determine the amount of porcelain insulation

Ron_04

hi everyone.....i want to know tht how we determine the amount of porcelain insulation to be done in any electrical systems situated in a substation? plz help

yungman

For power line that is 60Hz, the important thing to worry is not the insulation strength first. Most problem is creepage. Creepage is the current conduct across the surface of an insulator. That's the reason you see the HV standoff on the power lines is not straight, they are shape such the diameter get bigger and then smaller and bigger and smaller along the length. This is to increase the length of the path from the HV point at the top to the anchor point at the base.

Back to your original question, dielectric strength of most good insulator is about 400V/mil give and take. If you design with 150V/mil, usually it is ok already. You want to be more conservative, lower the number. This is a very general statement, you need to look up the specific material you are using.

As I said before, unless you push the insulation thickness, most problem is the creepage. It is only when the frequency goes up, then the material make a big difference. That's where the whole topics of dielectric loss and loss tangent and all come in.

Ron_04

thank u very much for ur explanation......but i want to knw one thing tht is like in bushing of a transformer we do insulation to protect the transformer from surface current so why again creepage current.......can u xplain me the creepage current plz

yungman

I don't think that's creepage, sound more like eddy current. Creepage is conduction on the surface due to potential difference. Dirt, moisture on the surface promote creepage. The distance depends on humidity and other factors. I don't remember the ball part distance. But it is a lot larger than insulation thickness.

I remember when I designed for CE requirement, say if I have a HV wire connect to one point. I need to prevent any human from touching even the insulation of the HV cable 3" from the exposed junction. From my experience, arcing in HV system was mostly from creepage. We trouble shoot where the arc was by turning off all lights and induced arcing. You can actually the lighting shoot across the surface.

Ron_04

ok thanx for the xplanation...but one more foolish question is how can creepage current flow through insulation? i mean to say how can a insulator conduct a current?

yungman

Creepage don't go through insulation. It only travel on surface of the insulator. When I design HV pcb, I lot of time, I route groove to create a longer path of creepage on the FR4.

I really have not look into the detail theory of creepage, I just know by real life experience. I believe is the stuffs deposit on the surface that break down and conduct. For example, solder flux residue, finger print oil, moisture in air all degrade the performance. I believe people use Teflon material not because of the insulation strength, it is still about 400V/mil. It's the surface of Teflon, things don't stick on the teflon so it's better to handle the creepage.

jim hardy

OP asked about electric utility insulators?

Yungman is exactly right.
The individual ceramic discs are mushroom shaped so that rain runs over top to edge and drips off leaving underside dry so it retains insulating capability.

Near the ocean or on a foggy morning inland you can hear high voltage insulators "sizzle" as the deposits absorb moisture and become conductive. A good rain will wash them clean.

search on 'insulator wash system'

there's some brave folks who wash them while energized..
http://www.wilorton.com/HotLineIns.htm

yungman

I never look at an power line insulator close up, I hope they have smooth shinny surface so it is less susceptible to things sticking onto the surface.

Ron_04

got it....thnx