Grid-connected Predictive Direct Power Control Of Three-phase Voltage Source PWM Rectifier

Three phase pulse width modulation(PWM)rectifier has numerous advantages,including sinusoidal line current,high power factor,bidirectional power flow,and controllable dc-link voltage.Because of that,it has been widely used in industrial applications,such as the adjustable speed drives,active power filter,renewable energy systems,and high voltage direct current transmission.In this thesis,a two-level three-phase voltage source rectifier(VSR)is taken as subject investigated,the operation principle of PWM rectifier is analyzed and its mathematical model is deduced.Besides,the grid-connected predictive direct power control(P-DPC)of three-phase PWM rectifier is deeply studied.This thesis reviews the research status of direct power control(DPC)strategy,analyzes the operation principle of PWM rectifier,and establishes the mathematical model of three-phase PWM rectifier based on current control in αβ and dq coordinate respectively.According to the instantaneous power theory,the mathematical model of PWM rectifier based on power control is derived.This thesis establishes the control model of conventional dead-beat predictive direct power control(DP-DPC)in different coordinates.What’s more,this thesis analyzes the operation principle of conventional DP-DPC and the corresponding theoretical analysis shows that the conventional DP-DPC cannot eliminate the steady-state errors.To deal with this problem,a feedback correction method based on the internal model principle is introduced to compensate for the instantaneous power reference value at next control period by accumulating and summing the instantaneous power errors.Combined with the control model of PWM rectifier,the improved DP-DPC can realize that instantaneous active and reactive power strictly follows their reference values.The finite control set model predictive direct power control(FCS-MPDPC)is studied in this thesis.The one-vector-based FCS-MPDPC cannot obtain satisfied steady-state performance.For example,the current harmonic content is high and power ripples are large.Therefore,the conventional FCS-MPDPC is improved by introducing a duty cycle control.This method named two-vector-based FCS-MPDPC adopts two voltage vectors during one control period and the usage of zero vector can adjust active and reactive power gently.In order to further improve the steady-state performance,this thesis proposes a three-vector-based FCS-MPDPC strategy,which adopts two active voltage vectors and one zero voltage vector during one control cycle.Compared with the two-vector-based FCS-MPDPC,the control strategy can maintain a fast dynamic response performance,operate at a fixed-switching frequency and obtain an excellent steady-state performance,such as low power ripples and less current harmonics.However,in order to get the best voltage vector,the FCS-MPDPC needs to calculate the power slopes corresponding to eight voltage vectors and predict active and reactive powers for many times.The control algorithm is relatively complicated.In order to simplify the control procedures of the FCS-MPDPC,a low-complexity MPDPC(LC-MPDPC)is presented in this thesis.In this control strategy,the negative conjugate of complex power is selected as a control variable,leading to a fast vector selection.By predicting the negative conjugate of complex power value in synchronous dq frame for one time,the LC-MPDPC can be able to obtain the best voltage vector.Compared with the FCS-MPDPC,the proposed control strategy reduces the control complexity and computational burden while achieving good dynamic and steady-state performance.In addition,this thesis also researched on the grid-connected P-DPC under unbalanced network conditions.If the conventional control strategy is still applied under unbalanced grid voltages,the grid-side current harmonic content would be high and the active and reactive power fluctuation would be large.To this end,this thesis adopts a novel instantaneous power theory and redefines the instantaneous reactive power.Based on the novel instantaneous power,DP-DPC and FCS-MPDPC under unbalanced grid voltages are proposed.These two control strategies can achieve fast dynamic responses and obtain good steady-state performance under balanced grid voltages condition,and can work normally under the condition of unbalanced grid voltages.In order to verify the effectiveness of the proposed P-DPCs,an experimental platform for two-level three-phase VSR is established,including the control board with the main control chip TMS320F28335,drive circuit,signal sampling and conditioning circuit.Besides,the algorithm programming is written.The effectiveness of the proposed P-DPCs are confirmed by both simulation and experimental results.