PURPOSE: To eliminate heat generation and power loss by suppressing loss at ordinary times and stepping down the secondary voltage effectively at fusing on the primary side by using such reactors as will become secondary and tertiary fixed burdens to cope with reactance of relays inferior in power factor.

CONSTITUTION: A neutral point of primary coils 4, 5, 6 in Y-connection is earthed, reactors 14, 16 in the value corresponding to a reactor 15 of a burden 20 for tertiary coils 10W12 are connected among phases U, W, V for secondary coils 7W9, and further a low-voltage relay 18 is connected to the reactors 14, 16 in parallel. Then, a secondary burden 19 is connected to the secondary coils 7W9, and thus a voltage on the tertiary coils 10W12 is stepped down to protect the relay 18 at fusing on the primary side. Further, the reactors 14W16 which become secondary and tertiary fixed burdens are used to cope with the reactance of relays inferior in power factor, thereby suppressing a loss at ordinary times and stepping down a voltage on the secondary side at fusing on the primary side to protect the relay 18.