THREE-PHASE VOLTAGE FREQUENCY DETERMINATION METHOD Russian patent published in 2015 - IPC G01R23/00 

FIELD: electricity.

SUBSTANCE: according to the method for determination of frequency F the digital signals of all three phases Ua(t_i), Ub(t_i), Uc(t_i) of industrial three-phase voltage measured in the moments t_i, where i - the whole value, digitized with the period dt = (t_i-t_i-1) are used. And the value dt is much less than the period T of the maximum frequency Fb = 1/T of the measurement range, dt<<T. Meanwhile the projection Ux(t_i) to the axis X of the rotating field U(t_i), created by three phases Ua(t_i), Ub(t_i), Uc(t_i) of industrial three-phase voltage, using the formula $$Ux\left(t_i\right)=\left(Uc\left(t_i\right)-Ub\left(t_i\right)\right)\cdot \frac{\sqrt{3}}{2},$$ the projection Uy(t_i) to the axis Y of the rotating field U(t_i) - Uy(t_i) = (2·Ua(t_i)-Ub(t_i)- Uc(t_i))/2, the module of the rotating field U(t_i) - $$U\left(t_i\right)=\sqrt{\left[U^{2}x\left(t_i\right)+U^{2}y\left(t_i\right)\right]},$$ are detrmined. The dependence from the time t_i of the phase increment dφ_i of the rotating field U(t_i) within the interval dt = (t_i-t_i-1) is determined according to the formula: |dφ_i| = |φ(t_i)-φ(t_i-1)| = arccos{[Ux(t_i)·Ux(t_i-1)+Uy(t_i)·Uy(t_i-1)]/[U(t_i)·U(t_i-1)]}, and the sign of dφ_i is determined according to the following algorithm: if |Ux(t_i)|≤| Uy(t_i)|, then the sign of dφ_i is equal to the sign of the value Uy(t_i)·[Ux(t_i-1)-Ux(t_i)], if |Ux(t_i)>|Uy(t_i)|, then the sign of dφ_i is equal to the sign of the value Ux(t_i)·[Uy(t_i)-Uy(t_i-1)]. The average value of the frequency F(t_i) for the period n·dt at the moment t_i is determined by the formula $$F\left(t_i\right)=\left({\displaystyle \sum _{k=0}^{n-1}d\phi \left(t_{i-k}\right)}\right)/\left(2\pi \cdot n\cdot dt\right),$$ where n is integer value.

EFFECT: improvement of accuracy of determination of three-phase voltage frequency.

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