Electrical Protection is considered as the heart of power system. Protection system should be such robust to isolate the fault in minimum possible time to minimize the equipment damage. The design of electrical protection is based on the following basic considerations:
Adequacy
The selected protection should be adequate to cover all types of faults and abnormal conditions. The operating principles of the selected relay scheme should ensure that the scheme would be responsive to all possible abnormal conditions.
High Speed of Operation
The protective relays shall have high speed of operation. This is important from the consideration of minimizing the damage and also for improving the power system stability. High speed operation should also be consistent with requirement of selectivity i.e. only minimum necessary equipment be disconnected from service.
Stability
Unit protection should remain inoperative and stable for external faults. Other protection should operate with proper discrimination for external faults. Any mal-operation of relay or loss of discrimination may result in tripping the large unit out of service and will have very adverse repercussions on the operation of the total system. The stability of the protective scheme is to be ensured by proper choice of CT, PT parameters and relay settings.
Redundancy
Redundancy means to use extra protective relay to perform the same job. This is done to ensure that fault is isolated even in case of failure of one relay. For this purpose it is common practice to use two relays of different make and model so that their working principle may be different.
Also to achieve complete redundancy, it may be necessary to provide, for each of the redundant scheme, separate cores of instrument transformers, DC power source & communication channel and trip relays. The trip contact of lockout relays and trip coils of circuit breakers may have to be duplicated for each scheme to the extent possible.
Reliability
The protective relying should not fail to operate in the event of fault in the protected zone. The maximum fault clearance time for 400 kV and 220 kV systems shall be 100 ms and 160 ms respectively.
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