Method and apparatus for measuring volt-amps reactive power using synthesized voltage phase shift
First Claim
1. A method for measuring reactive power in an electrical load in an electrical system, said method comprising the steps of:
- electromagnetically coupling a voltage probe across the electrical load;
electromagnetically coupling a current probe to the electrical load;
measuring a time-varying voltage across the electrical load using said voltage probe, said voltage probe outputting a voltage signal representing said time-varying voltage;
measuring a time-varying current flowing through the electrical load using said current probe, said current probe outputting a current signal representing said time-varying current;
sampling substantially simultaneously and instantaneously said voltage signal and said current signal at a sampling frequency to produce a plurality of voltage samples representative of said voltage signal and a plurality of current samples representative of said current signal;
shifting the phase of frequency components of said plurality of voltage samples by -90°
C. relative to said plurality of current samples for substantially all frequency components having a frequency less than one-half of said sampling frequency to produce a plurality of quadrature voltage samples; and
multiplying each sample of said plurality of quadrature voltage samples by each corresponding sample of said plurality of current samples to produce a plurality of reactive power samples, each of which is a measure of instantaneous reactive power in the electrical load.
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Abstract
In the present invention, true volt-amperes reactive power is measured by sampling instantaneous voltage values of voltage and substantially simultaneously instantaneous values of current of an arbitrary waveform, multiplying the voltage samples by the discrete Hilbert transform (corrected by a sign inversion) to obtain voltage samples of all frequencies shifted by -90° over the bandwidth of the signal of interest and then multiplying this quadrature voltage sample by the corresponding current samples to obtain a true value for VARs, i.e., proportional to the instantaneous volt-amperes reactive power under all conditions. The sampling rate and the period of sampling determine the bandwidth and accuracy of the sampling technique. The invention can be implemented in software or in suitable hardware components capable of sampling and delaying the signal of interest.
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Citations
22 Claims
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1. A method for measuring reactive power in an electrical load in an electrical system, said method comprising the steps of:
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electromagnetically coupling a voltage probe across the electrical load; electromagnetically coupling a current probe to the electrical load; measuring a time-varying voltage across the electrical load using said voltage probe, said voltage probe outputting a voltage signal representing said time-varying voltage; measuring a time-varying current flowing through the electrical load using said current probe, said current probe outputting a current signal representing said time-varying current; sampling substantially simultaneously and instantaneously said voltage signal and said current signal at a sampling frequency to produce a plurality of voltage samples representative of said voltage signal and a plurality of current samples representative of said current signal; shifting the phase of frequency components of said plurality of voltage samples by -90°
C. relative to said plurality of current samples for substantially all frequency components having a frequency less than one-half of said sampling frequency to produce a plurality of quadrature voltage samples; andmultiplying each sample of said plurality of quadrature voltage samples by each corresponding sample of said plurality of current samples to produce a plurality of reactive power samples, each of which is a measure of instantaneous reactive power in the electrical load. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. A meter for measuring reactive power in an electrical load in an electrical system, comprising:
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a voltage probe electromagnetically coupled across the electrical load, for measuring a time-varying voltage across the electrical load, wherein said voltage probe outputs a voltage signal representing said time-varying voltage; a current probe electromagnetically coupled to the electrical load, for measuring a time-varying current flowing through the electrical load, wherein said current probe outputs a current signal representing said time-varying current; sampling means coupled to said voltage probe and to said current probe, for sampling substantially simultaneously and instantaneously said voltage signal and said current signal at a sampling frequency to produce a plurality of voltage samples and a plurality of current samples; shifting means coupled to said sampling means, for shifting the phase of frequency components of said plurality of voltage samples provided to said shifting means by said sampling means by -90°
C. relative to said plurality of current samples for substantially all frequency components having a frequency less than one-half said sampling frequency, to produce a plurality of quadrature voltage samples; andcombining means coupled to said sampling means and said shifting means, for multiplying each of said plurality of quadrature voltage samples provided to said combining means by said shifting means by each corresponding sample of said plurality of current samples provided to said combining means by said sampling means to produce a plurality of reactive power samples, each of which is a measure of instantaneous reactive power in the electrical load. - View Dependent Claims (10, 11, 12, 13, 14, 15, 16)
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17. A meter for measuring reactive power in an electrical load in an electrical system, comprising:
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a voltage probe electromagnetically coupled across the electrical load, for measuring a time-varying voltage across the electrical load, wherein said voltage probe outputs a voltage signal representing said time-varying voltage; a current probe electromagnetically coupled to the electrical load, for measuring a time-varying current flowing through the electrical load, wherein said current probe outputs a current signal representing said time-varying current; a sampling circuit coupled to said voltage probe and said current probe, said sampling circuit sampling substantially simultaneously and instantaneously said voltage signal and said current signal at a sampling frequency to produce a plurality of voltage samples and a plurality of current samples; a shifter coupled to said sampling means, said shifter shifting the phase of frequency components of said plurality of voltage samples provided to said shifter by said sampling means by -90°
relative to said plurality of current samples for substantially all frequency components having a frequency less than one-half said sampling frequency, to produce a plurality of quadrature voltage samples; anda multiplier coupled to said sampling circuit and said shifter, said multiplier multiplying each of said plurality of quadrature voltage samples provided to said multiplier by said shifter by each corresponding sample of said plurality of current samples provided to said multiplier by said sampling circuit to produce a plurality of reactive power samples, each of which is a measure of instantaneous reactive power in the electrical load. - View Dependent Claims (18, 19, 20, 21, 22)
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Specification