Control and stability analysis of islanded microgrids based on inner control loops approach

Islanded microgrids are developed nowadays as a new model for the generation, consumption, and supply of electrical energy and services in remote areas in an efficient, sustainable, flexible, and environmentally friendly manner. Among others, they are considered as good technical solutions for the electrification of developing countries, especially in (but not limited to) rural areas. However, the stable operation of islanded microgrids, which consist mainly of renewable generation units with different and non-predictable characteristics, remains a major challenge. This is due to the power electronic interfaces used by the renewable distributed energy sources connected into the microgrids, and the small size of microgrids. Adapted control strategies are required to ensure the integrity and stability of islanded microgrids and to maintain the magnitude and frequency of the system voltage within the allowable range. This thesis proposes a non-vertical hierarchical control structure composed of the level-zero control and level-one control. Level-zero control is the closest control level with the fastest dynamics. Its purpose is the close control of the distributed energy sources comprising the microgrid so as to maintain their operating point. The proposed level-zero control structures allow an inverter-based source to be controlled as a constant current source in grid-following inverter mode injecting controlled active and reactive current into the microgrid and, as a constant voltage source in grid-forming inverter mode controlling microgrid voltage magnitude and imposing the system frequency. The level-one control is performed through a distributed hybrid control approach without communication allowing the control of the voltage magnitude and frequency, and power sharing within islanded microgrids powered by multiple distributed energy sources. To accomplish these tasks, the proposed level-one control strategy implements inverters in grid-forming control mode and grid-following control mode. The proposed control strategy decouples power-sharing control from the references of the voltage and current applied to the inner voltage and current control loops. This allows the inverters with the fastest dynamics to be most closely controlled. In addition, the proposed level-one control strategy is more efficient than the conventional droop control approach, for performing instantaneous power sharing independently of the frequency control and output impedance of the inverter. The active and reactive power are adjusted with respect to the control role of each interface of the distributed energy sources connected to the microgrid. The dynamic response of the proposed hierarchical control structure is analyzed through small and large disturbances using eigenvalues analysis and time-domain simulations, respectively. The obtained simulation results prove the effective performance of the proposed control strategy.