Oil pressure and gas pressure HV cables

Pressure cables are used above about 66 kV, where solid (paper) cables fail because voids form in the insulation and cause ionisation (partial discharge). In pressure cables the voids are removed or suppressed by keeping oil or gas under pressure.

Oil-filled (oil pressure) cables

Low-viscosity oil is kept under pressure (about 1–3 atm or more) and fills the voids in the paper insulation. Oil ducts let the oil flow along the cable, and oil reservoirs (feeding tanks) at intervals take care of expansion and contraction with load.

• Conductor-channel type (single core): the oil channel is at the centre of a hollow stranded conductor.
• Sheath-channel type: channels are made in the metallic sheath.
• Three-core filler-space type: oil ducts are in the filler spaces between cores.

   Single-core oil-filled cable
      ___________________
     /  lead sheath      \
    |  paper insulation   |
    |   (oil impregnated) |
    |   ( conductor  )    |
    |   (  [oil duct] )   |
     \___________________/

Advantages: no voids, higher working stress, smaller size, faults can be found by oil leak. Disadvantage: costly, needs reservoirs and careful laying.

Gas pressure cables

The cable is placed in a steel pipe filled with an inert gas (nitrogen) at about 12–15 atm.

• External pressure cable: the cable has a thin triangular lead sheath that acts as a diaphragm; the gas pressure outside compresses the cable so any voids close.
• Internal (impregnated) gas pressure cable: gas at high pressure is inside the insulation, so the ionisation voltage of the small voids rises and discharge does not occur.

      Steel pipe (N2 at ~14 atm)
     _____________________
    /   o        o        \
   |       o              |   o = cores with
   |    (3 cores)         |       thin lead sheath
    \_____________________/

Advantages: higher current rating and voltage (up to 275 kV), no oil leak problem, better thermal performance. Disadvantage: high cost of steel pipe and gas system.