Ground Fault Current Contributions

[mgtrp]

Hi all,

I'm in the process of preparing settings for a transmission line protection relay, and as part of this am simulating faults in various locations using our load flow/fault calculation software (ETAP).

With reference to the simplified network diagram in the attachment, the operation of the line protection for a line-ground fault in the location shown is that T4 protection will see the fault, trip the T4 LV CB (no high-side CB), and operate a high-speed grounding switch on the transmission line to give a bolted line-ground fault. The protection relay that I'm setting then sees this bolted ground fault and trips its own local circuit breakers.

When I simulate a ground fault in the location shown with G4 in service (i.e., before the T4 protection relay has tripped the T4 LV CB), the ground fault current seen by my protection relay is relatively high - about 170A - compared with about 80A seen by T4's protection.

When I simulate the same fault with G4 out of service (i.e., after the T4 has tripped the T4 LV CB), two things happen:
1/ The overall ground fault current increases to 325A
2/ The ground fault current seen by my protection relay collapses, with virtually of it the ground fault current (302A) returning via T4, while only 23A returning via T1, T2 and T3.

This seems somewhat counter-intuitive to me - removal of a source resulting in increased ground fault current, and a massive shift in the way the currents flow. Therefore, I drew out a rough sequence network for each case, also shown in the attachment. In case 1 (G4 in service), I would expect a higher fault current due to there being more parallel paths for the fault current to flow, with the zero sequence current splitting in proportion to the combined [ZTX + ZT1/ZT2/ZT3] zero sequence impedance to the ZT4 zero sequence impedance. In case 2 (G4 out of service), I would expect a lower fault current, with the zero sequence split remaining the same.

Am I missing something, and about to have a learning moment, or is my software leading me down the wrong path? I've already checked the data entered for each system element (or at least as much seems relevant - my sketch is a simplified version of a small portion of a large network model), and there don't seem to be any data entry errors.

Thanks in advance!

 

[HornTootinEE]

Do you have a much lower fault impedance once your grounding switch closes vs. before? That is to say, if your software is assuming some fault impedance then when T4 trips and the ground switch closes the fault impedance goes to zero which could increase the fault current, even after removing the G4 source.

Just a thought.

 

[PHovnanian]

How are you modeling loads and regulation in ETAP?

Prefault load flows determine source voltages. For the G4 out of service, the G1, G2, G3 voltages will be higher and so will the subsequent fault currents.

 

[mgtrp]

The fault impedance is the same for each case. Pre-fault voltages are fixed at 100% of nominal, so these shouldn't be different between the two cases.

I've created a heavily simplified ETAP model and get results that agree with what I expected for the two different scenarios, so there is presumably an issue in how I've created the either the ETAP software (unlikely) or the way I've built the model (almost certainly!) rather than my understanding of the what should happen. I'll work with the ETAP helpdesk to work out what the issue is.

Thanks for your replies!

 

[mgtrp]

For what its worth, problem solved - I had placed the T4 LV CB between the transformer and the LV bus, which caused ETAP some difficulties. A definite case for backing up your software calculations with some hand calculations.

 

[HornTootinEE]

Yes, and prime example of the need for some knowledge/expierience/understanding running the software. Corporate managers like to think you can put a monkey behind the simulation software and it's all a done deal. Pay an intern $15/hr to run studies rather than hire an engineer.