Power Systems : Protective Relays

By Mohd. Irshad|Updated : August 13th, 2021

Complete coverage of syllabus is a very important aspect for any competitive examination but before that important subject and their concept must be covered thoroughly. In this article, we are going to discuss the fundamental of Power Systems: Protective Relays which is very useful for SSC JE Exams


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.

Functions of protective Relay:

  • To sound an alarm or to close the trip circuit of a circuit breaker so as to disconnect Faulty Section.
  • To disconnect the abnormally operating part so as to prevent subsequent faults. For e.g. Overload protection of a machine not only protects the machine but also prevents Insulation failure.
  • To isolate or disconnect faulted circuits or equipment quickly from the remainder of the system so the system can continue to function and to minimize the damage to the faulty part. For example – If machine is disconnected,immediately after a winding fault, only a few coils may need replacement. But if the fault is sustained, the entire winding may get damaged and machine may be beyond repairs.
  • To localize the effect of fault by disconnecting the faulty part from healthy part, causing least disturbance to the healthy system.
  • To disconnect the faulty part quickly so as to improve system stability, service continuity and system performance.
  • Transient stability can be improved by means of improved protective relaying.
  • To minimize hazards to personnel.

Terminologies of 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.

Classification of Relay on the basis of technology

  • Electromagnetic Relay 
  • Static Relay 
  • Micro-Processor Based Relay

Classification of Relay on The Basis of Their Function

  • Over-Current Relay
  • Under-Voltage Relay
  • Impedance relay
  • Under Frequency relay
  • Directional relay

Other Category of Type of relay

Based on Characteristic

  • Definite time Relays.
  • Inverse definite minimum time Relays (IDMT)
  • Instantaneous Relays
  • IDMT with Instantaneous.
  • Stepped Characteristic
  • Programmed Switches
  • Voltage restraint over current relay

Based on Logic

  • 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

  • Current Relays
  • Voltage Relays
  • Frequency Relays
  • Power Relays etc.

Based on Operation Mechanism

  • Electro Magnetic Relay
  • Static Relay ⇒ (i)Analog (ii)Relay Digital Relay (iii) Numerical /Microprocessor Relay
  • Mechanical relay: These relay are subdivided into following Category of relay 

      (a) OT Trip (Oil Temperature Trip)
      (b) WT Trip (Winding Temperature Trip)
      (C) Bearing Temp Trip etc.

Float Type
     (a) Buchholz
     (b) OSR
     (c) PRV
     (d) Water level Controls etc.     

  • Pressure Switches.
  • Mechanical Interlocks.
  • Pole discrepancy Relay

Classification of Protective Scheme

1. Over-Current Protection

2. Distance Protection

3. Differential Protection

4. Carrier-Current Protection

1-Over Current Relay:

A relay that operates or picks up when it’s current exceeds a predetermined value (setting value) is called Over Current Relay.

Over-current protection protects electrical power systems against excessive currents which are caused by short circuits, ground faults, etc. Overcurrent relays can be used to protect practically any power system elements, i.e. transmission lines, transformers, generators, or motors.

Overcurrent includes short-circuit protection. Short circuits can be Phase faults, Earth faults, Winding faults, Differential and distance protection.

Over-current protection is useful for the following:

  • Detect abnormal conditions
  • Isolate the faulty part of the system
  • Speed Fast operation to minimize damage and danger
  • Discrimination Isolate only the faulty section
  • Dependability / reliability
  • Security / stability
  • Cost of protection / against the cost of potential hazards

Type of Over Current Relay:

  1. Instantaneous Over Current (Define Current) Relay
  2. Define Time Over Current Relay
  3. Inverse Time Over Current Relay (IDMT Relay): Moderately Inverse, Very Inverse Time, Extremely Inverse
  4. Directional over Current Relay.

Application of over current relay: Motor protection, Transformer protection, Line protection, Distribution protection

Distance Protection Relay

  • Distance protection is widely used in the transmission network, it is also called impedance protection because relay operates with respect to fault impedance of the transmission line (Z=V/I).
  • It calculates the apparent impedance of a line with the help of voltage & current input connected to the relay. If measured impedance falls below set impedance trip command is issued to clear the fault.
  • Measuring element is Current Transformer and Voltage Transformer. Usually, relay having 4 zones of the transmission line. Zone1, Zone2, Zone3 will be forward zones (Towards the Line) and zone 4 will be Reverse zone (Towards the Source).

Distance Relay are further classified as

  • Impedance Relay 
  • Reactance Relay
  • Mho Relay

Differential Protection Relay

  • It is used for transformer and generator protection. It simultaneously compares the phaser difference & magnitude of the current entering & leaving the protected zone. Differential protection is unit protection, relay works on the principle of Kirchoff's Current Law.
  • Measuring element is Current Transformer. The differential current measured between the incoming current and Outgoing current must be negligible current during stable and through fault condition.
  • In case of in-zone fault or unstable condition (due to CT saturate) relay will sense the differential current and issue the trip signal.

Carrier-current protection

  • For long overhead lines, the power line itself may be used as the interconnecting channel between the terminal equipment.
  • Carrier-current protection is the most widely used scheme for the protection of Extra High Voltage (EHV) and Ultra High Voltage (UHV) power lines.
  • The carrier signal is directly coupled to the power line itself which is to be protected.
  • Carrier-current protection is faster and superior to distance protection schemes and is more reliable when used for long transmission lines, although the terminal equipment are more expensive and complicated.


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Mohd. IrshadMohd. IrshadMember since Apr 2020
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