Mutual coupling of different voltages 1

[Mbrooke]

Ok, this is what modeling and standards have taught me: when dealing with a single circuit transmission line Zone 1 MHO is set at about 85% of the line while Zone 2 MHO is set at 125% of the line. Now, when dealing with a double circuit transmission line (two circuits on one pole) Zone 1 is set at 80% and Zone 2 at 160%. The reason for this adjustment is that mutual coupling causes zone 1 to over reach and zone 2 to under reach when compared to a single circuit line.

However, I am uncertain exactly what happens when dealing with different voltages, ie 345kv on one side 115kv on the other. This setup is usually found in narrow easements where capacity needs to be increased. My question is, how does this mutual coupling effect MHO and Quad elements?

 

[davidbeach]

Firstly, you only have to be concerned with elements to respond to zero sequence quantities. No worries about phase elements.

For truly parallel lines the tendency is to under reach until the remote end opens and then the reach expands. The generalized case is that the element will tend to under reach if the zero sequence current of the coupled line(s) flows in the same direction as it does in the protected line and that the element will tend to over reach if the flow is opposite.

Lines that are strongly mutually coupled but only weakly electrically coupled can produce unexpected results.

Odd line layouts can produce positively weird results. I've got a situation where a ground fault a bus beyond the end of the line produces a lower apparent impedance than a ground fault on the bus at the end of the line.

If you're willing to use fault cases to come up with k0M/k0A and k0M1/k0A1 values rather than just using the line self impedance you can somewhat counteract the tendencies mentioned above.

 

[Mbrooke]

In such a case would I be better off with Quad ground elements instead of MHO ground elements? Or do both give the same results?

 

[davidbeach]

As far as I know they both suffer similar issues.

 

[Mbrooke]

Figured as much. I will play around with the k0M/A factors and see what I get.

 

[JJHorak]

The voltage difference makes me wonder how much a fault at the end of say the 345kV line creates a current in the 115kV line, and vice versa. If there is low Io in the parallel line for a fault on the parallel line, then the parallel line would have low effect on the reach of the relays on the protected line and reaches would not be affected as much as when the parallel lines terminate at a common bus at each end. If the parallel line has lots of effect on the reach, then a quadrilateral shape has a bit more capability to be shaped around a specific problem on the RX plane than a mho circle, but you will need special software to accurately model what is going on, while Z0 and mutual coupling is one of the hardest values to have high faith in.
What software do you use for modeling your T-lines?

J. Horak, P-R Engineering, Colorado

 

[cranky108]

If you are unsure of the zero-sequence impedance, why are you trusting ground distance? Ground over-current has worked well for many years.

 

[Mbrooke]

I am going to admit something here... I am not 100% sure how to coordinate ground over current. To me over current is only used during a loss-of-potential as a final backup. But I am willing to learn.

 

[davidbeach]

If ground overcurrent worked well, even sort of half-arsed well, we would never have made the transition to ground distance. If we're going to misoperate for a ground fault it's going to be one of the places where we still have ground overcurrent rather than ground distance. That change has never caused us to have any regrets.

 

[Mbrooke]

@ David I agree. Ground over current might work well where generation is consistent and fault current remains the same at each buss, but in most power systems that varies to much in order for ground over current to coordinate 100% of the time. Distance elements pin point the location of the fault regardless if the bus goes from strong to weak or weak to strong.

 

[JJHorak]

With complex enough software models of the system, ground overcurrent is still used. I concur it would be nice to bypass the tedious checking of their coordination for a zillion different outages. I guess till all the old EM relays go away, they remain even in new uP relays, at least in the companies I have been working for.

J. Horak, P-R Engineering, Colorado

 

[Mbrooke]

What advantage does keeping or using ground over current have? I have to admit I am clueless on this subject.

 

[HamburgerHelper]

DavidBeach,

What sort of issues and how often were you running into problems due to using ground overcurrent? We use only ground overcurrent in a system with a lot of mutual coupling and it doesn't seem to be an issue. Our protection philosophy is very conservative so maybe there are issues but not enough to force a change. Any move to ground distance probably is going to come due to some NERC standard requiring the checking of coordination.

 

[davidbeach]

We're a mostly urban system, with a lot of 115kV on the top of poles shared with distribution. Lines are relatively short and all are completely redundant, so except at the winter and summer peaks we can take pretty much any line segment out of service for what ever project; some is our own work for our own purposes, some is road widening projects, some is clearance for cell antenna installations/maintenance, there are a multitude of reasons. Every one of those switching operations changed the balance of source impedances; not only for the line being switched but possibly for several other lines as well.

Ground distance, particularly for zone 1, doesn't give a whit about source impedance, either fore or aft. Back in the ground overcurrent days we could come up with phase distance reaches within a couple of hours, but might spend days tweaking ground overcurrent settings and having to adjust surrounding stations to make things work. Now ground is nearly as easy to set as phase. Mutuals make it somewhat more difficult, but we're still able to spend more time looking for odd corner cases now that we're not sweating bullets over simple N-1 conditions.

Realizing what we can achieve by using the fault studies to select independent k0M/k0A and k0M1/k0A1 the mutual problem seems to be less of an issue than when we were setting k0M/k0A = k0M1/k0A1 and calculating both based on the line's self impedance.

Our experience, on our system, is the ground distance is vastly easier to set and not misoperate than ground overcurrent ever was. Others may have other experiences.

Once PRC-027 becomes effective I'm going to guess that a lot of people who've been on the fence will jump into the ground distance camp. Anything based on line impedance is not subject to PRC-027. So, ground distance zone 1 set at 85% of the line doesn't fall under PRC-027. Ground distance zone 2 set at 125% of the line impedance doesn't fall under PRC-027. But each and every ground overcurrent element set to trip is within the scope of PRC-027. Have fun.

 

[Mbrooke]

What does PRC-027 require or mandate that forces ground step distance?

 

[davidbeach]

PRC-027 will mandate periodic review of coordination and documentation of the coordination study. Protective elements that aren't affected by changes in source impedance (changes in fault current levels) are not included in the scope. It won't mandate any changes in protection design, but could make changes in approach attractive to reduce the overall scope.

 

[Mbrooke]

Got it- something that people should be doing as with elements that can be effected by generation dispatch or outage contingency.