This system of distribution is also adopted in crowded areas and road crossings. There are two main systems of Transmission and Distribution of Electric power. These are
- Overhead system
- Underground system.
In the overhead system, the conductors are supported on concrete or steel poles or towers and are bound suitably to the insulators, and are open to the atmosphere. In the case of an underground system, a thickly insulated bunch of conductors called “cables” are buried underground. In the developed countries, this system is mostly used for distribution as well as transmission of electric power. But in developing countries, in comparison with underground cable systems, the overhead system is adopted as it is cheaper. But in unavoidable circumstances such as
(i) Thickly populated areas to avoid the risk of shocks.
(ii) Around the plants and substations where maintenance activities restrict the use of overhead lines, the underground cable system is most suited.
Generally, the underground cable system is nowadays becoming popular to its advantages even in developing countries where new localities are being developed and no problem of replacing the old overhead lines is there. Moreover, recent improvements in the design and manufacturing processes the cables are capable to be used for an underground system for small and moderate distances even for high-voltage transmission purpose too.
Requirements of Cables:
There are various types of cables used as per the voltage and type of service. But in any of the types, the following are the requirements of the cable:
(i) Conductors inside the cable may be of copper or aluminum. If the copper conductor is used, it must be coated with tin (tinned copper) which avoids the chemical reaction of copper with insulation.
(ii) Stranded type conductors: Instead of one thick conductor of a big cross-section, many conductors of a small cross-section bunched together is preferred. This gives flexibility to the cable.
(iii) The current carried by cable conductors produces heat and therefore, the temperature increased. The size of the conductor should be such that the cable is not over-heated.
(iv) There is a limit for voltage drop in the cable line. depends on voltage drop This on the “current x resistance” of the conductors. This L.R. drop should be within the permissible limits.
(v) Voltage rating decides the quality and thickness of the insulation covering on the conductors. This insulation shall suit the requirements so that it should not be damaged and provide safety and reliability for service.
(vi) The outermost covering i.e. ‘serving must be mechanically tough so that it should not be scratched while pulling of cable and laying of the cable.
(vii) Additional mechanical strength shall be provided by metallic armouring.
If the above requirements are fulfilled, the underground cable gives reliable and long time service.
Cables are broadly classified into two categories:
(a) As per the type of insulating material used in manufacturing the cables.
(b) As per the voltage rating of the service.
|Sr. No.||Types||Voltage Limit|
|1.||L.T. (Low Tension) Cables||Upto 1000 volts (1 kv)|
|2.||H.T. (High Tension) Cables||Upto 11,000 volts (11
|3.||S.T. (Super tension) Cables||From 22 kV to 33 kV|
|4.||EHT (Extra high tension) Cables||From 33 KV to 66 kV|
|5.||ESV (Extra Super voltage) Cables||Beyond 132 kV|
Further classification of cables depending upon a number of cores:
- Single-core cable
- Two (double) core cable
- Three core cable
- Four core cable
Depending upon the operating voltage, 3-phase cable may have 3-single cores or combined 3-cores.
Let us first know the general constructional details of a cable.
As shown in Fig. 5.2.2, the conductor is surrounded by different layers for specific purpose. Let us discuss it in detail.
- Cores or conductors: Conductors are made of Cu or Al and generally stranded to afford flexibility for pulling / laying. If Cu is used, it is tin coated.
- Insulation: The material used may be (i) Rubber (ii) Impregnated paper, (iii) Varnished cambric. The thickness of the insulation is decided according to how much voltage it has to withstand.
- Metallic sheath: Metal sheath, either of aluminum or generally lead surrounds the insulation. This protects the conductor and acids, alkalies, insulation from moisture, dampness, etc.
- Bedding: In order to avoid corrosion of aluminum or lead sheath, the sheath is rapped (covered) with “hessian” tape or jute fibers material. This also safeguards the sheath from metallic armoring from mechanical injuries.
- Armoring: There is the possibility of damaging the cable during handling, pulling, or laying. In order to avoid this and protect the cable one more layer in the form of round galvanized steel wires or strips/tape is provided. This layer is “Armouring”.
- Serving: It is a layer of impregnated hessian tape. This provides protection to the prior layer i.e. armouring. A coating of limewash is applied to this tape to avoid sticking to the armour.
Some cables may not require armoring. The layers of bedding, armoring, and serving are the protective layers to the cable.
Insulation is a very important part of an underground cable. The satisfactory operation and working and life of cable depend upon characteristics of insulation used. For a proper choice of the type of insulation, the selection is of great importance.
Following properties are required in the insulation
- Resistivity (p) should be as high as possible to avoid leakage current.
- Its dielectric strength should be higher. This avoids the electrical break-down of cable.
- High resistance to moisture i.e. it should be non-hygroscopic so that it should not absorb moisture. To protect. the cable from this happening, the insulation is covered with a lead sheath.
- High thermal conductivity: So that it passes out the heat to the next layer i.e. sheath.
- High tensile strength: It should be capable of withstanding mechanical injuries while working with cables. It should not get damaged or break.
- Non-inflammable: It should be non-inflammable and possibly fireproof.
- Cost: It should be cheaper in cost, so the system is affordable.
- It should be unaffected by acids/alkalies.
Above all properties may not be present as a whole in an insulating material. Proper selection is made as per the situation where the cable is to be used.