MV cable insulation

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Sajid khan

Sajid khan

Senior Member
Location
Pakistan
Why MV/HV cable have higher insulation than LV cable.

I know this is due to the high voltage and hence a greator stress on the cable.

Can some one explain it in detail and by using equation.

Thanks in advance

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electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Sajid khan said:
Why MV/HV cable have higher insulation than LV cable.

I know this is due to the high voltage and hence a greator stress on the cable.

Can some one explain it in detail and by using equation.

Thanks in advance

Sent from my GT-I9300I using Tapatalk
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I dont think I can fully answer, but some comments: In theory I believe a dielectric'ss strength is proportional to its thickness. In practice though, there are several other things going on. First is the insulation needs sufficient strength to not get damaged in the field and/or during installation. This strength requirement gets larger with a heavier conductor. Take a look at the insulation thickness of #12 THNN VS say 4/0 THHN. Even though they both have the same voltage rating, the larger cable has much thicker insulation.

Also, At around 2400 volts, you can start to get surface tracking and corona discharge which can degrade insulation. This results in different construction practices such as an additional semiconducting layer and shielding in most conductors at or above 2.4KV. I understand the semiconducting layer part, "shorting out" the corona discharges but I am not entirely clear on the shielding. It is always said that is "evenly distributes the electrical stress" - which seems to make sense, but why cant we just use more insulation material? What changes? I have never heard a satisfactory explanation.
 
S

s_colligan

Member
Location
Washington state
I think the last two replies are correct, but may add some.

As electrofelon put, the dielectric strength is proportional to the thickness of insulation (and the dielectric constant of the material itself). The dielectric strength is a measure of the voltage it can withstand per thickness of insulation (after which it breaks down and fails). So technically I do think you could just add more insulation thickness to the wire, but the thicker the insulation material, the more heat it traps in the conductor. At some point, the heat that is trapped in the wire trumps (and in a bad way) the value added from a thicker insulator. So in general, the insulation value is proportional to the dielectric constant and thickness of insulation.

I come from the transmission/distribution sectors... I'd say most, if not all, transmission lines have no insulation due to this fact (they are lighter and can cool better), and some distribution lines are also bare.

Hope this helps...
 
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