LAUSR

To study the frequency coupling effect (FCE) of nonlinear inductors in the frequency domain, in this article, a multifrequency small-signal model is developed for the modified LCL-filtered grid-connected inverter. This model is realized by extracting sideband coupling units out of the harmonic state-space (HSS) model and superposing them to the perturbation frequency loop. Compared with the complicated reductant HSS model, the proposed multifrequency model provides an intuitive, efficient, and accurate way to describe the system. Besides, notions in classical control theory can be directly utilized with the multifrequency model. With this model, the current control loop of the inverter is studied. Besides, by introducing a simplified linear time-invariant (LTI) small-signal model (the FCE is ignored), it is found that the FCE effect is nonnegligible, especially in the frequency range around resonance points, which affects the stability of the differential-mode resonance (DMR) and common-mode resonance (CMR). The proposed multifrequency model accurately predicts the DMR and CMR stability, while the simplified LTI model fails. Furthermore, damping strategies are developed to suppress the DMR and CMR. Finally, simulations and experiments based on a 228 kW photovoltaic inverter are conducted to verify the analytical result.