Coordinated Primary and Secondary Frequency Support between Microgrid and Weak Grid

Dispersed wind power connected to the weak grid may cause the frequency instability. In this paper, a hierarchical controller applied to a microgrid (MG), including wind turbines (WT) and battery units (BU), is proposed to provide a coordinated frequency support to a weak grid by adjusting the tie-line active power flow according to the frequency-grid requirements. The coordination between MG local and central controllers provides the following features. 1) In case of required grid-frequency, the MG tie-line power flow will be controlled to be constant in each dispatching time interval. 2) In case of under-frequency, the coordination between WT virtual inertia and BU controllers will be used to participate in primary frequency regulation (PFR). Then, BU supplies power according to the secondary frequency regulation (SFR) commanded by the main-grid dispatch center. 3) In case of over-frequency, BU absorbs power to reduce the tie-line active power for PFR purposes. After that, the SFR uses pitch control coordinated with the battery charge control. A stability analysis model is established to deal with several transitions among different operation modes and the interaction between the weak grid impedance and the MG output impedance. Simulation results are presented to validate the proposed approach.