Types of Electrical Protection Relays or Protective Relays

Definition of Protective Relay

A protective relay monitors electrical quantities or equipment states and applies configured logic to detect a fault or abnormal condition in an electrical circuit. When its operating criteria are met, it can issue a trip, alarm, block or control output. In a tripping scheme, the output energises the circuit breaker trip path so the faulted element can be isolated.

Several terms describe when a relay function starts, resets and completes its operation.
Pickup Level of Actuating Signal:

Pickup is the threshold at which a relay element starts to operate for its defined actuating quantity, such as voltage, current, frequency, impedance or a calculated value. Direction and comparison conventions depend on the protection function.

In an electromechanical overcurrent example, increasing input produces more operating torque until the relay picks up. Static and numerical relays evaluate the same concept electronically.

Reset Level:
Reset is the value at which an operated element returns to its normal state as current, voltage or another input moves away from the operating region.

Operating Time of Relay:
Operating time is measured from the specified input condition to the specified change in the relay output. The test definition must identify the function, settings, input and output state.

For a simple electromechanical trip relay, that interval may end when an output contact closes. For a numerical relay, it may end when the configured binary trip output changes state.

Reset Time of Relay:
Reset time is the interval from the specified reset condition to the specified return of the relay output.

Reach of Relay:
Reach is the impedance boundary assigned to a distance-protection zone. The relay calculates apparent impedance from measured quantities and operates according to the zone characteristic, direction, timing and settings. Reach is related to line distance but is not a direct physical-distance measurement under every fault condition.

Protective relays can be grouped by technology, measured quantity, operating characteristic, logic and application. These categories overlap because one numerical relay may implement many functions.

Types of Relays

The lists below use several historical classification approaches. They are useful vocabulary, but they are not a complete modern relay taxonomy.

By implementation technology, relays may be electromechanical, static or numerical. An electromagnetic relay uses magnetic force or torque to move contacts. A protective relay is more than its output contacts: it also measures inputs, applies a characteristic and produces the required control response.

Mechanical sensing devices use movement, pressure, temperature, fluid flow or another physical effect to operate a contact or transducer. Their construction and purpose vary by equipment.

A static relay uses analogue electronic or semiconductor circuits, which may include devices such as a thyristor. A numerical relay samples inputs, performs calculations in software and communicates through binary or digital interfaces as configured.

The following frozen list groups relay functions by time-current or programmed operating characteristic:

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.

The next frozen list mixes protection principles, measured conditions and protected-zone applications. Many are functions within one numerical device rather than separate physical relay types.

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.

A simpler classification uses the principal measured or calculated quantity. The frozen examples are not exhaustive.

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

By role in the clearing scheme, protection is commonly described as primary or backup:

1. Primary relay.
2. Backup relay.

Primary protection is assigned to clear faults in its own zone with the intended clearing time. A backup relay or backup function covers failure of the primary scheme, trip path or interrupting device. Backup may use a time delay, a different zone or breaker-failure logic, so it is not universally just a slower duplicate. The frozen list identifies common failure points.

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.

Independent sensing, DC supply, trip paths, communications and relay hardware can reduce common-mode failure, but complete separation is not always practical. The required redundancy follows a documented reliability assessment and protection philosophy.
The frozen list gives examples of mechanical or equipment-mounted protective devices.

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

The frozen tables below are legacy examples, not universal schedules. Relay functions must be selected and coordinated from the equipment data, grounding, network configuration, fault study, utility criteria and governing standards.

Relays for Transmission & Distribution Lines Protection

Relays for Transformer Protection

The following frozen transformer points are legacy project rules, not current universal requirements. Use a transformer-specific protection study and applicable standard instead.

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.