Research On Hybrid Pulse Width Modulation Strategies For Rail Transit Traction Inverter

Low switching frequency will become an inherent property of rail transit traction converters for a long time because of the basic properties of high-power switching devices.In order to cooperate with the variable voltage and variable frequency control strategy of the AC motor and alleviate the deterioration of the harmonic current in a single modulation mode,a multi-mode modulation strategy is introduced into the inverter control.With the emergence of various optimized modulation schemes in recent years,the content of multimode modulation strategies has become more and more abundant.However,the smooth transition of the multimode modulation strategy has become a major problem due to the harmonic characteristics between different modulation modes and the differences in AC motor impedance characteristics under different working conditions.The mechanism of current and torque shock during the switching process remains to be explored.This paper studies the problem of smooth transition of multimode modulation strategy.Firstly,this paper analyzes the development history and development prospects of multimode modulation strategies,and expounds the research status and existing problems in related fields abroad.This part points out the research focus and significance of multimode modulation strategy smooth transition scheme,and explains the main research content of this article.Secondly,the next section introduces the basic principles and performance comparison of common modulation modes and introduces the content and scope of application of the multimode modulation strategy used in this article,and analyzes its advantages from the perspective of harmonic characteristics and torque characteristics.At the same time,this part focuses on the analysis of the voltage harmonic characteristics of each modulation mode and the harmonic changes during the switching of different types of modulation modes,and gives an intuitive multidimensional curve and specific mathematical expressions.It laid the foundation for the subsequent analysis.Thirdly,this article simply analyzes the basic working principle and performance advantages of the induction motor in order to further analyze the transient process of the motor during the switching process.Then,this paper obtains the T-type equivalent circuit through frequency and winding conversion,and then calculates it with specific motor parameters circuit impedance characteristics under different operating conditions.Finally,the article explains in detail the generation mechanism of current and torque shock during the modulation mode switching process based on the previous analysis of the harmonic variation law and motor impedance characteristics.The amplitude of the current and torque shock during the switching process is quantitatively analyzed by introducing the simplified circuit model of the motor and the d-q axis state equation.This paper proposes a smooth switching principle and engineering implementation scheme suitable for most multimode modulation strategies based on the above analysis.At the end of the article,the correctness of the conclusions presented in the article and the feasibility of the proposed smooth handover strategy are proved by a large number of simulations and experiments.