UNINTERRUPTIBLE POWER SUPPLY

PURPOSE: An uninterruptible power supply is provided to reduce lower transmission loss by controlling the output of a sub uninterruptible power supply part without using a transformer. CONSTITUTION: A sub uninterruptible power supply part(120) comprises a battery unit, a battery charge circuit unit for charging the battery unit with external input power, a rectifying unit for rectifying the external input power, and an inverter unit for changing the rectified power into AC power. A first switch(SW1) is serially connected to the sub uninterruptible power supply part. A variable impedance unit(130) is parallely connected to the sub uninterruptible power supply part and the first switch and controlled with the same impedance as the sub uninterruptible power supply part. A second switch(SW2) is parallely connected to the sub uninterruptible power supply part and the first switch. A controller(140) controls the first and second switches according to a normal mode, a power fail mode, and a device failure mode.