Impact of SOFC fuel cell source impedance on low frequency AC ripple

A fuel cell is a non-ideal voltage source with a wide varying source impedance, depending on output power scale and operating conditions, such as operating temperature, fuel pressure, quality of fuel, etc. A stationary fuel cell power system that consists of a single-phase dc-ac inverter tends to draw an ac ripple current at twice the output frequency, and the peak-to-peak amplitude of the ac ripple at the fuel cell output depending largely on the source impedance. In this paper, the ripple current propagation path is analyzed, and its linearized ac model is presented. Relationship between the peak-to-peak amplitude of ac ripple and the fuel cell source impedance is studied using equivalent circuit model, and the model is verified via simulation and experimental results. To reduce ripple seen by the fuel cell, an advanced active ripple control technique is integrated into the dc-dc converter. The effect of the control technique on various source impedances is experimented, and the test results between open-loop and closed-loop control techniques are compared. The dynamic response of the entire fuel cell power system under load transient condition is tested to verify the stability of the controller. The results indicate that the controller originally designed for steady state ripple reduction ripple reduction remains stable during severe load transients.
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