向无源电网供电的MMC-HVDC稳态运行区域分析

柔性直流输电能够为无源电网供电，连接无源电网时，MMC功率传输极限有待深入研究。基于开关函数推导了dq旋转坐标系下的MMC详细解析模型，定量分析调制信号、桥臂电流、子模块电容电压波动和自身容量限制等约束条件对换流器输送功率极限的影响。研究表明换流器的功率输送主要受最大调制比和电容电压最大允许波动约束，而且子模块电容过小会明显降低MMC功率输送极限。投入环流控制器可使MMC较大程度地摆脱子模块电容电压的限制，提高为无源电网供电的能力。最后在PSCAD/EMTDC上仿真验证了稳态运行区域计算方法的有效性。

Analysis of Steady Operation Region of MMC-HVDC Connected to Passive Network

Modular multilevel converter based high voltage DC transmission， MMC-HVDC， can power passive grids. When connecting to a passive grid， the MMC power transmission limit needs further study. Based on the switching function，the detailed analytical model is deduced in the dq rotating coordinate system， and the influence of the constraints of MMC modulation signal， bridge arm current， sub-module capacitor voltage fluctuation and its own capacity limitation upon the transmitted power limits of converter is quantitatively analyzed. It is concluded that the steady-state power range of the converter is mainly limited by the maximum modulation ratio and the maximum allowable fluctuation of the capacitor voltage， and the small sub-module capacitance will significantly reduce the MMC power transmission limit. The use of circulating current controller can make the MMC get rid of the limitation of the sub-module capacitor voltage to a large extent and improve the ability to power passive grids. Finally， the effectiveness of the calculation method of steady-state operating region is verified by PSCAD/EMTDC simulation.