An Adaptive Controller of a Hybrid Storage System for Power Smoothing With Enlarged Battery Lifetime

Abstract

The volatility of grid-coupled photovoltaics can cause local voltage deviations, while the impact on frequency becomes obvious in isolated weak grids. Thus, a standalone battery is usually proposed for smoothing purposes. However, the frequent cycles and abrupt power variations shrink its life and impair control performance. To this end, this study introduces a controller for a hybrid system composed of a superconducting magnetic energy storage (SMES) and a battery. The proposed method establishes an idling zone for battery to eliminate its short-term activity, while SMES handles any power mismatch. The zone limits are dynamically adjusted in case of power balance detection, while an adaptive saturation is applied to them for maximal SMES utilization and minimal battery degradation. When SMES current deviates from this zone, battery operates with an adaptive ramp rate (i.e., di/dt) depending on the state of charge of SMES, to further optimize its life. Additionally, to prevent unnecessary power circulation among SMES and battery, supervisory control loops are implemented. Finally, to evaluate this scheme against preceding controllers regarding battery life extension, a real-time approach is followed using a dedicated simulator, while a hardware-in-the-loop verification is presented using an actual controller.