BUSBAR SIZE FACTOR

Busbar sizing is done to check withstand ability of

–        Thermal Stress

–        Mechanical Stress

–        Deflection

Thermal Stress:

•          Flow of current causes thermal stress.

•          Temperature rise is checked for

–        Normal load current

–        Fault current.

Note: Sizing based on governing criteria

•          Steady state temperature depends on

–        Radiation heat loss

–        Convective heat loss

–        Solar heat gain by the conductor

Mechanical Stress

•          Mechanical stress on the busbar material is due to

Dynamic force

–        Electrodynamic force during short circuit condition

–        Wind force

Static force

–        Static conductor tension

–        Self weight

•          Adequacy of busbar material is decided when it’s bending stress / tensile stress are within the limiting value.

Deflection

•          Deflection of busbar happens

At Dynamic condition

–        Due to combination of electromagnetic force & Wind force

At Static condition

–        Due to self weight and sag

•          Check for deflection in case of rigid busbar is mechanical property which decides the limiting value

•          In case of strung busbar it is minimum electrical clearance decides the limiting value.