Voltage source converters are used for grid connection of photovoltaic energy

wind energy and reactive power compensation in the renewable energy generation system. An emerging kind of oscillation has been observed due to the interactions among converters and AC power grids. In this case

the impedance model analysis has become one of the most promising methods for solving this type of oscillation. However

most of the existing impedance models are focusing on the high-frequency dynamics of converter

while neglecting the impacts of converter outer-loop control and frequency coupling. Therefore

these models are inaccurate when analyzing the medium and low frequency dynamic characteristics (such as the subsynchronous oscillation) of converters. To overcome these defects

this paper proposes a frequency-coupled impedance model of voltage source converter. Both outer-loop control and complementary frequency coupling are considered in this model. And the correctness of the model derivation is verified by the time-domain simulation and identification of detailed electromagnetic model. Based on the proposed impedance model

the overall impedance model of the grid-connected system is further derived to evaluate the system stability. The time-domain simulation has also demonstrated the effectiveness of the frequency-coupled impedance model in the medium and low frequency oscillation analysis.