Distribution Voltage Control for DC Microgrids Using Fuzzy Control and Gain-Scheduling Technique

Installation of many distributed generations (DGs) could be detrimental to the power quality of utility grids. Microgrids facilitate effortless installation of DGs in conventional power systems. In recent years, dc microgrids have gained popularity because dc output sources such as photovoltaic systems, fuel cells, and batteries can be interconnected without ac/dc conversion, which contributes to total system efficiency. Moreover, high-quality power can be supplied continuously when voltage sags or blackouts occur in utility grids. We had already proposed a "low-voltage bipolar-type dc microgrid" and described its configuration, operation, and control scheme, through experiments. In the experiments, we used one energy storage unit with a dc/dc converter to maintain the dc-bus voltage under intentional islanding operation. However, dc microgrids should have two or more energy storage units for system redundancy. Therefore, we modified the system by adding another energy storage unit to our experimental system. Several kinds of droop controls have been proposed for parallel operations, some of which were applied for ac or dc microgrids. If a gain-scheduling control scheme is adopted to share the storage unit outputs, the storage energy would become unbalanced. This paper therefore presents a new voltage control that combines fuzzy control with gain-scheduling techniques to accomplish both power sharing and energy management. The experimental results show that the dc distribution voltages were within 340 V +/- 5%, and the ratios of the stored energy were approximately equal, which implies that dc voltage regulation and stored energy balancing control can be realized simultaneously.