of the electrical circuit from rest of the healthy circuit. Now let’s have a discussion on some terms related to protective relay.
Pickup Level of Actuating Signal:
The value of actuating quantity (voltage or current) which is on threshold above which the relay initiates to be operated.
If the value of actuating quantity is increased, the electromagnetic effect of the relay coil is increased and above a certain level of actuating quantity the moving mechanism of the relay just starts to move.

Reset Level:
The value of current or voltage below which a relay opens its contacts and comes in original position.
Operating Time of Relay:
Just after exceeding pickup level of actuating quantity the moving mechanism (for example rotating disc) of relay starts moving and it ultimately close the relay contacts at the end of its journey. The time which elapses between the instant when actuating quantity exceeds the pickup value to the instant when the relay contacts close.
Reset Time of Relay:
The time which elapses between the instant when the actuating quantity becomes less than the reset value to the instant when the relay contacts returns to its normal position.
Reach of Relay:
A distance relay operates whenever the distance seen by the relay is less than the pre-specified impedance. The actuating impedance in the relay is the function of distance in a distance protection relay. This impedance or corresponding distance is called reach of the relay.
Power system protection relays can be categorized into different types of relays.

Also you can read: How relays working ?

Types of Relays

Types of protection relays are mainly based on their characteristic, logic, on actuating parameter and operation mechanism. Based on operation mechanism protection relay can be categorized as electromagnetic relay, static relay and mechanical relay. Actually 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 electromagnetic relay these closing and opening of relay contacts are done by electromagnetic action of a solenoid.
In 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 thyristor. 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-

  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.

Mechanical Relay

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


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