Underground Cables
Preferred in big cities and densely populated areas.
A cable is basically an insulated conductor, externally protected against mechanical injury, moisture entry and chemical reaction.
It should be economically attractive as well as able to carry large chunk of power.
The type of cable used depends upon the voltage and service requirements.
Advantages of underground cables
Better general appearance
Less liable to damage through storms or lightning
Less chances of fault
Low maintenance cost
Less spacing between conductors resulting in low inductance and hence less voltage drops
No interference with communication lines
Injects capacitive reactive power and hence improves voltage regulation
Disadvantages of underground cables
Greater installation cost
Tapping of loads and service mains is difficult
Draw high charging current, leading to overvoltage under light load conditions
Difficult to locate the fault point and its repair is difficult and expensive
Introduce insulation problems at high voltages compared to equivalent overhead system
Construction of Cables
Core or Conductor - single core or multi-core depending upon the type of service required. - made of aluminium or copper and is stranded in order to provide flexibility to the cable.
Insulation - suitable thickness of insulation should be provided, depending upon the operating voltage. - impregnated paper, varnished cambric or rubber mineral compound.
Metallic Sheath - to protect the insulation material from moisture, gases or other harmful liquids in the soil - also protects the insulation from mechanical damage - lead or lead alloy, aluminium
Bedding
- to protect the metallic sheath from corrosion and from mechanical injury. - consist of paper tape compounded with a fibrous material like jute or hessian tape.
Armouring
- to protect the cable from mechanical injury while laying and handling it. - consists of one or two layers of galvanized steel wire or steel tape. Steel tapes are coated with preservative compound and are helically wound on the cable insulation.
Serving
- a layer of fibrous material is provided, similar to bedding - to protect the armouring from atmospheric conditions
Properties of Insulating Material High resistivity. High dielectric strength. Low thermal co-efficient. Low water absorption. Low permittivity. Non – inflammable. Chemical stability. High mechanical strength. High viscosity at impregnation temperature. Capability to with stand high rupturing voltage. High tensile strength
Classification of Cables
Low tension cables – upto 1 kV
High tension cables – upto 11 kV
Super tension cables – upto 33 kV 1) H-type or screened cables 2) SL type cables 3) HSL type cables
Extra high tension cables – beyond 33 kV 1) Oil filled cables 2) Gas filled cables
Low Tension Cables
Used for operating voltage of upto 1 kV
Electrostatic stresses developed are small and the thermal conductivity is also not of very much concern
Insulation used may be paper, impregnated with oil
PVC is also used, sometimes, to increase dielectric strength
May be single core or multi-core cables
Single core cable
- consists of a copper or aluminium stranded oval or round shaped core - usually no armour is provided to avoid excessive sheath losses
Multi-core cable
- generally 3 core, 4 core or 3 and 1/2 core cables - cores are either circular-shaped or oval or sector-shaped conductors - cores are insulated from each other and a belt of insulation is provided surrounding the cores
High Tension Cables
Used for 3-phase medium and high voltage distribution (11 kV)
Cross linked polyethylene (XLPE) is becoming more popular due to better dielectric strength and longer life, high resistance to corrosion
Usually of belted type - cores are not circular in shape
Super Tension Cables
H-type or Screened cables
- Designed by H. Hochstadter - No paper belt - Each core is insulated by layer of impregnated paper - The insulation on each core is covered with a metallic screen which is usually of perforated aluminum foil - The cores are laid in such a way that metallic screen make contact with one another - Basic advantage of H-TYPE is that the perforation in the metallic screen assists in the complete impregnation of the cable with the compound and thus the possibility of air pockets or voids in the dielectric is eliminated - The metallic screen increase the heat dissipation power of the cable
SL Type Cables (Separate Lead)
- Each core insulation is covered by its own lead sheath - It has two main advantages, firstly the separate sheath minimize the possibility of core-to-core breakdown. Secondly the, bending of cables become easy due to the elimination of over all sheath - The disadvantage is that the lead sheaths of S.L is much thinner as compared to H-Type cables, therefore greater care is required in manufacturing.
HSL Type Cables
- combination of H type and SL type cables - each conductor is insulated, sheathed with metallized paper and is then provided with lead sheath
Advantages of Screened cables over belted cables
Reduced core-to-core faults
Electric stresses in the cables are uniformly radial in all sections
No possibility of formation of voids within the electric field
The current carrying capability of cables is increased
Extra High Tension Cables Difficulties
3-core cables are uneconomical and impracticable due to their high weight and size and hence, single core cables are widely used.
There are greater sheath losses in single-core cables
Due to the possibility of voids in extra high tension voltage cables, there is a danger of breakdown of the dielectric
Due to the failure of the dielectric, there is a possibility of ionization and allied chemical reactions, which damage the insulation
Remedies
Pressurized type cables - In these cables, pressure is maintained above atmosphere either by oil or by gas. - Gas pressure cables are used up to 275KV. - Oil filled cables are used up to 500KV.
Oil filled cables
- an oil channel is formed at the centre of the core, which is filled with low viscosity mineral oil by means of oil reservoirs and feeding tanks placed at about every 500-600 m length along the route of the cable and maintained at constant pressure - usually single phase type - when oil gets expanded due to rise in temperature of the cable, the extra oil gets collected in the external reservoir, which sends it back during contraction of oil when the temperature falls under light load conditions
Single core oil filled cable
Single core sheath channel cable
Three core filler space channel cable
Advantages of Oil Filled Cables Oil filled cables have the following advantages over solid cables • Greater operating dielectric stresses • Greater working temperature and current carrying capacity • Better impregnation • Impregnation is possible after sheath • No void formation • Smaller size of cable due to reduced dielectric thickness • Defect can easily be detected by oil leakage
Disadvantages • Higher cost • Laying of cables and maintenance complicated
Gas Pressure Cables an inert gas like nitrogen is used to exert pressure on paper dielectric to prevent void formation.
Three core external gas pressure cable
Three core impregnated pressure cable
Laying of Underground Cables a. b. c.
Direct Laying Draw in system Solid system
Direct Laying • • • • •
This method is cheap and simple and is most likely to be used in practice. A trench of about 1.5 meters deep and 45 cm wide is dug. A cable is been laid inside the trench and is covered with concrete material or bricks in order to protect it from mechanical injury. This gives the best heat dissipating conditions beneath the earth. It is clean and safe method
Disadvantages • Localization of fault is difficult • It can be costlier in congested areas where excavation is expensive and inconvenient. • The maintenance cost is high.
Draw in System • In this conduit or duct of concrete is laid in ground with main holes at suitable positions along the cable route. • The cables are then pulled into positions from main holes. • It has very high initial cost • Heat dissipation conditions are not good • This method is suitable for congested areas where excavation is expensive and inconvenient • This is generally used for short lengths cable route such as in workshops, road crossings where frequent digging is costlier and immposible
Solid System • In this system the cable is laid in open pipes or troughs dug out in earth along the cable route. • The troughing is of cast iron or treated wood • Troughing is filled with a bituminous after cables is laid. • It provides good mechanical strength • It has poor heat dissipation conditions • It requires skilled labour and favorable weather conditions • It is very much expensive system
Grading of Cables • The stresses are maximum at surface of the conductor or inner most part of the dielectric and it goes on decreasing as outer most layer is reached. • The process of achieving the uniform electrostatic stresses on the dielectric of cables is known as Grading of cables. • The unequal distribution of stresses is undesirable because,if dielectric is chosen according to maximum stress the thickness of cable increases or either this may lead to breakdown of insulation. • The following are the two main methods of grading - Capacitance grading - Intersheath grading