Design of Hybrid Energy Storage Control System for Wind Farms Based on Flow Battery and Electric Double-Layer Capacitor

An energy storage system (ESS) in a wind farm is required to be able to absorb wind power fluctuations during gusts, and improve the power quality and stability. This paper puts forward a flow-battery and electric double-layer capacitor (EDLC) hybrid ESS to provide both large power and energy capacity. The hybrid ESS is connected to the point of common coupling. Flow-battery is directly connected to the ESS dc bus while the EDLC has a dc/dc IGBT converter interface. The dc bus voltage varies within a certain range determined by the variable battery terminal voltage. With the EDLC absorbing high-frequency power surges, battery power rating, degree of discharge, and power losses are all reduced. Therefore the battery in the hybrid ESS has low cost and high longevity; and the system overall efficiency is enhanced. Fluctuations in the active and reactive power of the wind farm are compensated with four-quadrant power operation of hybrid ESS. The energy storage is optimized by properly designing the bandwidth of flow-battery and EDLC power regulator. Simulation results show the effectiveness of the control strategy of the hybrid ESS.