The Study Of Several Problems In Multi-level Inverters SHEPWM

Cascaded Multilevel Inverter has caught more and more attention for its special advantage.Thevalue of dv/dt and current stress components can be controlled can be reduced,the total harmonicdistortion (THD) of output waveform and the voltage,meanwhile,maintaining a very high powerfactor.In structure, it’s easy to modular design, as a result, production cost will bereduced.Wherefore, study based on this topology has important significance.The research object was Cascaded Multilevel Inverter,in this paper,and the main researchcontents and innovation points are as follows:The structure and working principle of several kinds of typical multilevel inverter wereintroduced and both the advantages and disadvantage between different topology had been analyzedand compared.Study of Selective Harmonic Elimination Pulse Width Modulation (SHEPWM)based on H-bridge cascaded multilevel inverter had been done.In addition,the principle of Stepwaveform modulation SHEPWM method and the process of nonlinear harmonic model weredescribed in detail.On the basis of previous research,a uniform multiband SHEPWM mode has beenproposed to solve the modulation ratio limited problem in traditional trapezoidal waveformmodulation,who can broaden modulation index range from0to1.Solving of nonlinear equations without a doubt is the key of SHEPWM technique,Newtoniterative method and its improved algorithm based on Newton iterative method, had became a hottopic of research on SHEPWM,because it’s simple, rapid, but it also has a problem that its greatdependence on initial.A walsh domain harmonic elimination model,which is suitable for multibandmodulation,had been proposed to get the solutions of nonlinear equations.What is a pleasantlysurprised is that it obtains good results, and laid the groundwork for calculation of nonlinearequations online.For unit power imbalance issues existing in the h-bridge cascaded multilevel inverter,the morebalanced power switch solution was proposed,which is suitable for multiband modulation.The technique of calculated switching angles online,based on a seven level inverter,waspresented in this paper.The detailed steps are as follows:converting SHEPWM harmonic equationsto algebraic polynomial systems of equations,then Using function solve in the mathematicalsoftware Matlab to solve algebraic polynomial equations.Distributions of all the solutions formultiband modulation was analyzed and discussed.Finally,calculating switching point online hadbeen implemented,in control chips DSP2812,using linear interpolation method and Newton’siterative method.