•For convenience and to limit the extent of the power system that is disconnected when a fault occurs, power system is divided into several convenient protection areas called protection zones. Usually, the zones of protection overlap, so that no part of the power system is left unprotected. (See Fig.1)
For practical physical and economic reasons, this ideal is not always achieved, accommodation for current transformers being in some cases available only on one side of the circuit breakers. This leaves a section between the current transformers and the circuit breaker A (See Fig.2) that is not completely protected against faults. In the Figure 2 b, a fault at F would cause the bus bar protection to operate and open the circuit breaker but the fault may continue to be fed through the feeder. The feeder protection, if of the unit type, would not operate, since the fault is outside its zone.
•For practical physical and economic reasons, this ideal is not always achieved, accommodation for current transformers being in some cases available only on one side of the circuit breakers. This leaves a section between the current transformers and the circuit breaker A (See Fig.2) that is not completely protected against faults. In the Figure 2 b, a fault at F would cause the bus bar protection to operate and open the circuit breaker but the fault may continue to be fed through the feeder. The feeder protection, if of the unit type, would not operate, since the fault is outside its zone.
Figure 1 Division of power system into protection zones
Figure 2 CT locations
Figure 3 Overlapping zones of protection systems Selectivity
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