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The research presented in this paper shows that the stability of pulse-width modulation (PWM) inverters is strongly related to the short-circuit power, and especially to the frequency dependent grid impedance (fGI) at the point of common coupling (PCC). Based on two different cases, it is shown that inverters can lead to unstable behaviour in a weak grid, independent of the nominal voltage level. With a set of field measurements, it is shown, that the fGI is highly influenced by the number and type of connected inverters. It is discussed that commercial, power-grid simulation tools are not able to predict this kind of instability. Finally, some recommendations are made to ensure a stable operation of distribution grids with a large number of inverter-based grid elements.

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