Contents I. Introduction
Grid-connected inverters are commonly found in modern power systems [1], where wide-band oscillation issues arising from the interaction between inverters and the grid have attracted increasing attention [2]. Passivity theory is a widely used and intuitive tool for inverter control design, and stability assessment of the system [3], [4], especially when there exists uncertainties in the grid impedance. From the energy perspective, a passive component only dissipates or stores energy without generating it. Examples of passive components include resistors, inductors, and capacitors. When all subsystems within a system are passive, the system is stable, regardless of the complexity of the interactions among the subsystems [5]. Therefore, achieving passive grid-connected inverters through reasonable design is a highly desirable goal that prevents wide-band oscillations caused by adverse inverter–grid interactions. In the frequency domain, based on the different impedance representations, the passivity degree of an inverter at a specific frequency can be quantified by the real part value or the passivity index of its input admittance [5], [6].
