Pitfalls of Using Passivity Index to Guide Grid-connected Inverter Control Design in Low-frequency Region

The main purpose of this article is to elaborate on the pitfalls of using the frequency-domain passivity theories in grid-inverter interactions within the low-frequency range. It mainly covers the relationship between the passivity index and stability, considerations for control design guided by optimizing the passivity index, and issues with a negative infinite index. It is advised to exercise caution when applying passivity theory in the low-frequency range. These conclusions have been substantiated through numerical and experimental studies.