Transmission Line Protection: Distance Relays
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
Impedance Relay |
Reactance Relay
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.
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