@Oblss: Question- how are substation transformers grounded in your country? What is the primary connection of pole transformers? Do you have 3 or 4 wires on your MV circuit? What voltage do you use?
I've always known that in the EU transformers are almost always delta primary which lets you set ground pick to a very low value.
@Itsmoked: Others can explain it better than I can, but by reclose I mean reclose automatically. (ANSI 79) Meaning if the feeder trips out automatically on over-current it will close back in automatically after a few seconds. If the fault is still there the circuit will trip back out automatically. In my case the feeder won't close back in after the very first trip.
This is different from re-energizing a feeder manually after circuit repair or inspection.
A typical distribution transformer would be DYn, 20/0.4 kV with the LV side resistance grounded, thus the MV circuit is 3 wires.
As for the ground pick, I think that high impedance faults as the one described above are detected by negative sequence current measurements more accurately ...
If you have come across different approaches I would like to hear them, my scope of work was mainly in choosing the type of covered wire and related accessories but I am interested in a general approach.
Most of our distribution is multi-grounded, but in some areas, that is not very good, you know sand, or rock. But we have had problems balancing distribution circuits, so we do use a ground setting higher than 2/3. Actually we have seen large load changes from season to season, and around holidays. As a dual peaking utility, it is hard to predict between compressor AC and electric heat, and holiday lights to the max.
We also have a few old delta circuits, where the ground fault currents calculate to about 5 amps, which is hard to detect and it has started fires.
My old company had tried covered conductors years before I worked there. They looked bad as the insulation was pealing off, or hanging off the wire. That's my thoughts on it.
But now the company I work for only has 30 foot right-of-ways, so they can not trim outside of that, and we still get trees bending so much in the wind to come into contact. The customers want us to underground the old lines, but don't want us to dig up the tree roots, and don't like the green boxes.
Though we have more problems with animal contacts, where people feed the things. Tree wire does not fix this, and animal guards only work so-so.
A comment on negative sequence, most three phase solar inverters do not create negative sequence into a fault, so you can't depend on them for that, or those for directional support.
Are the animal guards secure? I've seen a lot of open or improperly installed animal guards. Also want to ask, how is the covered wire not working? Is it tracking through?
