交流不对称故障下的LCC-MMC混合直流系统输送功率提升策略研究

将常规两端直流输电系统逆变站的电网换相换流器(LCC) 替换为模块化多电平换流器(MMC)所构成的混合直流输电系统，可结合两种换流器的优点而具有广阔的应用前景。在研究其基本稳态控制特性的基础上，重点分析了交流电网不对称故障引起的直流输送功率下降及中断问题。通过分析混合直流系统的交流故障特征，发现交流不对称故障发生在整流侧时易引起直流电压下降甚至输送功率的中断，发生在逆变侧时易引起直流系统电压异常。鉴于此，提出了基于MMC典型控制的附加直流电压控制策略，在其调制范围内通过降低故障时逆变侧的参考直流电压以提高直流系统的输送能力。若检测到本站直流电压的交流分量大小超过限定值，则附加控制策略自动投入，无需依靠换流站间的通信。最后，通过PSCAD/EMTDC电磁暂态仿真验证了所提控制策略的可行性。

Research on power transmission enhancement strategy of LCC-MMC hybrid HVDC system under AC symmetric fault

The hybrid HVDC system, which adopts line commutated converter in rectifier side, and modular multilevel converter in inverter side, can combine the advantages of the two converters and has broad application prospects. Based on the study of the basic steady-state operation control characteristics, the DC transmission power drop and the interruption problem caused by the asymmetric fault of the AC power network are analyzed emphatically on the basis of the LCC-MMC hybrid HVDC transmission system. And the AC fault characteristics of the hybrid HVDC system are analyzed. It is found that the DC voltage drop caused by the asymmetrical fault is most likely to cause the interruption of the transmission power at the rectifier side, and cause the voltage anomaly at the inverter side. A supplementary DC voltage control strategy based on MMC typical control is proposed to improve the transmission capacity of the DC system by reducing the reference DC voltage of the inverter in the modulation range. If it is detected that the inverter side DC component of the DC voltage exceeds the limit value, the additional control strategy is automatically input, without relying on the communication between the converter stations. Finally, the feasibility of the proposed control strategy is verified by PSCAD/EMTDC electromagnetic transient simulation.