Communication-free equivalent grid impedance estimation technique for multi-inverter systems

Interactions between grid-connected inverters bring major problems, such as increased harmonic distortion and instability. Furthermore, as the existing literature on inverter-based online grid impedance estimation mainly focuses on one inverter unit, the effects of the interactions on the estimation accuracy have been neglected. This article initially investigates how the interactions among the grid-connected inverters impact the accuracy of the estimated fundamental and wideband grid impedances. Afterward, a communication-free approach for multi-inverter systems is proposed to avoid the possible interactions between the inverters while estimating the equivalent grid impedance. Hence, accurate estimation results are ensured. The presented method is implemented in the local controller of each inverter, relying only on local measurements. Thus, it is more practical as coordinating different communication protocols between inverters is challenging. Furthermore, it is a generic approach as it can be used for single-/three-phase inverters and for implementing fundamental/wideband impedance estimation algorithms. The proposed strategy also minimizes the power quality issues as it is a trigger-based approach. The simulation and experimental results validate the effectiveness of the proposed communication-free strategy to accurately coordinate impedance estimation in multiple grid-connected inverter systems.