Most of the electrical power generated at the power plants is transmitted and distributed through overhead lines. The power transmission and distribution through overhead lines employs conductors with appropriate spacing between them supported on towers or poles.
The spacing is provided to avoid electrical discharge between conductors by increasing the insulation. Compare to underground power transmission and distribution lines, overhead lines are much cheaper.
For transmitting power over long distances at higher voltages, overhead lines are the best choice. For the same power transmission, insulation cost and size of conductor required for underground cables is more. The erection cost is much lesser and the maintenance is convenient for overhead lines.
However, the use of overhead lines offers some drawbacks like, they are subjected to uncertain weather conditions which decrease the overall efficiency, the conductor can be damaged by high winds or by falling trees, causes external interferences, the occurrence of faults due to lightning or short circuits, etc. But, by the use of proper safety factors, the drawbacks can be overcome. Let us see the various components of overhead lines in the power system.
Components of Overhead Lines :
Conductors :
Conductors play an important role in the overhead transmission and distribution line, it carries electric power from generating station to receiving end station and then to consumers. The major part of the total capital is invested in conductors.
Therefore, the choice of conductor material and size of the conductor should be chosen properly. Depending upon the current to be carried and the span of the line, the composition of materials like copper, aluminum, or ACSR is chosen.
Insulators :
Insulators are provided on the supports (poles or towers) to support conductors such that necessary insulation is provided to supports from conductors. This further prevents leakage currents from conductor to earth through supports. Insulators also prevent short-circuiting between conductors and metalwork. The most commonly used insulating materials are porcelain, glass, and Stealite. The various types of insulators are,
- Pin-type insulators
- Suspension type insulators
- Strain insulators
- Shackle insulators and
- Stay insulators.
Supports :
The function of line supports is to support the conductors and to keep the conductors at a suitable level above the ground. Generally, poles or towers are chosen as supports. These are employed depending upon the working voltage and the regions where they are used. Poles or towers are of various types like,
- Wooden poles
- Steel poles
- RCC poles and
- Lattice steel towers.
Cross-arms and Clamps :
These are provided on pole structures to support the insulators and conductors. These are made up of either wood or steel angle sections.
Guys and Stays :
These are employed to resists the lateral forces at the termination or angle poles by fastening the braces or cables to the poles.
Lightning Arrestors :
It is a device used to provide protection against traveling waves or high voltage produce due to lightning by discharging the excessive-high voltage of the line to the ground.
Fuses and Isolating Switches :
These are used to isolate different parts of the transmission system.
Earth Wire :
Earth wire is made run on the top of the towers in order to protect the line against lightning.
Vee-Guards :
To ensure public safety, these are provided below the base overhead line along the street.
Guard Wires :
The guard wires are provided above or below transmission lines while crossing the communication lines and are solidly connected to the earth.
Miscellaneous Components :
Phase plates, bird guards, danger plates, barbed wires, vibration-dampers, top hampers, beads for jumpers, etc. The phase plates give information about various phases used, the barbed wire, and are wounded to poles at a height of 2.5 meters in order to prevent the climbing of unauthorized persons. However, danger plates are also provided at a height of 2.5 meters from the ground level.
Various Types of Conductor Material Used for Overhead Lines :
Copper :
Copper is the most commonly used material for the construction of overhead line conductors. It easily conducts electrical current and is easily available. There are three kinds of copper wire, hard-drawn copper, medium hard-drawn copper, and soft-drawn copper.
Among these, the hard-drawn copper is mostly used for the construction of conductors because it is less elastic in nature and has high mechanical strength.
The current density of copper is high and hence requires less cross-sectional area than others. Other advantages include durability, higher scrap value, and homogeneity. Owing to its advantages, all small conductions and heavy power cables are made up of copper. However, it is not preferred for overhead transmission and distribution lines due to its high capital cost.
Aluminum:
Aluminum is the most commonly used material for transmission and distribution lines in the power system. It is cheaper and lighter in weight than copper. Owing to its lightweight, the tower structures used need not be so strong as that of copper. It is mostly used for carrying heavy currents.
For such applications, a special type of configuration is used which is called ACSR conductors (Aluminium Conductor Steel Reinforced). In this configuration, small strands of aluminum, are twisted around a steel core. This not only increases the current carrying capacity but also increases flexibility and mechanical strength.
The main drawbacks of aluminum are, it has lesser conductivity (60% of that of copper), lesser tensile strength (45% of copper), low melting point, and difficulty to make joints. Even though aluminum suffers from drawbacks, it is universally used for transmission and distribution conductors.
Galvanized Steel :
Galvanized steel is the cheapest material of all and hence is used in areas where the economy is the primary factor. However, it suffers from some drawbacks like low conductivity, high resistance and hence it is used only for small power and short-distance applications. It is also used for applications that require longer spans.
Cadmium Copper :
Cadmium copper is used for applications that require long spans and a small cross-sectional area. Cadmium is used along with copper in order to increase tensile strength. Owing to its higher cost, it is used for some specific applications.
Copper Weld Steel :
Copper weld steel conductors are nothing but steel wire with a copper coating. This configuration is used in order to have advantages of both copper and steel i.e., higher conductivity, greater strength, and less cost. It is basically used for rural lines, guy wires, and overhead ground wires. The main advantage of this conductor is that the conductivity can be increased to any percentage by providing the necessary thickness of the copper coating.