Advanced Islanded-Mode Control of Microgrids

This thesis is focused on modeling, control, stability, and power management of electronically-interfaced distributed energy resource (DER) units for microgrids. Voltage amplitude and frequency regulation in an islanded microgrid is one of the main control requirements. To that end, first a mathematical model is developed for an islanded DER system and then, based on the developed model, amplitude and frequency control schemes are proposed for (i) balanced and linear loads and (ii) unbalanced and nonlinear loads. The proposed control strategy for unbalanced and nonlinear loads, utilizes repetitive control scheme to reject the effects of unbalanced and/or distorted load currents. Moreover, a new approach is proposed to maintain the effectiveness of the repetitive control under variable-frequency operational scenarios. The thesis also presents an adaptive feedforward compensation strategy to enhance the stability and robustness of the droop-controlled microgrids to droop coefficients and network uncertainties. The proposed feedforward strategy preserves the steady-state characteristics that the conventional droop control strategy exhibits and, therefore, does not compromise the steady-state power shares of the DER systems or the voltage/frequency regulation of the microgrid. Finally, a unified control strategy is proposed to enable islanded and grid-connected operation of DER systems, with no need to detect the microgrid mode of operation or to switch between different controllers, simplifying the control of the host microgrid. The effectiveness of the proposed control strategies are demonstrated through time-domain simulation studies conducted in the PSCAD/EMTDC software environment