Research On Nearest Level-Pulse Width Modulation Of Mixed Frequency Conversion Power Electronic Transformer

Power Electronic Transformer(PET)is regarded as the core equipment of energy internet and has a broad application prospect.However,the existing PET topologies have many problems,such as large number of power switches and high frequency transformers(HFT),high hardware cost,low power density and engineering difficulty.In order to solve these problems,a new type of mixed frequency conversion PET has been concerned.In the existing research,the mixing modulation strategy used in this topology has problems such as high frequency band distortion and excessive switching loss,which will affect the performance of hybrid frequency conversion PET.In this paper,the Nearest Level-Pulse Width Modulation(NL-PWM)strategy suitable for mixed frequency conversion PET is studied.The specific research contents are as follows:Firstly,the basic principle of mixed frequency conversion and the topology of mixed frequency conversion PET are introduced.The requirements of mixed frequency conversion PET for modulation strategy and the defects of existing mixed-frequency modulation strategy are analyzed,and a NL-PWM strategy is chosen as the modulation strategy of mixed frequency conversion PET.Then,the implementation method of NL-PWM strategy in mixed frequency conversion PET is proposed.Using the proposed implementation method,the output of high frequency square wave voltage is directly realized by calculating the number of levels.Then,according to the characteristics mixed frequency conversion PET,the voltage balancing strategy of sub-modules based on voltage sorting method is proposed to realize the voltage balance of the mixed frequency conversion PET in the presence of high frequency current.Secondly,the existing NL-PWM method has the problem of narrow pulse in the implementation process.The generation mechanism of narrow pulse in traditional NL-PWM modulation is analyzed,and an improved NL-PWM method which can eliminate narrow pulse is proposed.The improved NL-PWM method generates step wave voltage based on round function,and changes the PWM reference voltage by adjusting the phase of step wave jump point to avoid the generation of narrow pulse.Then,the harmonic characteristics of the improved NL-PWM are analyzed based on the double Fourier transform and Fourier series.Through the derivation of the output voltage spectrum expression,it is shown that the improved NL-PWM can eliminates the narrow pulse,and its harmonic characteristics are better than that of traditional NL-PWM.Then,the problem of error pulse in the implementation process of NL-PWM is studied.the generation mechanism of error pulse is clarified,and the adverse effect of error pulse on the output voltage quality of NL-PWM is analyzed.Then,the step wave delay update strategy is proposed,which can avoid the generation of error pulses by delaying the jump time of step wave voltage to near the peak and valley value of the triangular carrier.The harmonic characteristic analysis of NL-PWM output voltage after eliminating the error pulse shows that the step wave delay update strategy can effectively eliminate the adverse effects caused by the error pulse.The analysis of amplitude error and phase delay of the fundamental frequency component of the output voltage shows that the step wave delay update strategy does not significantly increase the error between the output voltage and the theoretical value.Finally,an experimental platform of mixed frequency conversion PET is developed,the controller with DSP and FPGA is designed,and the control program of the experimental platform is developed.The application effect of mixed frequency conversion PET NL-PWM method in the actual system is analyzed.The experimental results show that the proposed NL-PWM implementation method in this paper can make the mixed frequency conversion PET run stably,and the proposed voltage balancing strategy has good voltage balancing effect;the proposed improved NL-PWM can effectively avoid the generation of narrow pulse and it has excellent harmonic characteristics;the step wave delay update strategy can effectively eliminate the error pulse of NL-PWM.