Study On Vienna Rectifier Based On Model Predictive Direct Power Control

In recent years,environmental protection and energy issues have become the focus of attention around the world.Wind power,solar energy and other clean renewable energy have been vigorously promoted once again,and the use of wind power,solar energy,electric vehicles need a large number of converters,so improving the efficiency of converter power,reducing the generation of harmonics to ensure the quality of power grids and the safe operation of the system has become a research hotspot of scholars all over the world.In this paper,three-level Vienna rectifier is taken as the research object,and two predictive control methods based on direct power model are proposed according to its structural characteristics.The research work mainly includes the following aspects:Firstly,the structure of Vienna rectifier is introduced.Taking a voltage range as an example,the working conditions of eight switching states are analyzed in detail,and its working principle is explained.In addition,according to the structural characteristics of Vienna rectifier,the corresponding mathematical models in three coordinate systems are established.On the basis of understanding the working principle of Vienna rectifier and establishing its mathematical model,a double closed-loop control strategy based on model predictive direct power inner loop and sliding mode variable structure outer loop is proposed,in which the input active power,reactive power and active power of AC side are equal to that of DC side when the system works at unit power factor.The inner loop of the system uses model predictive control to eliminate the modulation steps and simplify the complexity of the algorithm.The design process of the sliding mode controller for the outer loop of the voltage is described in detail.The causes of the chattering caused by sliding mode variable structure control are analyzed,and the corresponding improvement measures are put forward according to the specific situation of the system.The corresponding evaluation function is constructed by predicting the two capacitor voltages on the DC side.The neutral point potential balance control is realized.In addition,aiming at the problem that the switching frequency of traditional model power predictive control is not fixed,an improved scheme is proposed: the command voltage vector is obtained by model predictive control,and the switching frequency is fixed by improved space vector modulation strategy.Traditional space vector modulation strategies need to calculate the corresponding action time of each basic vector in each sector and arrange its order of action.A large number of trigonometric function operations will also increase the complexity of the system.In order to reduce the calculation of vector action time and simplify the sequence of vector action,a method of 60 degree coordinate transformation of command voltage vector is put forward,and the law between major sectors and small sectors is found out by drawing.Then,the method of distinguishing and normalizing is carried out according to odd and even large sectors.Finally,the corresponding simulation model is established under the environment of Matlab/Simulink,and the corresponding test platform is built.The performances of the two control strategies in the process of stable operation and sudden load change are verified by simulation and experiment.