Research On New Topology And Modulation Method Of Switched-Capacitor Multilevel Inverters

With the advantages of low voltage stress on power switches,high output waveform quality and low electromagnetic interference,multilevel inverters are widely applied in the variable frequency speed control system,the renewable energy generation system,and so on.However,traditional multilevel inverters lack the voltage boosting ability.Therefore,boost circuits are required in the application with higher output voltage,increasing the complexity of the circuit topology and making the system bulky and heavy.Recently,switched-capacitor multilevel inverters have become a research focus because the capacitors are used to boost the input voltage and the voltage stress of the devices is low.Besides,more voltage levels can be generated with fewer components.Nevertheless,switched-capacitor multilevel inverters also possess many shortcomings,such as large demand for capacitance,low utilization of the capacitor voltage and imbalanced capacitor voltage.Consequently,research on the new topology and the modulation method of switched-capacitor multilevel inverters is presented in this dissertation.The main work is summarized as follows.(1)The load is powered by the switched-capacitor multilevel inverter through the voltage levels that are composed of capacitor voltages and the DC power source voltage,and the capacitor voltages drop.Consequently,it is essential to quickly and effectively charge capacitors in the inverter topology design.The RC capacitor charging mode formed by the equivalent resistance of devices in existing switched-capacitor multilevel inverters has several disadvantages.Specifically,the charging speed cannot be flexibly controlled and there is a sudden change in the charging current.Aiming at these problems,a single-phase nine-level inverter is proposed,and the top voltage level is twice the input voltage.In this topology,a small inductor is connected in series between the upper and lower capacitors.The positive and negative poles of the DC power source are respectively connected to the negative pole of the upper capacitor and the positive pole of the lower capacitor by two groups of reversely series-connected switches,and the inductor freewheeling circuit is composed of the switch anti-parallel diode between them.The capacitors can be flexibly charged by the DC power source through the inductor,which possesses many advantages such as fast charging speed and controllable charging current.In addition,this charging method can be realized when most voltage levels are generated,so that the capacitors can be charged in time and the voltage drop is reduced.(2)When the traditional phase opposition disposition pulse width modulation method is applied to switched-capacitor multilevel inverters,capacitors are discharged for a long time in a cycle and the variation of the capacitor voltages is large.Aiming at this problem,a modified phase opposition disposition pulse width modulation method with optimized voltage levels is proposed.In this method,the voltage level is reset to charge the capacitor when the variation of the capacitor voltage could be large.There exists a period of time in each switching cycle when the capacitor with low voltage can be charged timely and quickly,and the duration of the capacitor discharging state is greatly shortened.Moreover,the variation of capacitor voltages and the demand for capacitance are effectively reduced.The simulation model and experimental platform are built to verify the feasibility of the proposed topology and modulation method.(3)There is a large capacitor voltage drop caused by the bus current that flows through the capacitor when the bus voltage approaches the positive or negative peak value.Aiming at this problem,a capacitor voltage control method based on the model predictive control is proposed.A discrete capacitor voltage prediction model using the capacitor voltage,the bus current and other related parameters at the beginning of the capacitor charging process is established.The minimum average deviation between the predicted capacitor voltage and the operating voltage within the interval between two capacitor charging processes is taken as the optimization goal,and the optimal capacitor target voltage after the current charging process is obtained.This target voltage is taken as a reference for the capacitor charging process.Simulation and experimental results show that the average and maximum deviation of capacitor voltages are reduced by about 50%.(4)In view of the voltage imbalance problem caused by the fact that the existing three-phase switched-capacitor multilevel inverters adopt capacitor series charging mode and the capacitor voltage cannot be adjusted independently,a three-phase seven-level inverter topology with 1.5 voltage gain is proposed.The neutral point of the DC-link capacitors is connected to the negative(positive)pole of the upper and lower floating capacitors respectively by two groups of reversely series-connected switches,and the positive(negative)pole of the upper and lower floating capacitors is connected to the output port respectively by the double bridge circuit.Two sets of redundant circuits that can generate the same voltage level with different floating capacitors are constructed.Thus,the appropriate circuit can be selected to balance the floating capacitor voltages under the condition of meeting the voltage level output requirements.Besides,not only the positive pole of the upper DC-link and floating capacitors but also the negative pole of the lower DC-link and floating capacitors are connected by two groups of reversely series-connected switches.The corresponding parallel-connected charging circuit of DC-link and floating capacitors is constructed,which can balance the DC-link capacitor voltage by connecting the floating capacitor in parallel.Digital simulation results show that the proposed topology could control the capacitor voltage flexibly and has the advantages of balanced capacitor voltage and reduced demand for capacitance.