送端交流不对称故障下LCC-MMC-HVDC穿越控制策略研究

混合直流输电系统将LCC-HVDC和VSC-HVDC进行优势互补,其发展面临的一个问题是:当送端交流系统发生不对称故障时,LCC-MMC混合直流输电系统将面临输送功率跌落甚至中断和直流侧二倍频波动的问题。首先阐述了LCC-MMC-HVDC的拓扑结构、数学模型和基本控制策略,在分析系统在送端交流不对称故障情况下暂态特性的基础上,提出集功率续传和二倍频波动抑制为一体的穿越控制策略:基于主动降压控制的功率续传策略通过改变逆变侧MMC运行点减小输送功率跌落幅度;三次谐波注入法增大了逆变侧直流电压的可调节范围;直流电压波动抑制策略中,逆变侧直流电压作为唯一控制变量,有效降低送端交流系统负序分量对系统逆变器及受端系统造成的影响。最后,在PSCAD/EMTDC中建立仿真模型,算例仿真结果验证了所提出穿越控制策略的有效性。

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.