To avoid unnecessary power failure change-over operation, prevent the capacitance of a capacitor, installed at a supply output end for a load device, from being increased, and reduce the size and cost of the relevant uninterruptible power supply apparatus.
When an instantaneous power failure due to very short-time voltage drop, such as commutation failure, occurs, a parallel inverter is operated according to a current control signal based on a reference voltage Vout* synchronized with the frequency of supply voltage by an output current control unit 4b. Thus, in an instantaneous power failure, a supply current to a load device is all supplied from the parallel inverter. When a normal power failure is detected by power failure detection for longer-time power failure than instantaneous power failure, supply voltage to a load device 2 is controlled to a predetermined value according to a voltage control signal by an output voltage control unit 7a.
Yoshiaki Naito
Cui Shu Tetsu
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