Differential protection, also called zone protection or unit protection, is used for detecting the fault in a particular equipment or section. Differential protection is used for the protection of large-rating alternators, transformers, and motors.
Differential protection is based on the principle of sensing the incoming and outgoing current and finding out the current difference. Principally, if the equipment is healthy and there is no fault inside the equipment, the incoming and outgoing current is equal. The difference in current shows a fault inside the equipment, and the differential relay trips the upstream breaker to isolate the faulty section.
The differential relay must trip only if a fault exists in a particular equipment, section, or zone. In the event of a through-fault condition, the relay should not trip.
The differential protection scheme is shown in the image below.
Two current transformers of PS class, CT1 and CT2, are mounted at the incoming and outgoing sides of the equipment to measure the current. Both CT1 and CT2 output are given to a differential protection relay. The relay measures the current and calculates the difference of the current. When the differential current exceeds the set point, the relay outputs a trip command that trips the breaker.
The protection class CTs 5P10,5P20,10P10 are used for overcurrent protection in feeders. Here, note that in differential protection, PS class CT is used. Never use a protection class CT for differential protection. Let us understand why protection class CT is unsuitable for differential protection.
If we use 5P10 protection class CTs for differential protection, theoretically, CTs should be saturated if the fault current through CTs flows more than 10 times. However, there are chances that one of the CTs saturates and the other keeps working in its linear zone. This will cause differential tripping, though there is no fault inside the transformer or electrical equipment. Therefore, in a nutshell, we can say it is difficult to determine at what fault current the protection class CTs will saturate. Therefore, the reliable operation of unit or zone protection can not be ensured with the use of protection class CTs.
The PS class CT falls in the category of special protection class CT. Both the PS class CTs used for zone or unit protection have identical saturation characteristics, and their knee point voltage(KPV) is the same. The designer takes the data like lead resistance, relay burden, and system fault level from the user before designing PS class CT. Both CTs used for differential protection operate at the same operating point of the magnetization curve under the through fault condition, and thus, the differential relay does not operate under through fault condition.
The PS class CT is designed considering the system fault level and burden of the CTs. These parameters decide the knee point of the PS class CTs,
The knee point voltage PS class and protection class CT formula are given below.
