Which relay is also called angle impedance relay?

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PGCIL Diploma Trainee EE Official Paper held on 27 October 2018
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  1. Impedance relay
  2. Reactance relay
  3. Mho relay
  4. Frequency relay

Answer (Detailed Solution Below)

Option 3 : Mho relay
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Detailed Solution

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Mho Relay:

A Mho relay measures a component of admittance |Y| ∠ θ. But its characteristic when plotted on the impedance diagram is a circle passing through the origin. It is also known as angle impedance relay.

The relay is called Mho relay because its characteristic is a straight line, when plotted on an admittance diagram.

Important Points:

  • Impedance relay is a voltage restrained overcurrent relay.
  • Reactance relay is an overcurrent relay with directional restraint.
  • Mho relay is a voltage restrained directional relay.
  • Reactance relay is suitable for the protection of a short transmission line because its operation is independent of arc resistance.
  • The relay which is selected for a long transmission line should be less affected due to power swings. Hence Mho relay is preferred.
  • Impedance relay is suitable for medium transmission lines.

Additional Information Mho Relay: Also known as a polarized distance relay or admittance relay, it is inherently directional and operates based on both the magnitude and angle of impedance. The term "Mho" is derived from the unit of admittance (the inverse of ohm), and it is characterized by a circular or oval operating characteristic on the impedance plane. The Mho relay is directionally sensitive and thus incorporates the angle of impedance in its operation, making it an angle impedance relay.

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