Abstract: An uninterruptible power supply is provided that is capable of turning off an AC switch made up of thyristors as quickly as possible by using a power converter. A power is supplied from the utility power supply 3 to the load 9 through the AC switch 1 made up of antiparallelly connected thyristors. Under this condition, in the event that the power failure detection means 12 detects a power outage, if the current detection means 11a detects a current of a positive polarity, the reference sinusoidal voltage generation circuit 11 generates the high synchronous sinusoidal voltage Vref2 which is higher, by the voltage difference command value &Dgr;V, than the synchronous sinusoidal voltage Vref1 in phase with the utility power supply.

Abstract: An uninterruptible power system capable of reducing strain in an output voltage thereof as compared with the prior art. A DC voltage control circuit of a current control system obtains an input current command value on the basis of a multiplied value between a differential voltage between a DC voltage command and a DC-side voltage of a power converter and an output of a reference voltage forming circuit. A current control circuit outputs, as a current control command, a value obtained by subtracting a value of a load current flowing to a load and a value of a current flowing from an AC side into the power converter from the input current command value. An AC current flowing through an AC switch is used as the input current command value during a period of time for which an abnormal voltage detection circuit detects momentary abnormality.

Abstract: An uninterruptible power system capable of reducing strain in an output voltage thereof as compared with the prior art. A DC voltage control circuit of a current control system obtains an input current command value on the basis of a multiplied value between a differential voltage between a DC voltage command and a DC-side voltage of a power converter and an output of a reference voltage forming circuit. A current control circuit outputs, as a current control command, a value obtained by subtracting a value of a load current flowing to a load and a value of a current flowing from an AC side into the power converter from the input current command value. An AC current flowing through an AC switch is used as the input current command value during a period of time for which an abnormal voltage detection circuit detects momentary abnormality.