Type of oil used in Filled cables
During 1960’s Dodecylbenzene (ddb) was commonly used. More recently alkylates like Decylbenzene (db) and branced Nonylbenezene (nb) is using because these are having low viscosity and the ability to absorb water vapour liberated during ageing at cellulose.
Gas pressure Cables: In gas pressure cables, an inert gas like nitrogen is used to exert pressure on the paper dielectric to prevent void formation.
Gas pressure cables are mainly available in two forms, namely
i) External gas pressure cable.
ii) Internal gas pressure cables
The external gas pressure cable. The construction of the cable is similar to that of an ordinary solid type except that it is of triangular shape and its thickness is 75% of solid type. The triangular shap section reduces the weight and thermal resistance. It acts as a pressure membrance. The sheath is protected by a thin metal tape. The cable is laid in a gas tight steel pipe.
Features:
1. The pipe is filled with day nitrogen gas at 12 to 15 atmospheres.
2. In Comparison with normal cables these cables carry 1.5 times the load current, 2 times the operating voltage.
3. The maximum potential gradient is 10kv/mm (or) 100 kv/cm
4. The dielectric power factor at 15c is 0.6 percent.
5. The nitrogen in steel pipes help in quenching any flame.
6. Maintenance cost in small.
7. The cost of the cable is very high.
Specifications of Cables
While selecting the underground power cables the following are the main important factors to the specified, namely
1. Voltage range upto which the cable is intended to use.
2. Number of cores.
3. Current carrying capacity of cable.
a) Continuous Current rating for L.T. cable
b) Continuous and short circuit current ratings in case of H.T. Cables.
4. Insulation compound used
5. Sheathing
6. Armouring
7. System on which the cables is used.
a) Earth neutral.
b) unearthed neutral system.
8. Finishes.
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