Abstract: A direction and a magnitude of a current flowing between an external alternating current power source (20) and a load (30) are detected by a current transformer (11). A voltage generated to a secondary winding (11B) of the current transformer as a result of detection is superimposed upon a voltage of the alternating current power source (20), and is supplied to the synchronous power source (S). The synchronous power source (S) generates a voltage having an instantaneous value which is the same as that of the voltage supplied thereto, and applies the generated voltage to the load (30) in parallel. Since the voltage applied by the synchronous power source (S) has a value for lowering the current detected by the current transformer (11), there is substantially no current flowing between the alternating current power source (20) and the load (30).

Abstract: A direction and a magnitude of a current flowing between an external alternating current power source (20) and a load (30) are detected by a current transformer (11). A voltage generated to a secondary winding (11B) of the current transformer as a result of detection is superimposed upon a voltage of the alternating current power source (20), and is supplied to the synchronous power source (S). The synchronous power source (S) generates a voltage having an instantaneous value which is the same as that of the voltage supplied thereto, and applies the generated voltage to the load (30) in parallel. Since the voltage applied by the synchronous power source (S) has a value for lowering the current detected by the current transformer (11), there is substantially no current flowing between the alternating current power source (20) and the load (30).

Abstract: The transfer switch (6) supplies the DC voltage supplied from the DC power source section (5), to the commercial electricity system (8) in such a way as to provide the DC voltage with the same polarity as the AC voltage generated by the commercial electricity system (8) according to the control of the commercial voltage monitor section (7) when the absolute value of the voltage of the commercial electricity system (8) is higher than the voltage threshold level. When the absolute value of the AC voltage is lower than the voltage threshold level, the supply of the DC voltage from the DC power source section (5) is interrupted. The generator (2) and the commercial electricity system (8) are isolated from each other by the isolation transformer (4).