Why Diffrential and Ref relays are used together although Diffrential relay ca

Why Diffrential and Ref relays are used together although
Diffrential relay can protect Pri and Sec of Transformer

Question Posted / vishal

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Why Diffrential and Ref relays are used together although Diffrential relay can protect Pri and Se..

The normal bias setting in a differential relay is 20%.
So, complete earth fault protection is not possible with
differential relay. That is why you need a seperate earth
fault relay with sensitive settings like 5%.

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