Battery energy storage system (BESS) is one of the effective technologies to deal with power fluctuation and intermittence resulting from grid integration of large renewable generations. In this paper, the system configuration of a China’s national renewable generation demonstration project combining a large-scale BESS with wind farm and photovoltaic (PV) power station, all coupled to a power transmission system, is introduced, and the key technologies including optimal control and management as well as operational status of this BESS are presented. Additionally, the technical benefits of such a large-scale BESS in dealing with power fluctuation and intermittence issues resulting from grid connection of large-scale renewable generation, and for improvement of operation characteristics of transmission grid, are discussed with relevant case studies.