Swinging Bus Operation of Inverters for Fuel Cell Applications With Small DC-Link Capacitance

For reliability reasons, the employment of small film capacitors instead of electrolytic ones is an interesting alternative for the dc-link in single-phase inverters for fuel cell applications. Due to the low capacitance that can be accomplished at an acceptable cost using this technology, there are large low-frequency voltage fluctuations (100/120 Hz and harmonics) in the dc-link caused by the double-frequency power transfer. By allowing these variations in the bus, the capacitor bank absorbs the current ripple from the inverter to avoid detrimental oscillations in the fuel cell. Traditional control strategies for inverters are usually designed to operate with nearly constant input voltage and are not able to effectively handle large (e.g., ) low-frequency input voltage fluctuations. This paper introduces the analysis of a swinging bus in the context of fuel cell standalone applications (i.e., voltage-source inverters) and proposes a nonlinear control approach to operate inverters with very large input voltage swing: the natural switching surface (NSS). Under the proposed scheme, the inverter presents excellent dynamic and steady-state characteristics, even at moderate switching frequency (e.g., 3.6 kHz). In order to illustrate the superior performance of the NSS, a comparison to a proportional-resonant (PR) controller is performed. Unlike the linear compensator, the NSS is able to reject the large bus voltage oscillations and achieve high-quality output voltage with low total harmonic distortion (THD). Simulation and experimental results are provided to illustrate the behavior of the swinging bus and to validate the NSS control scheme under the proposed demanding operating conditions.