Multiple-Time-Scales Hierarchical Frequency Stability Control Strategy of Medium-Voltage Isolated Microgrid

In this paper, an islanded medium-voltage (MV) microgrid placed in Dongao Island is presented, which integrates renewable-energy-based distributed generations (DGs), energy storage system (ESS), and local loads. In an isolated microgrid without connection to the main grid to support the frequency, it is more complex to control and manage. Thus, in order to maintain the frequency stability in multiple time scales, a hierarchical control strategy is proposed. The proposed control architecture divides the system frequency in three zones: (A) stable zone, (B) precautionary zone, and (C) emergency zone. In this way, dynamic stability control that copes with disturbances in short-time scale is implemented by microgrid central controller within Zone B and Zone C. Meanwhile, steady-state stability control to solve the peaks and valleys problem of loads and DGs in long-time scale is executed by the microgrid energy management system within Zone A. Furthermore, based on the developed complete small-signal state-space model, sensitivity analysis of the eigenvalues is conducted in order to reveal the dynamic stability margin of the MV microgrid, and to identify the proper range of the control parameters of Zone B. Theoretical analysis, time-domain simulation, and field test results under various conditions and scenarios in the Dongao Island microgrid are presented to prove the validity of the introduced control strategy.