Tuesday, 25 February 2020

How Distance Relay Works?- Operation, Types, Advantages, Limitations.

Distance relay

  • Relay is an electrical switch which is used to open or close a circuit with electromagnetic relay or electronically. It operates manually as well as automatically.
  • So the distance relay is another relay for the protection of the devices. Distance relay is used for the protection of transmission line &amp, feeder.
  • In a distance relay, instead of comparing the local line current with the current at far end of line, the relay compares the local current with the local voltage in the corresponding phase or suitable components of them.

Principle of operation of distance relay

Distance Relay Protection
Distance Relay Protection 

  • The basic principle of measurement involves the comparison of fault current seen by the relay with the voltage at relaying point, by comparing these two quantities.
  • It is possible to determine whether the impedance of the line up to the point of fault is greater than or less than the predetermined reach point impedance.
  • There are two types of torques
 1. Restraining torque
2. Operating torque

  • The relay trips when operating torque greater than restraining torque.

Types of distance relay

Distance relays are classified depending on their operating characteristic in the R-X plane
  • Impedance Relay 
  • Mho Relay
  • Reactance Relay


  • The torque equation T, for such a relay the current actuates the operating torque and the voltage actuates the restraining torque, with the usual spring constant K4.
  • Considering K2 to be negative (as it produces the restraining torque) and neglecting the torque component due to spring, the equation represents a circle in the R-X plane.

Impedance Relay Protection
Impedance Relay Protection

Disadvantages of Impedance Relay

1. It is not directional.
2. It is affected by the Arc resistance
3. It is highly sensitive to oscillations on the power system, due to large area covered by its circular characteristic.


  • The reactance relay is basically a directional restrained over current relay. 
  • The characteristics resembles a horizontal line parallel to the R-axis with constant X value. The portion below the line gives the operating zone of the relay.

Disadvantages of Reactance Relay

1. The reactance relay is designed to measure only reactive component of the line reactance.
2. The fault resistance has no affect on the reactance relay.


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