Electricity plays an important role in the modern society because of its versatility with respect to input energy form. Electricity also offers total environmental, enhancement opportunity as compared to other energy rout age pattern by the consumer. However, the rapid advancement in the field of electronics and its innumerable applications in residential, commercial, industrial and agricultural sectors, the demands for quality power supply has increased tremendously. The primary requirement of a planning engineer is to give power supply to the consumer in a reliable manner at a minimum cost, taking into consideration the constraints and the criteria such as reliability, economics, environment, society impacts and value of electricity. Therefore, availability of quality power depends on the extent of its requirement by the usage sectors mentioned earlier, the expected degree of reliability etc.
A cable is basically an insulated conductor and is used for underground (UG) transmission and distribution of electricity. Insulation is used to cover the conductor and provides isolation from the surroundings. It should possess high resistance, high dielectric strength, high mechanical strength and long durability.
UG cables are generally used in densely populated areas such as cities and metros, where there are high density of automobiles; high raised commercial, residential buildings and places; where vital installations of uninterrupted power supply such as water supply system, hospitals and IT services, etc. UG cables are also preferred in industries, sub-stations, railways and road crossings, servicing of residential installations and other similar locations.
UG cables help in ensuring uninterrupted power supply, which are hitherto uncommon in the Overhead (OH) system. The main hindrance in ensuring uninterrupted power supply in the OH system is the unavailability of space in developed urban areas. Falling of tree branches on the OH lines and short circuiting of conductors due to heavy winds and gales, rains, accidents due to bare conductors, failure of supporting structures, etc. Although UG cable system at high voltages is expensive than OH line system, the benefits reaped are higher. UG cables system provides high reliability, least interruptions due to line faults and good safety. In addition to this, the I2R losses are quite low compared to OH system due to the absence of steel wires. Its advantages such external protection is provided against mechanical injury, moisture entry and chemical reactions.
The type of cable used at a specific location is determined by the mechanical considerations and the voltage at which it is required to operate. The grade of insulation of the UG cable depends on the voltage levels. Hence, the size of the cable is dependent on the voltage levels. Commonly used UG cables and the basic insulating material structure are appended below:
Class 1A: PVC Cables
It is a synthetic material and obtained as white odorless tasteless, chemically inert, non-inflammable and insoluble powder. It is chemically combined with plastic compound. Gel is used over the conductor to form the insulation cover. It has a maximum cont. temperature rating of 75°C, dielectric strength of 17 kV/mm and dielectric constant of 5.
Class 1B: Paper Insulated Lead Cable
It is most popular and highly advantageous as insulating medium due to higher thermal conductivity, dielectric strength, better thermal withstanding capacity capability, low cost and long durability. Paper is dried and impregnated with insulating oil and taped over the conductor. Dielectric strength of this insulator is around 20 kV/mm with a maximum contact temperature of 80°C and dielectric strength is 3.5
Class 1C: Cross Linked Polyethylene Cable (XLPE)
Specially treated low density polyethylene results in cross linking of carbon atoms and the compound is a new material having extremely high melting point with light weight, small dimension, low dielectric constant and high mechanical strength due to high thermal resistance and very low moisture absorption. These cables can be directly laid on the soil bed and easily worth for voltages up to 33 kV. Depending on the type of cores, the cable is further classified as 1 core, 3 core, and 3 ½ core. Commonly used conductor materials are aluminium and copper. However, aluminium cables are more common because of expensiveness of copper.
The UG cables are classified based on operating voltage levels, as mentioned below.
• L.T. Cable, M.V. Cable, H.T. cable for voltages ranging from 11 kV to 33 kV.
• EHT (Extra High Tension) cable
Low Voltage (LV) cables are directly laid in ground and hence, the cheapest, simplest and extensively used method. High Voltage (HV) cables are usually laid on the brick bedding inside trenches, which are buried at a depth of 0.75-1 m. Additional cables can be laid in the same trench by a minimum clearance of 0.3m between the trench and the cable. The trench is refilled with sand or soil. Details of cable laying confirming to established standards and code of practice is explained below:
i. The cable drums shall be stored on a well-drained, hard surface, so that the drums do not sink in the ground causing rot and damage to the cable drums. Paved surface is preferred, particularly for long term storage.
ii. The drums shall always be stored on their flanges, and never on their flat.
iii. Both ends of the cables especially of PILCA cables should be properly sealed to prevent ingress/ absorption of moisture by the insulation during storage.
iv. Protection from rain and sun is preferable for long term storage for all types of cables. There should also be ventilation between cable drums.
v. During storage, periodical rolling of drums once in, say, 3 months through 90 degrees shall be done, in the case of paper insulated cables. Rolling shall be done in the direction of the arrow marked on the drum.
vi. Damaged battens of drums etc., should be replaced as and when it may be necessary.
i. When the cable drums have to be moved over short distances, they should be rolled in the direction of the arrow marked on the drum.
ii. For manual transportation over long distances, the drum should be mounted on cable drum wheels, strong enough to carry the weight of the drum and pulled by means of ropes. Alternatively, they may be mounted on a trailer or on a suitable mechanical transport.
iii. For loading into and unloading from vehicles, a crane or a suitable lifting tackle should be used. Small sized cable drums can also be rolled down carefully on a suitable ramp or rails, for unloading, provided no damage is likely to be caused to the cable or to the drum.
Operating problems with cables
● Cables required high charging current and reactive power for operation. The reactive power is capacity in nature and can affect at lightly loaded conditions due to Ferranti effects of rising of sending end voltage of the cable.
● Flow of charging current causes heating of cables and reduces the lower current capability. Sometimes, higher dielectric loss may further rise in the temperature.
● Switching of cable capacitive current may give rise to over voltages.
● Although the purpose of using OH line and cables being the same, use of UG cables is limited due to large charging current for long distance transmission.
● The insulation required by cable is high, heat dissipation is less than OH lines.
UG cables have greater safety and less interference with communication lines, with better outlook.
The limitations of UG cable are:
● Expensive compared to OH lines
● Difficulty in fault detection
● Restoration of power supply takes longer time during break down.
Precautionary Measures for better reliability and safety:
● For ensuring longer life of the cable, UG cable should never be overloaded for longer duration – and always advisable to restrict the loading to about 70% of the rated capacity.
● Before carrying out maintenance of the UG cable, care should always be taken to discharge the static charges stored in the cable.
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