Busbars are installed in all voltage distribution devices of the power system. The main function of busbars is to collect, distribute and transmit electric energy. Since busbars transmit huge electric power during operation, they pass large load currents and bear large heat and electrodynamic effects during short circuit. Therefore, the material, cross-sectional shape, cross-sectional area, arrangement method, connection and fixing method of busbars must be strictly calculated, analyzed, compared, and reasonably selected to meet the requirements of safe and economical operation.
1) In distribution devices, aluminum busbars are widely used; in some areas containing corrosive gases or strong earthquakes, copper busbars can be used due to technical requirements; in some high-voltage and low-current devices, steel busbars can also be used in parts that require high mechanical strength.
2) In indoor distribution devices of 35kV and below, rectangular cross-sectional aluminum busbars are mostly used; in 110kV indoor distribution devices, tubular busbars are generally used; in outdoor distribution devices of 35kV and above, circular cross-sectional busbars are mostly used.
3) The rectangular cross-section busbar is used because the rectangular cross-section has a longer perimeter than the circular cross-section under the same cross-sectional area, has a larger heat dissipation area, and consumes less metal. In order to enhance the heat dissipation conditions and reduce the influence of the skin effect, the thickness of the rectangular cross-section should be relatively small. Usually, the ratio of the side lengths of copper and aluminum rectangular cross-section busbars is 1:5~1:12, and the maximum cross-sectional area is 10×120=1200 (mm2). When the working current is very large, if the rectangular busbar with the largest cross-section cannot meet the requirements, several rectangular busbars can be used in parallel.
4) For large-capacity generator busbars, when the working current is very large and a single rectangular busbar cannot meet the requirements, the following measures are generally taken:
a. Use multiple rectangular busbars in parallel. Multiple rectangular busbars in parallel are to parallel multiple rectangular busbars with the same cross-section and fix them to the support insulator with hardware. The distance between each busbar is generally equal to the thickness of a busbar to ensure their heat dissipation. Since the distance between each busbar is very small, the stress under the action of short-circuit current may be very large. In order to reduce the stress, a gasket needs to be installed every 300~500mm between each rectangular busbar in the same phase.
The arrangement of rectangular busbars and their characteristics are as follows:
The three-phase busbars are arranged vertically, which has the advantages of good heat dissipation conditions and strong bending resistance; the disadvantage is that the height of the space is increased.
The three-phase busbars are arranged horizontally, which has the advantages of good heat dissipation conditions; the disadvantage is poor bending resistance.
The three-phase busbars are arranged horizontally, which has the advantages of strong bending resistance; the disadvantage is poor heat dissipation conditions.
Since the heat dissipation conditions of multiple rectangular cross-section busbars in parallel are poor, the allowable current is not proportional to the increase in the number of busbars. For example, when there are 3 busbars in each phase, the current in the two side busbars each accounts for 40% of the phase current, while the middle one only accounts for 20%, which reduces the utilization rate of the metal. Therefore, in the AC circuit, there are no more than 2 rectangular busbars per phase, and no more than 3 in some cases.
b. Use slotted busbars. The trough busbar is a hollow busbar structure that is approximately square and consists of two trough-shaped large-section busbars. This structure has good heat dissipation conditions, high metal utilization, and the working current can reach 10~12kA.
5) Use water-cooled busbars. The water-cooled busbar uses the good thermal conductivity of water to cool the busbar. Generally, a round copper tube is used as the busbar, and water is passed through the tube for cooling. The current carrying capacity of the busbar is several times higher than that of an ordinary round busbar. It is used in power plants with water-cooled generator sets. Its advantage is high cooling efficiency; its disadvantage is complex structure and high water quality requirements.
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