Purpose of using Differential Protection in Transformer

Transformers are being protected from over current and earth fault protection, though we use differential protection. The purpose behind this is the over current and earth fault protection senses the fault that is out of the zone of transformer also, but differential protection only senses fault within its specified internal zone. This specified zone we can consider where protection type PS class current transformers are installed on high voltage and low voltage side of transformer. 

Suppose there is any internal fault in transformer tank where oil is filled, then there is buchholz relay protection is given for detection of fault and to isolate the transformer. But in case of fault at points of bushing up to placement of CT, buchholz relay can not detect that fault. Differential protection can detect fault internal faults as well as this types of faults occurring at bushing points. The time taken by differential protection compared to buchholz relay is also very less and it can quickly isolate the transformer in case of faults.

Generally this type of protection is being used for transformers above 5 MVA capacity.