Modern practice is to adopt definite distance method of protection applied in 3 zones (steps). A number of distance relays are used in association with timing relays so that the power system is divided into a number of zones with varying tripping times associated with each. In developing an overall Scheme of Distance Protection, it is necessary to provide a number of relays to obtain the required discrimination. loss of synchronism protection, loss of excitation protection, or impedance automatics like fault locator. Impedance relays measure and evaluate the magnitude and angle of impedance and therefore these relays are adjusted to the power line parameters. The former gives an unduly long time delay in fault clearance at the generating station end when there are more than four or five sections and the pilot-wire system becomes too. To reach these demands, high-speed protection arrangements for electric transmission and distribution networks that are appropriate for use with the automatic reclosure of circuit breakers are under constant research.
[PDF Version]