Frequency-Adaptive Filtering of Low-Frequency Harmonic Current in Fuel Cell Power Conditioning Systems

Based on one of the most researched multiinput dc-dc converter topologies for renewable energy systems, the multi-input dual-active bridge (DAB) dc-dc converter, the effectiveness of harmonic current absorption by the energy storage branch in fuel cell power conditioning systems is critically evaluated. The closed-loop output impedances of the converter under single-voltage-loop and dual-loop controls are derived and compared. It is shown that both control strategies can effectively reduce the converter's closed-loop output impedance, thus favoring the flow of harmonic current and prevent it from being drawn from either the fuel cell branch or the dc-link capacitor. However, as shown by experimental results, the use of conventional PI control alone still produces noticeable voltage ripple on the dc voltage bus due to harmonic current being drawn from the dc-link capacitor. Proportional-resonant control is proposed to effectively compensate for the dc-bus voltage variation by generating an extremely low impedance path for harmonic current flow at specific frequency. An analog-based frequency tracking circuit is further proposed to adjust the resonant frequency for compensating the effect of harmonic frequency variation.