Impact of grid impedance and their resonance on the stability of single-phase PV-inverters in low voltage grids

This paper studies the behavior of grid-connected single-phase photovoltaic inverters in low voltage grids. The interaction of the inverter control, the grid-side filter and the power grid can lead to harmonic instabilities. The term is used, since its origin is not depending on the active or reactive power at fundamental frequency but an interaction of the inverter control with the impedance of the power grid at frequencies in the harmonic range. In power electronics, the power grid impedance is typically assumed as an RL-equivalent so that a critical RL-combination above which the inverter becomes instable can be calculated by using the Nyquist criterion. Neglecting resonances in a power grid can result in an erroneous assessment of stability based on the Nyquist criterion. This is of importance since recent field measurements indicate that the origin of harmonic instabilities of PV-inverters is primarily related to resonances in the grid impedance. This paper studies the behavior of a single-phase PV-inverter based on a simulation model for different impedance conditions of the power grid to illustrate the impact of the grid impedance and its resonances. The results are compared with laboratory measurements of commercially available PV-inverters.