Digital power metering system for reducing spectral leakage when determining the frequencies present in a power signal
A digital power metering system reduces spectral leakage when determining the frequencies present in a power signal by a technique that includes sampling the signal and performing a transformation from the time domain to the frequency domain. The system initially measures the fundamental frequency of the power signal, and samples the power signal at a known sampling rate to produce digital sample values representing the power signal. The system then determines whether the known sampling rate is within a preselected range of a rate that is coherent with the measured fundamental frequency. If the answer is negative, then the system resamples the signal at a rate that is coherent with the measured fundamental frequency to produce resample values representing the signal, so that the frequencies present in the power signal can be determined from the resample values.
1. A digital power metering process for reducing spectral leakage when determining the frequencies present in a power signal by a technique that includes sampling said signal and performing a transformation from the time domain to the frequency domain, said process comprising
measuring the fundamental frequency of said power signal,
sampling said power signal at a known sampling rate to produce digital sample values representing said signal,
determining whether said known sampling rate is within a preselected range of a rate that is coherent with the measured fundamental frequency, and if the answer is negative then
resampling said signal at a rate that is coherent with said measured fundamental frequency to produce resample values representing said signal, so that the frequencies present in said power signal can be determined from said resample values.
2. The digital power metering process of claim 1 wherein said sampling of said power signal is effected by an A/D converter.
3. The digital power metering process of claim 1 wherein said resampling is effected by interpolation.
4. The digital power metering process of claim 1 wherein said resampling is effected by linear interpolation.
5. The digital power metering process of claim 1 wherein said resampling includes reconstructing said power signal and then resampling the reconstructed waveform.
6. The digital power metering process of claim 1 which includes determining the frequencies present in said power signal by use of said resample values in a Fast Fourier Transform (FFT).
7. A digital power metering system for reducing spectral leakage when determining the frequencies present in a power signal by a technique that includes sampling said signal and performing a transformation from the time domain to the frequency domain, said system comprising a microprocessor programmed to
measure the fundamental frequency of said power signal,
sample said power signal at a known sampling rate to produce digital sample values representing said signal,
determine whether said known sampling rate is within a preselected range of a rate that is coherent with the measured fundamental frequency, and if the answer is negative then
resample said signal at a rate that is coherent with said measured fundamental frequency to produce resample values representing said signal, so that the frequencies present in said power signal can be determined from said resample values.
8. The digital power metering system of claim 7 which includes an A/D converter for sampling of said power signal.
9. The digital power metering system of claim 7 wherein said microprocessor is programmed to resample by interpolation.
10. The digital power metering system of claim 7 wherein said microprocessor is programmed to resample by linear interpolation.
11. The digital power metering system of claim 7 wherein said microprocessor is programmed to resample by reconstructing the waveform of said power signal and then resampling the reconstructed waveform.
12. The digital power metering system of claim 7 wherein said microprocessor is programmed to determine the frequencies present in said power signal by use of said resample values in a Fast Fourier Transform (FFT).