Design And Control Strategy Research Of Modular Multilevel Converter

With the development of power electronics, multilevel converter attracts more and more attention especially in the field of high-voltage and large-capacity. Modular Multilevel Convert (MMC) is an emerging multilevel topology which has broad prospect in medium and high voltage applications. Its modular structure not only ease of integration and installation, but also let power devices work at lower voltage level, thus greatly improving the installed capacity and voltage rating. Until now, the study of MMC around the world is still in its initial stage. Although researchers have made some progress, the mechanism of its nature and the capacitor voltage control method as well as the internal circulating current control method is still unclear.This paper begins with the working principle of MMC, proposing the equivalent voltage source circuit and its four-quadrant operation mechanism, based on these analyses, calculating MMC device parameters such as the DC bus voltage, the leg inductance and sub-module capacitor. Independent current control of positive and negative legs was also put forward in the paper, which achieved instantaneous current control and MMC’s internal circulating current control. By analyzing the relationship between the components of leg current and the active power absorbed or issued by the equivalent voltage source, this paper deduced a new multi-hierarchy method to balance the capacitor voltages. On one hand, in order to control the average voltage of all sub-modules in a phase leg, this method controls the active power absorbed or issued by each leg through adjusting the DC component or AC component of the leg current, and the leg output voltage is seen as a voltage source at this time. On the other hand, in order to balance each capacitor voltage in a leg, this method controls the active power absorbed or issued by each cascaded sub-module through adjusting the output voltage of each sub-module, and the leg current is seen as a current source at this time. Regulators involved in the multi-hierarchy method were also deduced. Simulation and experimental results testified the validity of this method. In addition, this paper studied the advantages and disadvantages of several MMC sub-module capacitor pre-charging strategy then proposed an optimized pre-charging strategy. The paper also analyzed how to control the DC bus voltage when MMC works under rectifying operation, how to control AC side voltage when MMC works at inverter status, as well as integrated control of a back-to-back MMC system. In the process of MMC experiments platform designing, the paper compared parallel control structure with serial control structure based on EtherCAT technology. The paper also constructed an experiment platform based on DSP and FPGA, which worked with MMC hardware-in-the-loop simulation platform. The experiment results verified the proposed control strategy.