Research On Modular Multilevel Power Electronic Transformer And Its Voltage Balance Control Stratege

Power electronic transformer is considered to be one of the key equipment of the smart grid in the future with the advantages of small volume,light weight,low pollution(insulating oil),fault isolation and other potential functions,such as reactive power compensation,active filter,voltage regulation,etc.Combined with power electronic transformation,high frequency coupling technology and advanced digital control technology,not only can the power electronic transformer realize isolated transformation of voltage levels,but also output highly controllable voltage.In addition,the power electronic transformer can provide ac and dc interfaces for distributed energy,which makes energy access,management and distribution are easier to implement.Modular multilevel power electronic transformer consisting of multiple and series-parallel modules is one of the scheme for current mains tream power electronic transformer,which can effectively solve the problem that the commercial IGBT voltage grade cannot meet the voltage grade of middle or high voltage distribution network.The rectifier stage using cascaded H bridge rectifier can reduce system loss,volume and weight of filter,and the high equivalent switching frequency can be implemented at a relatively low switching frequency,in addition multilevel features can further reduce the harmonic content ininput current.The dual active bridge converter combined with soft switch technology was adopted in isolation level which can achieve high efficiency and high power density of voltage grade transformation.Although modular multilevel power electronic transformer has above advantages,the balance between the rectifier voltage and the power of isolation level become more difficult because of massive use of series-parallel modules.The above problems were studied in this dissertation.Then,two novel control methods aimed at the unbalanced voltage of rectifier stage are proposed.The first voltage balance control strategy is to modif y the active and reactive power duty ratio of each rectifier-stage module by using the voltage feedback,which can realize the dc voltage balance and control the reactive power balance of modules at the same time,in addition eliminate the coupling between the basic dual loop controller and the voltage balance controller.The second is a novel multidimensional-space vector modulation strategy by comparing the size of the dc voltage and combining with the range of input current direction and modulation index,which can allocate the optimal switch state for each module.The modulation strategy can achieve very fast and wide range of voltage balance.The feasibility and effectiveness of the proposed two control methods are provedby simulation and experimental results.Finally,the experiment prototype of rectifier-stage was designed.The design of hardware circuit and process of software control are introduced,and a series of experiments testing on its basic characteristics are conducted.