Research On Topology And Control Strategy Of Low-Voltage Multilevel Converter

In recent years,with the development and popularization of new energy generation,distributed generation and microgrid technologies,the demand for low-voltage converters is increasingly high.In order to meet these requirements,the converter must be designed using a high-step multi-level topology.However,the traditional high-step multi-level converter,which is suitable for high-voltage and large-power applications,has many disadvantages,such as large number of devices,high cost,complex control strategy and so on.The main work of this paper is to develop a new type of high-order multi-level topology for low voltage applications,which has the characteristics of small number of devices,high efficiency,low cost,small size,simple and effective control strategy-The main contents of this paper are as follows:1.Firstly,some basic works have been done for multi-level converters.In this paper,the research status of multilevel converter topology is summarized,and the structure characteristics and working principles of various commonly used topologies are analyzed.The advantages and disadvantages of various common control strategies are studied.Combined with the structural characteristics of high-order multilevel converter,a modular design method is used to design a multilevel converter experimental platform.The design process and parameter calculation method of each hardware circuit module of the experimental platform are presented.The software framework of DSP+CPLD is designed,which provides a good platform for the later research works.2.In order to reduce the number of switching devices,a novel seven-level converter topology is proposed in ? section.The control method of DC bus capacitor voltages and flying capacitor voltage under space vector modulation strategy is proposed and the stable operation region of this method is obtained.Comparison with other seven-level converters,the number of switching devices used in this topology is the least.Each phase bridge arm consists of only 10 switching devices and one flying capacitor,which reduces the power losses and makes the converter have higher conversion efficiency.However,due to the limitation of the operation range of DC bus capacitor voltage,the converter is only suitable for reactive power transmission applications such as SVG and APF,which limits the application scope of the proposed topology.3.A novel nine-level topology with wide stable operation area,low number of devices and suitable for low voltage applications,is proposed in the IV section.Each bridge of the converter requires only 10 switching devices and two flying capacitors.In this paper,the structure,characteristics,working principle and the selection of flying capacitor are analyzed.The control of flying capacitor voltage is realized by using PD-PWM method combined with priority control method.On this basis,the DC bus capacitor voltage balance control is realized by changing the reference voltages value of the flying capacitors in the positive and negative half cycle of sinusoidal wave.The effectiveness of the converter and the control strategy is verified by simulation and experiment.The expansion characteristics of the topology are evaluated.The simulation and experimental results show that the operation range of the converter is not affected by the load power factor and has a wide stable operation area.The obvious advantages of the converter in the number of switching devices,the number of on-state switches in the current path,the number of gate driver,the number of flying capacitors,the number of DC power supply and the efficiency prove that it is a competitive solution for low voltage multilevel converters.4.For the proposed nine-level converter,an improved model predictive control strategy is proposed,which can control the output power,DC bus capacitor voltages,flying capacitor voltages,common-mode voltage and switching losses of the converter.The prediction model of multiple variables in the proposed nine-level converter is analyzed,the implementation process of the traditional model predictive control strategy in the proposed nine-level converter is described.The problems of large computational load and difficulty in real-time calculation are discussed.Aiming at the problem of large computational burden of traditional model predictive control algorithm,an optimization algorithm of model predictive control based on multi-step prediction is proposed.By using deadbeat predictive direct power control strategy,the computational complexity is reduced.Finally,simulation and experiment verify the control effect of the control strategy on the output power,DC bus voltages,flying capacitor voltages,common-mode voltage and switching losses of the proposed grid-connected converter.