Study On Model Predictive Current Control Strategy Of LCL-APF

With the increasing popularity of power electronic devices in the power grid,the problem of harmonic pollution has become more and more serious.Active power filter is an important means of harmonic control,and its research on current control strategies is widely studied.In this paper,the model predictive current control strategy of the three-phase three-wire LCL-APF is studied.The specific contents include the following aspects.Firstly,the mathematical model of LCL three-phase three-wire active power filter is established.The DC side bus voltage,DC side capacitance and AC side inductance are calculated.The main circuit parameter design is completed and the filter performance of different parameters of LCL filter is discussed.In order to achieve resonance suppression,passive damping is chosen primarily.Secondly,the model predictive current control strategy of LCL-APF,the closed-loop PI controller of DC side bus voltage and the long-field model predictive current control are studied.The current prediction model of LCL-APF is established.Then the model predictive control is used to complete the tracking of command current,and the feasibility of the above prediction model is verified by simulation.In order to improve the compensation current control accuracy of APF,it is necessary to design the DC bus voltage closed-loop PI controller to keep the DC voltage steady and improve the accuracy of current compensation.As the traditional single-step model prediction algorithm has certain conservativeness,the model predictive current control strategy with long prediction field needs to be designed,so that the compensation current of APF can accurately track the command current within a long prediction range and accomplish multi-step optimal control.After that,the calculation method of the predicted current reference value is improved and the direct prediction method is designed to increase the prediction accuracy.Besides,the active damping part is added in the predicted current reference value to achieve the resonance suppression.For the traditional reference current linear prediction method,it is difficult to accurately predict the higher harmonics.The APF reference harmonic current is divided according to the order,and then the predicted harmonics seperately and summed for each harmonic.Aiming at the virtual damping strategy of LCL-APF,the filter capacitor parallel virtual damping method is adopted,and the damping part is added to the predicted current reference value to achieve resonance suppression.Finally,this paper studies the multi-objective cost function,which enables APF to implement model predictive current control with multiple functions.By adding a secondary term other than current tracking to the cost function,the appropriate switching vector is chosen to achieve the current control on the premise of reducing the cost function.The filter capacitor voltage and the switching frequency are respectively used as secondary control items.The compensation current emitted by the APF can respectively achieve the resonance suppression under undamped conditions of the LCL filter and switching frequency reduction.