Oil filled Cables

 In case of oil filled cables, the channels or ducts are provided within or adjacent to the cores, through which oil under pressure is circulated.

       The Fig. 1 shows the construction of single core oil filed cable. It consists of concentric stranded conductors but built around a hollow cylindrical steel spiral core. This hollow core acts as a channel for the oil. The oil channel is filled in a factory and the pressure is maintained in the oil by connecting the oil channel to the tanks which are placed at the suitable distances along the path of the cable.

Fig. 1 Conductor channel single core oil filled cable

       The oil pressure compresses the paper insulation, eliminating the possibility of formation of voids. When cable is heated, the oil expands but expanded oil is collected in the tank. While when cable is cooled, extra oil is supplied by the tank to maintain the oil pressure. In this type of cable the oil channel is within the conductor, hence it is called single core conductor channel oil filled cables. The other construction of the cable is similar to that of solid type cables.

       Another type of single core oil filled cable is the sheath channel oil filled cable. In this type, the conductor is solid with a paper insulation. While the oil ducts are provided between the dielectric and the lead sheath.

       The construction of sheath channel oil filled cable is shown in the Fig. 2. The laying of such cables must be done very carefully.

Fig. 2  Sheath channel single core oil filled cable

       The three core oil filled cables use the shielded type construction. The oil channels are located in the spaces which are normally occupied by the filler material. The three oil channels are of preforated metal ribbon tubing. All the channels are at earth potential. The construction is shown in the Fig. 3. As the pressure tanks are required all all along the route of these cables, the lengths of these cables are limited. Leakage of oil is another serious problem associated with these cables. Automatic signalling units are located to indicate the fall in oil pressure in any of the phases.

Fig. 3  Oil filled three core cable

1.1 Advantages 

       The various advantages of oil filled cables are,

1. The thickness of insulation required is less hence smaller in size and weight.

2. The thermal resistance is less hence current carrying capacity is more.

3. The possibility of voids is completely eliminated.

4. The allowable temperature range is more than solid type cables.

5. Reduced possibility of earth fault. This is because in case of any defect in lead sheath, oil leakage starts, which can be noticed before earth fault occurs.

6. Perfect impregnation is possible.

1.2 Disadvantages

       The disadvantages of oil filled cables are,

1. The initial cost is very high.

2. The long length are not possible.

3. The oil leakage is serious problem hence automatic signalling equipment is necessary.

4. The laying of cable is difficult and must be done very carefully.

5. Maintenance of the cables is also complicated.