The busbars in a switchgear are divided into two parts: the main busbar, a horizontal busbar connecting all the panels, which collects and distributes current; and the branch busbars, a vertical busbar connecting the electrical components within each panel. Busbars are made of aluminum or copper. Common busbar structures include rectangular, trough, and tubular.
Single-piece rectangular conductors offer advantages such as low skin effect coefficient, good heat dissipation, simple installation, and convenient connection. They are generally suitable for circuits with operating currents ≤2000A. Multi-piece rectangular conductors have a larger skin effect coefficient than single-piece conductors, resulting in increased additional vibration losses. Therefore, current carrying capacity does not increase exponentially with the number of conductors. Especially when there are more than three conductors per phase, the skin effect coefficient increases significantly. In practical engineering, multi-piece rectangular conductors are suitable for circuits with operating currents ≤4000A. For operating currents above 4000A, slotted or tubular conductors should be used, as they facilitate AC current distribution. Slotted and tubular conductors have a low skin effect coefficient, more uniform current distribution, better heat dissipation, and high mechanical strength, but they are more expensive and inconvenient to install.
The cross-sectional area of the busbar should take into account the current load current and the possibility of future expansion. The main busbar current is supplied by the incoming switchgear, whose load current is predetermined when the circuit breaker parameters are selected. Therefore, the cross-sectional area of the branch busbar should, in principle, be selected based on the rated current of the circuit breaker, unless the actual load current is very small and future load increases are unlikely. In this case, a smaller cross-sectional area busbar should be selected.
If there is only one incoming line, the cross-sectional area of the main busbar can be equal to or slightly larger than that of the branch busbar in the incoming switchgear. If there are two incoming lines, the situation becomes more complex. In this case, the two incoming switchgear are typically placed at the left and right ends of the main busbar. This provides a more balanced current distribution, and the cross-sectional area of the main busbar can be selected based on more than the sum of the currents of the two incoming lines. If the two incoming switchgear are located close together, the load currents flowing through certain segments of the main busbar will be combined, and the cross-sectional area must be selected based on the sum of the currents of the two incoming lines. This situation should be avoided. Therefore, when there are two incoming switchgear panels, the main busbar cross-sectional area should be designed within the current range from the larger incoming current to the sum of the two incoming currents. The specific cross-sectional area should be determined based on the layout of the incoming and outgoing switchgear panels and after analyzing the current distribution.
The selection of busbar specifications for switchgear should consider the following:
1) Selecting the cross-sectional area based on the conductor's allowable current carrying capacity due to long-term heat generation.
2) Verifying thermal stability.
3) Verifying dynamic stability.
4) Verifying conductor resonance.
