What are Underground Cables? Why are they used extensively in power distribution?
- Underground Cables are designed to be placed beneath the surface of the earth and later to be used for communication, power distribution, or any other intended purposes. These cables are designed to protect against moisture, rodents, cable-seeking backhoes, and other earth-moving equipment.
- Underground cables are used for power applications where it is impractical, difficult, or dangerous to use the overhead lines. They are widely used in metropolitan cities.
- The underground cables have several advantages over the overhead lines. They have smaller voltage drops, low chances of faults, and no need for earth wire.
Construction of Underground Cable - The underground cable may be 3 core/3 phase or 6 core /3 phase. The conductor is generally copper or aluminum material while armoring material is steel or lead.
- Bonding is also provided at both ends of the underground cable to provide a path to leakage current which is generated due to magnetizing effect of adjacent conductors. Bonding reduces heating inside the cable so that the cable can be utilized for higher voltages.
- Electric field strength is not uniform between the conductor and the sheath. It decreases from the inner to the outer side ie. towards the sheath.
Grading of Underground Cables:
- To make the Electric field strength uniform two methods are adopted. First is the Most economical size method second is the grading of cables. In the MES method, the ratio of the outer radius to the inner radius is taken to 2.718.
- Grading of underground cable is done in two ways. The first one is Capacitance grading in which dielectric materials of different dielectric constants like XLPE, PVC, Rubber, and paper are used to make a uniform electric field. The second one is Inter-sheath grading in which several inter sheath of the same dielectric material is provided and the electric field is maintained uniformly by proving different voltages to different inter sheaths with the help of an external transformer. They are used up to 33 KV voltages only.
Underground Cable
