Re: can you help me about reactive power harmonics measurement by FFT
- From: Steve Underwood <steveu@xxxxxxx>
- Date: Sat, 01 Mar 2008 11:20:40 +0800
alpere wrote:
Hi all, thanks for your relevance.
I can measure current RMS value, voltage RMS value, active power, reactive
power, power factor on three phase systems.
What results do your reactive power and power factor measurements give for non-sinusoidal current waveforms?
But i can't measure current harmonics, voltage harmonics, active power
harmonics, reactive power harmonics.
Why? You said you are using a Fourier transform. What more is needed?
For harmonics measurement we must use the DFT. And to do it fastly, we
must use the FFT.
Fine.
This subject is realm of DSP.
True.
But the question is:
What way do I use to measure the harmonics?
The output of the FFT. What else? There are various ways to reduce the calculations by tuning the FFT bins to be exactly on the harmonic lines of the power waveform. You appear to have the compute power to do an FFT with a lot of samples. That's fine.
And i want to know: Why energy analyzer shows only 1., 3., 5., ..., 19. (odd harmonics)
harmonics value?
This is common, and a little bogus. Traditional loads would only produce odd harmonics in the current waveform. There really wasn't much point in looking at the even harmonic frequencies. Modern electronic loads can be much more creative in the current waveform they produce. Monitoring the even harmonics makes sense these days, but power industry practice changes very slowly.
I have samplings of current and voltage.
Samplings form is array of 16 bits integer that array_current(256),
array_voltage(256).
I assume the 256 here is the number of samples per mains cycle, right?
And, i have to convert from those arrays to amplitude_1stHarmonic_current,
amplitude_3stHarmonic_current etc. with FFT.
And, i have to convert for voltage harmonics, current harmonics. Active
and reactive power is derived from current and voltage.
Active power is usually calculated as the dot product of the DC corrected raw voltage and current samples. No Fourier transform is involved there. What you do for reactive power is debatable, as people argue about what reactive means for non-sinusoidal waveforms. There are IEC metering specs which say you delay the voltage waveform by 90 degrees at the fundamental, and take the dot product of that and the current waveform. Again, no Fourier transform is involved.
Maybe what you are actually looking for is the harmonic by harmonic active power.
Regards,
Steve
.
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