This paper proposes a novel control method for accurate sharing of load current among the Distributed Energy Resources (DER) and high power quality operating in islanded acmicrogrids. This control scheme is based on hierarchical structure comprising of decentralized primary controllers and a centralized secondary controller. The controllers in the primary level use GPS timing technology to synchronize their local time with a common time reference. In this context, proportional current sharing is achieved by adjusting the reference voltage of each DER unit according to a voltage-current (V-I) droop characteristic. The droop coefficient, which acts as a virtual resistance, is adaptively changed as a function of peak current. This strategy not only simplifies the control design but also enables faster dynamics and higher accuracy of current sharing especially at high loading conditions. The secondary controller produces compensation signals at fundamental and dominant harmonics to improve the voltage quality at a sensitive load bus.Experimental results are presented to validate the efficacy of the proposed method.
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