Research On Control Strategies Of Modular Multilevel Converter

In recent years, with the increasing application of power electronics in the field of motor drives, reactive-power compensation, active power filters and distributed power generation systems, high-voltage power electronics converters have received widespread attention, and the multilevel converter has become an effective alternative for high-voltage power conversion due to its reduced line harmonics and low voltage stress. Among the numerous multilevel topologies, the modular multilevel converter (MMC) is considered as the most competitive voltage source converter (VSC) operating in the range of high power and high voltage levels with high efficiency due to its modular nature, voltage/power expansion capability, very low output harmonics, simple redundancy and flexible front-end configuration.Firstly, the operation principles and hardware design methods of MMC are given, and the common modulation schemes and voltage balance strategies are discussed. Afterwards, the above-mentioned analysis is verified on the basis of the simulation and experimental models.Next, a precise circulation model of MMC is established with the consideration of submodule voltage disturbance. Based on the improved circulation model, the coupling effects between different harmonic orders and the equivalent harmonic current control model are detailed analyzed. On the basis of the traditional suppression method applying secondary resonant controller, a plug-in repetitive controller is proposed to achieve complete elimination of the circulating harmonics. The operation principles and stability analysis are discussed as well as the controller parameters design method based on the prototype electrical parameters. The simulation and experimental results are also given and compared along with the paper to validate the theoretical analysis.It should be noted that a practical MMC system with hundreds of submodules will lead to various problems like significant delays caused by the measurements and signals transmission, and complex control and communication systems. These defects increase the system cost and reduce the real-time control performance. To weaken the above-mentioned impacts, this paper presents a novel control scheme utilizing voltage observation strategy to cancel the module voltage feedbacks, which reduces the system cost and improve the real-time control performance of the system. The observation precision and control stability of the proposed method are systematically analyzed. Finally, the simulation and experimental results are given to verify the correctness of the theoretical analysis, and the effectiveness of the proposed scheme.