Method and apparatus for measuring volt-amps reactive power using synthesized voltage phase shift

A meter for measuring reactive power in an electrical load in an electrical system, comprising:

• 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.degree. 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;
• and
• combining 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.

A meter for measuring reactive power in an electrical load in an electrical system, comprising:

• 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.degree. 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;
• and
• a 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.