A method for operating an energy storage system and an energy storage system are disclosed. The energy storage device is coupled to a first switching device of the energy storage system via a live wire, the first switching device is coupled to a current transformer of the energy storage system via the live wire, and the current transformer is configured to access the external power grid. By switching on the first switching device, the energy storage device can be controlled to access the external power grid, enabling charging or discharging of the energy storage device through the external power grid. Thereafter, based on a difference value between an on-load electrical parameter and a static electrical parameter of the live wire, the connection status of the current transformer is analyzed to set the operating mode of the energy storage system.