Stability analysis and control of medium-voltage micro-grids with dynamic loads

The concept of micro-grid is gaining widespread acceptance in distributed power networks. Medium-voltage (MV) micro-grids will be subjected to high penetration level of dynamic loads (e.g. line-start induction motor (IM) loads). The highly-nonlinear IM dynamics that couple the active power, reactive power, voltage and supply frequency dynamics challenge the stability of MV droop-controlled micro-grids. However, detailed analysis, and more importantly, stabilization of MV micro-grids with IM loads are not reported in current literature. To fill-out this gap, this paper presents the stability analysis and stabilization of MV droop-controlled micro-grids with IM loads. The proposed model accounts for the impact of supply frequency dynamics associated with the droop-control mechanism to accurately link the micro-grid frequency dynamics to the motor dynamics. The complete small-signal model is used to assess the impact of the IM dynamics on the micro-grid stability as compared to the static load case. To stabilize the micro-grid system in the presence of IM loads, a two-degree-of-freedom active damping controller is proposed to stabilize the newly introduced oscillatory dynamics.