This paper proposes the seamless mode transfer technique for the parallel operation of grid-connected inverters. To meet the microgrid sources, the inverter of distributed generator is defined as a voltage source under the islanded mode to supply the power to the full local load demand without connection to the grid. On the other hand, if the grid is restored from its pre-fault or the strategic islanding is no more necessary, the inverter changes the operation mode to the grid-connected mode. During this mode transfer between the grid-connected and islanded modes, the load voltage should be kept constantly to meet the power quality for the continuous operation of sensitive load and to increase the load lifespan. In this paper, the PR based seamless mode transfer method is adopted with considering two following conditions. Firstly, the seamless mode transfer sequence for three-phase inverter operation is studied. Secondly, the operation technique for parallel inverters is added under the islanded mode that the load power can be supplied from the DG based parallel inverters. In this paper, the design and control of parallel inverters using the droop control based indirect current control is analyzed in detail for both islanded mode and grid-connected mode, respectively. Finally, the validity of the proposed seamless mode transfer technique is verified though the PSiM simulation results.
