Transmission Line | Source: Altium Resources

The impedance relays also called distance relays are employed to provide protection to transmission lines connected in a network as they are economic and possess several technical advantages. They are comparatively simple to apply, operate with extremely high speed, and both primary and backup protection features are inherent in them. Moreover, they can be easily modified to work as unit schemes by coordinating them with power line carrier facilities and are suitable for high-speed reclosing. 

The impedance relay is made to respond to the impedance between the relay location and the point where the fault is incident. The impedance is proportional to the distance to the fault, (hence the name 'distance relay') and is therefore independent of the fault current levels.

Type of Distance Relays

  • Impedance relay 
  • Reactance relay 
  • Mho relay 
  • Modified impedance relay 

Impedance Relays

The operation of the impedance relay is independent of the phase angle between V and I. The operating characteristic is a circle with its center at the origin, and hence the relay is non-directional. 

The directional unit of the relay causes separation of the regions of the relay characteristic shown in the figure by a line drawn perpendicular to the line impedance locus. The net result is that tripping will occur only for points that are both within the circles and above the directional unit characteristic.

Impedance Relay

Reactance Relay

Reactance relay measures V/I Sin (theta). Whenever the reactance measured by the relay is less than the set value, the relay operates. The operating characteristic on the R-X diagram is shown in the figure below,

Reactance Relay

The resistance component of impedance has no effect on the operation of the reactance relay, the relay responds solely to the reactance component of impedance. This relay is inherently non-directional. The relay is most suitable to detect earth faults where the effect of arc resistance is appreciable.

Related Article: Switch-on-to-Fault Schemes in the Context of Line Relay Loadability

Mho Relay

This is a directional impedance relay, also known as admittance relay. It's characteristic on the R-X diagram is a circle whose circumference passes through the origin as illustrated in the figure showing that the relay is inherently directional and it only operates for faults in the forward direction.

MHO Relay

Modified Impedance Relay

Also known as offset Mho relay whose characteristic encloses the origin on R-X diagram as shown in fig This offset mho relay has three main applications: 

  • Busbar zone backup 
  • Carrier starting unit in distance/carrier blocking schemes.
  • Power Swing blocking.

Title: Handbook for Protection Engineers
Author: Mohammed Fasil
Document: PDF | pages 47-52 | Download 

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