Wednesday, August 10, 2011

Belted cables

         As mentioned earlier, these are used for the voltage levels upto 11 KV. The construction of belted cable is shown in the Fig.1.
Fig. 1  Belted 3 core cable

       The cores are not circular in shape. The core are insulated from each other by use of impregnated paper. The three cores are grouped together and belted with the help of a paper belt. The gaps are filled with fibrous material like jute. This gives circular cross-sectional shape to the cable. The belt is covered with lead sheath which protects cable from moisture and also gives mechanical strength. The lead sheath is finally covered by jute like fibrous compounded material.
       The electrical field in single core cable is radial while it is tangential in case of three core cables. Hence the insulation is subjected to tangential electrical stresses rather than radial one. The paper has good radial strength but not tangential strength. Similarly paper resistance along the radius is much larger than resistance along tangential path. The same is true for dielectric strength also. The fibrous material is also subjected to the tangential electrical stresses, for which, the material is weak. Hence under high voltage cases, the cumulative effect of tangential electrical stresses is to from space inside the cable due to leakage currents. Such air spaces formed inside the insulation is called void formation. This void formation is dangerous because under high voltage, spaces are ionized which deteriorates the insulation which may lead to the breakdown of the insulation. Hence the belted cables are not used for the high voltage levels. Another disadvantage of the belted cable is large diameter of paper belt. Due to this, wrinkles are formed and gaps may be developed if the cable is bended. To overcome all theses difficulties, the screened type cables are used.

4 التعليقات:

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