The differential protection also called zone protection or unit protection is used for detecting the fault in a particular equipment or particular section. The differential protection is used for protection of large rating alternators,transformer and motors.
The principle of differential protection is to sense the incoming and outgoing current and finding out the difference of current. Principally, if equipment is healthy and there is no fault inside the equipment, the incoming and outgoing current is equal. The difference in current shows fault inside the equipment and the differential relay trips the upstream breaker to isolate the faulty section.
The differential relay must trip only in case there is fault in particular equipment,section or zone. The differential protection relay should not trip in case of through fault condition.
The differential protection scheme is as given below.
Two current transformers CT1 and CT2 are mounted at the incoming and outgoing side of the equipment to measure the current. Both the current transformers are wired in a differential protection relay. The differential protection relay measures the current, and calculate the difference of the current. The realy output a trip command to breaker if found difference in the current.
For feeder protection, protection class CTs 5P10,5P20,10P10 are used. The protection class CT can not be used in place of PS class current transformer. Let us understand Why protection class CT can not be used for differential protection?
If the 5P20 protection class CTs are used for differential protection at incoming and outgoing side of the transformer. In case of fault, If 20 times of rated current of CT flows through it, the current transformer will not saturate because the accuracy limit factor of the current transformer is 20. The CT may function well above the 20 times current, however it is difficult to predict at what current the CT will get saturate.
The protection class CT installed at incoming side of the zone protection lets say saturate at 21 times current and the CT installed at outgoing side of the zone protection saturate at 22 times current of the rated current of CT. In a faulty condition the difference of current will be sensed once the relay sense the saturation of one of the CTs. Thus, the differential protection can not be reliably ensured with use of protection class CTs.
The PS class CT is special protection class CT. Both the PS class CTs used for differential protection have the identical saturation characteristics and their knee point voltage are the same. The designer takes the data like lead resistance,relay burden and system fault level from user before design of PS class CT. In through fault condition, both the CTs operate at same operating points of BH curve and thus differential relay remains inoperative under through fault condition.
The knee point voltage (KPV) of current transformer depends on.
System fault level
Total burden of current transformer
The knee point voltage of the protection class CT is;