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An energy storage system (ESS) in a wind farm is required to be able to absorb wind power surges during gusts, and have sufficient energy storage capacity to level wind fluctuations lasting for longer periods. ESS using a single technology, such as batteries, or supercapacitors, will have difficulties providing both large power and energy capacities. This paper proposes a flow-battery supercapacitor hybrid ESS, which takes advantage of the two complementary technologies to provide large power and energy capacities. The flow-battery is directly coupled to the WTG dc bus while the supercapacitor has a dc/dc IGBT converter interface. The dc bus voltage varies within a certain limit determined by the variable battery terminal voltage. With the supercapacitor absorbing high frequency power surges, the 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 improved.