Improving CFI of LCC-HVDC Systems Through Voltage Compensation Using UPFC

This paper proposes a voltage compensation scheme (VCS) to improve the commutation failure immunity (CFI) of line-commutated converter-based high voltage direct-current transmission (LCC-HVDC) systems with an unified power flow controller (UPFC). A voltage compensation control system was designed for the UPFC, which enables the UPFC to inject three-phase voltages in series with the receiving-end ac source to compensate the distortion and magnitude drop of the inverter-side ac voltage in the cases where external disturbances occur. In this way, satisfactory commutation voltage is provided for the inverter station of a LCC-HVDC system and CFI of the LCC-HVDC system is improved. Simulation studies were undertaken on the CIGRE LCC-HVDC benchmark system in the cases where single-phase-to-ground faults occurred in the receiving-end ac grid. CFI of the test system having the proposed VCS, a commutation failure prevention method (CFPREV), and a direct-current predictive control strategy (DCPC) implemented respectively was investigated.