Enhanced Control Strategy for FESSs: A Derivative-Free Fast Terminal Sliding Mode Approach with Reduced-Order Error based ESO

Flywheel energy storage systems (FESSs) are primarily employed for smooth and efficient DC voltage regulation, but their performance can be compromised by rapid speed variations and DC-side load disturbances, which challenge conventional DC-link voltage control methods. To tackle this issue, this paper presents a derivative-free fast terminal sliding mode control (DFTSMC) strategy, enhanced by a reduced-order error based extend state observer (REESO). First, the REESO is designed for estimating the total disturbance in voltage control loop. Then, a low-pass filter and new reaching law are introduced to the proposed DFTSMC for mitigating sliding mode control chattering. Finally, the proposed REESO is integrated into the DFTSMC for estimating the total disturbance and reducing the sliding mode gain. Meanwhile, the voltage error differential signal in DFTSMC is calculated by the REESO for addressing the noise issue caused by differential calculation of traditional TSMC. Compared with the existing observer based on TSMC methods, the proposed scheme is an error-driven method, and can achieve fast and robust voltage tracking with smaller chattering level. Thus, the proposed scheme can achieve satisfactory voltage control performance with fast dynamic, robustness and small chattering level. Experimental results from a 3.75-kW FESSs validate the effectiveness of the proposed control approach.