Load Sharing Control of Parallel Inverters with Uncertainty in the Output Filter Impedances for Islanding Operation of AC Micro-Grid

Parallel connection of inverter modules is a solution to increase reliability, efficiency and redundancy of inverters in Micro-Grid system. Proper load sharing among parallel inverters is a key point. The circulating current among the inverters can greatly reduce the efficiency or even cause instability of the system. In this paper, a control strategy for improving the load sharing performance in order to reduce the circulating current among parallel inverters in islanding micro-grid is proposed. The control strategy includes an estimator for filter parameters, which compensates for any possible uncertainty or drift of their value. Also a double loop feedback control, with an outer voltage and inner current control, is adopted that provides excellent operation at transient and steady-state conditions. Finally, the proposed control and estimation algorithms are confirmed through simulations of two 2kW parallel connected single phase inverters.