A photovoltaic inverter (PVI) may enter operational instability region based on grid supply conditions, such as the presence of voltage waveform distortions and high grid impedance, especially when the actual power output of the PVI is low. This paper considers evaluation and modelling of one commercially available 3-phase PVI, for which laboratory tests showed instability when it was operating with a low power output and connected to a distorted voltage supply. A full switching model of the considered PVI is developed using simulated and measured frequency-based impedance plots, which are also used to calculate approximate values of the PVI output filter, as well as the type and parameters of its control. Afterwards, the operational stability of the PVI is evaluated using impedance-based and Nyquist stability criteria.