Stability of grid-connected PV inverters with large grid impedance variation

Photovoltaic (PV) inverters used in dispersed power generation of houses in the range of 1-5 kW are currently available from several manufacturers. However large grid impedance variation is challenging the control and the grid filter design in terms of stability. In fact the PV systems are well suited for loads connected in a great distance to the transformer (long wires) and the situation becomes even more difficult in low-developed remote areas characterized by low power transformers and long distribution wires with high grid impedance. Hence a theoretical analysis is needed because the grid impedance variation leads to dynamic and stability problems both in the low frequency range (around the current controller bandwidth frequency) as well as in the high frequency range (around the LCL-filter resonance frequency). In the low frequency range the possible variation of the impedance challenges the design of resonant controllers adopted to mitigate the effect of the grid harmonic distortion on the grid current. In the high frequency range the grid impedance influence the frequency characteristic of the filter and the design of passive or active damping (to ensure stability) becomes more difficult. In this paper both topics are addressed and discussed with simulation and experimental results.