POWER WAVEFORM HARMONIC ANALYSIS USING SUMMED SAMPLES

• US 20080218153A1
• Filed: 03/07/2007
• Published: 09/11/2008
• Est. Priority Date: 03/07/2007
• Status: Active Grant

• Current Assignee: Eaton Intelligent Power Limited (Eaton Corporation PLC.)
• Original Assignee: Eaton Power Quality Corporation (Eaton Corporation PLC.)
• Inventors: Tassitino, Frederick, Patel, Pratik Shirish
• Granted Patent: US 7,502,237 B2
• Time in Patent Office: Days
• US Class Current: 324/76.21
• CPC Class Codes: G01R 19/2513 Arrangements for monitoring electric power systems, e.g. power lines or loads; Logging
• CPC Class Codes: G01R 23/20 Measurement of non-linear distortion
• CPC Class Codes: G01R 29/04 Measuring form factor, i.e. quotient of root-mean-square value and arithmetic mean of instantaneous value; Measuring peak factor, i.e. quotient of maximum value and root-mean-square value

Abstract

Provided are methods and apparatus for characterizing a power waveform. Some such methods include sampling the power waveform to generate a plurality of samples and, for each of the samples, recursively processing the sample to generate an aggregate signal energy value, processing the sample to generate a harmonic component signal energy value associated with at least one target harmonic frequency, and accumulating the aggregate signal energy value and the component signal energy value with previously generated aggregate signal energy values and component signal energy values to generate an accumulated aggregate signal energy value and an accumulated harmonic component signal energy value. Methods also include characterizing the power waveform responsive to the accumulated aggregate signal energy value and the accumulated harmonic component signal energy value.

Claims

1. A method of characterizing a power waveform, the method comprising:

sampling the power waveform to generate a plurality of samples;

recursively performing the following for each of the samples;

processing the sample to generate an aggregate signal energy value;

processing the sample to generate a harmonic component signal energy value associated with at least one target harmonic frequency; and

accumulating the aggregate signal energy value and the harmonic component signal energy value with previously generated aggregate signal energy values and harmonic component signal energy values to generate an accumulated aggregate signal energy value and an accumulated harmonic component signal energy value; and

wherein the method further comprises

characterizing the power waveform responsive to the accumulated aggregate signal energy value and the accumulated harmonic component signal energy value.

2. The method of claim 1, wherein processing the samples to generate harmonic component signal energy values associated with at least one target harmonic component comprises performing bin-limited Discrete Fourier Transform (DFT) computations for the plurality of samples.

3. The method of claim 1, further comprising estimating a cycle timing of the power waveform and wherein sampling the power waveform to generate a plurality of samples comprises adjusting a sample period for the sampling of the power responsive to the estimated timing.

4. The method of claim 3:

wherein estimating a cycle timing of the power waveform comprises estimating a period of a first harmonic of the power waveform; and

wherein adjusting a sample period for the sampling of the power responsive to the estimated timing comprises adjusting the sample period to maintain a fixed number of sample periods within the estimated period of the first harmonic.

5. The method of claim 1:

wherein processing the sample to generate an aggregate signal energy value comprises computing a squared value of the sample;

wherein processing the sample to generate a harmonic component signal energy value associated with at least one target harmonic component comprises computing real and imaginary values for the at least one target harmonic frequency; and

wherein respectively accumulating the aggregate signal energy values and the component signal energy values to recursively generate an accumulated aggregate signal energy value and an accumulated harmonic component signal energy value comprises;

accumulating the squared values to generate the accumulated aggregate signal energy value; and

accumulating the respective real and imaginary values to generate respective accumulated real and imaginary values.

6. The method of claim 5, further comprising generating a target harmonic peak energy value by dividing the square root of the sum of the accumulated real values squared and the accumulated imaginary values squared by a quantity of samples.

7. The method of claim 1, wherein generating the aggregate signal energy value comprises computing a squared value of each sample and wherein the accumulating the aggregate signal energy value comprises adding each aggregate signal energy value to a sum of previously generated aggregate signal energy values.

8. The method of claim 1, wherein a quantity of sample values accumulated in a sample set is constant.

9. The method of claim 1, wherein generating the harmonic component signal energy value comprises multiplying the each of the plurality of samples with a cosine of a value that is responsive to the at least one target harmonic component and wherein accumulating the component signal energy value comprises adding the component signal energy value to previously generated component signal energy values.

10. The method of claim 1, wherein characterizing the power waveform comprises generating a total harmonic distortion percent as a first value divided by a second value, wherein the first value comprises a square root of a sum of the accumulated aggregate signal energy value and a square of the accumulated harmonic component signal energy, wherein the second value comprises the accumulated harmonic component signal energy.

11. An apparatus, comprising:

a sample generator circuit configured to generate samples of a power waveform signal;

a signal energy determiner circuit configured to recursively;

determine an aggregate signal energy value;

to determine

a harmonic real signal energy value associated with at least one target harmonic frequency and a harmonic imaginary signal energy value associated with the at least one target harmonic frequency;

to

accumulate the aggregate signal energy value with a sum of previously generated aggregate signal energy values; and

to

accumulate the harmonic component signal energy value with a sum of previously generated harmonic component signal energy values; and

a power waveform characterizer configured to characterize the power waveform responsive to the accumulated aggregate signal energy value and the accumulated harmonic component signal energy value.

12. The apparatus of claim 11, wherein the sample generator circuit comprises an analog-to-digital converter configured to generate a fixed quantity of sample values per sample set.

13. The apparatus of claim 11, further comprising:

a power waveform cycle timing estimator configured to estimate a period of a first harmonic of the power waveform; and

a sampling period regulator configured to adjust a sampling period responsive to the estimated cycle timing to maintain a fixed quantity of sample periods within the estimated period of the first harmonic, wherein a quantity of first harmonic periods in a sample set comprises an integer value.

14. The apparatus of claim 11, wherein the power waveform characterizer is configured to calculate percent total harmonic distortion and the aggregate signal energy values comprise a square of each of the samples.

15. The apparatus of claim 11, wherein the harmonic real signal energy values comprise a cosine function of each of the plurality of sample values and the harmonic imaginary signal energy values comprise a sine function of each of the plurality of sample values.

16. The apparatus of claim 11, further comprising an alert generator configured to generate an alert signal when a power waveform property value determined by the characterizing logic traverses a threshold value.

17. The apparatus of claim 11, wherein the electrical power apparatus comprises a power distribution apparatus and wherein the power waveform is characterized for each of a plurality of power distribution outputs.

18. The apparatus of claim 11, wherein the power waveform characterizer is configured to calculate a fundamental energy of the input signal corresponding to the energy of the input signal at a first harmonic frequency and a total energy of the input signal corresponding to the total energy of the input signal.

19. The apparatus of claim 18, wherein the power waveform characterizer is further configured to calculate a percent total harmonic distortion responsive to the fundamental energy and the total energy.

20. A method of characterizing a power waveform, comprising:

synchronizing a sampling frequency to an integer factor of a first harmonic frequency;

sampling, at the sampling frequency, the power waveform to receive a fixed quantity of a plurality of sample values;

summing selected harmonic components of the plurality of sample values;

summing aggregate signal energy values of the plurality of sample values; and

calculating total harmonic distortion responsive to the sums of the selected harmonic components and sums of the aggregate signal energy values.