Uninterruptible power supply device

An uninterruptible power supply device comprising:

• a plurality of uninterruptible power supply units connected in parallel with one another between an AC power supply and a load;
• a current detector configured to detect a load current;
• and
• a controller configured to select first to n-th uninterruptible power supply units required to supply the load current, and to select an (n+1)th uninterruptible power supply unit, from the plurality of uninterruptible power supply units, based on a result of detection by the current detector, n being a positive integer,
• each of the plurality of uninterruptible power supply units comprising:
• an inverter configured to perform power conversion between the AC power supply and the load;
• and
• a switch configured to switch an electrical connection state between the inverter and the load,
• each of the first to n-th uninterruptible power supply units being configured to operate the inverter in a state in which the inverter is electrically connected to the load by the switch to supply, to the load, a shared current which is 1/n of the load current,
• the (n+1)th uninterruptible power supply unit being configured to continue to operate the inverter in a state in which the inverter is electrically connected to the load by the switch to output, to the load, a counter voltage having a value in accordance with output voltages of the first to n-th uninterruptible power supply units, and thereby to stand by in a state where no current flows between the (n+1)th uninterruptible power supply unit and the first to n-th uninterruptible power supply units and between the (n+1)th uninterruptible power supply unit and the load,
• each of the remaining uninterruptible power supply units other than the first to (n+1)th uninterruptible power supply units among the plurality of uninterruptible power supply units being configured to stop the inverter in a state in which the inverter is electrically disconnected to the load by the switch, and thereby to stand by in a state where no current flows between the remaining uninterruptible power supply unit and the first to (n+1)th uninterruptible power supply units and between the remaining uninterruptible power supply unit and the load.