A Recovery Method for HVDC Systems Following AC System Faults

Commutation Failure (CF) is one of the most common faults in Line Commutated Converter based High Voltage Direct Current (LCC-HVDC) system. When a severe AC fault occurs, CF will deteriorate into a continuous commutation failure (CCF), which is extremely harmful. In order to suppress the CCF of HVDC systems, this paper proposes a recovery method after AC system faults. Firstly, we analyze the action logic of conventional voltage dependent current order limit (VDCOL), and show that the improper reactive power flow (RPF) from AC systems to DC systems is one of main reasons that the HVDC systems have a poor recovery performance (even the CCF). Then, the inherent relations among the DC current, AC voltage, and the RPF are explored. Furthermore, we propose a recovery method instead of VDCOL to calculate DC current instruction signal in real time during AC fault recovery. In this method, the unreasonable RPF is greatly mitigated and the constant extinction angle control is considered. Finally, simulation results show that our proposed method can highly improve the dynamical performance of HVDC systems during fault recovery period and avoid the CCF.