need someone smarter than me

[kooman]

Guys we are trying to figure out a power factor problem that we have on our system, we have feeder A and feeder B, both comming out of same sub, 7200/12,470 wye. Feeder A has great power factor and feeder B has crappy power factor, feeder B has 5 blocks of 4/0 urd primary that leaves the sub and then goes overhead from that, feeder A is all overhead, they loop around and come together at a switch junction that is normally open. OK, now if we put all the load onto feeder A and open up the section of underground then everything is fine with the power factor. so it seems like the underground is obviously causing the poor power factor but why and how do we fix it? the only thing i that might have something to do with it is the underground is all in one conduit together. any help would be great.

[BigClive]

Is it a leading or lagging power factor? If it was a leading factor then I'd be suspicious about the capacitance between adjacent underground cables.

[kooman]

Is it a leading or lagging power factor? If it was a leading factor then I'd be suspicious about the capacitance between adjacent underground cables.

i do believe it is in fact leading but i will check for sure tomorrow, that does make sense too. i got to thinking, and we have several short runs of underground feeders comming out of subs in the same conduit but they are all non concentric wire with a seperate conductor for the neutral, and the run of cable that is giving us the problem is jacketed with concetric, could that add to the problem too?

[Pootnaigle]

Ummmmm Harmonics can affect power factor and i would suspect that in this case that 5 blocks of Underground primary being in the same conduit may have something to do with your problem.

[wudwlkr]

Underground cable by the nature of its construction acts just like a capacitor, i.e. two closely spaced conductors separated by an insulator. Long runs of URD that don't have no or very minimal loading can generate a lot leading VARs. Saw a case once a number of years ago where a distribution co-op built over two miles of 25kV URD to feed a new mobile home park that was being built. They wanted to keep the cable energized so if a fault developed they would know about it. Was over a year before the park was finished and load started to be put on the cable. Had well over 2 megavars of reactive current generated during that time and they had to pay a substantial power factor penalty to the power supplier as a result.

[JohnnyBGood]

Underground cable by the nature of its construction acts just like a capacitor, i.e. two closely spaced conductors separated by an insulator. Long runs of URD that don't have no or very minimal loading can generate a lot leading VARs. Saw a case once a number of years ago where a distribution co-op built over two miles of 25kV URD to feed a new mobile home park that was being built. They wanted to keep the cable energized so if a fault developed they would know about it. Was over a year before the park was finished and load started to be put on the cable. Had well over 2 megavars of reactive current generated during that time and they had to pay a substantial power factor penalty to the power supplier as a result.

+1 exactly correct. Only way to get rid of it is to load up the UG circuit.

[BigClive]

The concentric conductor would not help, since the capacitance between the centre conductor and outer layer would be much higher than just two adjacent cables.

Without a load this capacitive coupling of current is going to produce a very visible amount of current, but even with a load that loss is still going to be there.

It might be worth seeking advice about this from the cable supplier.

[kooman]

thanks for all the help guys, sounds like the problem isnt going to have an easy fix, time to let the engineers earn their money on this one.