Power-Smoothing Control Strategy for a Slope-Track Gravity Energy Storage System in Wind-Farm Applications

With the increasing penetration of renewable energy, smoothing wind-farm-level power fluctuations at the point of common coupling has become an important requirement for maintaining grid stability. However, the dynamic power-tracking mechanism of large-inertia slope-track solid gravity energy storage systems remains insufficiently understood. This paper presents a theoretical and control-oriented study of a slope-track solid gravity energy storage system by establishing a low-speed-branch electromechanical coupling model, deriving its steady-state power-speed characteristics, and formulating several feedforward-feedback electromagnetic torque control laws. Pure feedforward control, power feedback, speed feedback, and a gain-scheduled speed feedback law are analyzed within a unified framework. The results show that, for a given power command and under sufficient torque-control authority, increasing the moving load or slope angle enhances the gravity-driven torque and reduces the required operating speed, acceleration, and cumulative displacement. This mechanism indicates that systems with heavier loads or steeper slopes have an advantage in smoothly responding to wind-power fluctuations at a fixed power scale. Frequency-response analysis further shows that the low-speed branch is suitable for slow and medium time-scale power smoothing, whereas rapidly varying commands remain constrained by the closed-loop response time and inertial correction terms. To demonstrate a finite-track implementation of the theory, a single-track multi-unit scheme with 30 standard load units is designed and numerically tested for wind-farm-side smoothing and slow-varying command tracking. This analysis provides a theoretical basis for prototype design and parameter selection before costly full-scale experimental validation.