Grid-connected renewable hybrid system for uninterruptible DC load maintenance

Uninterruptible Power Supplies (UPS) are strongly requested when critical loads are considered. The main problem associated with the conventional UPS consists in the employment of battery banks, since they are not renewable and, when long autonomy is required, heavy and bulky. This work proposes a 1 kW UPS based on Renewable Energy Sources (RES) - Photovoltaic (PV) array, Wind Turbine (WT) and Fuel Cell (FC) - able to operate on grid-connected or stand alone mode. The power flow among the sources, load and utility is dynamically defined taking into account the dc bus voltage level and the operational mode. PV and WT work providing the maximum available power, on the grid-connected mode, or only the dc load demand power, on the stand alone mode. The utility operates consuming active power if the dc bus voltage is higher than the reference, or generating active power, otherwise. FC is activated only in the stand alone mode, if the dc bus voltage decreases to a critical value. Simulation results are presented in order to validate the concepts.