Patent ID: 6985824

Claim:
A frequency measuring apparatus comprising: a voltage measuring part for measuring the voltage of an electric power system at timings which are obtained by equally dividing one period of a reference wave by 4N (N being a positive integer); a chord length calculation part for calculating, at each of (4N+1) timings, voltage vectors directed to points represented by complex numbers on a complex plane, each complex number consisting of a real part, which is one of the voltages measured at a first timing comprising any of the (4N+1) timings, and an imaginary part, which is a voltage measured at a second timing delayed by 90 electrical degrees from the first timing, the chord length calculation part further calculating, at each of the (4N+1) timings, the length of a chord connecting a tip end of one of the voltage vectors calculated at a third timing, comprising any of the (4N+1) timings, to a tip end of another of the voltage vectors calculated at the last timing before the third timing; a voltage root-mean-square value calculation part for calculating, at a fourth timing comprising each of the (4N+1) timings, a voltage root-mean-square value from those of the voltages which are measured at past 4N timings, from the fourth timing and at the fourth timing; a rotational phase angle calculation part for summing, at a fifth timing, comprising each of the (4N+1) timings, those of the chord lengths which have been obtained at past 4N timings from the fifth timing and at the fifth timing, and calculating, based on a sum of the chord lengths and the voltage root-mean-square value, a phase angle between one of the voltage vectors calculated at a sixth timing comprising any of the timings and another voltage vector calculated at a timing preceding the sixth timing by one period of said reference wave; and a frequency calculation part for calculating the frequency of the electric power system from the phase angle thus calculated.