Research On Topologies And Control Strategies Of Hybrid Asymmetric Multilevel Inverter

Multilevel inverter has been focused on because it can make low voltage power switching elements working at high voltage and power fields reliably. And it has more advantages, such as low harmonic component, reducing EMI and etc. It has been used in high voltage motor driving, static synchronous compensator (STATCOM) and flexible AC transmission system (FACTS) practically. However, conventional multilevel inverter used now has some disadvantages , for example, input transformer too complex with large volume, more power switching elements and low reliability. A new hybrid asymmetric multilevel inverter (HAMI) has been presented to avoid these advantages. Having the same voltage and levels with less power switching elements and more simple, HAMI is a trend of multilevel inverter. The research of HAMI is on the initial stage and more problems are needed to be considered and solved in practical applications. So the research of control strategies, topology and improved method has more important theoretical significance and applicable value.Based on the research of the topology and control strategy of HAMI , the thesis focuses on the problemes existing when control strategies ared used, and improved control strategies are presented. The improved topologies of HAMI are presented to add output voltage level, improve the reliability and simple the control method, and detailed analysis of control strateges is given.Firstly, a general topology of HAMI and mathematic model of asymmetric multilevel inverter are presented to give a base of the HAMI research. The output power of cells with different number of cascaded cells and ratio of DC of asymmetric multilevel inverter is analyzed with hybrid modulation. A novel hybrid modulation method is proposed to control the power provided by cells not negative, as well as can provide more output voltage and levels, meanwhile, the influence on the inverter output voltage of overmodulation is reduced to minimum. The novel control strategy can extend the DC of HAMI with unidirectional rectifier, reduce the number of cascaded cells and cost of inverter. And a general design method is studied.Secondly, the output common-mode voltage is investigated deeply when using hybrid modulation and third harmonic intruded hybrid modulation method. A novel hybrid modulation method is proposed to eliminate the high frequency PWM in common-mode voltage based on the different influence on common-mode voltage of cells. The distinctness between conventional space vector PWM (SVPWM) and SVPWM used in hybrid asymmetric multilevel inverter is discussed. Based on the different influence on common-mode voltage of voltage vectors, a new SVPWM method to eliminate common-mode voltage completely and partially is proposed by selecting different voltage vectors.Two novel HAMI topologies are proposed based on the analysis of five-level inverter presented now. The new topology use less switching elements when the output voltage level is same throughing cascaded the new five-level cell each other or cascaded with H-bridge cell. And SHEPWM, multi-carrier PWM and hybrid control strategy are presented when different topology and ratio of DC voltage are used. And detailed methodes to realize the control strateges are presented.Finally, a novel NPC/H bridge HAMI with single power topology is proposed aiming at the difficulty of controlling the constant voltage of capacitors in sub-inverter and invariable voltage of neutral point in main-inverter when used in motor driving. A assistant leg is used to control the neutral point voltage in main-inverter independently, it can much reduce the complexity of control and has the ability of re-constructing to maintain working when faulty of switching elements occurs. A hybrid control strategy is used and the main-inverter works at fundamental frequency while sub-inverter working at high frequency using duty cycle modulation (DCM) method. A PI control method is proposed to control the capacitor voltage in sub-inverter based on the analysis of the influence on capacitor voltage of switching angles variety in main-inverter. And redundant states selection (RSS) method is used to control capacitor voltage in sub-inverter meanwhile. The little variety of capacitor voltage and stabilization of the driving system are testified by simulation and test.