A comprehensive AC fault ride-through strategy for HVDC link with serial-connected LCC-VSC hybrid inverter

An HVDC link with line-commutated converter (LCC) as a rectifier at the power sending end and with a serial-connected LCC and voltage source converter (VSC) hybrid inverter (SLVHI) at the power receiving end, has been adopted for the forthcoming Baihetan HVDC engineering project in China. To realize the AC fault ride-through (FRT) of SLVHI, a new strategy based on DC chopper (DCC) is proposed in this paper. Firstly, the mathematical model is built to investigate the VSC DC overvoltage mechanism after SLVHI commutation failure (CF) and related factors. Secondly, a modified DCC topology of SLVHI is designed to adjust the unbalanced power dissipation based on voltage drop depth. Thirdly, an enhanced voltage-dependent current order limiter (VDCOL) is proposed to deal with the unbalanced power after DCC switch-off. With the cooperation between modified DCC and enhanced VDCOL, the proposed FRT strategy can realize CF mitigation and VSC DC overvoltage suppression simultaneously. Finally, the inverter side AC FRT performances of the HVDC link with SLVHI were studied using PSCAD software/EMTDC algorithm. The simulation results validate the effectiveness and superiority of the proposed strategy, with better CF mitigation and VSC DC overvoltage suppression abilities than offered by other existing FRT strategies under different fault scenarios.