Multimodal Joint Frequency Regulation Control for Thermal Power–Hybrid Energy Storage Systems Under AGC Time-Sharing Assessment

In thermal power (TP) units equipped with hybrid energy storage (ES) systems, automatic generation control (AGC) command allocation and storage power control directly affect frequency regulation performance, assessment results, and operational sustainability. Existing strategies usually do not distinguish between AGC assessment periods and non-assessment periods. Therefore, ES devices may still charge or discharge when their output no longer contributes to the AGC assessment, resulting in unnecessary energy loss, reduced ramping capability, and poor state-of-charge (SOC) recovery. In addition, the available reserve capacity of TP units and the residual capacity of different storage devices are not fully utilized. To solve these problems, this paper proposes a joint frequency regulation control strategy for a thermal-hybrid ES system considering AGC time-sharing assessment characteristics and multimodal coordination. First, the AGC assessment process is analyzed according to the requirements of regulation rate, regulation time, and regulation accuracy. The response process of the thermal-hybrid ES system is divided into the initial response stage, acceleration stage, steady regulation stage, and assessment-ending stage. For upward and downward AGC command changes, an assessment-oriented optimal response trajectory is established. Based on the real-time output of the thermal unit, the overall frequency regulation reference command of the hybrid ES system is obtained. Second, a multimodal coordinated control strategy is developed. During the ramping stage, the operating mode is determined according to the SOC states of the battery and flywheel ES systems. The regulation command is allocated between the two devices according to their SOC margins, power characteristics, and charging or discharging constraints. Smooth correction factors are introduced to reduce excessive battery charging and discharging, while a nonlinear correction is used to prevent the flywheel from frequently operating outside its safe SOC range. During the non-assessment stage, a SOC recovery strategy is further designed by using the reserve capacity of the thermal unit and the remaining capacity of the storage devices. A case study is conducted using typical monthly AGC command data and actual response power data from a TP plant in Hubei Province. The rated capacities of the battery and flywheel are set to 2.0 MWh and 0.2 MWh, respectively. Compared with single-storage, full-compensation, proportional allocation, and partial multimodal control schemes, the proposed strategy achieves the best overall performance. The average regulation rate index, regulation time index, regulation accuracy index, comprehensive evaluation index, and AGC assessment success rate reach 1.99, 0.99, 0.99, 1.91, and 99%, respectively. The proposed strategy also obtains the highest net benefit of 12.023 million yuan.