HIERARCHICAL CONTROL OF AC MICROGRIDS

Microgrids are a group of localized electrical resources mainly using renewable resources as a main source of power, which can operate independently or in collaboration with utility grid. When connection of a microgrid is concerned, switching from an islanding to grid-connected mode is always a difficult task for a microgrid mainly due to transients and mismatching in synchronization. Hierarchical control structure of a microgrid eradicates this issue by separating the control structure in multiple levels. This thesis explains different levels of hierarchical control strategies, which constitute primary control, secondary and tertiary control. The primary control is based on droop control including output virtual impedance, secondary control performs restoration of voltage and frequency performed by primary and tertiary control maintain the power flow between the micro grid and external utility. In first step, this thesis covers the technical overview of traditional control methods of power converters and then the latter part consists of detailed description of all three levels of hierarchical control with synchronization and power flow analysis. Various types of primary controls, like with and without communication, and improvements to droop control are discussed and compared. In the end, concepts explained in previous chapters, are done in practice and simulated results are discussed.