Control.com - Nerds in Control

In case of differential protection, say for generator, we know that current entering a phase must be equal to current leaving that phase. Now for example, there is a short cct between phase R & y at the point where current is entering the phase, just after CTs. So, now what will cause diff protection to operate? I mean due to short cct, will the current leaving these two phases will become zero? Which CTs will face maximum current, ones near the fault point or the others?
Will the current will flow in the reverse direction from the point where this short cct has happend? Please explain

Speedtronic, see:

http://control.com/thread/1026242938

Phil Corso

I suppose you are talking about a two-phase short circuit. With a high impedance grounding in the generator (to reduce short circuit current in case of a phase to ground fault). The differential protection protects for two-phase and three-phase faults only.

The differential protection protect only against faults between the associated current transformers. The basic algorithm sums the vectors of the currents in such a way than the sum is zero when there is no fault inside the differential zone during normal operation. When a short cct occurs inside the differential zone the sum is different from 0 (there is a margin, the minimum pick up, for usual errors during normal operation). The contribution to the fault from the generator flows from the neutral side to the point where the fault is. And about the contribution of the electrical network it flows from the network to the faulty point. As the differential protection sums the currents it will trip in any case (with the contribution of both sides in different directions the sum is different of zero or with the contribution of just one side when the other side is disconnected(generator or network).

The maximum contribution to the fault usually is from the neutral side but the truth is it depends on the the venin equivalent circuit of the electrical network at the point of connection.

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