Autonomous operation of hybrid AC-DC microgrids

This paper investigates on the active power sharing of an autonomous hybrid microgrid. Unlike existing microgrids which are purely ac, the hybrid microgrid studied here comprises a dc and an ac sub-grid, interconnected by a power electronic interface. To avoid complexity and save cost, the power interface should preferably be a simple traditional six-switch converter, capable of processing energy flow efficiently through only a single well-proven conversion stage. With this simple configuration, the main challenge then is to manage the power flow among all the sources distributed throughout the two sub-grids, which certainly is tougher than previous efforts developed for only an ac or a dc microgrid. This wider scope of control has not yet been investigated, and would certainly rely on the coordinated operation of dc sources, ac sources and interfacing converter. For the latter, it appears as a load to one sub-grid, and source to the other, inferring that the conceptualization of demand-droop control is important, in addition to the usual supply-droop control. Combining the two droop concepts then leads to the formulation of an appropriate normalization technique for merging system information sensed from both sub-grids, before the merged quantity is used for deciding on a proper power flow control action. The proposed concepts have already been tested in simulation in this paper, and would subsequently be tested in experiment for the four scenarios that the hybrid microgrid will operate in. Upon proven, the hybrid microgrid concepts are likely to benefit renewable or nonconventional distributed generation, where both ac and dc sources are present.