To obtain a system and a method for transmitting a power between a 1st AC power system with a 1st wattage and a 1st frequency and a 2nd AC power system with a 2nd wattage and a 2nd frequency.

A power with a 1st wattage and a 1st frequency is supplied to a 1st winding of a rotary transformer 102 and a power with a 2nd wattage which is different from that of the 1st wattage is generated in a 2nd winding of the rotary transformer 102. Furthermore, a differential power which is caused by a mechanical power transmitted to the rotatable shaft 113 of the rotary transformer 102 is recovered by a power recovery system 103 and supplied to a 2nd AC power system 24. The recovered power is added to the 2nd AC power system 24, and the power with the 2nd wattage is increased.