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A battery storage system connected to a micro-grid through a Voltage Source Inverter (VSI) can have at least two operating modes (dual-mode). In the first one the VSI is connected to a synchronous generator which could be considered as a large capacity grid or infinite busbar. The second operating mode is obtained when the VSI is feeding an isolated load and several small micro-sources. Therefore, for a proper operation of this system, the VSI has to be controlled in order to operate in the grid-connected or stand-alone modes, with seamless transfer between them. This paper proposes a master-slave control strategy for the main energy storage VSI operating in a microgrid. The proposed control strategy achieves seamless transfer between these two situations with minimal communication requirements between the VSI and the micro-grid controller, and allows the VSI to operate in parallel with virtually any type of generator or inverter. The control scheme is formulated for a micro-grid operated using conventional droop strategy. A control strategy is proposed for each of the VSI control levels. The control strategies have been experimentally tested in a full practical micro-grid implemented in a small village located in the Chilean Andes. Experimental results are presented and analyzed in this paper.