Direct Torque Control Of Cascaded Inverter Based On SVPWM

At present, the cell series high voltage inverter is more and more widely applied in thefield of speed regulation and energy savings. The development of frequency conversiontechnology for high power has great significance on energy savings.The cascaded multilevel inverter is based on a number of basic cells(i.e.,H-bridgeinverter) connecting in series to form a single-phase or three-phase inverter. Each basic cellcan produce square voltage wave, and create more level steps of the wave to approximate thesine output voltage through the superposition of the output waveform synthesis. The inverteris constrcted by the basic cell in series to complete the high voltage power output. Comparedwith non-cascaded circuits, the switch component’s power and voltage stress of cascadedinverter are not limited. Meanwhile it can decrease dv/dt and achieve improved harmonicperformance for the modulation of cascade inverters. As the advantages mentioned above, inrecent years, the cascaded multilevel inverter has attracted great attention in the applicationareas such as high power converters, AC drives, and power active harmonic filters. It isbecoming the first choice in the high voltage energy conversion applications.Topology of multilevel converter and technique of multi-level PWM is studied in thispaper. And the mathematic model of cascaded multilevel circuit is presented based on itsprinciple analysis. Due to the large number of voltage space vectors and redundant switchingstates, there is a problem of computational complexity in multilevel converter, and so theutilization of multi-level inverters with the SVPWM method can not be realized. To resolvethe problem, a novel SVPWM technique is proposed, and it is further analyzed in detail in thispaper.Direct torque control(DTC) is a high performance ac speed regulating method aftervector control. But the application of traditional DTC in cascaded inverter is very complicated.The state of inverter switch is determined by hysteretic comparison and voltage vectorselection table. Due to the huge number of switch states and the difficult establishment of switch table, the switching frequency is unstable and the torque ripple is very large.This paperpresents the DTC method based on PI controller and a novel SVPWM method.System simulation models based on different modulation methods is constructed withMatlab/simulink and verified by experiments. Simulation and experiment results demonstratethe effectiveness of this method and provides a new idea for the practical application ofmultilevel SVPWM method.