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What is the use of Z transform?

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The purpose of the restraining coil in a biased differential relay is 

(A) To reduce CT saturation during fault. 

(B) To increase the sensitivity of the relay. 

(C) To match the transformation ratio. 

(D) To limit the spill current through the relay during heavy external fault.

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The purpose of the restraining coil in a biased differential relay is To limit the spill current through the relay during heavy external fault.

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