Multitask Fuzzy Secondary Controller for AC Microgrid Operating in Stand-Alone and Grid-Tied Mode

A common method for load sharing is using frequency and voltage droop controllers because there is no need to have communication links in the microgrid. However, the droop control strategy has a drawback, that is, it imposes voltage and frequency deviations, which can be fixed by adding a secondary control loop. On the other hand, if the microgrid operates connected to the grid, a tertiary controller for regulating the injected power must be added. This paper proposes a multitask fuzzy secondary controller (FSC) applied to AC microgrids acting only on the secondary control layer. The proposed controller allows stand-alone and grid-tied operations, regulating the voltage and frequency in the secondary and the active and reactive powers, simultaneously, without a tertiary controller. Stability analysis are presented showing that the FSC keeps the microgrid stable while operating within a proposed operation range. A microgrid simulation test bed based on power inverters is presented to validate the FSC operation and to compare to conventional hierarchical controller. The proposed FSC fixes the deviations in the same way as the conventional controller during the islanded operation tests and allows the microgrid to inject power into the grid.