UNINTERRUPTIBLE POWER SUPPLY

A bidirectional chopper selectively performs a charging operation and a discharging operation, the charging operation being an operation in which a part of DC power generated by a converter is stored in a power storage device, the discharging operation being an operation in which DC power of the power storage device is supplied to an inverter. A control circuit controls the bidirectional chopper to perform the discharging operation when a power failure of a power system occurs. The control circuit controls the bidirectional chopper based on a system frequency detected by a frequency detector when the power system is normal. The control circuit controls the bidirectional chopper to perform the charging operation in response to an increase in the system frequency, and to perform the discharging operation in response to a decrease in the system frequency.