Research On Model Predictive Control For Active Power Filter Without Harmonic Detection

In low-voltage three-phase four-wire power supply system,the problems of harmonic and neutral current caused by various non-linear and asymmetric loads are becoming increasingly prominent.As an effective equipment to compensate the harmonic and unbalanced current,three-phase four-wire active power filter(APF)has received a lot of attention.In this thesis,three-leg split-capacitor-type APF was used as the research object,and its current tracking strategy based on model predictive control without harmonic current detection was studied.Firstly,to simplify the control system and reduce cost,under the comprehensive compensation target for harmonic and reactive current,a grid current control strategy without harmonic detection for APF was studied.By analyzing the power transfer process between the AC and DC side of APF,it is pointed out that the output of DC-side voltage control can be used as reference current in the strategy without harmonic detection.The equivalent form of harmonic detection link in traditional APF was deduced,and then the influence of detection error was discussed,which proves that harmonic detection is not necessary under comprehensive compensation target.Accordingly,the direct control strategy for grid current was given through further simplification.At last,two control strategies,one containing harmonic detection,the other without harmonic detection,was compared from four aspects.Secondly,considering the advantages of simplicity and fast response,the finite control set model predictive control(FCS-MPC)was introduced into the current tracking control strategy of split-capacitor-type APF.The system mathematical model in 3D coordinate system was established,and then the principles about the coupling elimination of three-phase currents and the suppression of unbalanced current were analyzed.Two kinds of prediction models,which taked compensation current and grid current as the control object respectively,were deduced.Simultaneously,the corresponding evaluation functions under single target and multi-objective control were established.Then,the control flows under two models were compared.In addition,the system stability condition determined by control delay and inductance mismatch coefficient under the model predictive control was derived.To ensure the stability and balance of DC-side capacitor voltages,the double-loop PI control was adopted,and the controller parameters were designed.Thirdly,in view of the problems of high sampling frequency and non-fixed switching frequency under conventional model predictive control,a model predictive control method with fixed switching frequency was given.Refering to the SVPWM principle,by spatial division and the orientation of grid voltage vector,several basic voltage vectors were determined as output in each control cycle.And the action time of each vector can be calculated according to current prediction model.After final pulse modulation,this method can realize the fixed switching frequency and obtain better compensation performance.Furthermore,the system control flow and current tracking process using this control method were analyzed and compared with the conventional model predictive control method.Finally,after designing the main parameters of APF system,the current control strategy studied was analyzed by simulation.The simulation results show that the DC-side voltage control can automatically compensate the active detection error under comprehensive compensation target,which verifies the feasibility of the strategy without harmonic detection.And model predictive control has a fast dynamic response.Compared with the strategy containing harmonic detection,the steady-state compensation effect of APF is better under the strategy without harmonic detection.In addition,the influence of sampling frequency,multitarget weight coefficient,control delay and inductance mismatch coefficient on system compensation performance were analyzed respectively,which provides a reference for further optimization of control performance.The simulation results also show that the stability and balance control performance of DC-side voltage are good,which verifies the validity of doubleloop PI control.Compared with the simulation results of conventional model predictive control,it is shown that the dynamic response speed of APF is almost the same under the model predictive control with fixed switching frequency,but the steady-state compensation precision is higher and the stability to parameters changes is better.