Research On Control Strategy Of Three-phase Cascaded Grid-connected Inverter

With the requirement of social modernization and sustainable development,the clean and renewable energy power generation technology,such as solar energy,wind energy and so on,has been developing rapidly.Grid-connected inverter,as the interface between renewable energy system and power grid,directly affects the output power quality of power generation system.Therefore,the research on grid-connected inverter has become a hot spot.Compared with the two-level grid-connected inverter,the three-phase cascaded grid-connected inverter has smaller voltage difference,lower switching frequency and voltage regulation,and lower total harmonic distortion of the output current.Compared with cascaded H-bridge topology,when the number of switch tubes is the same,the output voltage of the former is higher,which can be directly used in high-voltage and high-power applications,reducing the economic cost.Finite control set model predictive control(FCS-MPC)is composed of three parts: prediction model,rolling optimization and feedback correction.Compared with pulse width modulation and carrier phase shifted sinusoidal pulse-width modulation,the scheme eliminates the modulator and is easy to realize grid-connected control and has the advantages of fast dynamic response and so on.In this paper,FCS-MPC is applied to the topology of the three-phase cascade.In view of the problem of the time consumed in the process of sampling and calculation in traditional FCS-MPC,two sampling period prediction values are used to compensate the time delay error caused by the sampling and calculation;Because the current value of the second sampling time is related to the current value of the previous sampling time,and the error of the current at the time of prediction is unavoidable,the current feedback compensation is carried out for the second time;aiming at the problem that the traditional control strategy is computationally large,it is simplified by the correspondence between current and voltage;The absolute value of the difference between the reference variable and the predicted value is chosen as the value function,and the number of switch changes is taken as an influencing factor in the value function.Based on the above improved control strategy,under the ideal power grid condition,the three-phase current is transformed by 3/2,the forward difference,the discrete predictive model and the value function of the grid-connected inverter are established to predict control.The positive and negative sequence components of the second-order complex vector filter can be quickly and precisely separated according to the control requirements of realizing stable power or less current harmonics or both,and then the positive and negative sequence components of the grid voltage are obtained,and then get the grid connected current reference value,so as to construct the discrete predictive model and the value function to carry on the forecast control.Based on the above theoretical analysis and research,a simulation model is built on the Matlab 2010a/ Simulink simulation platform,and the simulation results show that the improved control strategy can improve the performance of the cascaded inverter.The output current of the inverter can track the reference current very well,and the total harmonic distortion of the grid-connected current is smaller.Under the unbalanced power grid,the output current balance can be guaranteed or the output power can be stabilized and so on,and the dynamic and steady-state performance are better.