Modular Multilevel Converter Control Strategy Design And Equipment Research

Modular multilevel converter(MMC) as the most promising topology one of the multilevel topologies applied to high voltage and large power, since it has been put forward, scholars in modulation strategy, modeling and analysis, restraining circulating current and DC side voltage fluctuation suppression of its were studied extensively. MMC in high voltage direct current transmission and high-voltage reactive power compensation field has broad application prospects. This paper on the establishment of the MMC mathematical model of bridge arm, design the bridge arm of the corresponding control strategy and its effectiveness is verified, through the design of the main circuit and control circuit built single-phase test prototype, made a test of control strategy and performance analysis.In this paper, several common topologies of multilevel technology are introduced, and the characteristics of various topologies are analyzed.Focus on the introduction of modular multilevel converter, summarized the research hotspots of modular multilevel converters,including mathematical modeling, modulation strategy, circulation analysis and control its strategy, DC side capacitor voltage fluctuation suppression strategy.The basic structure and working priciple of MMC are analyzed.The mathematical model of MMC system is established, and simplified the circuit structure reasonably.On the basic of the simplified circuit,starting in the switch function of the upper and the down bridge arm,the relationship between total voltage fluctuation and leg voltage and the bridge arm current is detailed derived and analysed,derived the upper bridge arm and under the bridge arm current formula.An individual bridge arm current control strategy is put forward.The mathematical model of the three-phase MMC is established,on the basis a, control strategy based on the PI controller is designed. And also designed a PIR circulating current suppressor which is an improvement on the traditional PI controller to suppress the even harmonic component of circulating current.The acquisition method of active current command, reactive current control command and the capacitor voltage balance control strategy between the sub modules of the MMC system are studied.The individual bridge arm current control strategy of MMC-STATCOM is researched and simulated. The working principle and advantages of MMC-STATCOM are analyzed,and it is pointed out that MMC-STATCOM is equivalent to an controllable source which amplitude and phase are variable, frequency equal to the frequency of the grid.By controlling the size of the amplitude and phase of the output voltage can control the size and nature of the reactive power absorbed from the system, so as to achieve the effect of reactive power compensation. In the PSIM simulation environment built a simulation model of the MMC-STATCOM system, a simulation analysis of the control strategy proposed in this paper is achieved. The simulation results show that the theoretical analysis and the proposed control strategy are correct and effective.The software and hardware design method of single phase four level MMC is introduced in this paper. Including the selection of important components and the structure of the control circuit, focusing on the sampling circuit, the zero crossing circuit design, hardware protection circuit including DC side overvoltage protection and SO protection circuit design, etc. The correctness of software and hardware circuit design is verified by the single phase four levels MMC test.