Mutual coupling of different voltages
Mutual coupling of different voltages
(OP)
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?
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?






RE: Mutual coupling of different voltages
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.
RE: Mutual coupling of different voltages
RE: Mutual coupling of different voltages
RE: Mutual coupling of different voltages
RE: Mutual coupling of different voltages
What software do you use for modeling your T-lines?
J. Horak, P-R Engineering, Colorado
RE: Mutual coupling of different voltages
RE: Mutual coupling of different voltages
RE: Mutual coupling of different voltages
RE: Mutual coupling of different voltages
RE: Mutual coupling of different voltages
J. Horak, P-R Engineering, Colorado
RE: Mutual coupling of different voltages
RE: Mutual coupling of different voltages
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.
RE: Mutual coupling of different voltages
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.
RE: Mutual coupling of different voltages
RE: Mutual coupling of different voltages
RE: Mutual coupling of different voltages