Research On STATCOM Based On Modular Multilevel Converter

With the development of National Smart Grid, FACTS will be widely used in the future. STATCOM, as the important device of FACTS, has drawn more and more attention. Focused on high-voltage area, the research emphasis of modular-multilevel-converter (MMC) has been proposed through the comparison and analysis between normal multilevel topologies, the application of which in STATCOM has been researched carefully in this paper.Basic principle and control strategy of STATCOM has firstly been introduced, dynamic detection method of reactive current in single-phase is proposed, also the method based on dqO conversion in three-phase is analyzed. Feasibility of the above methods is tested by Matlab. Topology structure, working principle and operation mode of sub module have been technically studied for MMC. By establishing the math model, relationship between the input and output has been got, as well as the influencing factor of SM capacitor voltage. To ensure the constant voltage of the capacitor at dc-side, average and balance hierarchical control method have been adopted and stability of this kind of method in the system has been strictly proved. The mathematical model in three-phase static coordinating system and two-phase synchronous rotating coordinating system of MMC-STATCOM are established in order with the equivalent circuits given. Feasibility of the proposed STATCOM is proved by open-loop transfer function through Routh criterion. Direct PI control of dc-side voltage and error stack are employed. While in ac-side, because of the difficulty to solve the coupling of the component, a kind of state feedback decoupling control method has been used with good dynamic tracking effect. To use Saber and Matlab software for collaborative simulation, the working characteristics have been tested.Based on the theory and simulation study, power circuit, driving element and sampling part of the hardware design of the system have been set up, while the DSP chip TMS320F28335has been used as the key control. Software design, including data acquisition and MMC control, is to realize A/D sampling, PWM output and so on. According to the prototype, experimental results have shown the favorable sinusoidal multilevel output performance of MMC and good reactive compensation effect of the STATCOM.