Improved Controller Design for a Microgrid Fuel-Cell Based Energy Storage System

Energy storage systems are one of the essential components of a renewable energy based microgrid. However, for a fuel-cell based energy storage systems, the constraints imposed by the internal dynamics of a fuel-cell must be taken into account by its power interface converter system. In particular, a fuel-cell requires a constrained rate-of-change-of-current because of its slow dynamic capability. This introduces a slew rate nonlinearity between the primary and secondary feedback loops within a conventional cascaded control system, which can adversely impact the transient performance of a classical proportional integral (PI) controller. Hence such a controller cannot ensure safe operating conditions for a fuel cell, and may in fact damage it. This paper now investigates the design of an improved current regulated compensator for a fuel-cell energy storage system that takes the slew rate non linearity into account during microgrid load transients, while avoiding the need for more complex variable gain control system architectures. The resulting cascade control system for the fuel-cell dc-dc converter is implemented in the discrete time domain.