Friday, September 30, 2011

Protective Zones

Protective Relaying (Part7)       
       In a protective relaying scheme, the circuit breakers are placed at the appropriate points such that any element of the entire power system can be disconnected for repairing work, usual operation and maintenance requirements and also under abnormal conditions like short circuits. Thus a protective covering is provided around each element of the system.
       A protective zone is the separate zone which is established around each system element. The significance of such a protective zone is that any fault occurring within a given zone will cause the tripping of relays which causes opening of all the circuit breakers located within the zone. The various components which are provided with the protective zones are generators, transformers, transmission lines, bus bars, cables, capacitors etc. No part of the system is left unprotected. The Fig. 1 shows the various protective zone, used in a system.
Fig. 1  Protective zones
       The boundaries of protective zones are decided by the locations of the current transformers.
       In practice, various protective zones are overlapped. The overlapping of protective zones is done to ensure complete safety of each and every element of the system. The zone which is unprotected is called dead spot. The zones are overlapped and hence there is no chance of existence of a dead spot in a system. For the failures within the region where two adjacent protective zones are overlapped, more circuit breakers get tripped than minimum necessary to disconnect the faulty element. If there are no overlaps, then dead spot may exist, means the circuit breakers lying within the zone may not trip eventhough the fault occurs. This may cause damage to the healthy system. The extent of overlapping of protective zones is relative small. The probability of the failures in the overlapped regions is very low, consequently the tripping of too many circuit breakers will be also infrequent. The Fig. 2 shows the overlapping of protective zones in the primary relaying.
 Fig. 2  Overlapping zones in primary relaying
       It can be seen from the Fig.2 that the circuit breakers are located in the connections to each power system element. This provision makes it possible to disconnect only the faulty element from the system. Ocassionally for economy in the number of circuit breakers, a breaker between the two adjacent sections may be omitted but in that case both the power system sections are required to be disconnected for the failure in either of the two.
       Each protective zone has certain protective scheme and each scheme has number of protective systems.

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