An Improved Synchronous Firing Control on Suppressing LCC-HVDC Commutation Failures Caused by Single-Phase-Grounding Faults

An existing synchronous firing control is mainly composed of phase-locked loop (PLL), equidistant pulse control (EPC) and area minimum control (AMC), which can accurately predict the firing time in the steady state, while the accuracy deteriorates under AC faults. In this article, in order to alleviate commutation failures in LCC-HVDC transmission project caused by single-phase-grounding (SPG) fault, the phase offset and the commutation margin under SPG faults are analyzed, and an improved synchronous firing control on suppressing LCC-HVDC commutation failures caused by SPG faults is proposed. First, the forward zero-crossing point of the grid-side non-fault line voltage is used to generate synchronous phases during the fault. Second, the phase offset of commutation voltage that is most likely to make the valve fail to be shut down was determined under SPG faults. Third, the phase offset was used to correct the EPC and the AMC respectively. Finally, in the Three Gorges Shanghai DC project model, the fault tests were carried out in the strong and the weak AC systems respectively. The results show that the improved control can improve the ability of LCC-HVDC projects to suppress commutator failure.