Abstract

Fuel cell systems appear as a good alternative for stationary and portable power generation. This is mainly due to their high energy conversion efficiency and simple and quiet operation. Since the output of the fuel cell is low voltage DC, fuel cell systems supplying power to AC loads are usually constructed connecting the fuel cell terminals to a DC-DC converter and the output of this converter to an inverter (Fig.1). It can be shown that in such a system under normal single phase or un-balanced three phase operation a large second harmonic ripple current is produced in the inverter DC-link. This current in turn is mirrored to the input terminals of the DC-DC converter and amplified according to its voltage gain. Therefore the current drawn from the fuel cell stack is typically not purely DC and has a large second harmonic (120 Hz in 60 Hz grids) current ripple. [1-8]. On the other hand, fuel cells have an internal impedance which is function of loading and frequency, and characterized by having a larger magnitude for lower frequencies (< 1kHz). Thus if the stack is connected to a power conditioner in order to supply AC loads (Fig. 1) its operation is affected due to the presence of a large second harmonic current component resulting in reduced energy conversion efficiency, additional heating, and reduced power availability (up to 20%). [2-4] To mitigate this issue it is required to eliminate or reduce the magnitude of the low frequency current component circulating through the fuel cell stack. Traditionally, the converter topologies of choice used to construct the DC-DC converter in the power conditioning unit are of the voltage source type which require extensive filtering either passive or active to achieve this goal. Instead, in this paper the use of current source topologies is investigated; which as will be shown offers numerous advantages.