Switch from ground overcurrent to ground distance 1

[HamburgerHelper]

For transmission, has anyone completely replaced their ground overcurrent schemes with ground distance protection. How difficult was it? Were there extra studies needed? What scenarios were looked at to validate the settings so they didn't underreach or overreach due to mutual coupling or infeed? I would like to if possible replace our ground overcurrent elements with ground distance due to our models never being 100% correct and not being forced to recheck coordination.

 

[davidbeach]

50 ohms was a rather arbitrary number that has worked reasonably well over time. Most ground faults with known resistance turn out to be less than 15 ohms, or a few hundred ohms. One case of a tree leaning on a line resulted in something much closer to 100 ohms and never flashed over to some much lower value. That one could have used a different setting criteria, but otherwise 50 seems to work.

 

[HamburgerHelper]

Davidbeach,

For 138 or 345 kV?

 

[davidbeach]

115kV and 230kV.

 

[Mbrooke]

What I've found, though, is that if you're working with lines that have particularly difficult conditions due to mutual coupling that calculating the k values you can get settings that are more sensible. On a line with no mutual coupling, and k set based on the line impedance, your ground and phase reaches are numerically identical - the whole point of k. Where mutual coupling is present you can't do that; either the reaches are numerically different or the coverage of the line is different. I like to have both zone 1 reaches the same. So, you can calculate the k0M1 and k0A1 values such that the reaches match at some point on the line, typically 85% in my cases. Then set k0M and k0A based on the line impedance or some other specific fault case; the remote bus for instance. This means that zone 1 and zone 2 (and higher) see different apparent impedances for the same fault.

Question, why not just set different reaches for the ground MHO but keep the K values the same? (ie phase Zone 1 at 80%, phase zone 2 125%; ground zone 1 85% and ground zone 2 165%) What advantage does a varied ground compensation factor have over modified reaches in mutual coupling?

My favorite reach anomaly is the pair of lines where a ground fault at the second bus out produces a lower apparent impedance than a ground fault at the first bus. Those are set with an impedance base k value. Next time they need to be touched I'm going to try the fault based selection of different k values and see if I can make the ground reaches much more like the phase reaches.

Im not following this. What conditions are present that result in one bus over reaching and another not over reaching? I am having trouble picturing the scenario.

 

[davidbeach]

It's a pretty perverse set up. Two parallel lines from A to B, call it 20 miles. At the 10 mile point the lines from A to B pass, but don't enter, station C belonging to a different company. There are also a pair of lines from C to B that are in the same right of way as the A to B lines.

For a fault at B all four lines have zero sequence current flowing in the same direction and the mutual coupling increases the apparent impedance. For a fault at C, as seen from A, the counter flow of zero sequence currents in the A-B and C-B lines decreases the apparent impedance.

 

[Mbrooke]

Ahhh, I see it now. Makes perfect sense- and to be honest it might no all be that perverse. Some POCOs do have this exact setup through out the system. Good to know however if I ever encounter it in the future. Will have to keep that in mind- and +1 for line differential lol.

 

[marks1080]

Distance all the way. Get your models fixed; it's very common for transformers to get modeled incorrectly. However, keep in mind, when working with ground values you need to look at k0 (zero-sequence) compensation. I'm not sure if modeling this is ever done 100% correctly. When you start to create your settings with apparent impedance in mind (not the physical system impedance) you'll see that setting distance elements really is as much of an Art (if not more so) than a Science. This may take some getting used to if you're used to setting o/c protections.

I don't know if this has been mentioned yet, but practically, the major difference from a high level perspective is the need for a VT or some voltage source, which is required for a distance protection. In our system I wouldn't expect to see a transmission element without ground distance, but on lower voltages the choice between adding a VT or not is often what decides whether a line gets distance or o/c protection.