Overhead Lines vs Power Cables - Question on Insulation

[Fjolvar]

Hello I have a very basic question.

I'm studying power cable engineering right now and I've learned about the role of insulation in a cable to reduce dielectric losses and charging current losses, but I don't understand what this means exactly. Here are my questions..

1. Is the insulation basically reducing the voltage from building up outside of the conductor and creating leakage current?

2. I'm trying to understand what would happen without insulation in a power cable. Why do we need insulation? And why do overhead power lines just have bare wires without any insulation? How does this effect power losses?

3. Why can overhead transmission lines transmit at higher voltage levels that power cables?

Any help would be greatly appreciated. I am preparing for an exam and this is my weakspot :(

 

[anorlunda]

I'm trying to understand what would happen without insulation in a power cable. Why do we need insulation? And why do overhead power lines just have bare wires without any insulation? How does this effect power losses?

With no insulation, the wires would short circuit to Earth ground (soil or water.) Insultation provides high resistance (ideally infinite resistance) to current flow. In a cable, it is provided by the jacket. For overhead lines, it is provided by the air in the long distance between wire and ground.

Why can overhead transmission lines transmit at higher voltage levels that power cables?

If the overhead wire is say, 20 meters in the air, the electric breakdown voltage is 20* 3 x 10 6 V/m, or 60 MV. The actual voltage is much less than that, probably more than 100 times less than that.

Compare the 3 x 10 6 V/m breakdown strength of air with the cable insulation. Then consider the thickness 20m overhead versus 0.02 m for cable.