Sending-end voltage fluctuation mechanism and suppression under commutation failures in LCC-HVDC systems

Inverter commutation failures (CFs) in LCC-HVDC systems can cause severe sending-end voltage fluctuations. However, owing to the reliance of analysis methods on average-concept-based power quantities, the transient behavior of the sending-end voltage during inverter CFs remains elusive, hindering the advancement of its suppression strategy. Given this, this paper proposes a power-free quantitative analysis method and a targeted suppression strategy for sending-end voltage fluctuations. Specifically, from the transient perspective, the expression for the instantaneous sending-end voltage is derived, which reveals the decisive role of the amplitude term and phase term of the outfeed-current-induced voltage drop. On this basis, in the constructed space of the amplitude term K, phase term ψ and RMS sending-end voltage U_p, the operating curves of the DC system after inverter CFs are analyzed in detail. According to the variations in the dominant terms that lead to voltage fluctuations, the postfault process is divided into five stages, and a corresponding targeted control strategy is proposed. Simulations under different fault conditions in PSCAD/EMTDC verify the correctness of this quantitative analysis method and the effectiveness of the proposed suppression strategy.