Research On Control Method And Application Of Cascaded H-bridge Multilevel Inverter

With the continuous development of power electronics technology,there is an increasing demand for high voltage levels in industrial applications.The Traditional two-level inverters are not suitable for high-voltage and high-power applications due to the limitation of the voltage withstanding and output voltage harmonic performance of power devices.The emergence of multi-level inverters reduces the voltage stress of power devices and the harmonic content of output voltage without considering the problem of series voltage equalization of power devices.It has become a research hotspot of high-voltage and high-power conversion technology.This paper mainly investigates the cascaded multilevel inverter,and the main contents and innovations include:For the traditional cascaded seven-level inverters,the characteristics of traditional modulation strategy are analyzed and an optimized modulation strategy is proposed.The single-pole frequency doubling SPWM modulation technology is used to optimize the PD modulation strategy so that the equivalent switching frequency remains unchanged and the maximum switching frequency of the switch tube is reduced to 1/2 of the original one.On this basis,the trigger pulse signals of the respective units are rotated in units of 1/4 cycle,the output power balance of each cell is realized after 3/4 cycles.All the switching tubes of each unit have the same working stress and uniform heat dissipation,which improves the service life and reliability of the system.For the hybrid cascaded seven-level inverter,the output characteristics of its traditional hybrid modulation strategy are analyzed,and an improved hybrid modulation strategy is proposed.The problem of current backflow and energy feedback is solved,and the maximum switching frequency of the switch tube is reduced to 1/2 by using the single-pole frequency modulation technology,which improves the service life and reliability of the system.For the hybrid cascaded seven-level inverter,a novel step-wave carrier hybrid modulation strategy is proposed to avoid the problem of current backflow and energy feedback.By analyzing the relationship between the amplitude of the fundamental voltage of each unit and the firing angle,A power balance control method is proposed to achieve output power balance of each cascade unit in one output cycle,which solves the problem of inconsistent DC power utilization.For the cascaded photovoltaic grid-connected inverter,a power balance control method is proposed.The modulation signal of each unit is corrected by using multiple duty cycle feed-forward compensation amounts,and the power balance control under independent MPPT is realized,so that the inverter can stably output the maximum power under normal and uneven illumination conditions and the grid-connected current waveform quality is good.For the leakage current problem of cascaded photovoltaic grid-connected inverter,a common mode equivalent model of cascaded H-bridge photovoltaic grid-connected inverter is established.The main factors affecting common mode leakage current are analyzed.An improved method is proposed.The shifting carrier modulation strategy solves the leakage current problem of the system by keeping the common mode voltage constant during the switching process.