System and method for frequency compensation in an energy meter

• US 6,128,584 A
• Filed: 11/30/1998
• Issued: 10/03/2000
• Est. Priority Date: 11/30/1998
• Status: Expired due to Term

• Current Assignee: Elster Electricity LLC (Honeywell International Inc.)
• Original Assignee: ABB Power T&D Limited (ABB Limited)
• Inventors: Hemminger, Rodney C., Holdsclaw, Scott T., Hubbard, Vick A.
• Primary Examiner(s): Hoff, Marc S.
• Assistant Examiner(s): BUI, BRYAN
• Application Number:
  US09/201,640
• Time in Patent Office: 673 Days
• Field of Search: 702/60-62, 702/64-67, 702/73-75, 324/141, 324/142, 361/79, 361/85-87
• US Class Current: 702/75
• CPC Class Codes: G01R 21/133 by using digital technique
• CPC Class Codes: G01R 23/16 Spectrum analysis; Fourier analysis

Abstract

A system and method for improving the ability of an electronic meter to make measurements on signals to determine content of different frequencies, and harmonics of the fundamental frequency, of AC signals (voltages and currents). The line frequency is determined and compensated for prior to performing frequency-dependent parameter measurements or determining frequency-dependent parameters.

Claims

1. A method of measuring frequency dependent electrical parameters by an energy meter in an electrical system that provides electrical energy having a varying frequency, comprising the steps of:

measuring a frequency of the electrical energy;

selecting a reference waveform having a positive zero crossing;

synchronizing two ideal waveforms with said reference waveform, said two ideal waveforms each having an ideal frequency;

obtaining an input signal waveform; and

determining a magnitude of a signal of said ideal frequency within said input signal waveform,

wherein said two ideal waveforms are a function of said frequency and are approximately 90 degrees out of phase with one another, one ideal waveform representing an in-phase component and the other ideal waveform representing a quadrature component.

2. The method according to claim 1, wherein the step of measuring said frequency comprises the steps of determining a time interval between a plurality of samples of a signal waveform and counting a number of samples between a plurality of zero crossings of said signal waveform.

3. The method according to claim 1, further comprising the step of determining an angle of said signal of said ideal frequency within said input signal waveform with respect to said reference waveform.

4. The method according to claim 1, further comprising the steps of:

multiplying said input signal waveform by said in-phase ideal waveform to produce an in-phase product for each sample of a plurality of samples in a line cycle, for at least one line cycle;

adding said in-phase product for each said sample to an in-phase summing accumulator to produce an in-phase summing value;

multiplying said input signal waveform by said quadrature ideal waveform to produce a quadrature product for each sample of said plurality of samples in a line cycle, for said plurality of line cycles; and

adding said quadrature product for each said sample to a quadrature summing accumulator to produce a quadrature summing value.

5. The method according to claim 4, further comprising the step of determining an in-phase quantity and a quadrature quantity.

6. The method according to claim 5, further comprising the step of counting a number of samples in said plurality of line cycles to determine a sample count,

wherein said in-phase quantity=said in-phase summing value/said sample count, and said quadrature quantity=said quadrature summing value/said sample count.

7. The method according to claim 5, wherein said magnitude and angle of said signal of said ideal frequency within said input signal waveform are responsive to said in-phase quantity and said quadrature quantity.

8. The method according to claim 7, further comprising the step of determining said magnitude of said signal of said ideal frequency by a square root of the sum of the squares of said in-phase and quadrature quantities multiplied by a scale factor that is a function of the RMS value of one of said ideal waveforms.

9. The method according to claim 7, wherein said step of determining said angle comprises determining said angle in accordance with the table:

space="preserve" listing-type="tabular">______________________________________ Sign of in-phase Sign of quadrature Angle calculation quantity quantity (in degrees) ______________________________________ + + Arctan(quadrature/in- phase) + 0°

- + Arctan(quadrature/in- phase) + 180°

- - Arctan(quadrature/in- phase) + 180°

+ - Arctan(quadrature/in- phase) + 360°

. ______________________________________

10. A system for measuring frequency dependent electrical parameters by an energy meter in an electrical system that provides electrical energy having a varying frequency via a service type, comprising:

means for measuring a frequency of the electrical energy;

means for selecting a reference waveform having a positive zero crossing;

means for synchronizing two ideal waveforms with said reference waveform, said two ideal waveforms each having an ideal frequency;

means for obtaining an input signal waveform; and

means for determining a magnitude of a signal of said ideal frequency within said input signal waveform,

wherein said two ideal waveforms are a function of said frequency and are approximately 90 degrees out of phase with one another, one ideal waveform representing an in-phase component and the other ideal waveform representing a quadrature component.

11. The system according to claim 10, wherein said means for measuring said frequency comprises means for determining a time interval between a plurality of samples of a signal waveform and means for counting a number of samples between a plurality of zero crossings of said signal waveform.

12. The system according to claim 10, further comprising means for determining an angle of said signal of said ideal frequency within said input signal waveform with respect to said reference waveform.

13. The system according to claim 10, further comprising:

means for multiplying said input signal waveform by said in-phase ideal waveform to produce an in-phase product for each sample of a plurality of samples in a line cycle, for at least one line cycle;

means for adding said in-phase product for each said sample to an in-phase summing accumulator to produce an in-phase summing value;

means for multiplying said input signal waveform by said quadrature ideal waveform to produce a quadrature product for each sample of said plurality of samples in a line cycle, for said plurality of line cycles; and

means for adding said quadrature product for each said sample to a quadrature summing accumulator to produce a quadrature summing value.

14. The system according to claim 13, further comprising means for determining an in-phase quantity and a quadrature quantity.

15. The system according to claim 14, further comprising means for counting a number of samples in said plurality of line cycles to determine a sample count,

wherein said in-phase quantity=said in-phase summing value/said sample count, and said quadrature quantity=said quadrature summing value/said sample count.

16. The system according to claim 14, wherein said magnitude and angle of said signal of said ideal frequency within said input signal waveform are responsive to said in-phase quantity and said quadrature quantity.

17. The system according to claim 16, further comprising means for determining said magnitude of said signal of said ideal frequency by a square root of the sum of the squares of said in-phase and quadrature quantities multiplied by a scale factor that is a function of the RMS value of one of said ideal waveforms.

18. The system according to claim 16, wherein said means for determining said angle comprises means for determining said angle in accordance with the table:

space="preserve" listing-type="tabular">______________________________________ Sign of in-phase Sign of quadrature Angle calculation quantity quantity (in degrees) ______________________________________ + + Arctan(quadrature/in- phase) + 0°

- + Arctan(quadrature/in- phase) + 180°

- - Arctan(quadrature/in- phase) + 180°

+ - Arctan(quadrature/in- phase) + 360°

. ______________________________________

19. An apparatus comprising storage means that stores software that measures frequency dependent electrical parameters in an electrical system that provides electrical energy having a varying frequency via an energy meter and performs the steps of:

measuring a frequency of electrical energy;

selecting a reference waveform having a positive zero crossing;

synchronizing two ideal waveforms with said reference waveform, said two ideal waveforms each having an ideal frequency;

obtaining an input signal waveform; and

determining a magnitude of a signal of said ideal frequency within said input signal waveform,

wherein said two ideal waveforms are a function of said frequency and are approximately 90 degrees out of phase with one another, one ideal waveform representing an in-phase component and the other ideal waveform representing a quadrature component.

20. The apparatus according to claim 19, wherein said software performs the steps of determining a time interval between a plurality of samples of a signal waveform and counting a number of samples between a plurality of zero crossings of said signal waveform.

21. The apparatus according to claim 19, wherein said software further performs the step of determining an angle of said signal of said ideal frequency within said input signal waveform with respect to said reference waveform.

22. The apparatus according to claim 19, wherein said software further performs the steps of:

multiplying said input signal waveform by said in-phase ideal waveform to produce an in-phase product for each sample of a plurality of samples in a line cycle, for at least one line cycle;

adding said in-phase product for each said sample to an in-phase summing accumulator to produce an in-phase summing value;

multiplying said input signal waveform by said quadrature ideal waveform to produce a quadrature product for each sample of said plurality of samples in a line cycle, for said plurality of line cycles; and

adding said quadrature product for each said sample to a quadrature summing accumulator to produce a quadrature summing value.

23. The apparatus according to claim 22, wherein said software further performs the step of determining an in-phase quantity and a quadrature quantity.

24. The apparatus according to claim 23, wherein said software further performs the step of counting a number of samples in said plurality of line cycles to determine a sample count,

wherein said in-phase quantity=said in-phase summing value/said sample count, and said quadrature quantity=said quadrature summing value/said sample count.

25. The apparatus according to claim 23, wherein said magnitude and angle of said signal of said ideal frequency within said input signal waveform are responsive to said in-phase quantity and said quadrature quantity.

26. The apparatus according to claim 25, wherein said software further performs the step of determining said magnitude of said signal of said ideal frequency by a square root of the sum of the squares of said in-phase and quadrature quantities multiplied by a scale factor that is a function of the RMS value of one of said ideal waveforms.

27. The apparatus according to claim 25, wherein said software determines said angle in accordance with the table:

space="preserve" listing-type="tabular">______________________________________ Sign of in-phase Sign of quadrature Angle calculation quantity quantity (in degrees) ______________________________________ + + Arctan(quadrature/in- phase) + 0°

- + Arctan(quadrature/in- phase) + 180°

- - Arctan(quadrature/in- phase) + 180°

+ - Arctan(quadrature/in- phase) + 360°

. ______________________________________

28. A method of measuring frequency dependent electrical parameters by an energy meter in an electrical system that provides electrical energy having a varying frequency, comprising the steps of:

selecting a reference waveform having a positive zero crossing;

synchronizing two ideal waveforms with said reference waveform, said two ideal waveforms each having an ideal frequency;

obtaining an input signal waveform; and

determining a magnitude of a signal of said ideal frequency within said input signal waveform,

wherein said two ideal waveforms are approximately 90 degrees out of phase with one another, one ideal waveform representing an in-phase component and the other ideal waveform representing a quadrature component.

29. The method according to claim 28, further comprising the step of determining an angle of said signal of said ideal frequency within said input signal waveform with respect to said reference waveform.

30. The method according to claim 28, further comprising the steps of:

multiplying said input signal waveform by said in-phase ideal waveform to produce an in-phase product for each sample of a plurality of samples in a line cycle, for at least one line cycle;

adding said in-phase product for each said sample to an in-phase summing accumulator to produce an in-phase summing value;

multiplying said input signal waveform by said quadrature ideal waveform to produce a quadrature product for each sample of said plurality of samples in a line cycle, for said plurality of line cycles; and

adding said quadrature product for each said sample to a quadrature summing accumulator to produce a quadrature summing value.

31. The method according to claim 30, further comprising the step of determining an in-phase quantity and a quadrature quantity.

32. The method according to claim 31, further comprising the step of counting a number of samples in said plurality of line cycles to determine a sample count,

wherein said in-phase quantity=said in-phase summing value/said sample count;

and

said quadrature quantity=said quadrature summing value/said sample count.

33. The method according to claim 31, wherein said magnitude and angle of said signal of said ideal frequency within said input signal waveform are responsive to said in-phase quantity and said quadrature quantity.

34. The method according to claim 33, further comprising the step of determining a magnitude of said signal of said ideal frequency by a square root of the sum of the squares of said in-phase and quadrature quantities multiplied by a scale factor that is a function of the RMS value of one of said ideal waveforms.

35. The method according to claim 33, wherein said step of determining said angle comprises determining said angle in accordance with the table:

space="preserve" listing-type="tabular">______________________________________ Sign of in-phase Sign of quadrature Angle calculation quantity quantity (in degrees) ______________________________________ + + Arctan(quadrature/in- phase) + 0°

- + Arctan(quadrature/in- phase) + 180°

- - Arctan(quadrature/in- phase) + 180°

+ - Arctan(quadrature/in- phase) + 360°

. ______________________________________

36. A system for measuring frequency dependent electrical parameters by an energy meter in an electrical system that provides electrical energy having a varying frequency via a service type, comprising:

means for selecting a reference waveform having a positive zero crossing;

means for synchronizing two ideal waveforms with said reference waveform, said two ideal waveforms each having an ideal frequency;

means for obtaining an input signal waveform; and

means for determining a magnitude of a signal of said ideal frequency within said input signal waveform,

wherein said two ideal waveforms are approximately 90 degrees out of phase with one another, one ideal waveform representing an in-phase component and the other ideal waveform representing a quadrature component.

37. The system according to claim 36, further comprising means for determining an angle of said signal of said ideal frequency within said input signal waveform with respect to said reference waveform.

38. The system according to claim 36, further comprising:

means for multiplying said input signal waveform by said in-phase ideal waveform to produce an in-phase product for each sample of a plurality of samples in a line cycle, for at least one line cycle;

means for adding said in-phase product for each said sample to an in-phase summing accumulator to produce an in-phase summing value;

means for multiplying said input signal waveform by said quadrature ideal waveform to produce a quadrature product for each sample of said plurality of samples in a line cycle, for said plurality of line cycles; and

means for adding said quadrature product for each said sample to a quadrature summing accumulator to produce a quadrature summing value.

39. The system according to claim 38, further comprising means for determining an in-phase quantity and a quadrature quantity.

40. The system according to claim 39, further comprising means for counting a number of samples in said plurality of line cycles to determine a sample count,

wherein said in-phase quantity=said in-phase summing value/said sample count;

and

said quadrature quantity=said quadrature summing value/said sample count.

41. The system according to claim 39, wherein said magnitude and angle of said signal of said ideal frequency within said input signal waveform are responsive to said in-phase quantity and said quadrature quantity.

42. The system according to claim 41, further comprising means for determining a magnitude of said ideal frequency by a square root of the sum of the squares of said in-phase and quadrature quantities multiplied by a scale factor that is a function of the RMS value of one of said ideal waveforms.

43. The system according to claim 41, wherein said means for determining said angle comprises means for determining said angle in accordance with the table:

space="preserve" listing-type="tabular">______________________________________ Sign of in-phase Sign of quadrature Angle calculation quantity quantity (in degrees) ______________________________________ + + Arctan(quadrature/in- phase) + 0°

- + Arctan(quadrature/in- phase) + 180°

- - Arctan(quadrature/in- phase) + 180°

+ - Arctan(quadrature/in- phase) + 360°

. ______________________________________