Types of Electrical Protection Relays or Protective Relays

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Key learnings:
  • Protective Relay Definition: A protective relay is an automatic device that senses abnormal conditions in electrical circuits and triggers actions to isolate faults.
  • Types of Protective Relays: Protective relays are categorized by their mechanism (electromagnetic, static, mechanical) and function (time-based, current, voltage).
  • Operating Principles: Protective relays operate by detecting abnormal signals, with specific pickup and reset levels to start or stop their action.
  • Application in Power Systems: Primary and backup protective relays are critical for continuous and safe operation of electrical power systems.
  • Failure Modes: Understanding common failures in protective relays helps enhance system reliability and prevent prolonged downtimes.

Definition of Protective Relay

A protective relay is an automatic device that detects abnormalities in an electrical circuit and closes its contacts. This action completes the circuit breaker‘s trip coil circuit, causing the breaker to trip and disconnect the faulty section from 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.

When the actuating quantity increases, so does the electromagnetic effect on the relay coil. Once this quantity reaches a specific level, the relay’s moving mechanism begins to activate.

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.

Operating time is the duration from when the actuating quantity exceeds the pickup level to 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.

Protective relays can be categorized based on their operating mechanisms into electromagnetic relay, static, and mechanical types. 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:

  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 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,

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

SLLines to be protectedRelays to be used
1400 KV
Transmission Line
Main-I: Non switched or Numerical Distance Scheme
Main-II: Non switched or Numerical Distance Scheme
2220 KV
Transmission Line
Main-I : Non switched distance scheme (Fed from Bus PTs)
Main-II: Switched distance scheme (Fed from line CVTs)
With a changeover facility from bus PT to line CVT and vice-versa.
3132 KV
Transmission Line
Main Protection : Switched distance scheme (fed from bus PT).
Backup Protection: 3 Nos. directional IDMT O/L Relays and
1 No. Directional IDMT E/L relay.
433 KV linesNon-directional IDMT 3 O/L and 1 E/L relays.
511 KV linesNon-directional IDMT 2 O/L and 1 E/L relays.

Relays for Transformer Protection

SLVoltage Ratio and
Capacity of Transformer
Relays on HV SideRelays on LV SideCommon Relays
111/132 KV
Generator Transformer
3 nos Non-Directional O/L Relay
1 no Non-Directional E/L Relay
and/or standby E/F + REF Relay
– –Differential Relay or
Overall differential Relay
Overflux Relay
Buchholz Relay
OLTC Buchholz Relay
PRV Relay
OT
Trip Relay
WT Trip Relay
213.8/220 KV
15.75/220 KV
18/400 KV
21/400 KV
Generator Transformer
3 nos Non-Directional O/L Relay
1 no Non-Directional E/L Relay
and/or standby E/F + REF Relay
– –Differential Relay or
Overall differential Relay
Overflux Relay
Buchholz Relay
OLTC Buchholz Relay
PRV Relay
OT
Trip Relay
WT Trip Relay
3220 /6.6KV
Station Transformer
3 nos Non-Directional O/L Relay
1 no Non-Directional E/L Relay
and/or standby E/F + REF Relay
3 nos Non-Directional O/L RelayDifferential Relay
Overflux Relay
Buchholz Relay
OLTC Buchholz Relay
PRV Relay
OT Trip Relay
WT Trip Relay
4Gen-volt/6.6KV UAT3 nos Non-Directional O/L Relay3 nos Non-Directional O/L RelayDifferential Relay
Overflux Relay
Buchholz Relay
OLTC Buchholz Relay
PRV Relay
OT Trip Relay
WT Trip Relay
5132/33/11KV upto 8 MVA3 nos O/L Relay
1 no E/L Relay
2 nos O/L Relays
1 no E/L Relay
Buchholz Relay
OLTC Buchholz Relay
PRV Relay
OT Trip Relay
WT Trip Relay
6132/33/11KV above 8 MVA &
below 31.5 MVA
3 nos O/L Relay
1 no Directional E/L Relay
3 nos O/L Relay
1 no E/L Relay
Differential Relay
Buchholz Relay
OLTC Buchholz Relay
PRV Relay
OT Trip Relay
WT Trip Relay
7132/33KV, 31.5 MVA & above3 nos O/L Relay
1 no Directional E/L Relay
3 nos O/L Relay
1 no E/L Relay
Differential Relay
Overflux Relay
Buchholz Relay
OLTC Buchholz Relay
PRV Relay
OT Trip Relay
WT Trip Relay
8220/33 KV, 31.5MVA &
50MVA 220/132KV, 100 MVA
3 nos O/L Relay
1 no Directional E/L Relay
3 nos O/L Relay
1 no Directional E/L Relay
Differential Relay
Overflux Relay
Buchholz Relay
OLTC Buchholz Relay
PRV Relay
OT Trip Relay
WT Trip Relay
9400/220KV 315MVA3 nos Directional O/L Relay
(with dir.highset)
1 no Directional E/L relay.
Restricted E/F relay
3 nos Directional O/L Relay for
action
3 nos Directional O/L Relay
(with dir.highset)
1 no Directional E/L relay.
Restricted E/F relay
Differential Relay
Overflux Relay
Buchholz Relay
OLTC Buchholz Relay
PRV Relay
OT Trip Relay
WT Trip Relay
Over Load (Alarm) Relay

Points to be remembered in respect of protection of transformers

  1. No Buchholz relay for transformers below 500 KVA capacity.
  2. Transformers up to 1500 KVA shall have only Horn gap protection.
  3. 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.
  4. Transformers above 8000 KVA shall have individual HV and LV circuit breakers.
  5. The relays indicate above shall be provided on HV and LV.
  6. LAs to be provided on HV and LV for transformers of all capacities and voltage class.
  7. OLTC out of step protection is to be provided where master follower scheme is in operation.
  8. Fans failure and pumps failure alarms to be connected.
  9. Alarms for O.T., W.T., Buchholz (Main tank AND OLTC) should be connected.
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