The Impedance Relay Developments and Application

This paper reviews the developments made in the impedance relay since its introduction in 1923. A brief review is given of its construction and operation. Special emphasis is placed on the methods of obtaining proper potential for the restraining elements without the use of high-tension potential transformers. The use of potential from condenser type bushings is discussed. To make possible the use of low-tension potential, methods of compensating for the voltage drop through power transformers and for the shifting of the secondary neutral point have been developed. These methods are described. Some special applications of the relay are also discussed.