Research On New Hybrid AC And DC Output Three-level Converter

Nowadays,the widely used power electronic equipment can only complete a single link of energy conversion.However,in practical application scenarios,two or more energy conversion functions are usually required.The multi-stage structure has the advantages of wide operating range,power decoupling and simple control.The number of switching devices,driving circuits and radiators is multiplied,resulting in a doubling of the control cost and volume,especially for application scenarios of strict requirement for the power electronic equipment occupying space,multi-stage converter is difficult to apply and the single-stage structure shows the advantages.Whereas,there are still some problems in the single-stage structure,such as low device utilization rate,poor output waveform quality and high voltage stress.To solve the above problems,a new hybrid AC and DC output three-level converter(HO-TLC)is proposed in this thesis.The HO-TLC is composed of 15 power switching tubes(IGBT or MOSFET)and 2 capacitors,with 3 input/output ports,including one group of AC port and two groups of DC ports.The operating mechanism and switching state of HO-TLC are analyzed.According to the power flow direction of the converter port,HO-TLC is divided into two operating modes.The advantages and disadvantages of the proposed topology and similar circuits are compared in terms of the number of switches and the voltage stress borne by the switches.The modulation strategy of HO-TLC is studied,and the coupling relationship between AC output period and DC output period in a switching cycle is analyzed by using space vector modulation based on time-sharing idea.In order to deal with the problem of voltage imbalance at midpoint,a capacitor voltage balance algorithm based on beat free control is added.The feasibility of carrier modulation(CBPWM)method is analyzed theoretically and the relationship between the modulated wave expression and the switching sequence is deduced.In order to improve the dynamic response performance of the proposed HO-TLC,the voltage and current double closed-loop control is added to the DC conversion link of the HO-TLC.In addition,the influencing factors of transmission efficiency and feasible domain of DC boost ratio are analyzed,and the availability of modulation method and control program are verified by simulation.In order to further explore the application potential of HO-TLC,the HO-TLC topology was simplified and the HO-TLC with split source was formed,which is called hybrid AC and DC output split source converter(HO-SSTLC).The system modeling of the HO-SSTLC is carried out,the continuous time model and the discrete time model are derived,and finally the state space equation of the converter is obtained.HO-SSTLC adopts the control mode of finite set model predictive control and PI control.The output result of the converter under the control mode and the output result of time-sharing modulation strategy are compared and analyzed.Finally,the corresponding hardware experiment platform was built,and the prototype of low-power three-level multi-port converter was controlled by FPGA.The experimental results of AC/DC port output voltage and current and DC side link capacitance voltage were analyzed.The analysis results proved the feasibility of the proposed HO-TLC topology and the effectiveness of the modulation strategy.