Answer / rukmagoud

Differential protection requires two cts for its connection.
If one of the CT reaches its Knee point voltage , then the
CT Enters into Saturation. so when Ct is in Saturation
state, its characteristic changes, i.e the current will
not be proportional to primary current thereby
differential current comes into picture in differential
relay , there by there is a chance of maloperation of relay.
This usually comes when there is any through fault.

rukmagoud
APGENCO/NAGARJUNASAGAR

Answer / ramesh p

the differential relay will get maloperation when one of
the CT get saturation

Answer / girish kumar samal, hindalco,h

DIFFERENTIAL PROTECTION FAIL. CT DOES NOT GIVE SECONDARY
CURRENT ACCORDING TO PRIMARY. EOR WHICH DIFFERENTIAL RELAY
OPERATE.

Answer / tony

Normally the knee point voltage must be

Vk > 2*Vs

Where Vs is the calculated max. voltage appears across the relay.

Beyond knee point the chara of CT is no longer proportional to the primary current and this cause faulty measuring by relay.Thus if we have higher Vk we can easily with stand higher Vs(fault equivalent seen by relay)

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