Oil Filled 400kV HVDC Cables: How Do They Work, Oil Pressure, Circulation, DC vs AC

Hi

OIL FILLED HVDC CABLES
I need to find out more about HVDC cables.

I recently worked on a line for 400KV HVDC and the cable was oil filled.
Please advise.
1. How is oil-filled better than any other material and how does it work
2. I understand that the oil is kept under pressure. What pressure?
3. Does the oil circulate through the cable and is it cooled at certain sections
4. Why is DC superior to AC for HVDC underground cables? I always understood that AC was used on OHLs to step up and down the voltage easier to reduce energy losses

Please advise

Cheers

DC is superior to AC for long HVDC underground cables because DC can carry more power than AC can for the same conductors (RMS current limit) and insulation (Peak voltage limit). Naive calculation says Pdc = sqrt(2) * Pac per conductor (Experts can correct this). DC does not couple energy into the surroundings the way AC does. In particular, the line capacitance and inductance have no effect on DC transmission but generate reactive power on AC transmission. DC can be built with two conductors, while AC requires three conductors.

DC can easily interconnect unsynchronized AC systems. Pairs of converter stations with zero-length DC lines between them are sometimes built for this purpose.

Hi Peter
Thank-you very much for taking the time to give your explanation which helps to understand the issue a little better. I think I grasp your explanation of the effect of inductance and capacitance regarding the varying AC and the double and triple conductor
However:
1. I do not understand this: .(RMS current limit) and insulation (Peak voltage limit). Naive calculation says Pdc = sqrt(2) * Pac per conductor (Experts can correct this)
2. Would it be possible to have an explanation of the oil properties and function of the oil in the cables.
Thank-you

1. The RMS (root-mean-square) current Irms is limited by the Irms*Irms*Rconductor heating of the conductor. For DC the current is constant, so Idc can equal Irms of the AC.

The peak voltage Vpeak from conductor to ground, is limited by the insulation of the conductor. For DC the voltage is constant so we can let Vdc = Vpeak. For AC the peak voltage is sqrt(2) * Vrms, so Vrms must be no more than Vpeak/sqrt(2), so the RMS AC voltage Vrms can be no more than Vpeak/sqrt(2).

Thus, the RMS current limit per conductor is the same for DC and AC. DC can run sqrt(2) times the RMS voltage in each conductor, so transmits sqrt(2) times as much power per conductor as AC.

Another consideration, is that AC transmission lines require three conductors (for three-phase AC power) per circuit, while DC transmission lines require two conductors per circuit. This means that, per circuit, AC and DC transmit similar amounts of power.

2. Oil is used as insulation for the conductor. If the oil is circulated through the cable (as I expect it is), the circulation provides some cooling of the cable. More important, is that a momentary insulation breakdown of liquid oil is self-healing. Solid insulation is not self-healing; once solid insulation breaks down the insulation must be replaced.

Again, those more expert than I, can make corrections and expand my explanations.