For voltages beyond 66 kV, the electrostatic stresses in the cables exceed the acceptable values and solid cables become unreliable. This occurs mainly because voids are created when voltages exceed 66 kV.
Hence, instead of solid cables, we use Pressure cables. Typically, such cables are either oil filled or gas filled.
Oil Filled Cables:
In this type oil ducts are provided for oil circulation. The oil channel is formed at the center by stranding the conductor wire around a hollow cylindrical steel spiral tape.
In single core sheath channel oil filled cable the conductor is solid similar to that of solid cable and is paper insulated.
Oil is circulated under suitable pressure through ducts provided for such purpose. This oil supply and pressure are maintained through reservoirs kept at proper distances.
The same oil is employed for impregnation of paper insulators. The oil pressure compresses the layer of paper insulation and prevents the possibility of formation of void.
When the temperature of cable increases the oil gets expanded and the extra oil collects in the reservoir. When temperature falls during light load periods, the oil from the reservoir flows to the channel.
Oil ducts are provided in metallic sheath and filler space as shown below. filled 3 core cable oil ducts are provided in the filler space.
Gas Filled Cables:
Pressurized gas (usually dry nitrogen) is circulated around cables in an air-tight steel pipe. Such cables are cable of carrying higher values of load current and can operate at higher values of voltage. But the overall cost is more.
The construction of cable is similar to that of a ordinary solid type except that it is of triangular shape.
The sheath is protected by a thin metal tape. This cable is laid in a gas tight steel pipe.
The pipe is filled with dry nitrogen gas at 12 to 15 atmospheres.
The gas pressure produces radial compression and closes the voids that may have formed between the layers of paper insulation.
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