Method and apparatus for digitally measuring electrical energy consumption

• US 5,061,890 A
• Filed: 04/25/1990
• Issued: 10/29/1991
• Est. Priority Date: 08/07/1989
• Status: Expired due to Term

• Current Assignee: Vectormetrics, Inc.
• Original Assignee: Digital KWH, Inc.
• Inventors: Longini, Richard L.
• Primary Examiner(s): Wieder, Kenneth A.
• Assistant Examiner(s): BURNS, WILLIAM
• Application Number:
  US07/514,255
• Time in Patent Office: 552 Days
• Field of Search: 324/107, 324/142, 324/103 R, 324/117 R, 364/569, 364/576, 364/577, 364/481, 364/483, 364/485, 364/487
• US Class Current: 324/107
• CPC Class Codes: G01R 21/133 by using digital technique

Abstract

A method and apparatus for digitally measuring electrical energy consumption from a transmission line carrying alternating current. Instead of actually measuring current from a transmission line conductor, the time derivative of the magnetic field induced by current flowing in the conductor is measured thereby obtaining a reading which is proportional to the time derivative of the current flow. Using an appropriate microcomputer, the time derivative of the magnetic field (the magnetic field rate of change) is represented with an algorithmic series for expressing cyclic phenomena, such as the Fourier series of low order. An approximation of the power consumption is then computed by converting the time derivative of the current as expressed in the algorithmic series and a voltage measurement from the conductor mathematically to a power reading, and the power reading is summed to obtain a number value proportional to the energy consumption. Even harmonics and high frequency harmonics may be eliminated in the algorithmic expression of the current derivative to further simplify computer computations and required hardware, and the magnetic rate of change pickup and the voltage pickup are preferably filtered to remove unwanted noise.

Claims

1. The method of measuring electrical energy consumption from a plurality of transmission line conductors carrying alternating current, comprising the steps of, measuring the time derivative of the magnetic field induced by an electrical current flowing in a transmission line conductor thereby obtaining a reading proportional to the time derivative of the current flow, mathematically approximating the time derivative of the current flowing in the conductor by expressing said magnetic field rate of change with an algorithmic series for expressing cyclic phenomena, measuring the voltage between conductors of the transmission line, computing an approximation of the power being consumed through the transmission line by converting the time derivative of the current as expressed in an algorithmic series and the voltage measurement mathematically to a power reading, and summing the power reading to obtain a number value proportional to the energy consumed through said conductors.

2. The method of claim 1 wherein said algorithmic series is a low order Fourier series.

3. The method of claim 2 including the step of converting the measurement of the time derivative of the magnetic field to digital value prior to the step of mathematically approximating.

4. The method of claim 1 wherein the step of computing an approximation of the power includes the steps of, integrating said algorithmic representation to obtain a representation of the current, computing an estimat future or recent past value or values of said approximated current in said algorithmic series for a selected future or recent past voltage reading time or times, measuring said voltage at said selected future or recent past time or times, and multiplying said measurement of voltage or voltages and future or recent past current or currents.

5. The method of claim 1 including the step of comparing an algorithmic series reading or group of readings of the derivative of the magnetic field around said conductor, when taken, to one or more values thereof already computed and thereby detecting the deviation of the computed from actual magnetic field derivative current readings, and utilizing said deviation to accordingly adjust coefficients of the algorithmic series for corrections.

6. The method of claim 5 wherein the step of comparing is characterized in that a group of said readings of the time derivative of the magnetic field and said value or values already computed are assumed to be zero for the purposes of said comparison such that said group of readings is used directly to obtain new coefficients of the algorithmic series.

7. The method of claim 5 wherein said algorithmic series is a low order Fourier series.

8. The method of claim 4 wherein said algorithmic series is a Fourier series.

9. The method of claim 8 wherein the step of mathematically approximating the current flowing in the conductor by representing said time derivative of the magnetic field with a Fourier series includes the step of eliminating even harmonics in the series representing the current wave form.

10. The method of claim 8 wherein the step of mathematically approximating the current flowing in the conductor by representing said time derivative of the magnetic field with a Fourier series includes the step of eliminating high frequency harmonics in the series representing the current wave form.

11. The method of claim 1 including the step of comparing an algorithmic series reading or group of readings of the time derivative of the magnetic field around said conductor, when taken, to the value or values thereof already computed, summing deviations each with a weighting factor, and utilizing said sum of weighted deviations to accordingly adjust coefficients of the algorithmic series for corrections.

12. The method of claim 11 wherein the step of comparing is characterized in that a group of said readings of the time derivative of the magnetic field and said value or values already computed are assumed to be zero for the purposes of said comparison such that said group of readings is used directly to obtain new coefficients of the algorithmic series.

13. The method of claim 2 wherein the step of computing an approximation of the power includes the steps of expressing said voltage measurement by representing it with a Fourier series, separately multiplying the coefficients of sines and the coefficients of cosines for each harmonic in the current and voltage algorithmic series expressions, and the step of summing includes the steps of summing the multiplication results and dividing the sum in half.

14. The method of claim 13 wherein said algorithmic series are low order Fourier series.

15. The method of claim 2 wherein the step of computing an approximation of the power includes the step of eliminating even harmonics in the Fourier series expression of the current derivative.

16. The method of claim 2 wherein the step of computing an approximation of the power includes the step of eliminating high harmonic content in the Fourier series expression of the current derivative.

17. The method of claim 13 including the step of filtering noise from the measurement of the time derivative of the current flow and from the voltage measurement to give identical phase shifts prior to the step of computing, and, after step of computing, multiplying each harmonic coefficient by the appropriate factor to correct for both the attenuation of the current derivative measurement and the attenuation of the corresponding voltage measurement.

18. The method of claim 13 including the step of filtering noise from the time derivative of the current flow measurement and the voltage measurement with filters to give identical phase shifts and attenuation prior to the step of computing, multiplying, the voltage output harmonics and the time derivative of the magnetic field output harmonics by factors required to compensate for the known attentuation to which each has been subjected as a result of said filtering.

19. The method of claim 18 wherein the step of multiplying each harmonic by the compensation factor comprises multiplication thereof by a reciprocal of the attenuation factor, thereby dividing each harmonic by the attenuation factor.

20. The method of claim 1 including the step of filtering noise from the measurement of the time derivative of the current flow and the voltage measurement to give identical phase shifts prior to the step of computing, and after the step of computing, dividing each algorithmic harmonic coefficient by the square of its attenuation to correct both the current derivative attenuation and the attenuation of the corresponding voltage measurement, which has not been put in harmonic form.

21. The method of claim 1 wherein the step of measuring the time derivative of the magnetic field includes the steps of utilizing a pick-up wire coil having a gapped ferromagnetic material core passing around the conductor and imposing a magneto-motive force on said core having a frequency which is not a harmonic of the frequency of the alternating current carried by said conductor, thereby removing effects of core retentivity.

22. The method of claim 21 wherein the magnitude of said magneto-motive force is small compared to the full scale magnitude of the measured time derivative of the magnetic field.

23. The method of claim 22 including the step of converting the measurement of the time derivative of the magnetic field to digital value prior to the step of mathematically approximating.

24. A digital electrical energy consumption meter for measuring electrical energy consumption from a plurality of transmission line conductors carrying alternating current comprising;

a magnetic field rate of change pick-up adapted for measuring the time derivative of the magnetic field induced by an electrical current flowing in a transmission line conductor thereby obtaining a reading proportional to the time derivative of the current flow, and voltage pickup measuring means adapted to measure the voltage between conductors of the transmission line;

computer means connected to said pick-up and to said voltage measuring means and programmed for representing said derivative of the current with an algorithmic series for expressing cyclic phenomena, for computing an approximation of the power being consumed by converting the time derivative of the current as expressed in an algorithmic series and the voltage measurement mathematically to a power reading, and for summing the power reading to thereby obtain a measurement number value proportional to the energy consumed by a load connected to the conductors.

25. The digital meter of claim 24 wherein said computer means is further programmed for integrating said algorithmic representation to provide an approximate mathematical representation of the current flowing in the conductor and for computing an estimated future or recent past value or values of said approximated current in said algorithmic series for a selected furture or recent past voltage reading time or times, for measuring the voltage with said voltage measuring means at said selected time or times and for multiplying said voltage or voltages and future or recent past approximated current or currents and summing the result.

26. The digital meter of claim 24 wherein said computer means is further programmed for comparing an algorithmic series reading or group of readings of the derivative of the magnetic field around the conductor, when taken, to one or more values thereof already computed, thereby detecting the deviation of computed from actual derivative current readings, and for utilizing said deviations to accordingly adjust the coefficients of the algorithmic series representing the current derivative for corrections.

27. The digital meter of claim 26 wherein said computer means is further programmed whereby a group of readings of rate of change of magnetic field around the conductor is taken and said value or values thereof previously computed are assumed to be zero for the purposes of said comparison such that said group of readings is used directly to obtain new coefficients of the algorithmic series.

28. The digital meter of claim 24 wherein said computer means is further programmed for:

comparing an algorithmic series reading or group of readings of the time derivative of the magnetic field around the conductor, when taken, to the value or values thereof already computed, summing deviations each with a weighting factor, and utilizing said sum of weighted deviations to accordingly adjust coefficients of the algorithmic series for corrections.

29. The digital meter of claim 28 wherein said computer means is further programmed whereby a group of readings of rate of change of magnetic field around the conductor is taken and said value or values thereof already computed are assumed to be zero for the purposes of said comparison such that said group of readings is used directly to obtain new coefficients of the algorithmic series.

30. The digital meter of claim 24 wherein said algorithmic series is a low order Fourier series.

31. The digital meter of claim 25 wherein said algorithmic series is a Fourier series.

32. The digital meter of claim 31 wherein even harmonics of said Fourier series are omitted.

33. The digital meter of claim 31 wherein high frequency harmonics of said Fourier series are omitted.

34. The digital meter of claim 24 wherein said magnetic field rate of change pick-up comprises a pick-up wire coil having a ferromagnetic material core for passing around a conductor.

35. The digital meter of claim 34 wherein said core is provided with at least one gap therein.

36. The digital meter of claim 35 including means to impose a magneto-motive force on said ferromagnetic core having a frequency which is not a harmonic of the frequency of the alternating current carried by the conductor, thereby removing effects of core retentivity.

37. The digital meter of claim 36 wherein the magnitude of said magneto-motive force is small compared to the full scale magnitude of the measured time derivative of the magnetic field.

38. The digital meter of claim 37 including an analog to digital converter connected for converting said measurement of the time derivative of the magnetic field to a digital value.

39. The digital meter of claim 24 including an analog to digital converter connected for converting said measurement of the time derivative of the magnetic field to a digital value.

40. The digital meter of claim 24 wherein said computer means is further programmed for expressing said voltage measurement by representing it with an algorithmic series for expressing cyclic phenomena, for separately multiplying the coefficients of sines and the coefficients of cosines for each harmonic in the current and voltage algorithmic series expressions, and for summing the multiplication results and dividing the results in half.

41. The digital meter of claim 40 wherein said algorithmic series are low order Fourier series.

42. The digital meter of claim 24 wherein said algorithmic series is a Fourier series and said computer means is further programmed for eliminating even harmonics in the Fourier series expression of the current derivative.

43. The digital meter of claim 24 wherein said algorithmic series is a Fourier series and said computer means is further programmed for eliminating high frequency harmonic content in the Fourier series expression of the current derivative.

44. The digital meter of claim 40 including noise filter means connected for electronically filtering unwanted noise from said magnetic field rate of change pickup measurement and from said voltage pickup measurement with identical phase shifts, said computer means further programmed for multiplying each harmonic coefficient by the appropriate factor to correct for both the attenuation of the current derivative pickup measurement and the attenuation of the corresponding voltage pickup measurement.

45. The digital meter of claim 40 including identical noise filter means connected for electronically filtering unwanted noise from said magnetic rate of change pickup measurement and from said voltage pickup measurement respectively, said computer means further programmed for multiplying the voltage output harmonic coefficients and the time derivative of the magnetic field output harmonic coefficients by factors required to compensate for the known attenuation to which each has been subjected as a result of said filtering.

46. The digital meter of claim 45 wherein said multiplication factors consist of the reciprocal of the attenuation factor, said computer means thereby being programmed to divide each harmonic by the attenuation factor.

47. The digital meter of claim 24 including noise filter means connected for electronically filtering unwanted noise from said magnetic field rate of change pickup measurement and from said voltage pickup measurement with identical phase shifts, said computer means further programmed for dividing each harmonic coefficient by the square of its attenuation to correct both the magnetic field rate of change pickup attenuation and the attenuation of the corresponding voltage measurement, which has not been put in harmonic form.