Research On The Suppression Strategy Of Parallel Circulation Of T-Type Three-Level Grid-Connected Inverter Based On FCS-MPC

Due to the difference in control signals of the grid-tied inverters and the fluctuation of filter specifications,the grid-tied inverter is prone to generating circulating current when running in parallel,causing adverse consequences such as grid-tied current distortion,power tube damage and even grid-tied system paralysis;hence,studying the parallel circulating current suppression control strategy of the inverters is of great relevance;T-type three-level inverter extensively applied due to their simple topology and convenient control.Therefore,this thesis examines the control strategy for parallel circulating current suppression of T-type three-level grid-tied inverters,as follows:Firstly,to realize the control targets of grid current following and circulating current suppression at the same time,he circulating current suppression control strategy of FCS-MPC based on multi-objective value function is adopted,that is,three constraint control items of grid-connected current,DC side midpoint potential and common mode voltage are set in the value function simultaneously;since choosing the optimal weight coefficients by conventional methods is difficult,this paper proposes a cascaded FCS-MPC circulation suppression strategy based on no weight coefficient.The three-level hierarchical idea is used to screen the switching quantity step by step,so that the dynamic response of the system is fast,the grid-connected current distortion rate is lower,and the circulation suppression effect is better.Secondly,in order to solve the problem of parameter perturbation caused by factors such as current change,temperature and aging of filter inductance,which leads to parameter mismatch of prediction model,the filter inductance of grid-connected inverter side is identified based on FRLS algorithm,and the identification results fed into the cascaded FCS-MPC circulating current suppression control strategy based on weightless coefficients to guarantee the accuracy of the prediction model on a real-time basis.Thirdly,for suppressing the intrinsic resonance spikes of the LCL filter,based on the calculation of the filter parameters,an active damping control based on the feedback of the high frequency portion of the capacitor voltage is applied to dwindle the resonance spike,thus guaranteeing the grid-tied current stability.Lastly,the control strategy was demonstrated by simulation in MATLAB/simulink simulation platform,Simulation results indicate that under the circulating current suppression control strategy based on the cascade FCS-MPC without weight coefficient,the distortion rate of the grid-tied current is 0.6%,and the circulating current amplitude is controlled within ±8mA.Compared with the traditional circulating current control strategy,the circulating current suppression control strategy designed in this paper can not only ensure the following of the grid-connected current but also quickly reduce the circulating current amplitude.After adding FRLS inductance dynamic identification to the above circulating current suppression controller,when the inductance parameter fluctuates from 3.5mH to 3mH,the distortion rate of the grid-connected current is 0.66%,and the circulating current amplitude is within ±8mA.Compared with no inductance identification strategy,the distortion rate of the grid-connected current is reduced by 0.08%,and the circulating current amplitude is reduced by 2mA.