MMC-HVDC的环流抑制和并网控制策略

Circulation current suppressing and grid-connected strategy of MMC-HVDC

为了使得柔性直流输电MMC-HVDC更加稳定地接入电网,并且有效地抑制其桥臂环流,提出了一种改进型的控制策略.首先,实时监测上、下桥臂电流以及子模块电容电压;通过计算得到环流抑制分量以及子模块电容电压平衡分量,将两者叠加到MMC的调制波中,从而有效地抑制桥臂二次环流分量,同时也降低了系统的开关损耗.然后,利用并网控制策略,独立地控制系统的整流侧和逆变侧的电力潮流,保证直流母线电压在负荷改变时保持恒定,实现了子模块电容电压的动态平衡.最后,搭建MMC-HVDC系统的仿真模型,仿真结果表明,桥臂环流被有效地抑制,直流母线电压保持恒定.同时,搭建了五电平实验样机,实验结果也有效地证明了控制策略的正确性.

In order to deal with the problem of circulation current suppression and grid-connected control for modular multilevel converter-high voltage direct current（ MMC-HVDC）,an improved control strategy is proposed. First,the arm currents and the sub-module voltages are monitored by real-time. The component of circulation current suppression and the component of sub-module voltage balance are calculated. The circulating current is effectively suppressed by overlaying the tw o components on the modulation waves. Thus,sw itching loss of the system is reduced. Secondly,the grid-connected strategy of the MMC-HVDC is adopted,and hence the rectifier and inverter of the system are controlled independently. Therefore,the voltage of the direct current bus is stable when the load is changing. The dynamic balance of the capacitors in the sub-modules is realized. Finally,the simulation model of the MMC-HVDC system is built. The simulation results show that circulating current is suppressed and the bus voltage is stable. Finally,the experimental prototype of 5-level MMC is built,and the correctness of the proposed control strategy is also verified by the test results.