Types of Electrical Protection Relays or Protective Relays

Definition of Protective 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 closes 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 return 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 the reach of relay.

Power system protection relays can be categorized into different types of relays.

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, 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 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:

• 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 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.

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

• 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.

List Different Protective Relays are used for Different Power System Equipment Protection

Now let’s have a look on which different protective relays are used in different power system equipment protection schemes.

Relays for Transmission & Distribution Lines Protection

Relays for Transformer Protection

Points to be remembered in respect of protection of transformers

• No Buchholz relay for transformers below 500 KVA capacity.

• Transformers up to 1500 KVA shall have only Horn gap protection.

• Transformers above 1500 KVA and upto 8000 KVA of 33/11KV ratio shall have one group control breaker on HV side and individual LV breakers if there is more than one transformer.

• Transformers above 8000 KVA shall have individual HV and LV circuit breakers.

• The relays indicate above shall be provided on HV and LV.

• LAs to be provided on HV and LV for transformers of all capacities and voltage class.

• OLTC out of step protection is to be provided where master follower scheme is in operation.

• Fans failure and pumps failure alarms to be connected.

• Alarms for O.T., W.T., Buchholz (Main tank AND OLTC) should be connected.

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