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Tuesday, 25 February 2020

What is Mho Relay- How it Works, and its Applications

Mho Relay


  • Mho relay is voltage controlled directional relay. Mho relay is the type of Distance Relay. This type of relays used for the long distance protection of transmission lines.
  • Its characteristics is straight line, when plotted in an admittance. Because of this it is called the Mho Relay.

Mho Relay Circuit Diagram
Mho Relay Circuit Diagram

Working Principle of Mho Relay

  • Due to the voltage elements controlling elements develops and operating torque is obtained in volt amperes.
  • Mho relay is used for long transmission lines. Because it can protect from power swing better than other simple impedance relay. It has directional property also which is essential in transmission line.
  • The Mho relay combines the properties of impedance and directional relays. Its characteristic is inherently directional and the relay only operates for faults in front of the relay location. 
  • In terms of the torque equation the relay characteristics can be obtained by making the constant K1 equal to zero. It is basically a voltage restrained directional relay and the torque
Characteristics Of Mho Relay
Characteristics Of Mho Relay
  • This relay has a larger coverage of R-X plane and therefore it is least affect ted by condition of power swing. The characteristics is shown below.

Applications Of Mho Relay

  • Mho relay is used for protection of EHV/UHV transmission lines. These are generally used for protection of long transmission line due to its stability in case of power swing. Further no separate directional unit is required in case of Mho relay, since these relay are inherently directional.


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