Research On The Circulating Current And Suppressing Strategy Of Modular Multilevel Converter

In recent years, the power electronic technology is developing rapidly. The device of high voltage and large capacity has been widely utilized in the field of reactive-power compensation, active power filters, high voltage DC transmission system and so on. Since the good scalability and low requirements for power electronic devices, MMC has a good research prospect in the field of high voltage and large capacity. In a word, it is significant to implement further study on MMC. In this thesis, research on circulating current and circulating current suppressing strategy of MMC is conducted. The main contents are as follows:The topology and working principle of MMC are analyzed. According to the output voltage characteristic of submodule and arm port, the equivalent mathematical models of the DC-side and AC-side for MMC are derived.To suppress the circulating current effectively, circulating current steady-state model of MMC is established, and the components of circulating current are given in this thesis. Then a general method used to suppress circulating harmonic current and circulating current close-loop control model of MMC are introduced. In order to compare with the circulating current suppressing strategies which are adopted in this thesis, the suppressing strategy based on proportional integral control is studied, which is analyzed and verified by simulation.To suppress the multiple even circulating harmonic currents, a simplified circulating current suppressing strategy is presented based on multi-resonant controllers. On the basis of the explicit analysis of the quasi-proportional-resonant controller parameters that influence the system stability, the secondary resonant controller parameters are designed in the continuous domain. In order to obtain more accurate parameters of controller, the different discretization method of resonant controller is discussed and the secondary resonant controller parameters are designed in the discrete domain. The circulating current suppressing strategy based on multi-resonant controllers is analyzed and verified by simulation.In order to solve the issue that number of resonant controller used and steady state performance of control system which has to reach a compromise, a circulating current suppressing strategy based on plug-in repetitive controller is proposed. In combination with the stability conditions of circulating current closed loop control system, the parameters design principle of repetitive controller are given, and the robustness of the closed-loop control system, the harmonic suppression performance and dynamic performance are analyzed. The circulating current suppressing strategy based on plug-in repetitive controller is analyzed and verified by simulation.Finally, the main circuit and structure of control system of MMC experiment platform are introduced in this thesis. The three kinds of circulating current suppressing strategy are verified in the platform. The experimental results verified the feasibility and effectiveness of circulating current suppressing strategies.