Research On Key Technology Of Switched Capacitor Virtual Synchronous Generator

Virtual synchronous generator(VSG)solves the problems of high penetration and low inertia when distributed power generation systems are connected to the power grid.However,the traditional grid-connected inverters used in virtual synchronous generators have many shortcomings and severely restrict the system efficiency and performance.However,the switched capacitor multilevel inverter possess the features that high efficiency,high power density,high step-up ratio,and low cost.Therefore,the application of switched capacitor multilevel inverters as grid-connected inverters in virtual synchronous generators is conducive to improving the efficiency and performance of the system and has broad application prospects.Chapter 2 proposes a single-stage switched capacitor seven-level inverter,and analyzes its topology and operating mode.The voltage balance of the capacitors is achieved by improving the operating mode.Based on the seven-level output voltage of the inverter,a unequal amplitude carier cascade PWM control strategy is proposed,which enables the inverter to obtain four times the boosting capability,some switches can work at the power frequency,and the performance of the output waveforms could be improved..In chapter 3,a two-stage switched capacitor nine-level inverter is proposed,and its topology and operating mode are analyzed.The control strategies of the switched capacitor module and the inverter module are designed separately,and the capacitor voltage balance and the nine-level AC output are realized.The capacitors in the topology are optimized and designed,which provides a reference for capacitance selection.An optimized unequal-amplitude inverse carrier cascaded PWM(PODPWM)control method is proposed,and based on this method,the switching angle is optimized to minimizes the THD of the output voltage.Finally,the loss of the switched capacitor module and the inverter module were calculated respectively,and the method to calculate the efficiency of the inverter was obtained.Chapter 4 analyzes the mathematical model of the traditional synchronous generator,and researches the principle of the virtual synchronous generator.The virtual governor and virtual excitation regulator are designed respectively.A control model of switched capacitor inverter based on VSG was established,and its closed-loop system was analyzed.In chapters 5 and 6,the single-stage switched-capacitor seven-level inverter system and the two-stage switched-capacitor nine-level inverter system are designed.Simulation models and experimental prototypes are built.The operating mode and control strategy of the inverters were verified,and the efficiency was tested and analyzed.A simulation model of switched capacitor type virtual synchronous generator was established,and the output waveforms of the inverter and the dynamic response characteristics of the system frequency were simulated and verified.