Most of the electricity generated by power plants is transmitted and distributed overhead lines. The transmission and distribution of power through overhead lines uses conductors supported on towers or poles with appropriate spacing between them. This spacing is provided to avoid discharges between conductors by increasing insulation. Compared to underground transmission and distribution lines, overhead lines are much cheaper.
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For transmitting electricity at higher voltages over long distances, overhead lines are the best choice. For the same power transmission, underground cables require more insulation costs and conductor sizes. Overhead costs are much lower and overhead lines are easy to maintain.
However, there are some disadvantages to using overhead lines, for example, they are subject to uncertain weather conditions that reduce overall efficiency, conductors can be damaged by high winds or fallen trees, causing external interference, faulty short circuits due to lightning or faults, etc. However, these drawbacks can be overcome by using appropriate safety factors. Let's look at the various components of overhead lines in a power system.
Components of overhead lines
Conductors
Conductors play an important role in overhead transmission and distribution lines, which carry electricity from the generating station to the receiving end and then to the consumer. A major part of the total capital is invested in the conductors. Therefore, the choice of conductor material and the size of the conductor should be selected appropriately. Depending on the current to be carried and the span of the line, the composition of materials such as copper, aluminum or ACSR is selected.
Protective wire
Protective lines are provided above or below the transmission line when crossing communication lines and are firmly connected to earth.
Lightning arrester
It is a device used to provide protection against traveling waves or high voltage caused by lightning by discharging excessive voltage from the line to ground.
Insulators
The insulator is set on a support (pole or tower) to support the conductor, thus providing the necessary insulation for the support with the conductor. This further prevents leakage currents from the conductor to earth through the support. Insulators also prevent short circuits between conductors and metalwork. The most commonly used insulating materials are porcelain, glass and Stealite. the various types of insulators are, the
Pin insulators
Suspension insulators
Strain insulators
Unbuckled insulators
Retention insulators.
Supports
The function of a line support is to support the conductor and keep the conductor at a proper level from the ground. Generally, poles or towers are chosen as supports. These are used depending on the operating voltage and the area of use. There are various types of electric poles or towers, such as.
Wooden electric poles
Steel poles
RCC poles and
Lattice steel towers.
Cross arms and clamps
These are provided on the pole structure to support insulators and conductors. These are composed of wood or steel angles.
Poles and brackets
Used to resist lateral forces on terminal or angular poles by securing brackets or cables to the pole.
Fuses and disconnect switches
These are used to isolate different parts of the transmission system.
Grounding lines
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Grounding wires are run at the top of the tower to protect the line from lightning strikes.
Vee-Guards
For public safety, these are provided underneath the base overhead lines along the street.
Miscellaneous components
Phase plates, bird protection plates, hazard plates, barbed wire, shock absorbers, top dampers, beads for jumpers, etc. The phase plates provide information on the various phases used, barbed wire, which is wrapped around a 2.5 meter high pole to prevent unauthorized climbing. However, there are also hazard plates at a height of 2.5 meters above the ground.
Various types of conductor materials used for overhead lines
Copper
Copper is the most common material used to construct overhead line conductors. It conducts current easily and is readily available. There are three types of copper wire, hard drawn copper, medium hard drawn copper and soft drawn copper. Of these, hard-drawn copper is mostly used to construct conductors because of its low elasticity and high mechanical strength.
Copper has a high current density and therefore requires less cross-sectional area than the others. Other advantages include durability, higher scrap value and homogeneity. Because of its advantages, all small conductors and heavy power cables are composed of copper. However, due to its high capital cost, it is not the preferred choice for overhead transmission and distribution lines.
Aluminum
Aluminum is the most common material used in power systems for transmission and distribution lines. It is cheaper and lighter in weight than copper. Because of its light weight, the tower structures used do not need to be as strong as copper. It is mainly used to carry heavy currents.
In this application, a special configuration called ACSR conductor (aluminum conductor steel reinforcement) is used. In this configuration, small strands of aluminum are wrapped around a steel core. This not only increases the current carrying capacity, but also increases flexibility and mechanical strength.
The main disadvantages of aluminum are its poor electrical conductivity (60% of copper), low tensile strength (45% of copper), low melting point, and difficulty in making joints. Despite its disadvantages, aluminum is commonly used as a transmission and distribution conductor.
Galvanized steel
Galvanized steel is the cheapest of all materials and is therefore used in areas where economy is a major factor. However, it has some drawbacks such as low conductivity, high resistance and therefore it is used only for low power and short distance applications. It is also used in applications where longer spans are required.
Cadmium copper
Cadmium copper is used for applications that require long spans and small cross-sectional areas. Cadmium is used together with copper to increase the tensile strength. It is used for some specific applications due to its higher cost.
Copper-welded steel
Copper welded steel conductors are nothing more than steel wires with a copper coating. This configuration is used to get the advantages of copper and steel, i.e. higher conductivity, greater strength and lower cost. It is basically used for rural lines, overhead wires and overhead grounding wires. The main advantage of this conductor is that its conductivity can be increased to any percentage by providing the necessary thickness of copper coating.
Equipment for the power overhead lines construction
The construction of overhead and cable power lines requires the use of various types of equipment. Individual facilities are selected depending on the type of line, its purpose and environmental conditions. This ensures correct operation and interaction of all parts of transmission and distribution lines. It is worth taking a closer look at what type of equipment and tools for the construction of power lines are commonly used by investors.
Cable power lines can be used in both densely built-up and industrialized areas. They are usually invisible to the user and can be used for direct and alternating current. A cable line consists of a multi-core cable, a multi-phase cable bundle or several cables connected in parallel with cable accessories. Cables are laid along a specific route and connect electrical devices.
The wires of overhead power lines are suspended on poles or towers. The supporting structures can be made of various materials, such as wood, concrete, steel or composites, depending on technical requirements and field conditions. Steel-aluminium or aluminium cables of various designs are also used here, for example, cables with reinforced core.
In the case of cable power lines, the equipment used must have the same degree of reliability as the cable itself. Appropriate electrical and mechanical properties are ensured by joints and cable heads. LV, MV and HV cable lines can be equipped with them. Their design depends on factors such as the type of cable, rated voltage, manufacturing technology and the number and cross-section of conductors. Pass-through and transition joints are often used. The former allow you to connect different sections of the same type of cable, while the latter create connections between different types of cables, for example three-phase cables with plastic insulation.
Other important elements of cable line equipment are connectors and terminals. They are used to permanently connect the conductors of copper and aluminium power cables in devices most often used indoors. This type of connectors can be used to create connections of single-wire and multi-wire cables installed under low and medium voltages. The installation methods depend on the voltage level and the design of the cable cores. In addition, manufacturers offer universal connectors and terminals that facilitate the connection of various types of conductors, taking into account the selected cross-section range.
Equipment for the power cable lines
The components used, including cable joints, can be made using tape, heat-shrinkable, cold-shrinkable or poured technology. Hybrid solutions are also becoming more and more popular, i.e. connectors and cable heads, which at the same time guarantee adequate electrical and mechanical strength, as well as effective sealing and protection against moisture. They are used to connect or create terminations of working and return conductors.
Cable accessories for connection to the overhead power line mainly include components such as cable heads, meters, surge arresters, as well as various types of holders for mounting cables on the pole structure. The heads are installed on previously prepared cables and terminated with a special clamp, which is screwed to the overhead wire holders or mounted using a pin and screw clamps.
Cable heads are used to connect medium, high and low voltage cables to switchboards, transformer stations and overhead power lines. They also function as cable terminations inside or outside buildings, in the so-called cable cabinets. Surge arresters are, as their name suggests, accessories that protect cables and connected devices against the effects of atmospheric or switching overvoltage.
Various types of lifting and assembly devices are used for the installation and maintenance of overhead and cable lines. These include lifts, winches, cranes and work platforms. Drones, i.e. remotely controlled flying vehicles, are also helpful in the construction of transmission lines. Thanks to them, it is possible to monitor the progress of work and verify the parameters of the power line under construction. The work is facilitated by hydraulic drum lifts, as well as cable ramps designed for unwinding and rolling cables, electric wires, optical fibres, cords and ropes.
Overhead lines construction cannot be complete without accessories such as hooks, hook screws, brackets, handles or hook shanks. They are located on installations such as low-voltage overhead lines. The screws are used to hang the handles and are screwed to the supporting structures. The holders are made of steel and intended for installing cables on seatposts and tubular booms. The hooks are used to hang through and guy holders on round poles, while the hook shanks allow for mounting standing insulators.
Equipment and tools for the construction of a power line are selected taking into account many factors, including the purpose of the line, its type, environmental conditions and standards applicable in the installation region. The individual elements can be used in various combinations, depending on requirements, for example in situations such as the expansion of installations or the design of substations. In this way, it is possible to ensure the expected efficiency and a high level of safety during the execution of works.
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