Short-circuit capacity of 35kV busbar

The minimum short-circuit capacity of a 35kV busbar is typically defined by its rated short-time withstand current (Icw) and peak withstand current (Icp), with common industry benchmarks around 31–50 kA for 1 second, depending on system design and IEC standards.

Key Parameters

Rated Short-

Time Withstand Current (Icw): This is the RMS current a busbar can sustain for a specified duration (commonly 1 second) without permanent deformation or insulation failure. For a 35kV busbar, typical values range from 31 kA to 50 kA for 1 second, depending on the switchgear and system fault levels (IEC 61439-1). Rated Peak Withstand Current (Icp): The peak instantaneous current the busbar can survive during the first half-cycle of a short circuit.

This is usually 1. 8–2 times the Icw, reflecting the asymmetrical nature of fault currents. Thermal and Electrodynamic Considerations: The busbar must withstand both the thermal energy from the fault (I²t) and the mechanical forces due to Lorentz forces between parallel conductors.

The thermal withstand ensures the conductor temperature does not exceed material limits (e.g., 250°C for copper), while electrodynamic withstand ensures mechanical integrity. Standards and CalculationIEC 60865-1 governs the calculation of electrodynamic forces and thermal stress on busbars.

IE

C 61439-1 specifies assembly-level compliance, including short-circuit withstand requirements. The adiabatic equation is used to calculate thermal withstand: Ith=k·St where Ith is the thermal short-circuit current, S is the cross-sectional area, t is the fault duration, and k is a material constant (e.g., 143 for copper).

Practical Design Considerations

The minimum short-circuit capacity must exceed the maximum prospective fault current calculated using IEC 60909 for the system. Busbar spacing, support insulators, and mechanical bracing must be designed to handle the electrodynamic forces during faults. For 35kV systems, typical switchgear designs adopt Icw values of 31–50 kA for 1 second and corresponding peak currents of 55–85 kA to ensure safety and compliance.

In summary, a 35kV busbar should be designed with a minimum short-circuit capacity of at least 31 kA RMS for 1 second, with a peak withstand current approximately 1. 8–2 times higher, while ensuring both thermal and mechanical stresses are within safe limits according to IEC standards.