Research On Model Predictive Control For 3-level Grid-Connected Photovoltaic Inverter

In recent years,thanks to advances in Digital Signal Processors(DSP)augmented with Field Programmable Gate Array(FPGA)platforms,some advanced control algorithms,such as Finite Control Set Model Predictive Control(FCS-MPC),have been in research for gridconnected inverters.Because of more advantages in fast dynamic response without overshoot,multi-objective and multi-constraint incorporation,FCS-MPC has attracted great attention.However,FCS-MPC,which is applied to control a grid-connected inverter,has shortcomings of variable switching frequency and low control accuracy,which not only affect the design of output filtering inductor for grid connected inverters,but also introduce more distortion into grid current compared with conventional linear control strategy.In this paper,FCS-MPC has been proposed to control a 3-level grid-connected photovoltaic inverter and the aforementioned problems are fixed through modifying the control set of 3-level inverter.Based on the fundamental principles of FCS-MPC,predictive models for grid current and capacitor voltage difference are established firstly,and the cost function is also built at the same time.Then,the time-delay caused by calculation and sample is analyzed and compensated with two-step prediction.Moreover,in order to reduce the online computational burden,basic voltage vectors are preselected and an offline table is accordingly built.To achieve a fixed switching frequency and high control accuracy,the cause of variable switching frequency and low control accuracy for FCS-MPC is analyzed,and then,a novel method using virtual voltage vector is proposed to solve these problems.The sequence of basic voltage vectors,which are used to synthesize the virtual voltage vectors,is redistributed to decrease the switching losses,and accordingly,the predictive model of capacitor voltage difference is rebuilt.Based on section division and the calculation of reference voltage vector,these synthesized virtual voltage vectors are preselected and an offline table is built so as to reduce online computational burden.And then,a circle named “reference voltage vector circle” is proposed to establish contact between composite coefficient(δ)and modulation indexes(M).A prototype of 3 level grid-connected inverter is established to validate the proposed method and the experimental results show that the proposed method has good static and dynamic performance with lower current distortion and higher current tracking accuracy for a wide range of modulation indexes.