Transmission ground distance relaying with distribution underbuild

[stevenal]

How do you handle the mutual coupling?

1. Ignore it.
2. Carefully model the distribution circuit(s) and mutual(s) in your fault program.
3. Model just the distribution neutral in your fault program.
4. Measure the line impedances including the mutual coupling with test equipment.
5. Use actual fault data to estimate the parameters.
6. Avoid using ground distance. (In favor of?)
7. Something else. (What?)

Thanks for sharing.

 

[stevenal]

I had the path wrong on G-H, it does share a path with F-G for part of it's length.

As far as G-H coupling with F-H, would a parallel path 2000 ft away for 2.4 miles have an effect?

 

[bacon4life]

Prefault current of 17 amps might be a branch/tree sitting on the line for a short duration before the fault evolves to low impedance.

Can you post the waveform recording? When the fault magnitude varies throughout the fault it is unclear to me which instant the relay uses to calculate the location.

Mutual coupling is inversely related to separation distance, so lines 2000 feet apart would have very tiny impact.

 

[stevenal]

Thanks.
Not much fault variability once it got beyond the "pre-fault" stage. I tried some calculating in Sychrowave Event, zoomed in on the pre-fault.

 

[stevenal]

The setting has been corrected in the relays, so the next fault should give the distance. Modelling the mutuals with the two other 115 kV lines gave no better results, so reach settings remain the same.

More recently, a voltage sensitive customer was affected by a line to ground fault on a 69 kV line, so I looked into the event there. Differential worked just as it should, but I went looking for trouble and found that Z3 picked up on one end and no zones picked up on the other end for a fault location that should have been seen as Z1 for both. On a trouble finding mission now, I looked at all the fault records I could find. All the line to ground faults failed to pick up on the intended zone. There was only one line to line fault, and it picked up properly in zone 1. It would seem that Z0 is the problem. SEL thinks that the poor SIR for these short lines over here at the ends of the grid is the problem, but I'm still hopeful we can model it better. I'm kinda leaning toward option 4 in the OP. Anyone here use Omicron equipment to measure line impedance? If so, how well did it work? Thanks.

 

[davidbeach]

SMUD has used the Omicron equipment and has found the results useful.

 

[bacon4life]

I have never used the Omicron. From looking at the ratings, I would not have realized it had a high enough withstand rating for testing parallel lines. The capacitively coupled voltage on our parallel 115 kV lines regularly exceeds 10 kV on isolated wires.