The stability theory based on sequence impedance model has been widely used in the stability analysis of grid-connected voltage source converter (VSC) systems. In order to describe the external impedance characteristics of VSC more accurately and improve the accuracy of stability analysis, this paper introduces the non-ideal characteristics of phase locked loop (PLL) and dc bus into VSC sequence impedance modeling. Firstly, the effects of these two factors on the impedance are analyzed, the results show that there exist frequency coupling terms caused by these two factors in the sequence impedance. Then, the sequence impedance model of VSC considering the frequency couplings is derived, and the accuracy of the model is verified by simulation. The effects of PLL parameters and dc link voltage controller parameters on the coupling terms are studied, the results show that the frequency coupling phenomenon is obvious in the low frequency range, and the coupling strength can be reduced by appropriately selecting parameters.
