Sunday, August 28, 2011

Screened Type Cables

       These are used for the voltage levels of 22 kV and 33 kV. The two types of screened cables are 1. H type cables and   2. S.L. type cables.
1.1 H-Type Cables
       The cable is designed by M.Hochstadter and hence the name given to it is H-Type cable. There is no paper belt in this type of cable. Each conductor in this cable is insulated with a paper, covered with a metallic screen which is generally an aluminium foil. The construction is shown in the Fig. 1. The metallic screen touches each other. Instead of paper belt, the three cores are wrapped with a conducting belt which is usually copper woven fabric tape. Then there is lead sheath. The conducting belt is in electrical contact with the metallic screen and lead sheath. After lead sheath there are layers of bedding, armoring and serving. The metallic screen helps to completely impregnate the cable which avoids the possibility of formation of voids and spaces. The conducting belt, the three metallic screens and lead sheath are at earth potential, due to which electrical stresses are radial in nature. This keeps the dielectric losses to minimum. Another advantage of metallic screens is increase in the heat dissipation which reduces the sheath losses. Due to these advantages, current carrying capacity of these cables increase. In special cases, the use of these cables cable extended upto the 66 kV level.
H-type cable

1.2 S.L. Cables
        The name S.L. stands for separate lead screened cables. In this cable, each core is insulated with impregnated paper and each one is then covered by separate lead sheath. Then there is a cotton tape covering the three cores together using a paper filler material. Then there are the layers of armouring and serving. the difference between H-type and S.L. type cable is that in S.L. type a common lead sheath covering all the three cores is absent while each core is provided with separate lead sheath. This allows bending of the cables as per the requirement. The construction of S.L. type cable is shown in the Fig. 2.
S.L. type cable

       The three cores in this type of cable are as good as three separate single core cables.
       The various advantages of S.L. type cable are,
1. Due to lead individual lead sheath, core to core fault possibility gets minimised.
2. The electrical stresses are radial in nature.
3. Due to absence of overall lead sheath, bending of cable is easy.
4. The dielectric  which gets subjected to electric stresses is paper which is homogeneous hence there is no possibility of formation of voids.
5. Metal sheath increases the heat dissipation which increases the current carrying capacity.
A combination of H-type and S.L. type cable called H.S.L cable also can be used.
The lamination of screened cables which are also called solid type cables are,
1. It uses solid insulation only like paper. When the conductor temperature increases, the paper gets expanded. This eventually stretches the lead sheath.
2. When the load on the cable decreases, it cools down and there is contraction of lead sheath. Due to this air may be drawn into the cable forming voids. This deteriorates the cable insulation.
3. Moisture may be drawn in alongwith the air which deteriorates the dielectric strength of dielectric.
4. Mechanical shock can cause voids. The breakdown strength of voids in much less than insulation. Hence voids can cause permanent damage to the cables.

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