Analysis and Mitigation of Low-Frequency Instabilities in Autonomous Medium-Voltage Converter-Based Microgrids With Dynamic Loads.

The microgrid concept is gaining widespread acceptance in near-term future power networks. Medium-voltage (MV) microgrids can be subjected to a high penetration level of dynamic loads [e.g., induction motor (IM) loads]. The highly nonlinear IM dynamics that couple active power, reactive power, voltage, and supply frequency dynamics challenge the stability of MV droop-controlled microgrids. However, detailed analysis and, more importantly, stabilization of MV microgrids with IM loads, are not reported in current literature. To fill in this gap, this paper presents integrated modeling, analysis, and stabilization of MV droop-controlled microgrids with IM load. A detailed small-signal model of a typical MV droop-controlled microgrid system, which is based on the IEEE Standard 399, with both dynamic and static loads is developed. The proposed model accounts for the impact of supply frequency dynamics associated with the droop-control scheme to accurately link the microgrid frequency dynamics to the motor dynamics. To stabilize the microgrid system in the presence of IM loads, a 2-degree-of-freedom active damping controller is proposed to stabilize the newly introduced oscillatory dynamics. The proposed supplementary active damping controller does not interfere with the steady-state performance, and yields robust control performance under a wide range of droop parameters and robust damping performance at small- and large-signal disturbances. A theoretical analysis, simulation, and experimental results are presented to show the effectiveness of the proposed control scheme.