Hierarchical control strategies in AC microgrids

Hierarchical ac microgrid control strategies are presented in this paper. All operating modes of ac microgrid are also discussed and ranked. There are two operating mode in an ac microgrid: grid-connected mode and islanding mode. Grid-connected mode is operated under utility grid connected with the ac microgrid whereas islanding mode is operated without the utility grid. Normally, energy conversion devices such as a dc-dc converter, an ac-ac converter and an inverter have a local control. For instance, a solar inverter will have a MPPT control mode; however, if the ancillary service for reactive power compensation due to over voltage is required, this inverter control mode will change from MPPT mode to constant voltage mode. Meanwhile, other devices can reduce supplied power to the grid to stabilize grid voltage. Therefore, hierarchical control strategy of ac microgrid is necessary to control and stabilize voltage and frequency in the microgrid. Hierarchical control can also offer high efficiency and more degree of freedom energy management in ac microgrid. Hierarchical control proposed in this paper consists of four ranks: current and voltage loop control for regulating voltage and power in each distributed generation (DG), primary droop control, secondary droop control and energy management and power flow control between ac microgrid and utility grid. The proposed hierarchical ac microgrid control can explain the relationship and interaction among control ranks. This can lead to a better understanding and design for ac microgrid applications.