An energy storage system, comprising: a power conversion system configured to produce a control signal Sf for regulating a frequency of power flowing from a power generation system to an electric-power system; and a battery system including a first battery rack for low-speed operations and a second battery rack for high-speed operations, wherein the battery system further comprises: a charge/discharge unit configured to perform a charging/discharging operation of the second battery rack; and a rack battery management system BMS configured to control the charging/discharging operation of the first and second battery racks using the control signal, and to control the charge/discharge unit, thus controlling a state of charge SOC of the second battery rack, wherein the control signal comprises: a charge control signal causing the power to be charged into the first or second battery rack when the frequency of the power flowing in the electric-power system exceeds a predetermined value; and a discharge control signal causing the first or second battery rack to be discharged, thus supplying power to the electric-power system when the frequency of the power flowing in the electric-power system falls to be less than the predetermined value, wherein, if the control signal is the charge control signal, the rack BMS performs control such that the power is charged into the first battery rack when the state of charge of the first battery rack is less than a first preset state of charge, and performs control such that the power is charged into the second battery rack when the state of charge of the first battery rack is equal to or more than the first preset state of charge, and wherein, when a state of charge of the second battery rack increases to exceed a second preset state of charge lower than the maximum state of charge as the power is charged into the second battery rack, the rack BMS controls the charge/discharge unit such that the state of charge of the second battery rack maintains the second preset state of charge, thus discharging the second battery rack. a power conversion system configured to produce a control signal Sf for regulating a frequency of power flowing from a power generation system to an electric-power system; and a battery system including a first battery rack for low-speed operations and a second battery rack for high-speed operations, wherein the battery system further comprises: a charge/discharge unit configured to perform a charging/discharging operation of the second battery rack; and a rack battery management system BMS configured to control the charging/discharging operation of the first and second battery racks using the control signal, and to control the charge/discharge unit, thus controlling a state of charge SOC of the second battery rack, wherein the control signal comprises: a charge control signal causing the power to be charged into the first or second battery rack when the frequency of the power flowing in the electric-power system exceeds a predetermined value; and a discharge control signal causing the first or second battery rack to be discharged, thus supplying power to the electric-power system when the frequency of the power flowing in the electric-power system falls to be less than the predetermined value, a charge/discharge unit configured to perform a charging/discharging operation of the second battery rack; and a rack battery management system BMS configured to control the charging/discharging operation of the first and second battery racks using the control signal, and to control the charge/discharge unit, thus controlling a state of charge SOC of the second battery rack, wherein the control signal comprises: a charge control signal causing the power to be charged into the first or second battery rack when the frequency of the power flowing in the electric-power system exceeds a predetermined value; and a discharge control signal causing the first or second battery rack to be discharged, thus supplying power to the electric-power system when the frequency of the power flowing in the electric-power system falls to be less than the predetermined value, a charge control signal causing the power to be charged into the first or second battery rack when the frequency of the power flowing in the electric-power system exceeds a predetermined value; and a discharge control signal causing the first or second battery rack to be discharged, thus supplying power to the electric-power system when the frequency of the power flowing in the electric-power system falls to be less than the predetermined value, wherein, if the control signal is the charge control signal, the rack BMS performs control such that the power is charged into the first battery rack when the state of charge of the first battery rack is less than a first preset state of charge, and performs control such that the power is charged into the second battery rack when the state of charge of the first battery rack is equal to or more than the first preset state of charge, and wherein, when a state of charge of the second battery rack increases to exceed a second preset state of charge lower than the maximum state of charge as the power is charged into the second battery rack, the rack BMS controls the charge/discharge unit such that the state of charge of the second battery rack maintains the second preset state of charge, thus discharging the second battery rack.