What is meant by distance relay?

A distance relay is a type of protection relay most often used for transmission line protection. Distance relays measure the impedance from the installation side to the fault location and operates in response to changes in the ratio of measured current and voltage.

What do you mean by RX diagram?

The R-X graph shows an apparent impedance which has a complex relationship to the actual Rf depending on the power system conditions. The fault position vs. true Rf graph can remove all of those effects and show the true performance no matter what the power system conditions are.

Why do we need distance relay?

Distance relays are used for both phase fault and ground fault protection, and they provide higher speed for clearing the fault. Thus, they eliminate long clearing times for the fault near the power sources required by overcurrent relay if used for the purpose.

What is a distance relay 21?

The goal of a distance relay (ANSI/IEEE code 21) is to trip its circuit breaker(s) if a fault occurs within its programmed “reach” and to ignore both normal operating loads and faults lying outside its reach.

Is distance relay a directional 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.

What is the use of distance relay?

Distance Protection Relay. Distance relays are used for both phase fault and ground fault protection, and they provide higher speed for clearing the fault. It is also independent of changes in the magnitude of the short circuits, current and hence they are not much affected by the change in the generation capacity and the system configuration.

What are the types of relay?

There is one type of relay which functions depending upon the distance of fault in the line. More specifically, the relay operates depending upon the impedance between the point of fault and the point where relay is installed. These relays are known as distance relay or impedance relay.

Why contacts of distance relay remain open during normal operation?

Under normal operating condition restraining torque is greater than deflecting torque. Hence contacts of this distance relay remain open. When any fault occurs in the feeder, under protected zone, voltage of feeder decreases and at the same time current t increases.

What is the impedance of relay?

This impedance between the relay and the fault depends on the electrical distance between them. The principal type of distance relays is impedance relays, reactance relays, and the reactance relays.