Explain over-current protection of transformer.
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Over-Current Protection of Transformer:


Simple over-current & earth fault protection of transformer against external shortcircuit and excessive overloads is shown in the above figure. The over-current and earth fault relays may be of inverse and definite minimum time type or definite time relays. The over-current relay has three elements; one for each phase winding. The normal range of current setting available on IDMT over-current relay is 50% to 200%. Due to excessive overload or any external fault, when over-current flows through the transformer winding, the corresponding phase CT supply the current to over-current relay. The relay depending upon its setting, sends trip command to the circuit breaker and the transformer is disconnected from load or fault.


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