Coordinating ground distance with directional ground overcurrent

[HamburgerHelper]

Are there any good rules of thumb or any good references on how to coordinate ground overcurrent with ground distance protection? This system I am working with just has instantaneous and toc curves for ground protection. I worked on a job for another utility and the toc curves were selected to not trip for a remote end fault for something like 0.5 seconds, giving time for the distance relaying to clear out the fault before the ground overcurrent protection had a chance to operate. For inline faults, I think the ground overcurrent curves sometimes beat the distance protection. At least from what I have read, ground overcurrent provides better sensitivity, may be faster or slower than ground distance, and better tolerates resistive faults. The reasons I would like to include distance protection is that it is not as dependent on how much fault current is available and that allows the scheme to be more independent to anything going on around it. There are probably relays in the system that have old ground overcurrent settings that have never been upgraded and probably should have as things were added to the system. Do you really get that much by having both ground overcurrent and ground distance? Is it too much of a headache to coordinate that it is better just to use distance protection for everything? How rare are high impedence faults at transmission level and should you even be worrying about them? Almost all the lines have DCB as well so maybe it doesn't even matter?

 

[davidbeach]

Ditch the direction ground overcurrent as fast as you can. We started using ground distance a few years ago and find it makes the coordination challenges far easier to deal with. At true transmission voltages, 230kV and above, the variations in source impedances are generally small and the lines generally longer than at the sub-transmission voltages (115kV and 57kV in our case). Coordinating ground overcurrents on the sub-transmission was an exercise in finding least bad settings, nothing good about it. Ground distance coordination is far easier. Where we have POTT we still use directional ground overcurrent in the KEYing equation. That way we can get the resistive coverage without the coordination issues. We also use quad ground and set the R component of the furthest zone to meet our resistive coverage goals if possible.