Overbuild distribution circuits

[magoo2]

I wondered if anyone had experience with overbuild distribution circuits like 25 kV on top and 12.5 kV on the bottom. We've had incidents from tree limbs and from vehicle accidents where the upper phase makes contact with the lower phase.

Short of separating the 2 circuits (including putting one underground), the only other solution that comes to mind is to convert the lower voltage to the same voltage as the upper one. I don't know of any other solutions so that's why I'm asking what others do.

Thanks in advance.

 

[wbd]

What you are describing is one of the risks of underbuilt circuits. I would suspect that the 25kV is a feeder that runs for some distance with no taps or transformers and the 12kV is where the distribution is, laterals and transformers.
One option would be to replace the 25kv run, if it is open wire, tangent construction with a spacer cable run.
Not knowing the exact configuration, I would guess the other option is to install surge arresters every few spans on the 12kV to clamp the overvoltage from the 25kV landing on it.
That's not as bad as my overbuild where I had 46kV land on 4160V.

 

[gobblerhuntr]

We have a 69kv and a 12 kv that crossed each other and are on the same pole in several places. The 69 burnt down across the 12 on day and we bought a BUNCH of tvs,vcrs, and various appliances. Sometimes you just have to have the different voltages close to each other, there is just nothing you can do about it.

 

[apowerengr]

Multi-circuit construction is very common, especially in areas where load density requires several circuits in a relatively small area. If I may as what are the conductors used on the two circuits in question and the vertical spacings between the decks? Mostly the only issues I've seen are the incorrect use of expulsion fuses on the upper deck - when the fuse is required to clear a fault the expulsed materials cause a trip on the lower circuit.

 

[magoo2]

We use a lot of 477 AAC and 249.6 AAAC. Spacing is based on using an 8-ft crossarm on top (44-inch spacing with the center phase on a ridge pin) and a 10-ft one on the lower circuit (with about a 30-inch spacing between the 2 conductors on the same side of the pole).

 

[matwong]

We normally installed so-called "broken wire guard" for these kind of circumstances. It is just something like a bracket (made of hot-dipped galvanised material) installed on the cross-arm beneath every phase of the HV conductors and were grounded(or earthed). It can be installed on the pole itself with different construction design. The idea is on any fallen conductor, prior to the conductor touches to the ground or line underneath, it will fall on these broken wire guard which will trigger the earth fault relay to operate immediately. This is quite common in British grounded system and not too sure whether is applicable to US system.

 

[oldfieldguy]

matwong--

I haven't seen the "broken wire guard" down here along the Gulf of Mexico in Louisiana, at least as you describe it. I used to work for one of the big electrical utilities.

I did hear talk about installing a copper conductor on the crossarms supporting some conductors. This was connected to the grounded conductor running up the pole, and the idea was the same as your broken wire guard, to insure that a falling conductor had a solid, relatively low impedance ground path to achieve reliable ground fault relay operation.

old field guy

 

[magoo2]

I did hear about the broken-wire guard, but it talked more about using it for areas where broken conductors are common and grounding was poor (i.e., low fault current). In many of our cases, a tree limb falls on the line and winds up bridging a phase from the higher and lower voltage circuit. No conductor break usually occurs so I don't see where this would help.