Study On Discrete Modeling And Bifurcation Phenomenon Of Three-Level Grid-Connected Converter

Three-level grid-connected converters are often used as interfaces between new energy generation and large power grids.Due to their inherent nonlinear characteristics,bifurcation and chaos may occur.They would lead the three-level converters to generate oscillation in lower frequency,or to output unpredicted value,or to output suddenly zero,even to collapse.In order to ensure the stable operation of the system,it is much significant to study and suppress the bifurcation phenomenon in the diode-clamped three-level grid-connected converter.Firstly,this paper analyzes the working principle of diode clamped three-level grid-connected converter,introduces SVPWM modulation algorithm and grid-connected current decoupling control.Based on the stroboscopic mapping method,a discrete model of the system is established.According to the discrete model,the Jacobian matrix and the bifurcation diagram method are used to analyze the high frequency bifurcation phenomenon in the system.According to the influence of controller parameter kp,grid side inductance L and switching frequency f on the bifurcation boundary,the system parameter is optimized.Secondly,in order to suppress the bifurcation phenomenon in the system and expand the system parameter stability domain,a washout-filter controller suitable for diode clamped three-level grid-connccted converter is designed,and a discrete based on the washout-filter controller system is established.The model gives the selection of the washout-filter controller parameters in the Jacobian matrix and the bifurcation diagram.Finally,a closed loop system of diode clamped three-level grid-connected converter is built in PSIM simulation software.Jacobian matrix and the bifurcation diagram is implemented in MATLAB.The correctness of the discrete model is verified and the bifurcation boundary is obtained.The effectiveness and rationality of the designed control algorithm are proved.Based on the 7.5kW experimental platform of diode clamped three-level grid-connected converter,the system of bifurcation control software is designed.The theoretical analysis in this paper is verified by experiments.