Transmission lines are very common in life, but many people in power-related industries do not know how to determine the voltage level through towers. Today we will talk about this~
According to the structure, transmission lines are divided into overhead transmission lines and cable lines. This article discusses overhead transmission lines, which are composed of line towers, wires, insulators, line fittings, cables, tower foundations, grounding devices, etc., and are erected on the ground. Transmission wires are connected by sections of transmission poles. For high-voltage grades, "iron towers" are used, and for low-voltage grades, such as those found in residential areas, "wood poles" or "cement poles" are generally used. Together they are collectively called "poles and towers."
Overhead transmission lines can be divided into AC transmission and DC transmission according to the nature of the current transmission. So how to distinguish between DC and AC transmission lines at a glance?
In fact, it is very simple. AC is three-phase electricity, and the number of transmission lines is 3 or a multiple of 3. DC transmission lines have only positive and negative poles, that is, two wires plus lightning protection wires.
Next, let’s go to the topic, how to identify the voltage level of the transmission line at a glance? Just "three look":
Split conductor is a wire erection method adopted by UHV transmission lines to suppress corona discharge and reduce line reactance, that is, each phase conductor is composed of several sub-conductors with smaller diameters. The more the number of split wires, the stronger the transmission capacity and the higher the voltage level.
The 1000kV UHV transmission line and the 800KV DC transmission line are divided into eight, which are eight split conductors.
750kV ultra-high voltage transmission lines generally use six-split conductors, and this voltage level is only used in China's Northwest Power Grid.
500kV transmission lines should be four-split conductors according to regulations, but some use six-split conductors.
220kV is generally double split
For voltage levels of 110kV and below, because corona is not serious, a single wire is generally used.
An insulator is a special insulation control, usually made of glass or ceramic, to increase the creepage distance. The insulator is in the shape of a disc, and a disc is considered as a piece of insulator, and the insulator string plays the role of isolating the wire and the tower. Each insulator can withstand a voltage of about 15-20 kV, so the voltage level can be judged according to the number of insulators. However, if you are in a high-altitude, dirty area, the number of insulators will increase.
1000KV: about 60 pieces
500kV: about 25 pieces
220kV: about 13 pieces
110kV: 7-9 pieces
35kV insulators are 3 pieces; 220 (380)V lines generally do not have insulators.
The safety distance between the transmission line and the ground is regulated. The higher the voltage level, the higher the height when the transmission line is erected. According to "Code for Design of Overhead Transmission Lines", the minimum distance between conductors in residential areas and the ground is generally:
110kV, 220kV, 330kV transmission lines: 10 meters
500kV: 15 meters
750kV: 20 meters
1000kV: 35 meters
It should be noted that the above figures are only approximate concepts and will be adjusted according to different terrains and climates.
In summary, regarding the voltage level of transmission and distribution lines, look at the number of wire splits, the number of insulators, and the line height. Have you learned?
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