Ride-through Control of LCC-MMC-HVDC Under Asymmetrical Faults at the Sending End

The hybrid HVDC transmission system combines the advantages of LCC-HVDC and VSC-HVDC together. One of the problems faced in its development is that when an asymmetrical fault occurs at the sending end, the LCC-MMC hybrid HVDC transmission system would have the problem of transmission power drop and even interruption and double frequency fluctuation of the DC side. This paper firstly describes the topology structure, mathematical model and basic control strategy of LCC-MMC-HVDC. Based on the analysis of the transient characteristics of the system in the case of asymmetric faults at the sending end, a ride-through control strategy is proposed. The power transmission strategy reduces transmission power drop amplitude by adjusting the operating point of MMC at the inverter side. The third harmonic injection method increases the adjustable range of DC voltage and DC voltage is the only control variable which is required in the DC voltage fluctuation suppression strategy, which could effectively reduce the influence of the negative sequence component of the sending end on the inverter and receiver end. Finally, a simulation model is established in PSCAD/EMTDC. The simulation results of the example verify the effectiveness of the proposed ride-through control strategy.