METHODS AND APPARATUS FOR MEASURING THE FUNDAMENTAL FREQUENCY OF A LINE SIGNAL

• US 20120310569A1
• Filed: 06/06/2011
• Published: 12/06/2012
• Est. Priority Date: 06/06/2011
• Status: Active Grant

• Current Assignee: Siemens Industry Incorporated (Siemens AG)
• Original Assignee: Siemens Industry Incorporated (Siemens AG)
• Inventors: Fan, Chun, Terricciano, Paul
• Granted Patent: US 9,037,429 B2
• Time in Patent Office: Days
• US Class Current: 702/64
• CPC Class Codes: G01R 19/2513 Arrangements for monitoring electric power systems, e.g. power lines or loads; Logging
• CPC Class Codes: G01R 23/15 Indicating that frequency of pulses is either above or below a predetermined value or within or outside a predetermined range of values, by making use of non-linear or digital elements (indicating that pulse width is above or below a certain limit)

Abstract

The present invention provides methods, apparatus, and systems of measuring frequency. The invention includes sampling a first number of full cycles of a signal at a rate based on a nominal frequency of the signal; converting the samples to digital samples; processing the digital samples to identify a first pair of digital samples with each one disposed on either side of a first zero-crossing and a second pair of digital samples with each one disposed on either side of a last full cycle zero-crossing; interpolating the first pair of digital samples and the second pair of digital samples to determine a position of the first and last zero-crossings relative to each other in time; and calculating a fundamental frequency of the signal based on the first number of full cycles sampled and the relative position of the first and last zero-crossings. Numerous additional aspects are disclosed.

Claims

1. A method of measuring frequency comprising:

sampling a first number of full cycles of a signal at a rate based on a nominal frequency of the signal;

converting the samples to digital samples;

processing the digital samples to identify a first pair of digital samples with each one disposed on either side of a first zero-crossing and a second pair of digital samples with each one disposed on either side of a last full cycle zero-crossing;

interpolating the first pair of digital samples and the second pair of digital samples to determine a position of the first and last zero-crossings relative to each other in time; and

calculating a fundamental frequency of the signal based on the first number of full cycles sampled and the relative position of the first and last zero-crossings.

2. The method of claim 1, further comprising:

sampling a second number of full cycles of a signal at a rate based on the calculated fundamental frequency of the signal;

converting the samples to digital samples;

processing the digital samples to identify a third pair of digital samples with each one disposed on either side of a third zero-crossing and a fourth pair of digital samples with each one disposed on either side of a final full cycle zero-crossing;

interpolating the third pair of digital samples and the fourth pair of digital samples to determine a position of the third and final zero-crossings relative to each other in time; and

calculating a final fundamental frequency of the signal based on the second number of full cycles sampled and the relative position of the third and final zero-crossings.

3. The method of claim 1 further comprising repeating the method of claim 1 using the calculated fundamental frequency of the signal as a basis for setting the rate at which the first number of full cycles are subsequently sampled.

4. The method of claim 1 further comprising iteratively repeating the method of claim 1 using each successive calculation of the fundamental frequency as a basis for setting the rate at which subsequent full cycles are sampled.

5. The method of claim 1 further comprising filtering the digital samples before the step of processing the digital samples.

6. The method of claim 5 wherein filtering the digital samples includes applying a band pass filter to a subset of the sampled digital samples determined based upon a settling time of the band pass filter.

7. The method of claim 6 wherein applying a band pass filter includes applying a direct form II realization of a band pass filter.

8. The method of claim 1 wherein interpolating the first pair of digital samples and the second pair of digital samples includes linearly interpolating the pairs of digital samples.

9. The method of claim 1 wherein interpolating the first pair of digital samples includes computing a zero-crossing position based upon the following equation:

10. A circuit for measuring frequency comprising:

an analog to digital (A/D) converter adapted to sample a first number of full cycles of a signal at a rate based on a nominal frequency of the signal and adapted to converting the samples to digital samples; and

a digital signal processor (DSP) coupled to the A/D converter and adapted to;

process the digital samples to identify a first pair of digital samples with each one disposed on either side of a first zero-crossing and a second pair of digital samples with each one disposed on either side of a last full cycle zero-crossing,

interpolate the first pair of digital samples and the second pair of digital samples to determine a position of the first and last zero-crossings relative to each other in time, and

calculate a fundamental frequency of the signal based on the first number of full cycles sampled and the relative position of the first and last zero-crossings.

11. The circuit of claim 10, wherein the A/D converter is further adapted to:

sample a second number of full cycles of a signal at a rate based on the calculated fundamental frequency of the signal; and

converting the samples to digital samples.

12. The circuit of claim 11, wherein the DSP is further adapted to:

process the digital samples to identify a third pair of digital samples with each one disposed on either side of a third zero-crossing and a fourth pair of digital samples with each one disposed on either side of a final full cycle zero-crossing;

interpolate the third pair of digital samples and the fourth pair of digital samples to determine a position of the third and final zero-crossings relative to each other in time; and

calculate a final fundamental frequency of the signal based on the second number of full cycles sampled and the relative position of the third and final zero-crossings.

13. The circuit of claim 10 wherein the DSP is adapted to use the calculated fundamental frequency of the signal as a basis for setting the rate at which the A/D converter subsequently samples additional full cycles of data.

14. The circuit of claim 10 wherein the circuit is adapted to iteratively repeat sampling data and calculating fundamental frequency using each successive calculation of the fundamental frequency by the DSP as a basis for setting the rate at which subsequent full cycles are sampled by the A/D converter.

15. The circuit of claim 10 wherein the DSP includes a filter adapted to filter the digital samples before the DSP processes the digital samples.

16. The circuit of claim 15 wherein the filter is a band pass filter adapted to be applied to a subset of the sampled digital samples determined based upon a settling time of the band pass filter.

17. The circuit of claim 15 wherein the filter is a direct form II realization of a filter.

18. The circuit of claim 10 wherein the DSP is adapted to interpolate the first pair of digital samples and the second pair of digital samples using linear interpolation of the pairs of digital samples.

19. The circuit of claim 10 wherein the DSP is adapted to interpolate the first pair of digital samples by computing a zero-crossing position based upon the following equation:

20. A method to synchronize voltage sampling to power line fundamental frequency comprising:

sampling a first number of full cycles of a power line signal at a rate based on a nominal frequency of the power line;

converting the samples to digital samples;

processing the digital samples to identify a first pair of digital samples with each one disposed on either side of a first zero-crossing and a second pair of digital samples with each one disposed on either side of a last full cycle zero-crossing;

interpolating the first pair of digital samples and the second pair of digital samples to determine a position of the first and last zero-crossings relative to each other in time;

calculating a fundamental frequency of the signal based on the first number of full cycles sampled and the relative position of the first and last zero-crossings; and

measuring at least one of voltage RMS and current RMS using a sampling rate based on the calculated fundamental frequency of the signal.