Analysing the Damping of Grid-Connected Inverter by Applying Impedance-Based Sensitivity Function

Stability issues have emerged in the grid interfaces of power electronics when the grid impedance is high or multiple devices are connected in parallel. Impedance-based stability criterion has demonstrated high applicability in the stability assessment, as the analysis can be performed based on the terminal impedances of the grid and the converter. Typically, impedance measurements are required to obtain the terminal impedances. However, extracting the stability margins and predicting the system dynamics from the measured impedances typically requires complex methods, such as transfer function fitting. This work proposes an extension to the impedance-based stability criterion, where the critical system damping and the critical resonant mode are extracted from the impedance data. An impedance-based sensitivity function is constructed from the terminal impedances, and the system damping factor is calculated from the sensitivity function. A second-order transfer function is constructed from the obtained damping factor and resonant frequency, which captures the critical resonant mode of the system. The method is validated in experimental stability analysis of a 2.7 kW grid-connected three-phase inverter, where the method accurately predicts the resonant dynamics of the system when the stability margins are low.