Defining Short-Time Withstand Current for DC Switchgear in HVDC Systems

This article proposes a methodology for characterizing short-time withstand current with high voltage direct current (HVDC) switchgear, which aims informing the ongoing standardization work on DC switchgear in IEC TC17. To maintain commonalty in methodology with AC switchgear standards, it is proposed that the DC short-time withstand stress is described with an equivalent RMS current and duration, in addition to a peak fault current. Considering much different DC system dynamics, a wide range of DC topologies and DC fault scenarios, it is proposed to utilise Joule integral to determine rated short-time withstand current. DC systems are classified in 3 topologies to account for the industry HVDC experience and the development of future DC grids: VSC HVDC, LCC HVDC, and DC systems with DC CBs. For each topology, short-circuit current waveform is characterized and expressions for rated parameters are derived, considering relevant operating conditions with worst-case stresses. The accuracy of analytical expressions, and the recommended numerical values are verified using detailed simulation models with relevant test cases and a range of operating conditions.