More Efficient AC Filterless Flexible LCC HVDC by Analyzing the Impact of Single-Phase Fault on Commutations

Single-phase fault is the most commonly occurred and severe AC fault that causes most of the Commutation Failures (CF) in LCC HVDC systems. It has long been regarded as having the same level of adverse impact on commutations of both 6-pulse bridges in the 12-pulse LCC HVDC scheme. This has resulted in the same level of investment on both 6-pulse bridges, for CF mitigation/elimination, e.g. firing angle advancement and equipment investment. However, due to different winding configurations of converter transformers, single-phase fault actually affects each 6-pulse bridge to different degrees. Based on the previously proposed AC Filterless Flexible LCC HVDC, this paper shows that, compared with the Y-Y bridge, the Y- $\Delta $ bridge has much smaller undesired phase advancement of the actual commutation voltage (commutation voltage at secondary side of converter transformer) under severe single-phase fault. As a result, the required voltage rating of controllable capacitors for Y- $\Delta $ bridge can be reduced, leading to considerable savings from reduced equipment costs and lower losses of controllable capacitors without compromising the commutation performance. The detailed theoretical analysis in this paper provides a valuable foundation for the development of more efficient CF mitigation/elimination strategies. Simulation results of 1) CIGRE HVDC benchmark system and 2) the same benchmark system but connected to a 39-bus inverter AC network are presented to validate the correctness of the analysis.