Uninterruptible power supply system

1. An uninterruptible power supply system including a plurality of uninterruptible power supply devices connected in parallel to a load, the uninterruptible power supply system comprising

a selection unit that selects, of the plurality of uninterruptible power supply devices, a necessary number of uninterruptible power supply devices to supply power to the load, wherein

each of the uninterruptible power supply devices performs

when this uninterruptible power supply device is selected by the selection unit, an operating action of supplying power to the load,

when this uninterruptible power supply device is not selected by the selection unit, a standby action of standing by without supplying power to the load, and

during the standby action, a diagnostic action of diagnosing whether or not a predetermined part of this uninterruptible power supply device has a failure, and

the selection unit changes uninterruptible power supply devices to be selected, such that the plurality of uninterruptible power supply devices have an equal operating time.

2. The uninterruptible power supply system according to claim 1 , wherein

each of the uninterruptible power supply devices performs, when the operating time of this uninterruptible power supply device exceeds a predetermined amount of time, the diagnostic action during the standby action.

3. The uninterruptible power supply system according to claim 1 , wherein

the selection unit changes, in a predetermined cycle, uninterruptible power supply devices to be selected, such that the plurality of uninterruptible power supply devices have an equal operating time.

4. The uninterruptible power supply system according to claim 1 , wherein

each of the uninterruptible power supply devices includes

a DC line that transmits DC power,

a capacitor connected to the DC line,

a first power converter that converts AC power supplied from an AC power supply into DC power and supplies the DC power to the DC line,

a second power converter that provides and receives DC power between the DC line and a power storage device, and

a third power converter that converts DC power received from the DC line into AC power and supplies the AC power to the load, and

the predetermined part is one of the first to third power converters.

5. The uninterruptible power supply system according to claim 4 , wherein

each of the uninterruptible power supply devices further includes

a first switch connected between the AC power supply and the first power converter,

a second switch connected between the second power converter and the power storage device,

a third switch connected between the third power converter and the load, and

a control circuit that controls the first to third switches and the first to third power converters, and

the control circuit

during the operating action, turns on the first to third switches and operates the first to third power converters,

during the standby action, turns off the first to third switches and stops the operation of the first to third power converters, and

during the diagnostic action, turns on at least one of the first and second switches and turns off the third switch, operates at least one of the first to third power converters, and diagnoses whether or not the at least one of the power converters has a failure.

6. The uninterruptible power supply system according to claim 5 , wherein

during the diagnostic action, the control circuit

sets a DC voltage on the DC line to a predetermined voltage value,

turns on the first switch,

operates the first power converter, and

diagnoses whether or not the first power converter has a failure based on a temporal variation in the DC voltage on the DC line.

7. The uninterruptible power supply system according to claim 5 , wherein

during the diagnostic action, the control circuit

sets a DC voltage on the DC line to a predetermined voltage value,

turns on the second switch,

operates the second power converter, and

diagnoses whether or not the second power converter has a failure based on a temporal variation in the DC voltage on the DC line.

8. The uninterruptible power supply system according to claim 5 , wherein

during the diagnostic action, the control circuit

sets a DC voltage on the DC line to a predetermined voltage value,

operates the third power converter, and

diagnoses whether or not the third power converter has a failure based on a temporal variation in the DC voltage on the DC line.

9. The uninterruptible power supply system according to claim 4 , wherein

when AC power is normally supplied from the AC power supply during the operating action, the AC power supplied from the AC power supply is converted into DC power by the first power converter, the DC power is converted into AC power by the third power converter, and the AC power is supplied to the load and stored in the power storage device by the second power converter, and

when AC power is not normally supplied from the AC power supply during the operating action, operation of the first power converter is stopped, DC power in the power storage device is supplied to the third power converter by the second power converter, and the DC power is converted into AC power and supplied to the load by the third power converter.

10. The uninterruptible power supply system according to claim 1 , wherein

each of the uninterruptible power supply devices stops, when this uninterruptible power supply device fails during the operating action, execution of the operating action.

11. The uninterruptible power supply system according to claim 1 , wherein

each of the uninterruptible power supply devices includes a notification unit, the notification unit providing, when the predetermined part of this uninterruptible power supply device is diagnosed as having a failure, a notification to that effect.

12. The uninterruptible power supply system according to claim 1 , having a hot swap function for replacing an uninterruptible power supply device that has been diagnosed as at least partially having a failure with a new uninterruptible power supply device, while supplying power to the load from uninterruptible power supply devices selected by the selection unit.