Modeling and analysis of a micro-inverter configuration for high power phosphoric acid fuel cell application

In this paper a micro-inverter configuration for utility and commercial scale fuel cell (FC) power systems is proposed. A conventional system uses multiple FC stacks connected in series and is interfaced to the utility grid through a centralized inverter. This system suffers from disadvantages such as the FC current being limited by the weakest stack and the requirement for high voltage semiconductor devices under light loads, due to the voltage vs. power characteristics of FCs. In the proposed configuration each FC stack is individually connected to a micro-inverter. Each micro-inverter contains an off-the-shelf 3-phase inverter fed by a three-level boost converter with four interleaved converters termed as stages. This new configuration enables independent operation of the FC stacks leading to increased energy processing and wide power range operation without the need for high voltage semiconductor devices. The interleaved three-level boost converter leads to increased efficiency through stage-shedding, depending on load level. The topology also makes FC systems more modular in design and control. This paper discusses the operation, modeling and design of the proposed topology for a 100 kW FC stack. Simulation results demonstrate that this topology can be attractive for commercial fuel cell power plants. A scaled-down laboratory prototype was designed and experimental results are also provided in this paper.