Does anyone know how to take on a DLG fault where Vbg and Vcg are unequal.

In the book the equation is Vbg=Vcg=Zf(Ib+Ic) but how would the problem be worked if Vbg and Vcg were not the same?

Wouldn't they have to be the same since they are connected, hence double line to ground fault?

I understand the concept well but just don't see how to solve it if they are not equal.

 

If the two lines are solidly grounded at the same physical location then the line voltages at the fault point will be the same. The individual line currents into the fault may differ if the line impedances on the two phases are different - for whatever reason. Also each line source voltage may be different - again for whatever reason. But the circuit conditions would force the same voltage at the double line fault point.

Another possibility is the fault being at the same physical location but the fault impedance on each line being different. Say one line is solidly grounded at the fault point and the other is conducting through an arcing path.

One can also envisage a situation where two lines are simultaneously grounded at different physical locations - so both the fault voltages and line currents would possibly be unequal. This would depend on the individual fault impedance at the two different locations.

The possible scenarios are manifold.

Is this a general question or do you have a specific problem / scenario in mind?

 

I thought about the impedance as well but I'm with you in thinking that if both lines were connected and grounded then the voltage potential would be the same.

My professor postulated a question about what if the 2 voltages were different. I was thinking you would have to introduce some impedance to 'balance' the fault so it would be possible to use sequence network and ultimately solve the problem via symm components and the equivalent network.

If he gives us a homework with that condition specifically I'll post the question but in the mean time I was more or less just thinking and wanted some input from others.