Time and frequency-domain evidence on power quality issues caused by grid-connected three-phase photovoltaic inverters

The amount of grid-connected inverters has been growing steadily over the past decade due to increase in renewable power generation. These inverters have been reported to degrade power quality in the grid in areas where the amount renewable power generation is large. However, the reasons behind the power quality issues are not yet extensively reported in the literature. The output impedance of single and three-phase PV inverters has been reported to resemble a negative resistance over a frequency range which depends on the selected control scheme and control parameters. Negative resistance can cause impedance-based interactions which may lead to instability, especially, when the inverter is connected to a weak grid which has large inductance. The main contribution of this paper is the experimental results which clearly show that the inverter has a significant role on the reported power quality issues. The inverter is shown to generate harmonic, interharmonic or even subharmonic currents depending on the processed power, component sizing, control parameters and the grid impedance. The paper also demonstrates that these phenomena can be accurately predicted by applying the well-known Nyquist stability criterion.