Open-Loop RHP Poles Issues in Online Stability Monitoring for Microgrid

Impedance-based stability analysis has been widely adopted for ac grid. Online stability monitoring, which requires a real-time assessment of the system stability margin, is advantageous to perform crucial tasks in the grid. Most related studies assumed no right-half-plane (RHP) poles in the minor loop gain to apply many popular criteria such as the forbidden-region-based criterion. However, this paper found that RHP poles may appear in a microgrid's open-loop impedance if its PV power or PV converters' phase-locked-loop (PLL) bandwidth exceeds a certain value. With or without open-loop RHP poles changes the characteristic loci dramatically in the complex plane, as such, many past stability-margin criteria become not applicable. This paper thus proposes other potential stability margin indexes for online stability monitoring applications.