Derivation of Requirements for Small-current Switching in Future Hvdc Substations

As a central component of any existing AC substation, disconnector switches are responsible for providing isolation between grid-connected and safely approachable sections of bus. Additionally, they can provide the capability of transferring load currents between different busses. Due to the excellent performance history in AC, possible challenges for disconnector switches in future HVDC substations were identified by comparing computed effects of DC stress to the standardized capabilities of existing AC disconnector switches. It was found that pure, time-constant DC voltages and currents will not lead to excessive stress onto disconnector switches. However, voltage and current ripples present in real HVDC systems can give rise to considerably different results. Ripples exhibiting a 1/f decay in magnitude up to the 30 harmonic were utilized in calculations. Similarly to AC, switching of sections of bus can lead to multiple restrikes or prestrikes, potentially resulting in very fast transients. The maximum voltage collapse exhibits a strong dependence on the magnitude of the voltage ripple. Transferring load currents between different busses has been shown to be possible under DC conditions. However, the recovery voltage as well as the extinction process itself depend heavily on the current ripple.