Extra High Tension Cables
This type of cables are mainly used beyond 66kv. The solid type cables are unreliable for beyond 66kv because there is a danger of breakdown of insulation due to the presence of voids and it can eliminated in pressure cables by increasing the pressure of compound. Hence, for beyond 66kv operating voltage pressure cables are used.
Pressure cables are mainly of two types, namely
a) Oil-Filled Cables.
b) Gas pressure Cables.
1. Oil Filled Cables:
a) A single core cable with central oil ducts. The oil channel is formed at the centre by standing the conductor wire around a hallow cylindrical steel spiral tape.
The oil under pressure is supplied to the channel by means of external reservoir. The oil pressure compresses the layers of paper so that it prevents void formation. The single core conductor channel oil filled cable. In this type it is so designed that when the oil gets, expanded due to increases in cable temperature, the extra oil collects in the reservoier and when the temperature falls, the oil from the reservoir flows to the cable channel.
The Constructional detail of a single core sheath channel oil filled cable. In this type of cable the conductor is solid similar to the L.T. cable but at the metallic sheath oil ducts are provided.
The 3 core oil filled cable. The filled cable. The oil ducts are located in the filler spaces. These channels are composed of perforated metal ribbon tubing and are at earth potential.
This type of cables are widely used in submarine cable as well as bridge and vertical shift Applications.
Advantages of oil Filled Cables:
The following are the main advantages of oil filled cables over solid cables.
1. Greater operation dielectric stress.
2. Greater working temperature and greater current carrying capacity.
3. Better impregnation.
4. No void formation.
5. Impregnation possible after sheathing.
6. Smaller size of cable due to reduced dielectric thickness.
7. A defect can easily identified by oil leakage.
Disadvantages of Oil Filled Cables:
1. Greater cost.
2. Lying of cables is complicated
3. Maintenance cost is more.
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