Optimized DC Fault Transient Control Strategy for Embedded Symmetric Unipolar LCC-HVDC

When adopted in the symmetric unipolar LCC-HVDC, the common phase-shifting restart strategy can lead to problems, including power transmission interruption and reactive power redundancy. To decrease the power fluctuation of the symmetric unipolar LCC-HVDC during the fault clearing, a unipolar grounding fault ride-through and restart strategy is proposed. During the ride-through and restart process, the faulted line's control strategy is changed to limit the fault current, while the line without fault maintains the original control strategy to maintain power transmission. With the proposed strategy, the power fluctuation and reactive power redundancy are decreased. Further, optimized methods for suppressing the commutation failure and the rectifier station's harmonics are proposed to improve the reliability of the proposed strategy. Electromagnetic simulation of the symmetric unipolar LCC-HVDC is performed based on PSCAD/EMTDC to verify the effectiveness of the proposed method.