UNINTERRUPTIBLE POWER SUPPLY SYSTEM COMPRISING ENERGY STORAGE SYSTEM

Disclosed is an uninterruptible power supply system comprising an energy storage system (ESS). The uninterruptible power supply system comprises an ESS and is connected to a grid, and further comprises: a first converter for converting an alternating current voltage of the grid into a direct current voltage; a second converter connected in series to the first converter, for converting the direct current voltage outputted from the first converter into an alternating current voltage and transferring same to a load; the ESS comprising a battery connected to a node between the first and second converter, for performing charging and discharging; and a PLC for receiving the operation statuses of the first and second converters, and on the basis thereof, controlling the operation of the uninterruptible power supply system, wherein the PLC determines the operation mode of the ESS by using the received operation statuses of the first and second converters.