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fault current in a 2 winding xfo

fault current in a 2 winding xfo

fault current in a 2 winding xfo

(OP)

I'm using a software for calculating fault currents in a network (don't wanna do this by hand). But I'm getting strange readings: the balanced 3 phases fault current is smaller than the single line-to-ground in a 675 kVa xfo. The xfo is Dyn1, with the secondary grounded. Is this normal?

RE: fault current in a 2 winding xfo

Yes.

RE: fault current in a 2 winding xfo

In a core-type delta-wye transformer, the zero sequence impedance is generally slightly lower than the positive sequence impedance.

Most reference sources cite Zo = 85% of Z1.  

So, without any other impedance, the line-to-ground fault current will be higher than three-phase.  

RE: fault current in a 2 winding xfo

Even if the zero sequence impedance were the same as the positive sequence impedance, the 1Ø fault would be higher than the 3Ø fault because the zero sequence impedance of the system ahead of the transformer is not included in the network.

Suggestion:  Do it by hand a couple times before using the software.  You will gain a better understanding of the results.

RE: fault current in a 2 winding xfo

Consider a simple network  consisting of a source with some internal impedance, and the DYn-transformer. The source is connected to the D-side.

When there is a line to earth fault on the secondary (Y) side, current flows in one secondary winding only. On the primary side, current flows also in one winding. One phase of the source feeds current to (or from) one primary winding only.

In the case of a three phase fault, there is current in all windings of the transformer. Each phase of the source feeds current to (or from) two primary windings, not one, as in the case of the earth fault. Thus, due to the internal impedance of the source, the current in one winding must be smaller than in the case of the line to earth fault. As a  consequence, the fault current is also smaller.

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