Admittance relay is _______ relay.

Admittance relay is _______ relay.

Right Answer is:



Distance Relays

Distance relays respond to the voltage and current, i.e., the impedance, at the relay location. The impedance per mile is fairly constant so these relays respond to the distance between the relay location and the fault location. As the power systems become more complex and the fault current varies with changes in generation and system configuration, directional overcurrent relays become difficult to apply and to set for all contingencies, whereas the distance relay setting is constant for a wide variety of changes external to the protected line.

There are three general distance relay types as shown in Fig. Each is distinguished by its application and its operating characteristic.

Impedance Relay

The impedance relay has a circular characteristic centered at the origin of the R-X diagram. It is nondirectional and is used primarily as a fault detector.

Admittance Relay

The admittance relay is the most commonly used distance relay. It is the tripping relay in pilot schemes and as the backup relay in step distance schemes. Its characteristic passes through the origin of the R-X diagram and is therefore directional. In the electromechanical design, it is circular, and in the solid-state design, it can be shaped to correspond to the transmission line impedance.

Reactance Relay

The reactance relay is a straight-line characteristic that responds only to the reactance of the protected line. It is nondirectional and is used to supplement the admittance relay as a tripping relay to make the overall protection independent of resistance. It is particularly useful on short lines where the fault arc resistance is the same order of magnitude as the line length. 

MHO Relay

MHO relay is a high-speed relay and is also known as an admittance relay. lt is well known that a long line is less stable than a short line; that is, a long line has a larger swing angle δmax compared to a short line. The short line has a higher Pmax than that of a long line.

Why is Mho’s Relay used for Long Transmission Line?

  • Mho relay comes in the category of the distance relay protection scheme.
  • The mho type relay is most suited for long lines because there are more chances of severe synchronizing power surges on the system.
  • MHO Relay is less affected by the power swing.
  • It does not need any additional equipment to prevent tripping during these surges.
  • The mho relay occupies the least space on an R-X diagram for a given line section and is, therefore, least affected by abnormal system conditions except for the line faults.
  • Since the mho relay is most affected by arc resistance, it is used for long lines.
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