Frequency Couplings in Impedance Modeling of Voltage Source Converters

A grid-connected voltage source converter (VSC) system may be unstable when the grid is weak. Sequence impedance has proven to be an effective tool for analyzing such problems. As a small signal analysis method, sequence impedance can be obtained by calculating the current response to a voltage perturbation at the same frequency. However due to the phase locked loop (PLL) and dc link voltage controller, the current response at a different frequency from voltage perturbation may be noticeable, which means there exist frequency couplings in the impedance modeling. This paper proposes an impedance model of VSC considering frequency couplings caused by these two factors and analyzes how these factors affect the impedance. The proposed model and analysis results are verified by simulation.