Insulation of bus bars at 35 kV 1

[qman5]

Hi all,

A project I now working on makes use of some outdoor equipment with some tight working clearances. A co-worker has proposed to insulate the buswork, which is essentially flat copper bus, to increase safety and decrease working clearances. I disagree with this proposal, but wanted some evidence before rebuking with my arguments.

I'm not familiar with insulated bus. I've heard of its use on low voltage equipment (600 V), but I'm unaware of use at higher voltage. I didn't think it existed for bus work, but some searching shows that 3M even makes a heatshrink product for insulating bus up to 35 kV.

I understand that a conductor over a certain voltage should have a shield to contain the electric field. In the case of an insulated bus without shielding, there would be no field containment so the risk of insulation breakdown is much more likely than with a shielded cable. Are there any other disadvantages?

Some other questions:


1. Over what voltage level must insulated cables contain a shield?
2. Is there any benefit to having an insulated bus, both in terms of safety and clearances?
3. Would insulated bus find its application outside of switchgear?

Thanks!

 

[dpmac]

Check the sleeving manufacturer product regarding outdoor use. We have used 3M BBI Heat Shrink many times with great results indoors (or outdoors but enclosed and heated)

 

[racookpe1978]

Is there any chance that inadvertant contact (people, ladders, items on a forklift or from above, or squirrels and rats) could touch the exposed conductors and surroundings?

 

[pwrtran]

Many insulated busbars are not fully rated insulation but insulation to prevent single phase fault migrates to a more damaging 3 phase fault due to the arcing and metal dust contamination released during a fault.

 

[Marmite]

The installation of heat shrink to outdoor 35kV busbars should not have any bearing on safety clearances. The conductor would be classed as covered, rather than insulated. It would have a beneficial effect on BIL, subject to testing, but it would be foolish to rely on it for life safety. What state will it be in after a few years in the sun, and exposure to the elements?
Would you rely on it if your life depended on it? Don't go there.
Regards
Marmite

 

[davidbeach]

As hinted at by Marmite, at those voltages - or any over about 600V - insulation isn't enough. There has to be a shield and a drain.

Take a 35kV conductor and place a perfect dielectric around it. Now place the slightest hint of conductive dust around that dielectric. What do you have? You've got a capacitor, that's what you've got. What's the impedance of a capacitor at the power system frequency? Pretty darned low, that's what it is.

That's why you never see an insulator at that voltage level that isn't tied to ground at one end. Cables have grounded shields; other insulators are mounted to grounded surfaces.

Cover a substation bus bar with a really good insulator but don't provide a grounded shield and you'll find the same voltage level on both sides of the insulator. Once insulated it might not be able to supply as much current upon initial contact as it would otherwise, but it will still do a great job of knocking you on your a$$.

I'm hard pressed to imagine an application for insulated (covered) bus is a substation without going all the way from air insulated buswork to GIS.

 

[crshears]

I concur with what davidbeach and Marmite wrote; in historic and current practice within the utility where I am employed, any instances where bus bar encapsulation is in use it is only for BIL improvement, with no change whatever to the limits of approach.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]