Hey guys,
I want to confirm my understanding on the differential protection relay scheme, from a Cram for Exam Vol. 2 problem.
In problem 4 of the Cram for Exam Vol. 2 book, the solution basically states that a differential protection relay scheme will not trip for a turn-to-turn fault in the machine winding, an open-circuit in the machine winding, and high currents caused by an external short circuit. It doesn't elaborate fully on why, but I think I understand it and I just want to confirm my understanding.
In a turn-to-turn fault, the current flowing on both sides of the winding will still be the same (or at least the same p.u. in the case of a transformer). Hence, both CTs will see the same current and the differential protection relay will not trip. The relay does not see a difference in current.
In an open-circuit in the machine winding, the current is basically 0 on both sides of the winding. Hence, both CTs see 0 current and there is no difference in current. So the differential relay does not trip.
In an external short-circuit fault (outside of the differential relay protection zone - outside of the two CTs), the same fault current (or the same per-unit fault current for a transformer) flows through both CTs. Hence, the relay does not detect a difference in current between the two CTs. The differential relay will not trip.
Is my understanding of this correct? Just want to make sure.
Thanks!
I want to confirm my understanding on the differential protection relay scheme, from a Cram for Exam Vol. 2 problem.
In problem 4 of the Cram for Exam Vol. 2 book, the solution basically states that a differential protection relay scheme will not trip for a turn-to-turn fault in the machine winding, an open-circuit in the machine winding, and high currents caused by an external short circuit. It doesn't elaborate fully on why, but I think I understand it and I just want to confirm my understanding.
In a turn-to-turn fault, the current flowing on both sides of the winding will still be the same (or at least the same p.u. in the case of a transformer). Hence, both CTs will see the same current and the differential protection relay will not trip. The relay does not see a difference in current.
In an open-circuit in the machine winding, the current is basically 0 on both sides of the winding. Hence, both CTs see 0 current and there is no difference in current. So the differential relay does not trip.
In an external short-circuit fault (outside of the differential relay protection zone - outside of the two CTs), the same fault current (or the same per-unit fault current for a transformer) flows through both CTs. Hence, the relay does not detect a difference in current between the two CTs. The differential relay will not trip.
Is my understanding of this correct? Just want to make sure.
Thanks!
