Seamless Transfer Scheme With Unified Control Core for Paralleled Systems

In distributed generation, grid-connected and islanded modes are essential, but usually they have different control cores to satisfy their respective control targets. Transferring between the cores may therefore lead to prominent disturbances, if the cores are complex and noticeably different in structures. To introduce seamlessness, some techniques have been recommended, but they usually lack unification of control blocks for the front dc-dc converter and rear dc-ac inverter. Moreover, when connected to the grid, some seamless techniques do not control the grid current directly. Hence, its dynamics may be slow, in addition to distortion caused by grid-voltage harmonics. When islanded, most techniques also use two control loops to regulate a drooped voltage across local loads. This double-loop structure works fine, but it has more state variables to change during a transfer of modes, which may not be as seamless. An alternative technique has thus been proposed here, whose purpose is to identify a simple unified control core, which will remain unchanged during the mode transfer. The transfer may then be viewed as a normal change of power reference, instead of an internal change of control structure. Hence, seamless operation may be ensured, as it is tested in simulations and experiments. These tests have additionally shown that the proposed scheme can regulate the grid current directly when connected to the grid,
without compromising the droop-voltage regulation when islanded.