Hierarchical Frequency and SOC Control of Power Grids with Battery Energy Storage Systems

A novel approach to modeling of and integrating the state-of-charge (SOC) of a battery energy storage system (BESS) into the load frequency control (LFC) of power systems is proposed. By considering the SOC as a state variable in the statespace model of the system, a hierarchical frequency and SOC control scheme is introduced. On top of the primary frequency control (PFC), which is the lowest hierarchy level, the SOC signal is used as the input to a novel primary charge controller (PCC) to prevent the BESS from discharging/charging beyond minimum/maximum allowable ranges. In the middle level of the hierarchy, the secondary frequency control (SFC) loop is augmented to restore the system frequency to its nominal value when the PCC is active. At the top level of the hierarchy, a novel secondary charge controller (SCC) is designed to charge the BESS to its maximum allowable range so that it can continue to contribute to frequency control. System stability is studied using Popov/circle criterion. It is also shown that with multiple distributed energy storage units, following the SOC-balancing principle and by sharing SOC information among the units, only one BESS shall be equipped with the proposed primary and secondary charge controllers. Effectiveness of the proposed hierarchical control in providing fast and stable frequency response under different load and generation conditions is validated through hardware-in-the-loop (HIL) studies.