A power system contains generators, transformers, bus bars, transmission and distribution lines etc. There is a separate protective scheme for each piece of equipment of the power system, such as generator protection, transformer protection, transmission line protection, busbar protection etc. Thus a power system is divided into a number of zones for protection. A protective zone covers one or at the most two elements of a power system. The protective zones are planned in such a way that they collectively cover the entire power system and no part of the system is left unprotected. The various protective zones of a typical power system are shown in figure 1.2.
A well designed and efficient relay system should be able to detect the fault point and cause the opening of the circuit breakers closest to the fault with minimum or no damage to the system. From the figure it may be seen that circuit breakers are located in the connections to each power system element in order to make it possible to disconnect only the faulty section. Thus if a fault occurs at bus bars on the last zone, then only breakers nearest to the fault viz. 10, 11, 12 and 13 should open. In fact opening of any other breaker to clear the fault will lead to a greater part of the system being disconnected. Hence selectivity is given by dividing the entire system into several protective zones. When fault occurs in a given zone, then only the circuit breakers within that zone will be opened. This will isolate only the faulty circuit or apparatus, leaving the healthy circuits intact.
There is a certain amount of overlap between the adjacent protective zones. For a failure within the region where two adjacent zones overlap, more breakers will be opened than the minimum necessary to disconnect the faulty section. But if there were no overlap, a failure in the region between zones would not lie in either region and therefore no breaker would be opened. Thus the overlapping between the adjacent zones is unavoidable. (The overlap is usually small and the probability of failure in this region is low. Therefore opening of too many breakers will be quite infrequent.)
Fig. 1.2 Zones of Protection