PURPOSE: To prevent an accident from being erroneously judged as an internal accident even if the second high harmonic suppression method is used by preventing differential current from flowing to the current transformer of a series transformer so that an iron core cannot be magnetically saturated when a system fails.

CONSTITUTION: A series transformer 3a consisting of a series coil winding 31a which is inserted in series into a power system to adjust the voltage phase of the power system or a voltage value and a phase and an excitation coil winding 32a for supplying voltage to the series coil winding is used to set the ratio of a sectional area Sc of an iron core main leg 331a of the series transformer 3a and a sectional area Sy of a yoke and a side leg according to the ratio of impedance XL of the power system to impedance XT of the series transformer.