L-G Fault Zero Sequence Current in un-faulted phases

[rockman7892]

I know that for a single L-G fault the ground fault current If on the faulted phase is represented as 3I0 and it is this 3I0 that can be measured and represented as ground fault current. I also understand that this 3I0 value comes from the contribution of both the pos sequence, neg sequence and zero sequence current on the faulted phase (I1=I2=I0=1/3IF).

What I am not clear on however is what if any zero sequence is present in the other two unfaulted phases during a L-G fault. I recall reading somewhere once that there is I0 current in the other two phases however they are offset by the negative sequence current in those phases?

If there is I0 current present in the other un-faulted phases can it be measured with CT/relaying on that phase or is it offset to the degree where it is not measurable?

Appreciate any insight in helping my understanding.

 

[davidbeach]

The zero sequence current in each phase is identical to the zero sequence current in the other two phases. The positive sequence current in each phase has the same magnitude as that in each of the other two phases with an angular difference such that A leads B leads C by 120 degrees each. Likewise, the negative sequence current in each phase has the same magnitude as that in each of the other two phases with an angular difference such that A leads C leads B by 120 degrees each.

In many fault cases, for a single phase fault, the positive, negative, and zero sequence currents of the faulted phase are in phase with each other and of equal magnitude. I1 + I2 + I0 = 3I0. The angular differences between the three sequence keep them from adding up like that in the other two phases. But look at the current into/out of a grounding bank for a fault elsewhere on the system. Just I0 in each phase, all on top of each other. But the three add to 3I0.

But you can only calculate the sequence currents; all three phase magnitudes and angles need to be known. But then, by definition, you can calculate each, and also by definition, the magnitude of any sequence in each phase will be the same as the other phases, though the magnitudes of the three sequences can be vastly different.