Imbalance Power Sharing Improvement in Autonomous Microgrids Consisting of Grid-Feeding and Grid-Supporting Inverters

Abstract

Imbalance power sharing problem in autonomous AC microgrids (MGs) is usually addressed by inverter-interfaced distributed generation units (IIDGs) with reliable DC power supply operating in voltage control mode (VCM); while IIDGs with unpredictable DC source (i.e. solar, wind, etc.) are rarely considered as a potential origin of contribution. This paper introduces an imbalance power sharing role for such units considering that they are usually generating intermittent, unreliable, and erratic power. To cope with the problem, negative virtual impedance and voltage unbalanced factor (VUF) concepts are used and applied to an autonomous inverter- interfaced MG, including two grid-supporting battery energy storage systems (BESSs) and a grid-feeding photovoltaic (PV) system. Specifically, a proportion of negative-sequence output current is added to the control structure of grid-feeding PV system which is obtained from the product of output current and VUF transformed into the synchronous reference frame with negative fundamental frequency. This strategy enables us to use the remaining capacity of a PV system for imbalance power compensation. Moreover, proper reactive power sharing between grid-supporting BESSs is achieved using virtual impedance strategy. Simulation studies conducted in MATLAB/SIMULINK software verify the excellent performance of the proposed approach in improving imbalance power sharing amongst IIDGs. Furthermore, the VUF values at IIDG outputs are kept less than the maximum allowable amount of 3%.