| Modular multilevel HVDC system(MMC HVDC) which based on MMC converter have a low requirement for power device, good scalability and can be easily extended to high voltage level and power level. In addition, as gate control switch, for instance IGBT, can be can be used in MMC which avoiding commutation failure problem, MMC has better controllability and flexibility. MMC is one of the important means to solve the challenges of modern power grid and has a broad application prospect in high-capacity long-distance transmission, clean energy such as wind power grid, improve system reliability, improve power quality, etc in the future.Firstly, the mathematical model of MMC, including switch model, the average model and small signal model are studied in detail and the derivation of the mathematical model is presented in this paper. Through introducing the switching function, bridge arm voltage and current state equations are acquired; By equalising the switch bridge arm state equation via switch cycle average operator,the averaging period average model is presented; On this basis,the MMC small signal model of inverter are deduced. The charging module method with the external power supply is verified by simulation and experiment respectively. Aiming at avoiding the shortcomings of the separately module charging methods, it analyzes the ac power charging method which is applicable to modular multilevel inverter and presents the simulation and experimental results.Secondly, the control strategy and the submodule equalizing capacitor voltage control strategy are studied in detail. The detailed design steps of the controller parameters are given according to control theory. Based on continuous mathematical model, it deduces the MMC converter station level control system structure and study detailedly the design method of relative controller for the optimized double closed loop control system. The control system has superior steady state and transient response characteristics by combine the feedforward-feedback control and steady inverse model. Time domain simulation results show that the system has good steady state and transient properties when MMC type HVDC-light system adapts the structure of the control system and its parameters design method a presentented in this paper.Finally, an three-phase modular multilevel inverter experiment platform had been developed. The detailed design process of the experiment platform is given from the aspects of hardware and software. At first, it gives the overall design thought train, importanthardware circuit diagram and software flow chart of experimental platform. A small power single-phase experiment platform is developed and the corresponding analysis was carried out on the experimental results. The experimental results prove the correctness of this paper research conclusion and laid a solid foundation for the further research. |
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