Low-cost Multi-Port DC Transfer Switch for HVDC System based on Line Commutated Converter

In high voltage direct current transmission systems based on line-commutated-converters (LCC), DC transfer switches (DTSs) are installed on the neutral bus for operation mode switching. Conventional DTSs employ SF6 switches combined with large-capacity capacitors (≥60μF) and highcapacity metal oxide varistors (MOV, ∼100MJ) for current commutation and energy absorption. The capacitor and MOV are the most expensive and voluminous part in LCC stations, where multiple DTSs (5 for rectifier station and 3 for inverter station) are typically configured, leading to significant capital expenditure and spatial footprint. This paper proposes a novel multi-port DTS (MP-DTS), validated by system simulations and prototype experiments. Key contributions are: 1) with vacuum switch used as the interrupter, controllable high-frequency oscillation circuit excited by a square-wave voltage source is proposed to replace the conventional SF6 switch, enabling capacitor downsizing from 60μF to 4.2μF; 2) based on the concept of sharing the large-capacity capacitor and high-capacity MOV, multi-port DTS topology (MP-DTS) is proposed to replace the multiple conventional DTSs. Comparison between the proposed MP-DTS and conventional DTSs shows that the proposed MP-DTS has benefits of superior interruption capability, low cost, and zero usage of SF6 gas.