Limits to Impedance Relaying

The paper reviews the role of impedance relaying in modern transmission line protection systems. The speed and reach of the first zone of an impedance relay are identified as being the most important attributes of a relay. The paper develops a mathematical model of the relaying process in the presence of transient phenomena which accompany faults. It is shown that there is an uncertainty associated with the impedance estimated by a relay and this uncertainty depends upon the speed with which these estimates are made. A quantitative relationship between the relay speed and its reach is obtained.