Busbar knowledge explanation

The function of busbars is to collect, distribute and transmit electric energy. In the power system, busbars connect various current-carrying branch circuits in the distribution device together, playing the role of collecting, distributing and transmitting electric energy.

Commonly used busbars are divided into two types: hard busbars and soft busbars.

1. Hard busbar
2. Busbar classification: Busbars can be divided into rectangular busbars, trough busbars, diamond busbars, tubular busbars, etc. according to their shapes.

(1) Rectangular busbars are the most commonly used busbars, also known as busbars. According to their materials, they are divided into aluminum busbars (aluminum bars) and copper busbars (copper bars). The advantages of rectangular busbars are easy construction and installation, small changes during operation, and large current carrying capacity, but the cost is relatively high.

The rated current of rectangular busbars when installed horizontally and vertically is different, so the heat dissipation is also different. The heat dissipation condition of vertical busbars is better, and the heat dissipation condition of flat-mounted busbars is worse, so the rated current of flat-mounted busbars is 5% to 8% less than that of vertical-mounted busbars: However, the mechanical stability of vertical-mounted busbars under electric force is worse.

(2) Both trough-shaped and diamond-shaped busbars are used in busbar bridges with large currents and in situations where high requirements are placed on thermal and dynamic stability.

(3) Tubular busbars are usually used in conjunction with plug disconnectors. Currently, most of the busbars used are steel tube busbars, which are easy to construct but have a small current-carrying capacity. Characteristics of tubular busbars: Due to the expanded diameter, the reactance is small. Compared with busbars of the same diameter, the operating current allowed to pass is much larger.

2. Comparison of the generatrix of rectangular and circular cross sections

Under the same cross-sectional area, the rectangular busbar has a larger circumference than the circular busbar, and the heat dissipation surface is larger, so the cooling conditions are good. In addition, when the AC current passes through the busbar, due to the influence of the skin effect, the resistance of the rectangular cross-section busbar is also smaller than that of the circular cross-section. Therefore, under the same cross-sectional area and the same allowable heating temperature, the rectangular cross-section busbar has a larger allowable working current than the circular cross-section busbar. Therefore, indoor distribution devices below 35kV mostly use rectangular cross-section busbars. In outdoor distribution devices above 35kV, circular cross-section busbars are mostly used to prevent corona. The smaller the radius of curvature of the busbar surface, the greater the electric field strength. The four corners of the rectangular cross-section are prone to cause corona phenomenon, and the circular cross-section has no electric field concentration phenomenon. In order to reduce the electric field strength and increase the busbar diameter, steel core aluminum stranded wire or tubular busbars are used in outdoor distribution devices of 110kV and above.

3. Hard busbar connection

Hard busbars are usually crimped or welded. Crimping is to crimp the busbars with screws to facilitate modification and disassembly. Welding is to press with electric welding or gas welding, which is mostly used in places where disassembly is not required. Soldering and binding are not allowed for hard busbars. When connecting copper and aluminum busbars, the copper busbars should be tinned or zinc sheets should be used as gaskets for crimping.

4. Function of the expansion joint for rigid busbar

Objects have the characteristics of thermal expansion and contraction. The busbar will change its length due to heat during operation. In order to avoid excessive stress and damage to the busbar and supporting insulators due to thermal expansion and contraction, expansion joints should be installed on the rigid busbar.

2) Soft busbar: including aluminum stranded wire, copper stranded wire, steel core aluminum stranded wire, expanded diameter hollow conductor, etc.

Flexible busbars are mostly used outdoors. The outdoor space is large, the conductor spacing is wide, and it has good heat dissipation effect, convenient construction, and low cost.

Regardless of the busbar selected, the following conditions must be met:

(1) The selected busbar must meet the requirements of continuous working current.

(2) For busbars with high average annual load, long busbars and large transmission capacity, the selection should be based on the economic current density.

(3) The busbar should be tested and qualified according to the corona voltage.

(4) Verified according to short-circuit thermal stability conditions.

(5) Pass the short-circuit dynamic stability test.