Underground Cables Underground cables

41.30. Underground Cables Underground cables

are used where overhead lines are not possible as in large cities despite the fact that in their case, cost per kW per km is much more as compared to overhead transmission lines. Another advantage of overhead system for distributors is that tap- ping can be made at any time without any dis- turbance, which is of great importance in rap- idly developing areas. However, underground

Underground cables are used where overhead transmission is difficult

cables are more advanta- geous for feeders which are not likely to be disturbed for tapping purposes because, being less liable to damage through storms or lighting or even wilful damage, they offer a safer guarantee of supply. But this advantage may be offset by the cost of trenching and expensive jointing necessary in case of repairs.

However, cables score over overhead lines in cases where voltage regulation is more important, because, due to very small spacing of their conductors, they have a very low inductance and hence low inductive drops.

1646 Electrical Technology

Cables may be classified in two ways according to ( i) the type of insulating material used in their manu- facture or ( ii) the voltage at which they transmit power. The latter method of classification is, however, more generally used according to which cables are divided into three groups :

1. Low-tension cables–up to 1000 V

2. High-tension cables–up to 23,000 V

3. Super-tension cables–from 66 kV to 132 kV For all cables, the conductor is tinned stranded

copper of high conductivity. Stranding is done to secure flexibility and the number of conductors in a core is

generally 3, 7, 19 and 37 etc. Except for 3-strand, all Fig. 41.44 numbers have a centrally-disposed conductor with all others surrounding it. A cable may have one or

more than one core depending on the type of service for which it is intended. It may be ( i) single-core ( ii) two-core ( iii) three-core and ( iv) four-core etc.

In Fig. 41.44 is shown a section through a twin-cored, high-tension lead-covered underground cable sheathed with a continuous tube of pure lead whereas Fig. 41.45 shows a section of a typical concentric type 2-core cable used for single-phase distribution. The cores are arranged concentri- cally. The outer core is arranged in the form of hollow tubing. Both cores are of stranded copper and paper-insulated and are protected by a lead-sheath.

In Fig. 41.46 is shown a section through a 3-core extra-high-tension paper-insulated lead- covered and steel-wire armoured cable.

The cores are surrounded by insulation or impregnated paper, varnished cambric or vulcanised bitumen.

The insulation is, in turn, surrounded by a metal sheath made of lead or a lead alloy and prevents the entry of moisture into the inner parts. On the sheath is applied the bedding which consists of two compounded paper tapes along with suitable compounded fibrous materials.

Next comes ‘armouring’ which is placed over the bedding and consists of either galvanized steel wires or two layers of steel tape. Armouring may not be done in the case of some cables.

Next comes ‘serving’ which consists of compounded fibrous material (jute etc.) placed over the armouring in the case of armoured cables or over the metal sheath in the case of unarmoured cables- in which case serving consists of two layers of compounded paper tapes and a final covering of compounded fibrous material.

Fig. 41.45 Fig. 41.46

A.C. Transmission and Distribution