Classification and composition of overhead lines

In the power grid, the power lines that consume electricity from power plants are considered generalized transmission lines. Lines can be divided into overhead lines and cable lines. The overhead line is composed of conspicuous towers and wires suspended in the air, exposed to the air. It is convenient to erect and maintain, and the cost is low, but it is easy to be affected by weather and environment (such as strong wind, lightning strike, pollution, ice and snow, etc.) , looks more scary.




Classification and composition of overhead lines

The so-called overhead line is relative to the cable, that is, the line that is erected by the tower wire to transmit electric energy at a certain height. For the ground insulation of the overhead lines of the high-voltage transmission and distribution network, the air is generally considered. Due to the rapid development of the city, the power distribution environment in the city has become more complicated. The distribution lines with bare conductors often have short-circuit grounding and lightning strikes, resulting The reliability of power supply is reduced, so at present, overhead lines below 10kv mostly use overhead insulated wires.

Classification of overhead lines

1. Transmission lines (also known as power supply lines)
The electric energy produced by the power plant, through the step-up transformer, the voltage is increased (usually at 110KV and above) and transmitted to the step-down substation with a long distance through the overhead line or cable line. The overhead line or cable line used to transmit electric energy is called power transmission. line.
2. Distribution lines
The line that changes the voltage to 10KV and below through a step-down substation, and then transmits power to each user through overhead lines or electric energy is called a distribution line. 3KV~10KV lines are called high-voltage distribution lines, also known as primary distribution lines; lines of 1KV and below are also called low-voltage distribution lines.
3. Direct distribution circuit
The line that directly transmits 10kv electricity to the user through cables or overhead lines from the generator without going through the transformer is called a direct distribution line.

Classification of distribution lines

1. According to the voltage level:

High voltage distribution network (35~110KV)
Medium voltage distribution network (6~10KV)
Low-voltage distribution network (220~380V)

2. Different divisions of distribution lines:

Overhead distribution network
Cable distribution network
Overhead Cable Hybrid Distribution Network

Compared with overhead lines, cable lines have the following


(1) It is not easily affected by the surrounding environment and pollution, and the reliability of power transmission is high.
(2) The insulation distance between lines is small, occupying a small area, and there is no interference with radio waves.
(3) When laying underground, it does not occupy the ground and space, which is safe and reliable, and it is not easy to expose the target.


(1) The cost is high, and the one-time investment cost is relatively large.
(2) The cable line is not easy to change and branch.
(3) It is difficult to find and repair cable faults, and personnel with high professional technical level are required to operate.

Basic requirements for distribution lines

(1) Reliable power supply: To ensure reliable and uninterrupted power supply to users, it is necessary to ensure the quality of line erection, strengthen operation and maintenance, management and repair work, and prevent accidents.
(2) Voltage quality: The quality of voltage directly affects the safe and economical operation of electrical equipment. Too low voltage not only reduces the processing and efficiency of the motor, but also often causes the motor to overheat and burn out.
(3) Economic power supply: In the process of power distribution, there must be power loss on the overhead line. The percentage of line loss in the total transmission power is called the loss rate, which is an important indicator to measure the economy of power supply. Distribution lines should continue to take various measures to reduce line loss, improve transmission efficiency, reduce transmission costs, and save maintenance costs on the existing basis to improve the economy of power supply.

 The composition of overhead lines

 Overhead wires are mainly composed of lightning conductors, wires, fittings, insulators, tower stay wires and foundations.
(1) Lightning conductor: used to protect overhead lines from being damaged by lightning and atmospheric overvoltage. Often the transmission lines are not equipped with lightning conductors. Most lightning conductors are galvanized steel strands. Due to the special needs of individual lines or line segments, steel-cored aluminum stranded wires or aluminum-magnesium alloy stranded wires are sometimes used as good conductors.
(2) Wire: It is the main component of the line and is used to transmit current. Generally, a single wire is used in the circuit. For ultra-high-voltage large-capacity transmission lines, due to the large transmission capacity, in order to reduce corona loss and corona interference, phase-split conductors are often used. Wires are roughly divided into two types, one we usually call bare wires, and the other is called insulated wires. Among them, bare wires are the most commonly used at present, and insulated wires are basically used in 10kv and below distribution network lines.

What is a bare wire?

Commonly used conductors are basically "round concentric stranded overhead conductors". This statement is normative and refers to most bare conductors. The most commonly used bare conductors are aluminium conductor steel reinforced (ACSR). The middle part of the aluminum steel cored wire (ACSR) is a steel core, which is mainly used to withstand various forces on the wire, and the outer part is an aluminum wire, which is mainly used to transmit electric energy, which is the core function of the wire.
In addition to the most commonly used steel-cored aluminum stranded wire, there are other wires, but the overall structure is basically the same. The similar part of the middle steel core is used for force, and the outside is used for power transmission. In addition, there are aluminum alloy wires with better anti-corrosion performance and so on.

(3) Fittings: Metal parts used to connect wires to insulator strings and fix insulators to tower poles.
(4) Pull wire: Balance the pulling force in all directions of the pole to prevent the pole from bending or falling.