SOLUTION

Distance relay: A relay that measures the impedance or a component of the impedance at the relay location is known as a distance relay. It is used for the protection of a transmission line. As the impedance of a line is proportional to the length of the line, a relay that measures the impedance or its component is called a distance relay.

Distance protection is a widely used protective scheme for the protection of high and extra-high voltage (EHV) transmission and sub-transmission lines. This scheme employs a number of distance relays that measure the impedance or some components of the line impedance at the relay location. The measured quantity is proportional to the line length between the location of the relay and the point where the fault has occurred. As the measured quantity is proportional to the distance along the line, the measuring relay is called a distance relay. Overcurrent relays have been found unsuitable for the protection of transmission lines because of their inherent drawbacks of variable reach and variable operating time due to changes in source impedance and fault type. Distance relays have been developed to overcome the problems associated with the use of overcurrent relays for the protection of transmission lines.