- on the basis of insulating material used in their manufacture,
- on the basis of voltage for which they are manufactured.
However, the method of classification of electrical cable on the basis of voltage more common, according to which they can be divided into the following categories:
- Low-tension cables — up to 1000 V
- High-tension cables — up to 11000 V
- Super-tension cables — from 22 kV to 33 kV
- Extra high-tension cables — from 33 kV to 66 kV
- Extra super voltage cables — beyond 132 kV
Electrical cable classification can be done on the basis of their construction as under:
- Belted cables — up to 22 kV
- Screened cables — from 22 kV to 66 kV
- Pressure cables — beyond 66 kV
These cables are used up to 22 kV. The construction of belted cable is shown in Figure. The cores are not circular in this case. The cores are insulated from each other by use of suitable insulating material. The three cores are grouped together and belted with the help of a paper belt.
The gaps are filled with fibrous material (like jute etc.). The paper belt is covered with lead sheath to protect cable from ingress of moisture. The lead sheath also gives mechanical strength to the cable. The lead sheath is finally covered with outer sheath.
The belted type cables are suitable only for low and medium voltages as the electrical stresses developed in the cables at these voltages are mostly radial in nature beyond these voltages insulation is subjected to tangential electrical stresses rather than radial one.
But electrical properties of impregnated paper (i.e. dielectric strength, insulation resistance etc.) along the radius are much larger than that of along tangential path.
Therefore in the case of belted cables, leakage current set up along the layers of paper insulation at higher voltages. This causes heating of cables and may result in the breakdown of cable.
Another disadvantage of the belted cables is a large diameter of the paper belt. Due to this, wrinkles are formed if the cable is bent this may lead to the breakdown of cable at higher voltages.
These cables are used for the voltage levels of 22 kV and 33 kV. But in special cases, their use can be extended up to 66 kV. There are two types of screened cables namely:
- H type cables and,
- L. type cables.
H – type Cables: There is no paper belt in this type of cables. Each conductor, in this case, is insulated with impregnated paper, covered with a metallic screen which is usually an aluminum foil. The metallic screen touches each other.
Instead of a paper belt, the three cores are wrapped with a conducting belt which is generally copper woven fabric tape. Then there is an inner sheath made of lead. After lead sheath, there are layers of bedding, armouring and the outer sheath.
In these cables, each core is in electrical contact with lead sheath and metallic screen and is at earth potential. This makes electrical stress inside the cable purely radial which results in reduced dielectric losses. Metallic screens increase the heat dissipation capacity of cable which reduces the sheath losses.
S.L. Cables: In S.L. (separate lead) cables each core is insulated with impregnated paper and each one is then covered by a separate lead sheath. Instead of an overall lead sheath, a cotton tape covering is used to wrap the three cores together. Then there are layers of armouring and the outer sheath.
The various advantages of S.L. type cable are:
- The absence of common lead sheath covering all the three cores increases the flexibility of the cable in this case.
- The lead sheath on each core increases the heat dissipation of the cable.
- The electric stresses are radial in nature.
For voltages over 66 kV, pressure cables are used. In these cables, a possibility of void formation is completely eliminated by increasing the pressure of insulating medium (i.e. oil or gas). As the insulating medium in these cables is under pressure, these cables are known as pressure cables. These are of two types namely:
- Oil filled cables
- Gas pressure cables.
Oil Filled Cables: In these cables, oil ducts are made within or adjacent to the cores, oil under pressure is circulated in these ducts. The cable is filled with oil and the pressure is maintained in the cable by connecting the oil channels to the tanks which are placed at the suitable distances along the path of the cable.
The oil pressure compresses the paper insulation and completely removes the possibility of void formation. Due to the elimination of voids, dielectric strength of insulation is such cables become very high. Operating voltage of these cables ranges from 66 kV to 230 kV.
As the pressure tanks are required all along the path of these cables, the lengths of these cables are limited. Oil leakage is another serious problem connected with these cables. Automatic signaling units are used to indicate the fall in pressure in any cable core. In this way, the complete system becomes very complicated and requires a high initial cost.
Gas Pressure Cables: In these cables, an inert gas like nitrogen at high pressure is used to create pressure in the cable. The gas pressure is about 12 to 15 atmospheres. Due to such a high pressure, radial compression of insulation takes place which eliminates the possibility of void formation and ionization.
The construction of cable is shown in Figure, it is triangular in shape and installed in a steel pipe. The pipe is filled with gas.
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