Sunday, August 28, 2011

Gas Pressure Cables

       In case of as pressure cables, an inert as like nitrogen at high pressure is introduced. lead sheath and dielectric. The pressure is about 12 to 15 atmospheres. Due to such a high pressure there is a radial compression due to which the ionization is totally eliminated. The working power factors of such cables is also high.
Fig. 1 Gas pressure cables

       The Fig. 1 shows the section of a gas pressure cable. The cable id triangular in shape and installed in the steel pipe. The pipe is filled with the nitrogen at 12 to 15 atmospheric pressure. The remaining construction is similar to that of solid type cable but the thickness of lead sheath is 75% of that of solid type cable. There is no bedding and serving. The pressure cable was firstly designed by Hochstadter, Vogel and Bownden.
       The triangle shape lead sheath acts as a pressure membrane. The shape reduces the weight and provides the low thermal resistance. The high pressure creates the radial compression to close any voids. The steel pipe is coated with a point to avoid corrosion. 
       During heating, the cable compound expands and a sheath with acts as a membrane becomes circular in such a case. When cable cools down the gas pressure acting via sheath forces compound to come back to the noncircular normal shape. Due to good thermal characteristics, fire quenching property and high dielectric strength, the gas SF6 is also used in such cables.
1.1 Advantages
       The various advantages of gas pressure cables are,
1. Gas pressure cables can carry 1.5  times the normal load current and can withstand double the voltage. Hence such cables can be used for ultra high voltage (UHV) levels. 
2. Maintenance cost is small.
3. The nitrogen in the steel tube, helps in quenching any fire or flame.
4. No reservoir or tanks required.
5. The power factor is improved.
6. The steel tubes used make the cable laying easy.
7. The ionization and possibility of voids is completely eliminated.
       The only disadvantages of this type of cables is very high initial cost.

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