DC Secondary Ripple Suppression Method for HCC-HVDC Under Asymmetric Grid Fault Conditions

The hybrid commutation converter (HCC) based high voltage direct current (HVDC) system features controllable turn-off capability, eliminating commutation failure (CF) caused by single-phase grounding faults in the AC grid. However, under severe asymmetric grid fault conditions, significant voltage and current secondary ripples occur in the DC lines and lead to sending end renewable energy systems destabilizing or even off-grid, posing a serious threat to the stable operation of HVDC systems. To address this issue, this paper first investigates the DC secondary ripple generation mechanism and its transmission process between the sending and receiving ends. It finds that negative-sequence voltage at the receiving end grid is converted into DC voltage ripple by converter modulation, propagating through the DC line and controller to induce fluctuation in the sending end converter bus voltage. Then, a method for suppressing DC secondary ripple for HCC-HVDC is proposed, leveraging the wide-range firing angle adjustment capability of HCC. The method involves actively injecting a double-frequency disturbance signal into the inverter firing angle command. This enables the converter to trigger asymmetrically and introduces a negative-sequence component into the fundamental frequency-switching function. The negative-sequence component couples with the positive-sequence voltage of the AC grid to offset the DC voltage secondary ripple. Finally, a DC secondary ripple suppression controller (DCSRSC) is designed based on the proposed method and embedded in the HVDC control system. The controller dynamically adjusts the inverter's firing angle based on real-time detection of AC negative-sequence voltage, effectively suppressing DC secondary ripple and enhancing the stability of DC power transmission. Hardware-in-the-loop experiments validate the effectiveness of the proposed method.