COMPUTATION METHOD OF DIFFERENTIAL VOLTAGE FOR AC VOLTAGE

PURPOSE:To enable the computation of a differential voltage of an AC voltage at a high accuracy by determining a compensation coefficient from amplitudes and a phase difference of two AC voltages measured with a breaker closed to compensate a amplitude difference and the phase difference based on a compensation formula. CONSTITUTION:When amplitudes Vgamma1 and Vgamma2 and a phase difference DELTAPHIgamma of an AC voltage measured 101 and 102 with a breaker 1 closed are computed by a differential compensation coefficient computing means 104 by a command of a differential voltage compensation coefficient computation command means 103, errors of the voltage measuring means 101 and 102 do not bring an amplitude difference DELTAVgamma and the phase difference DELTAPHIgamma to zero. Here, a required coefficient of a specified compensation type is computed 104 and stored beforehand based on the measured values and a differential voltage computing means 105 compensates computed values of a difference DELTAVR of the amplitudes DELTAVR1 and DELTAVR2 and the phase difference DELTAPHIR as when the breaker 1 is kept optional based on the compensation formula to output a compensation coefficient and the difference DELTAVRH of the amplitudes and the difference DELTAPHIR of the phases compensated. Thus, the computation of a differential voltage with limited error is made possible thereby enabling the measuring of the differential voltage of the AC voltage at a high accuracy.