Studies on fault impedance

[cranky108]

Does anyone know of any studies on the percentage of different fault impedances on transmission systems?

We should all know that phase to ground faults are the most common, but what percentage are high impedance and what percentage are low impedance?

Common thinking is most faults on transmission systems are low to mid impedance, but where did we get that from?

 

[davidbeach]

My experience in trying to make the model currents match the relay currents for faults at known locations is that nearly all faults need less than 5 ohms of resistance to make the model match the relay. When I need more than 5 it generally requires a few hundred ohms. Only one example comes to mind where it as in the 50-100 ohm range.

 

[HamburgerHelper]

How much does soil effect the impedance of the fault? I was going through a copy of Silent Sentinals that I picked up that mentioned possibly adjusting settings due to seasonal changes in ground resistivity. Do people in cold climates change the settings when the ground freezes? Does Apen oneliner use a rule of thumb for the impedance of the ground path? Is the ground path impedance usually so small that it can be ignored?

 

[davidbeach]

I can't see ever doing seasonal adjustments. I'd like to get through the whole system every few years. If it were truly possibly to know the annual range, it would be easy enough to set the different elements based on worst case time of year. But since it's nearly impossible to get one good rho value, I see nothing to be gained by trying to come up with two.

 

[cranky108]

We don't use season settings, and if we did it would be on the distribution system for holiday lighting.

What I have seen is while making coordination for low impedance ground faults, the coordination for high impedance ground faults seems to suffer. So I was wondering if there were any studies of typical fault ranges and chances for each range so I can pay more attention on the most likely events.

Another thing I have seen is that once you describe how the phase protection works, people tend to think they know all about protection. When the fact is more than 95% of faults are ground faults. So truthfully the phase protection is less important than ground protection, but phase protection effects tends to be more destablizing to the power system.

 

[piterpol]

We discovered high resistance faults on 400kV lines when we realized large MW consumption in comparison to MVAR.

 

[crshears]

Hey cranky, are you looking for some actual statistics? If so, I'll see what I can dig up for our utility.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]