Analysis of active impedance characteristics and harmonic deterioration of multiple grid connected inverters considering nonlinear factors

The harmonic problems caused by non-linear factors of the grid connected inverter (GCI) system are more complicated, including both non-characteristic harmonics emitted by the dead-time and the changes in harmonic impedance characteristics. Harmonics interact with the changing impedance, and even cause harmonic amplification and resonance. The harmonic deterioration of the multiple GCIs system is serious. To analyse the mechanism and way of harmonic deterioration in grid-connected system caused by nonlinear factors, the active impedance models of single inverter and multiple GCIs system including dead-time effect and digital control delay are established first. In view of this, the influence mechanism of non-linear factors on system stability is explored. The improved modal analysis method is used to traverse the network series parallel resonance caused by nonlinear factors. The results show that both the dead-time effect and digital control delay reduce phase margin of the system, resulting in the resonant frequency shift and the resonant peak increase. When the harmonic excitation source interacts with the complex network under the influence of nonlinear factors, it will lead to further deterioration of harmonics. Finally, based on the MATLAB and RT-LAB hardware-in-the-loop simulation platforms, a multiple GCIs system model is built to verify the correctness of the established active impedance model and harmonic deterioration analysis.