A high speed relay for short lines

TRANSMISSION of power over longer distances and the concentration of greater amounts of power over a single circuit have rendered the presence of a short circuit on the system a more serious matter, and a considerable effort therefore has been made to reduce the duration of the short circuits. The result has been the development of circuit breakers and relays operating to clear the disturbance in a very short time. The conventional type of induction overcurrent relaying, depending upon successively increasing time intervals to obtain proper selectivity between circuit breakers, is no longer suitable where the maximum speed is desired; and so a new form of relaying, known as distance relaying, has been developed for the general protection of transmission and distribution lines. Relays operating on the distance principle make use of the characteristic impedance or reactance of the circuit, as determined by comparing the voltage and current at the relaying point, to determine the distance from the relay to the fault. Hence, selectivity between circuit breakers can be obtained without the use of increasing time intervals in the time settings of relays as the generating station is approached.