级联型混合直流输电系统的自适应下垂控制策略研究

本文研究了受端级联型混合直流输电系统的控制特性，分析了受端柔性直流在下垂控制模式下对级联型混合直流控制特性的影响，明确了混合级联直流的总体UI特性曲线。相比于主从控制，采用下垂控制的MMC具有同时控制直流电压和直流功率的能力，不会出现功率反送现象。但在下垂控制作用下，MMC无法实现直流电压的准确控制。因此，本文提出一种受端级联型混合直流输电系统的自适应下垂控制策略，该策略可根据系统直流电流的变化，实时调节下垂特性，避免MMC的直流电压随直流电流的变化而产生波动。最后，基于PSCAD/EMTDC的仿真结果验证了所提控制策略的有效性，该策略可实时自适应调节下垂特性，实现直流电压的准确控制，提高了混合直流输电系统的稳定性。

Adaptive droop control strategy of cascaded hybrid HVDC system

In this paper, the control characteristic of cascaded hybrid HVDC system is studied. The influence mechanism of MMC inverters to the whole hybrid HVDC are investigated, and the complete UI curves of the hybrid HVDC are obtained in droop control modes. Compared with the master-slave control, MMCs in droop control have ability to control both DC voltage and DC power, and they will not change to rectifiers as the droop control has guaranteed that they are always in inverter operation. However, MMCs in droop control cannot control DC voltage accurately. Thus, an adaptive droop control strategy of cascaded hybrid HVDC system is proposed. The droop curve can be moved in real time adaptively according to DC current, which avoid the fluctuations of DC voltage due to the change of DC cur-rent. Finally, the simulation results verify the effectiveness of the proposed strategy based on PSCAD/EMTDC. This strategy can adjust the droop curves adaptively and realize accurate control of DC voltage, which improves the stability of the hybrid system.