Research on Stroboscopic Mapping Modeling and Bifurcation Characteristics of Grid-Connected Inverter Considering PWM Saturation

For the grid-connected inverter system, the sustained constant-amplitude oscillations often occur. At this time, the grid-connected current undergoes oscillation, and the PWM process appears saturation nonlinearity (i.e., overmodulation). However, the existing modeling methods cannot accurately model this hard nonlinearity, thereby explaining the phenomenon of grid-connected current oscillation. In this article, the step function is used to accurately represent the PWM saturation nonlinearity, and the accurate stroboscopic mapping model of three-phase grid-connected inverter system is established, and it is strictly proved that in a fundamental period, although the switching state type, sequence, and working time are different in any switching period of different sectors, the expression of stroboscopic mapping model is same and unique. Based on the precise stroboscopic mapping model, the bifurcation characteristics of the system when PWM saturation occurs are analyzed using the bifurcation diagrams and the root locus of Jacobian matrix, the accurate bifurcation boundary is obtained, and revealing the essential reasons for grid-connected current oscillation and its relationship with PWM saturation. The correctness of the theoretical analysis is finally verified by simulation and experiment.