Underground Cables
Underground cables are used to bring and carry electrical power.
Underground cables are used in places where the system for transmission or distribution to the overhead is interrupted.
There are many types of underground cables depending on the voltage.
Classification of Underground Cables
According to the mechanical structure and voltage, underground cables are classified as follows.
The cables are classified as follows
(i) Low voltage cable
(ii) High voltage cable
(iii) Super voltage cable
(iv) Extra high voltage cable
(v) Extra super high voltage cable
(i) Low Voltage Cable:- The operating voltage of this type of cable is up to 1 kilovolt. These are also called low tension cables. It does not have any electrostatic stress. Paper insulators are generally used in this type of cable. But sometimes resin is also used, which increases the viscosity and helps in preventing drainage. It is made of a circular core of strained copper or aluminium. The conductor is made insulated by impregnated paper. There is a lead sheath over the paper insulator.
(ii) High Voltage Cable:- The operating voltage of these cables ranges from 1 kilovolt to 11 kilovolts. These are also called high tension cables. It is used more because the 3-core cable is more economical. The following types of cables are generally used for electrical service.
(iii) Best Voltage Cable:- The operating voltage of these cables ranges from 11kV to 33kV. These are also called super tension cables.
(iv) Very High Voltage Cables:- The operating voltage of such cables ranges from 33 kilovolts to 132 kilovolts. These are also called very high-pressure cables. (E.H.T Cable-From 33 kv to 132 kV)
(v) Extreme Voltage Cables:- The operating voltage of these cables is above 132 kV. These are also called super high-pressure cables.
- Read more: Classification of Overhead Transmission Line
Underground Cable Structure
Single-core cable is used for low voltage, and multi-core cable is used for high voltage. Multi-core cables are tied together by vulcanized bitumen and impregnated paper. The structure of the multi-core underground cable is shown in the figure below.
The structure of the underground cable is shown. The main parts of a cable are
(i) Cores or conductors- A cable may contain one or more conductors or cores. Its value depends on the utility of the cable. For example, a three conductor cable is used in three-phase phases. These conductors are mainly made of tin, copper or aluminium.
(ii) Insulation – Each core or conductor is insulated with an insulation metal of suitable thickness. The thickness of this insulation depends on the potential that the conductor cable can carry. Impregnated paper, rubber, etc., are mainly used for insulating materials.
(iii) Metallic surface – To protect the cable from the effects of moisture, gas and other factors like acids and alkenes, a metallic surface lead (Sheath) and insulation surface aluminium is used, which is described in the above figure.
(iv) Bedding- A fiber material such as jute or hessian tape is bedding on the metallic sheath. The purpose of bedding is to protect the metallic sheath from mechanical damage caused by friction and armouring.
(v) Armouring- is done over the reading surface consisting of two or one layer of steel tape or steel wire with two galvanized steel. Its purpose is to protect the cable from mechanical damage during laying. This action cannot be performed on all cables.
(vi) Serving- To protect the armouring layer from atmospheric conditions, armouring is also done on the surface of fibrous material like jute-like beading. This action is called serving.
Advantages and Disadvantages of Underground Cable
Following are the advantages and disadvantages of underground cable
Advantages of Underground Cable
1. This type of cable is better from the point of view of security as it is underground.
2. There is less chance of fault due to friction, heat and other mechanical reasons as the cable is underground.
3. It also has a longer life span.
4. In this, the interference is low due to the circuit.
5. Their maintenance rate is low.
6. This type of cable has an excellent and high appearance.
Disadvantages of Underground Cable
1. The current carrying capacity and a potential drop in the underground cable are less.
2. Being underground, it takes more time for fault detection and repair.
3. Their initial cost is high, and flexibility is low.
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