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Transcript of Underground Cables
• In these cables the conductors are wrapped with oil impregnated paper, and then cores are assembled with filler material.
•These can be used for voltages up to 11KV or in some cases can be used up to 22KV
•High voltages beyond 22KV, the tangential stresses becomes an important consideration
•As the insulation resistance of paper is quite small along the layer, therefore tangential stress set up, hence, leakage current along the layer of the paper insulation.
• This leakage current causes local heating, resulting breaking of insulation at any moment .
1) Belted Cables
Low tension (L.T) ----- up to 1000V
High tension (H.T) ----- up to 11, 000V
Electrical Power (1)
Classification of Underground Cables
Cables of 3-phase
Super tension (S.T) ---- from 22KV to 33KV
Extra high tension (E.H.T) cables --- from 33KV to 66KV
Extra super voltage cables ------beyond 132KV
These can be used up to 33kv but in certain cases can be extended up to 66kv.
These are mainly of two types
S.L type cables
• Each core is insulated by layer of impregnated paper .
• The insulation on each core is covered with a metallic screen which is usually of perforated aluminum foil .
• The cores are laid in such a way that metallic screen make contact with one another.
• Basic advantage of H-TYPE is that the perforation in the metallic screen assists in the complete impregnation of the cable with the compound and thus the possibility of air pockets or voids in the dielectric is eliminated .
• Basic disadvantage is The metallic screen increase the heat dissipation power of the cable.
• Each core insulation is covered by its own lead sheath.
• It has two main advantages, firstly the separate sheath minimize the possibility of core-to-core breakdown. Secondly the, bending of cables become easy due to the elimination of over all sheath.
• The disadvantage is that the lead sheaths of S.L is much thinner as compared to H-Type cables, therefore for greater care is required in manufacturing.
3◘Pressurized Type Cables
• In these cables, pressure is maintained above atmosphere either by oil or by gas.
>Gas pressure cables are used up to 275KV.
>Oil filled cables are used up to 500KV
•Oil Filled Cables
• Low viscosity oil is kept under pressure and fills the voids in oil impregnated paper under all conditions of varying load
•There are three main types of oil filled cables ;
1)Self-contained circular type 2)Self-contained flat type 3)Pipe Type cables
of Oil Filled Cables ;
*Greater operating dielectric stresses *Greater working temperature and current carrying capacity
*Better impregnation ,Impregnation is possible after sheath *Defect can easily be detected by oil leakage
•In these cables an inert gas like nitrogen is used to exert pressure on paper dielectric to prevent void formation.
• These are also termed as Compression cables
•They insulated cores similar to solid type
•The cable is inserted in a pressure vessel which may be a rigid steel pipe, commonly known as pipe line compression cable.
•The nitrogen gas is filled in vessel at nominal pressure of 1.38 * 10 exp 6 N/ square meter with a maximum pressure of 1.725 * 10 exp 6 N/ square meter.
•Gas Pressure Cables