Passivity-Based Design of Passive Damping for LCL-Type Grid-Connected Inverters to Achieve Full-Frequency Passive Output Admittance

The frequency-domain passivity theory suggests that incorporating passive output admittance in grid-connected inverters (GCIs) can effectively mitigate system instability issues. This theory has been widely recognized and applied in design guidelines for active damping (AD) and passive damping (PD) methods tailored explicitly for LCL-Type GCIs. However, AD methods have a limitation in that they can only reshape the output admittance of GCIs to be passive (with a nonnegative real part) up to the Nyquist frequency. This limitation arises from the inherent characteristics of the digital control system. Similarly, existing passivity-based PD design cases primarily focus on addressing instability issues that occur below the Nyquist frequency. To overcome this limitation and achieve full-frequency passive output admittance, this article introduces a resistor-capacitor (RC) branch-based PD scheme. It provides a comprehensive design guideline that can be applied to different scenarios involving inverter-side or grid-side current control, regardless of whether AD methods are utilized. By incorporating this RC-PD scheme, the designed GCIs achieve full-frequency passive output admittance, effectively preventing potential harmonic oscillations both below and above the Nyquist frequency. Finally, comparative experimental results for different cases are demonstrated to verify the effectiveness and superiority of the proposed approach.