Research On The PWM Modulation Strategy Of H-bridge Multi-level Inverter And Control System

Multilevel inverters have many advantages, such as low switching frequency,low harmonic contents, and better output performance, so they are widely applied inthe field of high power ac speed regulation. This paper took three-level H-bridgefive-level inverter and electrical synchronous motor as research objects, and studiedon the aspects of topologies, modulation strategies, common-mode voltage, powerswitch devices, power balance between the unit modules, dead time effect and itscompensate strategies and so on for multilevel inverter and the control system ofthree-level H-bridge five-level inverter.Firstly, this paper analyzed the work mode and pulse width modulation strategies(carrier-based pulse width modulation strategies, CPWM and space vector pulse widthmodulation strategies, SVPWM) suitable for three-level H-bridge five-level inverter.Combined with H-bridge topology Aiming at the problems of switch time inaccordance of each switch device and unbalance output power between each bridge ofthe traditional carrier disposition strategy, the carrier band frequency vary modulationstrategy was studied, and simulative results verified the feasibility of the method.However, when this method applied to cascaded multilevel inverter, there existproblems of difficulty for programming realization, high switching loss and so on. Inorder to solve the shortcomings of the above modulation strategy for cascadedH-bridge multilevel inverter, this paper emphasizly introduced the carrier phase shiftpulse width modulation strategy. Analyzing its simulative output waveforms showedthat this method can effectively solve the problem of large switching loss, outputpower balance between bridges and unit module. Due to low DC voltage utilizationrate of above strategies, we can increase DC voltage utilization rate about15%byinjecting zero sequence component to the modulation waves.Owing to many trigonometric functions and complex formulae in the traditionalspace vector pulse width modulation, this paper introduced a generalized SVPWMalgorithm based on line voltage coordinate system. It’s easy to programmingrealization and extending to higher level. Then, common-mode voltage was analyzedbased on this method, and a common-mode voltage reduction7segment typeSVPWM algorithm was presented and verified both by simulation and experiment. Inaddition, the essence relationship between SPWM and SVPWM was stated based onthe modulation strategies of five-level inverter. What’s more, the dead band effect of five-level inverter when its output voltage’s amplitude and frequency are low wasanalyzed and a compensate strategy based on pulse and difference voltage coordinatedwas introduced. Simulative and experimental results verified its effectiveness.Secondly, synchronous motor flux oriented control principle and three flux orientedcontrol mathematical model of synchronous motor were analyzed. A three-levelH-bridge five-level synchronous motor vector control simulative platform was built inthe Matlab/Simulink simulation environment. Simulative results verified thefeasibility of motor load supplied by three-level H-bridge five-level converter.Finally, a three-level H-bridge five-level converter experimental platform wasbuilt, the detail process of main circuit devices type selection and software realizationflowchart of the control system were presented. Moreover, five-level control systemwas debugged and experimental verified by load of resistance-inductance and motor.