Grid-connected fuel cell system based on a boost-inverter with a battery back-up unit

In general, two power conversion stages are required when low-voltage unregulated fuel cell (FC) output is conditioned to generate AC power. In this paper, the boost-inverter topology that achieves both boosting and inversion functions in a single-stage is used as a building block to develop a single-phase grid-connected FC-system which offers high conversion efficiency, low-cost and compactness. The proposed system incorporates additional battery-based energy storage and a DC-DC bi-directional converter to support the slow dynamics of the FC. The single-phase boost-inverter is voltage-mode controlled and the DC-DC bi-directional converter is current-mode controlled. The low-frequency current ripple is supplied by the battery which minimizes the effects of such ripple being drawn directly from the FC itself. Moreover, this system can operate either in a grid-connected or stand-alone mode. In the grid-connected mode, the boost-inverter is able to control the active (P) and reactive (Q) power using an algorithm based on a Second Order Generalized Integrator (SOGI) which provides a fast signal conditioning for single-phase systems. Analysis, simulation and experimental results from a laboratory prototype are presented to confirm the validity of the proposed system.