To suppress the divergence of the frequency far apart from the system frequency even if an infinitesimal vibration voltage is retained by providing a lower limit limiter having a predetermined lower limit in output means of a standard value calculator.

The lower limit value is set to 0.5 (unit pu) by a lower limit limiter 9 provided in the output means of a standard value calculator 3. Thus, zero dividing is prevented in the calculation of an arcsine calculator 4, and the value of a denominator for standardizing the signal becomes 0.6 or more. Even if a voltage value is resultantly low at the AC three-phase voltage of a vibration voltage, the output value of the limiter 9, i.e., the input value of the calculator 4 is suppressed to 0.6. Thus, the output value of the calculator 4 becomes an infinitesimal value, and a voltage control oscillator 6 finally outputs a phase θ' not so apart from a system frequency. Thus, even if the voltage of the input signal is disordered by the voltage due to a fault, the lower the voltage is, the nearer the phase θ' near to the system frequency is output. Accordingly, the divergence of the frequency apart from the system frequency during the fault can be suppressed.