fault current in a 2 winding xfo 1

[unclebob]

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?

 

[stevenal]

Yes.

 

[dpc]

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.

 

[jghrist]

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.

 

[ijl]

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.