无源MMC站通过DC/DC变换器在线平滑接入直流电网的控制策略

Control Strategy for On-line Smooth Connection of Passive Modular Multilevel Converter Station via DC/DC Converter into DC Grid

基于模块化多电平换流器(modular multilevel converter,MMC)的直流电网已成为未来电网的重要发展方向,然而仍没有保证无源MMC站通过DC/DC变换器在线接入直流电网的平滑性的成熟方案。若解锁顺序或控制策略不当,都将产生较大的冲击电流,对原正常运行系统产生影响。首先,提出了无源MMC站通过DC/DC变换器接入直流电网前、后的有功功率参考值设计方法。然后,分析了DC/DC变换器和无源MMC站接入期间的等效电路,分析了解锁前后子模块电容电压的变化,提出了解锁时投入的直流电压匹配控制策略,可减小解锁时刻子模块电容充电带来的冲击电流。最后,以平滑接入为目标,给出了整体控制策略流程。在PSCAD/EMTDC仿真环境下搭建了含DC/DC变换器的五端对称双极直流电网模型,仿真验证了所提出控制策略的有效性。

The modular multilevel converter（MMC） based DC grid is becoming an important trend in the electrical power system. But there is no mature strategy on on-line smooth connection of passive MMC station via DC/DC converter into DC grid. If the unlock sequence or control strategy is not appropriate,there will be a higher inrush current and the original operation of the system will be affected. Firstly,the design method of active power reference value before and after the connection of DC/DC converter and passive MMC station is analyzed. Secondly,the equivalent circuit of DC/DC converter and passive MMC station is analyzed. The change of the capacitor voltage of the sub-module before and after delocking is analyzed,and the DC voltage matching control strategy is put forward to decrease the inrush current at the delocking moment. Finally,taking the smooth connection as target,the overall control strategy process is presented. A model of five-terminal symmetrical bipolar DC grid with DC/DC converter is built in PSCAD/EM TDC,and the simulation results validates the effectiveness of the proposed control strategy.