Dynamics of a microgrid supplied by solid oxide fuel cells

The paper presents a model for a solid oxide fuel cell (SOFC) stack operating at relatively low pressures. The model incorporates the electrochemical reaction dynamics and the major voltage losses in SOFCs, but is not concerned with the thermal dynamics. A DC-DC boost converter interfaces the SOFC stack to a DC bus, where an ultra-capacitor provides energy storage. The paper presents a control system for the converter which regulates the DC-bus voltage along with the fuel cell current. A control strategy is proposed for the inverter that interfaces the SOFC plant to the AC grid. The controller regulates the grid-side voltage and the active power delivered to the grid, and takes into account the phase-locked loop (PLL) dynamics. The results of an example in which two SOFC plants provide power to a microgrid are presented. The simulation includes disconnection from the main grid, autonomous operation, and re-synchronization with the main grid.