Researches Of The Finite Control Set Model Predictive Control Strategy With Fixed Switching Frequency In Three-Phase Inverters

The three-phase inverter has been widely used in today’s industrial application due to simple structure and convenient controller design.The finite control set model predictive control has a serious drawback in the practical application,which is the variable switching frequency.However,the harmonic distribution of power electronic system is often closely related to the switching frequency,therefore,the variable switching frequency not only makes the power quality of the system worse,but also makes the filter design extremely difficult.In this thesis,the finite control set model predictive controller is used to study how to realize the fixed switching frequency by controlling three-phase inverter.Firstly,the existing research achievements are summarized and the reason of variable switching frequency is explained.After that,the existing fixed switching frequency methods of finite control set model predictive control are clarified into several types,and the principles of typical methods are introduced.At last,the comparison their advantages and disadvantages is listed.Secondly,the discrete mathematical model of the inverter is proposed,which is the object of our research.Then,A fixed frequency method based on nonlinear optimization is proposed.By examining the minimum value of mean square root error,this method gets the action time of each vector and gives it to the modulator,so as to control each bridge to get the fixed switching frequency.And the issue that the output current cannot accurately track reference values is resolved.The comparison of simulation and experimental results verifies the effectiveness of the method.However,the dynamic performance of this method is weakened due to the introduction of modulator.Thirdly,a fixed frequency method based on virtual space vector is proposed.An alternative virtual space vector is calculated on-line,which avoids traversing the offline vector table and speeds up the operation of the control algorithm.Through simulation and experimental results,it is proved that the method has an effect on fixed switching frequency,low harmonic and fast operation speed.This method has better dynamic performance,nevertheless,its realization relies to the help of hardwares.Finally,based on the connection between PWM modulation and traditional FCS-MPC control method,a fixed frequency method without modulator is proposed.This method perfectly inherits the advantages of traditional FCS-MPC and has its typical characteristics.By adding a adjustment coefficient to the original cost function and increasing the sampling frequency appropriately,the required fixed switching frequency is realized.At the same time,two different adjustment methods are obtained,and the advantages and disadvantages of the two methods are analyzed.Simulation and comparison experiments verify the effectiveness and superiority of this method.