An active current ripple compensation technique in grid connected fuel cell applications

Fuel cell stacks represent a quite promising technology for the realization of small electrical generators for distributed generation. Key advantages of fuel cells over combustion based generators are the absence of polluting emissions and a high efficiency even on small size systems and partial loads. A typical application of distributed generators is in the residential field, where they are connected to single phase AC grids. In these cases a power unbalance is generated because the power drawn by the grid features a large ripple at twice the fundamental frequency, while the power produced by the stack is essentially constant. A large capacitor bank is then used to solve such a power unbalance, that however causes a ripple on the stack output current. This ripple is shortens the fuel cell lifetime, to worsen the stack efficiency and to reduce the available peak power. Active current ripple reduction techniques have been proposed in the past exploiting expensive active filters. An alternative approach is proposed in this paper where an active filter is integrated into the converter, without introducing extra power devices. Moreover, the auxiliary power supply is integrated in the main converter partially compensating the power unbalance. According to the proposed approach the size of the electrolytic capacitor bank can be largely reduced, thus improving the reliability, while lowering costs and size.