Simulation Study On Topology And Modulation Strategy Of High-voltage Inverter

As to the widely employed High-voltage High-capacity frequency converting governor, multilevel inverter has been regarded as one of the most significant problems in the Power Electronic Technology at the present time. Research of corresponding inversion technology from high frequency soft switching technique coming of age to low frequency multilevel conversion technique. The objective of the former is pursuing high frequency, reducing switching stress and improving the output voltage. But the objective of the latter is to make the multilevel into fundamental frequency by improving the structure. The former is suited for medium and small capacity inverter and the technology is nearly mature, while the latter has been used in medium or high voltage for high power applications and its technique is still in developing stage. Using the technology of multilevel inverter, the full controlled power electronic device with lower voltage withstand may be applied to high voltage and large capacity field and it could effectively control the high harmonic generation. The cascaded multilevel inverter can be easily expanding to higher voltage grade. But the topology and control strategy of multilevel inverter is comparatively complicated. How to improve electric circuit and make the multilevel into base frequency has been the research focus of low frequency multilevel conversion technology.On the basis of analyzing the present electric circuit of multilevel inverter firstly, the structure of inverter in this paper is decided to be cascaded inverter. In addition, these PWM control methods have been further analyzed and a concept of hybrid method is presented from analyzing of Phase shifted carrier PWM and Carrier disposition PWM. The results of simulation by MATLAB show that the component of high order harmonic decrease greatly and low order harmonic is relatively minor when the hybrid method is adopted. Based on the analysis of the PWM control technique of multilevel inverters, computer models were developed under the environment of MATLAB/SIMULINK and the single-carrier PWM modulation method was presented too. Spectrum of multilevel voltage was simulated and analyzed and its harmonic characteristics were compared with the other modulation methods. The simulation results illustrate that converter controlled under single-carrier PWM modulation method is better in harmonic content of phase voltage and line voltage than the conventional PWM modulation method. Then the simulation of motor frequency starting controlling system is made by the single-carrier PWM method. Under this method, operation characteristics of motor are studied on. The results showed that harmonic content of the output voltage and current and the torque ripple of motor are less by simulation. The results show the advantages of the control strategy of cascaded high voltage converter.