Phase to Ground Zero Sequence Fault Current - Half Magnitude?

[highvoltagetom]

Hi all, been reading the forum for a while but this is my first time posting. Thanks for all the information.

I am trying to make sense of some fault current waveforms from a fault just outside a transformer. By all appearances, this is a single-phase to ground fault. To confirm, I calculated the symmetrical components of the fault waveform (I can't upload pictures yet as I am a new user).

It is my understanding that a single-phase to ground fault should produce equal magnitude I0, I1, and I2, all in phase, because the fault puts the sequence networks in series. However, I am seeing equal magnitude I1 and I2, but a zero-sequence current I0 with roughly half the magnitude of I1 and I2. All are roughly in phase.

Does anyone have any insight into what this might tell me about the fault? I haven't seen a similar fault signature in any textbooks or online examples. My instinct is either this means it is not a "pure" phase-to-ground fault (i.e., there is some involvement from another phase), or it has something to do with the fact that these fault traces were taken at a slight distance through the line impedance, not directly at the fault itself.

Any insight is much appreciated.

 

[D. Castor]

I assume you are looking at fault waveform data from a digital relay. It should be possible to also look at the actual phase and ground current instead of the sequence currents. That may give you a better understanding of how the fault started. But keep in mind that many faults that start as line-to-ground will quickly escalate to three phase faults. Due to arc resistances, faults can manifest an infinite variety of current distributions and change rapidly during the fault.