On the stability analysis of multiple parallel inverters using the impedance multiplication effect

The aim of this paper is to analyze the stability of grid-connected photovoltaic (PV) generators when a large number of equally rated inverter modules is connected in parallel, as recently implemented in PV power farms. In grid-connected applications, the presence of parallel inverters results in the impedance multiplication effect, where the grid impedance seen by each inverter module is multiplied by the number of inverters. An existing stability analysis, based on a multi-input multi-output (MIMO) system, is compared with an equivalent and easier Norton-based impedance analysis. Moreover, the cancellation of the internal poles, associated to the internal converter stability, is investigated for the equivalent single inverter case. The proposed analysis has been verified by simulation and by using a 15kVA experimental setup. The obtained results confirmed the unstable mode predicted by the analysis.