Closed-loop Control Of Three-level Converter Based On Optimized Modulation

With the development of power electronics technology and the improvement of hardware level,various modulation theories and control methods have made considerable progress.Compared with the traditional two level converter,the three level converter has more advantages in medium and high-power applications.Three level converter have the lower switching frequency and current THD.Therefore,it has been widely used in medium and high-power applications.Experts and scholars have also done a series of researches on the control method and modulation technology of three level PWM converter.As a new control theory,model predictive control(MPC),has received extensive attention and research because of its simple control method and good control performance.As a kind of optimal modulation,the selective harmonic elimination PWM(SHEPWM)has also been applied in medium and high-power applications gradually.This paper focuses on the high quality closed-loop control performance of three level PWM converter in medium and high power field.Besides,reducing the low harmonic and the switching frequency of the converter is also another important goal.In order to reduce the switching frequency,a control algorithm which combines model predictive control(MPC)with the selective harmonic elimination(SHEPWM)is studied in this paper.And a unified control model of model predictive current control and the selective harmonic elimination is established.By judging the state of the system,the weight coefficients in the cost function are changed to make sure the system track the the MPC current references when the system is in the dynamic and track the voltage references of the SHEPWM in the steady state.This can ensure a fast dynamic response and a low switching frequency during the steady state.Beside it can eliminate the low order harmonics.The methods of solving the SHEPWM equations are discussed in this paper and the effect of the selective harmonic elimination switching angles in the different initial values is verified by Matlab simulation and experiment.The SHEMPC control algorithm,which combines the MPC and the SHEPWM is proved to be effective for converter with resistive load and grid-connected inverte.In addition,in order to reduce the low order harmonic of the three level PWM converter,this paper also studies the characteristics of the sliding discrete Fourier transform(SDFT)and uses it to extract the low order harmonic in the converter voltage.Then,the the sliding discrete Fourier transform is combined with the model predictive control,put the SDFT into the cost function of the model predictive control can reduce the low harmonics in the output voltage and improves the control performance.The feasibility and effectiveness of the proposed method are verified by simulation and experiment.