Research On The Structure And Control Method Of Hybrid Cascaded Multilevel Inverter

Multilevel converters have been widely used in the field of high voltage and high power due to its high power quality,high efficiency and high power.But with the increasing performance and functional requirements in this field,traditional converters are increasingly showing its shortcomings,such as the increasingly requirement of power devices,complicated structure and lack of function.The hybrid inverters can use less power devices to achieve more output voltage level than traditional inverters,which can simplify the converters' structure,reduce costs and volume.The definition of hybrid inverter is that the power electronic converters which are composed of power units with different voltage levels,different structures or switching devices with different power levels.This combination allows the hybrid inverter to inherit some characteristics from the original power cells,so as to achieve the purpose of improving its performance.In this thesis,two kinds of hybrid inverter are researched and the respective characteristics and control methods are given.One is a hybrid cascaded 7 level inverter and the other is a hybrid cascade multilevel STATCOM.The hybrid cascaded 7 level inverter uses less switching device to achieve a higher power quality,but the hybrid modulation method makes the current flowing backwards of the low-voltage cell which is not conducive to practical use.Hybrid frequency carrier modulation method can solve the problem of the power circulation.But it makes the switching frequency of the high voltage cell very low which has a great negative influence on power quality,so an improved hybrid frequency carrier modulation method is proposed.The improved method makes the high voltage cell run in monopole multiplier sinusoidal pulse width modulation,and improves its equivalent switching frequency without increasing the switching frequency.This method improves output power quality of the hybrid inverter,and retains the advantages of the hybrid frequency carrier modulation method which can eliminate the power circulation.Simultaneously,its multiple rectifying circuit is studied,which proves that there is no power circulation with this method.Finally,the simulation is carried out under MATLAB/Simulink to verify the proposed method.Hybrid cascade multilevel static synchronous compensator(HCMC-STATCOM)combined with the three-phase bridge inverter which has the advantages of simple structure and the cascade H bridge inverter which has the advantages of good power quality.Due to the three-phase bridge cell has the common DC side which can exchange the phase energy,it has certain unbalance current compensation ability.The compensation principle of HCMC-STATCOM is studied,and the zero sequence voltage which can balance the wave-shaping circuit DC side of HCMC-STATCOM is deduced,as a result the range of compensation unbalance current of HCMC-STATCOM can be derived.Compared with the star STATCOM with the same output voltage level,the HCMC-STATCOM has a wide compensation range.The control system of HCMC-STATCOM is studied,and the compensation current is controlled by the positive and negative sequence separate decoupling method.The DC side balance control strategy of three phase bridge cell and wave-shaping circuit is given.Finally,the model of HCMC-STATCOM is built in the MATLAB/Simulink simulation platform,and the simulation results are basically consistent with the theoretical analysis.