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Power Systems Protection Criteria
The three (3) basic criteria when designing protection schemes for power systems – Selectivity, Speed and Reliability.
Selectivity is the ability of the protection scheme to trip the minimum number of devices to isolate a given fault in the system. It also means to isolate the correct devices only and leave out the rest of the systems operating.
Speed means that the protection devices, such as relays and circuit breakers, must isolate the fault in timely manner. In North America, the requirement set out by the North American Electric Reliability Corporation (NERC) is 100mSec (or 0.1 Second). This requirement only applies to High Voltage systems of 115kV and above. No specific requirements applies to voltage levels below mentioned.
Reliability means that the protection system must be secure and dependable.
Power Systems Protection Zone
The main objective of defining zones of protection is simple – to make sure that there are no gaps or no part of the system is left unprotected.
Figure-1 above illustrates this. The zones of protection are overlapping. For example, the the Generator Zone overlaps with the Transformer Zone which overlaps with the Bus Zone and so on. In this manner, all parts of the system from Generator all the way down to the load is covered.
Dual Element Spot Network (DESN)
Dual Element Spot Network or DESN is a design concept that provides redundancy by duplicating the critical components of a network, such as transformers and buses, in order to provide flexibility for the system during fault conditions.
The idea is that, as in the case of Figure-2 above, whenever one of the transformers are out of service maybe due to maintenance or fault, the system allows the other transformer to carry all the loads by simply isolating the faulted components and switching all the loads to the healthy part of the system.
Figure-3 above illustrates a sample design for a 34.5/12kV Distribution Substation. There are two(2) power transformers and two(2) 34.5kV Gas Insulated Switchgear (GIS) and two(2) 12kV GIS.
Bus-Tie breakers are installed between the 34.5kV gears to allow the system to transfer the feed in case one of the transformers are commissioned or faulted. Similarly, a Tie-Breaker arrangement is also installed between the 12kV gears which also allows the loads to be transferred.
In this manner, it forms a ring or loop which interconnect all the four GIS gear. This design provides maximum flexibility and ensures that customers gets power in the event that one of the transformers are down.
Relay Pickup, Relay Dropout
When the actuating value applied to the relay increased to a point where it operates, this is called the Relay Pickup value. For example, if the relay is set to operate, say at 125% of 5A, then the pickup is at 6.25A. At that current value the relay will operate. On the other hand, when the value goes below that set point then the relay contacts will reopen, this is called the Relay Dropout.
Normally Open, Normally Closed
The contact is said to be Normally Open (N.O.) when the contacts are open when the relay has not operate. It is considered Normally Closed (N.C.) if it is closed when the relay has not operated.
In the case of circuit breaker auxiliary contacts, a Normally Open contact is one that is open when the breaker is in the Open Position while a Normally Closed auxiliary contact is one that is closed when the breaker is in the Open Position.