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 o...

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 electric...