Harmonics measurement instrument with in-situ calibration
View Patent ↗A portable harmonics measurement instrument performs in-situ self-calibration of its current transducers and their associated measurement channels and of its voltage probes and their associated measurement channels. In-situ self-calibration is performed immediately before making a measurement on the alternating current power distribution grid. In-situ self-calibration is performed by means of a built-in voltage reference source and a built-in current reference source, which generate highly-accurate voltage and current calibration waveforms with harmonic components.
1. A portable apparatus for measuring harmonic voltages and currents on an alternating current electric power distribution grid, said alternating current having a fundamental frequency, said apparatus comprising:
a. a plurality of current measurement channels, each such current measurement channel equipped with an associated current transducer; and
b. a plurality of voltage measurement channels; and
c. a current calibration source, said current calibration source having an output, and said current calibration source configured to provide a means of calibrating the current measurement channels and their associated current transducers for gain and phase, at frequencies higher than the fundamental frequency; and
d. a voltage calibration source, said voltage source having an output, and said voltage calibration source configured to provide a means of calibrating the voltage measurement channels for gain and phase, at frequencies higher than the fundamental frequency; and
e. said current calibration source and said voltage calibration source configured so that the current calibration source output and the voltage calibration source output are physically separated, and
f. said current calibration source and said voltage calibration source constructed as a physically embedded element of the portable apparatus.
2. The portable apparatus of claim 1 , in which the current calibration source is physically integrated as a plug-in module.
3. The portable apparatus of claim 1 , in which the voltage calibration source is physically integrated as a plug-in module.
4. A method for in-situ calibration of a portable measurement instrument for measuring voltages and currents on an alternating current power grid, said alternating current power grid having a fundamental frequency, and said instrument equipped with a plurality of current transducers with associated current measurement channels and a plurality of voltage probes with associated voltage measurement channels, said method comprising:
a. a method to generate a plurality of calibration currents and calibration voltages at a location near where measurements will be made, said calibration currents and calibration voltages having at least one frequency component higher than the fundamental frequency, and said method to generate the plurality of calibration currents and calibration voltages configured so that the calibration currents are physically separated from the calibration voltages; and
b. a method for the current transducers and their associated current measurement channels, and voltage probes and their associated voltage measurement channels, to make in-situ measurements of the calibration currents and calibration voltages; and
c. a calibration method for using the in-situ measurements of the calibration currents and calibration voltages to adjust for invariant and time-varying and location-varying sources of gain error and phase error in the current transducers and their associated current measurement channels, and in the voltage probes and their associated voltage measurement channels.