Variable Voltage Control of a Hybrid Energy Storage System for Firm Frequency Response in the U.K.

The National Grid in the U.K. proposed the firm frequency response (FFR) documents on recent developments regarding tendering options for active balancing mechanism units, specifically for batteries. However, frequent bipolar converting and instantaneous high power demand challenge battery lifetime and operation cost in the FFR. Combining a battery with a supercapacitor (SC) has several advantages, but the system cost may rise. Targeting the FFR service, this article presents a new variable voltage control within a semiactive battery SC hybrid scheme. In the proposed hybrid energy storage system, the dc bus voltage is controllable to improve the SC use ratio while reducing the converter's cost. A power management strategy integrating fuzzy logic with dynamic filtering method is proposed benefiting in broadening the filtering flexibility while reducing battery degradation. In addition, both the long-term simulations and scaled-down experiments are implemented to verify the proposed topology with the control strategy and power management by quantitative comparisons of battery degradations and SC use ratios.