Source-driven and resonance-driven harmonic interaction between PV inverters and the grid

Together with the rising share of renewable generation more and more modern inverters are installed in the electricity grid. Existing standards for the characterization of harmonic emissions turn out not be suitable anymore for future requirements; e.g. today's guidelines do not help to prevent detrimental phenomena like harmonic resonances in systems with high penetration of renewables. This paper starts with giving some theoretical considerations about the correlation between voltages, currents and impedances for a Thevenin equivalent description of an inverter-grid system. A new method of Thevenin modelling based on sets of independent measurements is presented. Finally the impedance-based stability criterion will be applied to a real measured resonance in a 44 MW PV power plant. In conclusion the paper discusses methods, which are suitable to get a better understanding of the harmonic behavior of inverters, which can help to avoid power quality problems in advance.