Introduction to Power cables
Power is supplied by electric cables, the electric cable configuration is either flat or round. Each one of them will be in different size and it is because each size cables are used for different operating conditions. The cables must be selected which will be suitable for the installation
What is underground wiring
In urban areas underground cables are used for bulk transmission of power, underground cables consist of one or more conductors covered with suitable insulation and surrounded by a protecting cover. In underground wiring, the cables may be buried directly under the ground or it may be installed in ducts buried in the ground.
The cable could have more than one core it is entirely depended upon the service which it is used for and the conductors in the cable will be of aluminum or copper core is provided with proper insulation and it is depended upon the voltage to be withstood by the cable most commonly used insulation materials are varnished cambric, impregnated paper or rubber mineral compound.
What are the properties needed for the insulating material
- High resistivity
- High dielectric strength
- Low water absorption
- Low permittivity
- It must be chemically stable and non-inflammable
- Low thermal co-efficient
- High mechanical strength
- High viscosity at impregnation temperature
- Capability to withstand high rupturing voltage
- High tensile strength and plasticity
Advantages of underground cables
Physical protection is high for underground cables low ambient temperature will be presented to the cables and it could result in high ampacities. A heat sink is provided by the soil and it would make a good impact on transient loading capabilities, it has a low voltage drop and has low maintenance.
Laying of underground cables
Underground cable reliability is depended upon the laying. if the underground cable is not properly laid it will affect the reliability. So proper laying must be done. There are three main laying methods for underground cables they are
- Direct laying
- Draw in system
- Solid system
Direct laying
This method is cheap and simple, in this method a trench of 1.5 meters deep and 45cm wide is dug and then the cable is laid inside the trench after that it is covered with concrete material or bricks so that it will be protected from mechanical injury.
This method is clean and safe and it gives the best heat dissipating conditions below the earth
Disadvantages of direct laying
- Fault localization is difficult
- It is costlier in congested areas
- High maintenance coast
Draw in system
In this method conduits or duct of concrete is laid in the ground and its main holes will be in suitable positions along the cable route. Then the cables are pulled into positions from the main holes.
Advantages of draw in the system
- It is suitable for congested areas where evacuation is expensive
- It is mostly used for short length cable route such as in road crossing where frequent digging is costlier
Disadvantages
- It has high initial coast
- Heat dissipation conditions are not good
Solid system
In this method, the cable is laid in open pipes or troughs dug out in earth along the cable route, troughing will be filled with bituminous after the cable is laid. It has good mechanical strength and heat dissipation condition is poor and it is really expensive, it needs skilled labor and favorable weather conditions
What are the limitations of underground lines
- It is highly expensive
- In underground cables the cable length is limited due to physical factors unlike overhead lines
- Higher the voltage shorter the line length must be
- Limiting effects will become important at transmission voltages especially 100,000 volts and above