Research On The Control Technology Of Cascade H-Bridge Without Grid Voltage Sensors Based On Natural Frame

With the continuous increase of the proportion of new energy distributed power generation in China’s power supply system,power conversion devices based on Pulse Width Modulation technology（PWM）have gained more and more research and development.The Cascade H-bridge（CHB）multilevel converters have been extensively used in photovoltaic power generation,active power filtering and other applications due to their advantages of straightforward modularization and expansion and low harmonic content.This article focuses on the research of CHB multilevel converter.Briefly describe the coordinate transformation is complicated,the performance is affected by the phase-locked loop（PLL）,the sensor cost is high in typical control methods for single-phase CHB multilevel converters and discuss a new method for CHB multilevel converters based on natural frame control without grid voltage sensor.The main research contents of the full text are as following:First of all,the working principle and four-quadrant operation mode are described for the full-bridge PWM converter,which is the component unit of the single-phase CHB multilevel converter.Then analyze and establish the mathematical model of the single-phase CHB multilevel converter.Subsequently,the principle of coordinate transformation of the mathematical model between different coordinate systems is explained.In addition,the realization principle of the PWM modulation strategy of the CHB multilevel converter is introduced.Secondly,in view of the aspects that can be improved in the current typical control strategy of CHB multilevel converters,a new type of natural coordinate control strategy suitable for single-phase CHB multilevel converters without PLL and coordinate transformation is discussed.In order to make the system adapt to the operation without the grid voltage sensor,the algebraic model analysis without the grid voltage sensor is carried out.Aiming for the high-frequency interference caused by the differential quantity di_sa/dt and the high-order harmonics caused by the switching function ladder wave,the concept of virtual flux linkage is introduced.The basic principles,advantages and disadvantages of various virtual flux linkage observers were analyzed and compared,and a virtual flux linkage observer with negative feedback was adopted.On the basis of the equivalent model of natural frame control,a new method of natural frame control that does not require the grid voltage sensor to obtain the grid voltage signal is proposed.A simulation model was established on MATLAB/Simulink2018a to verify the correctness of the proposed control method.Finally,in order to verify the feasibility of the control strategy,on the basis of theoretical research,the software and hardware design of the single-phase CHB converter system under the condition of voltage sensorless with TMS320F28377D as the main control chip was completed.The operation of the system without grid voltage sensor is verified from the dynamic and transient aspects respectively.The results show the feasibility and effectiveness of the proposed control method under the condition of voltage sensorless.