Probabilistic Stability Analysis for Commercial Low Power Inverters Based on Measured Grid Impedances

This paper presents a probabilistic stability analysis of commercial inverters. The set of chosen inverters consists of common types of rooftop inverters for photovoltaic (PV)-system applications in the low voltage grid for low power applications below 10 kW. The main aim is to assess the stability according to the Nyquist criterion based on a comprehensive number of real measured impedances in public distribution grids. This provides an enhanced and more general approach for the assessment and the analysis of the behavior of individual inverter designs with regard to possible instabilities. The state of the art assessment that uses only one single hypothetical grid configuration has the problem which grid parameters to choose. This paper proposes a probabilistic approach covering a wide range of typical grid impedances, which allows a more general statement on grid compatibility of particular inverters and its possibly problematic frequency ranges. This provides a more comprehensive tool for inverter manufacturers to analyze the robustness of their own inverter designs. The paper proposes a first assessment index to quantify the "robustness" of an inverter. In addition, the paper evaluates the impact of the power level on the stability, since inverters operate usually at lower than rated power.