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Substation Bus Design: Current Methods Compared with Field Result
Current standards define accepted methods of substation bus design. These standards also require certain strength and deflection criteria. This paper will examine commonly proscribed rigid bus design methodologies and compare them with actual field experience in the design of a 500kV switching station. Special attention will be given to required deflection criteria, methods commonly used to achieve these criteria, and results actually measured in the field.
The bus work in a substation are the structures that carry the electrical current. They are usually made of aluminum pipe or cable and are supported by porcelain or polymer insulators which are in turn supported by steel supports on foundations. Since the buses are typically energized at high voltages they must be separated from each other by distances that increase as the voltages increase. The air between the buses serves as the insulation between the voltages on each bus preventing electrical arcs from developing between the buses which would cause their destruction. This paper will consider the bus types used in this type of typical air-insulated substation using aluminum pipe bus. Gas insulated substations or substations using cable or materials other than aluminum will not be discussed.
Several forces, some electrically produced and some resulting from environmental effects, must be resisted by the Substation Bus Design. Short circuits occurring either near or remote from the substation cause high current flows for short periods of time in the buses. These high currents cause high magnetic fields between the buses which produce large forces that must be considered in design. Buses must also be designed for gravity forces, including the weight of ice during icing conditions, wind, and seismic forces.
The approach normally taken in design is to choose a minimum bus diameter dependent upon current carrying ability and corona limitations and then determining the maximum span between bus supports dependent upon both deflection and strength limits of the aluminum pipe used. Bus diameter may need to be adjusted due to required span lengths between bus supports. After an adequate span and bus diameter are chosen, the necessary cantilever strength of support insulators is calculated and insulator types are chosen. If sufficiently strong insulators are not available, or are determined to be too costly, additional supports may be added to reduce maximum bus spans.
|Substation Bus Design: Current Methods Compared with Field Result|