通过对采用传统dq两轴解耦控制器的模块化多电平换流器(modular multilevel converter, MMC)的分析,揭示了直流端口电压和子模块电容电压的耦合效应,及其所引起的直流端口等效电容效应和子模块电容电压偏差效应。将上、下桥臂电压之和作为可变的直流内电势,并将直流电流作为新的内环状态量,提出了具有三个内环状态变量的三轴解耦控制器。基于新引入的直流电流状态变量,可以使直流端口电压与子模块电容电压控制解耦,同时实现子模块电容的直接控制和直流端口电压/电流的快速灵活控制。基于所提出的内环三轴解耦控制器,针对不同运行场合需求设计了多种外环控制器,可以灵活实现直流电压、有功功率、无功功率、电容电压的控制,适应MMC的各种运行方式。对不同运行方式的MMC进行了仿真研究,验证了所提出的三轴解耦控制的有效性。

Through the analysis of the modular multilevel converter (MMC) with the traditional d-q axes decoupling controller, this paper reveals the coupling effect between the DC voltage and the sub-module capacitor voltage, which also results in the equivalent capacitance on the DC port and the deviation of the capacitor voltage. Through considering the sum of the upper and lower arm voltages as the control variable, the DC current is introduced as the third inner-loop state variable, and a three-axis decoupling controller with three inner-loop state variables is proposed. Applying the introduced DC current state variable, the control of the DC voltage can be decoupled from the sub-module capacitor voltage. The direct control of the sub-module capacitor voltage and the fast and flexible control of the DC voltage/current can be realized simultaneously. On the basis of the proposed inner-loop three-axis decoupling controller, various outer-loop controllers are designed for different operating modes of MMC, which can flexibly realize the control of DC voltage, active power, reactive power, and capacitor voltages. The feasibility and effectiveness of the proposed three-axis decoupled controller have been verified by using simulation results for different operating modes.