Adaptive Optimal Control for Parallel Grid-connected Inverters with LCL Filters

With the increasing penetration of renewable energy sources into modern power system especially in a microgrid form, power inverters are commonly employed as the interface to the utility grid. LCL-filters are commonly used in interfacing inverters, in order to reduce the harmonics around the switching frequency and its multiples. However, unlike the single gridconnected inverter system where the resonance frequency is mainly fixed by the inverter output LCL filter parameters, the parallel-inverter-based grid-interactive power system presents a more challenging picture where inverter interactions will excite complex resonances at various frequencies. In this paper, an adaptive optimal feedback control is proposed to address system uncertain multi-resonance problem in the parallel-connected inverter system with LCL filters. The proposed adaptive control will force the inverter by modifying state-feedback optimal control law adaptively to follow the desired system response, thus significantly improve the grid current quality under various grid conditions. Experimental results have verified the performance of the proposed adaptive control.