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To calculate THD, or Total Harmonic Distortion, as a percentage of the fundamental, here's how that works.
Take a waveform—a waveform that is not clean and sinusoidal means that it is distorted from harmonics. A Fourier analysis can be performed on one electrical cycle of that waveform, which will take all of the subfrequencies or the other frequencies associated with that waveform, break them down and rebuild the waveform based on all of those frequencies.
For example, when the waveform is analyzed, you may see a fundamental part at 60 Hz as well as 3rd, 5th, 7th, 11th and 13th harmonics. Those are typical harmonics that we might see on a power system. If you wanted to calculate the THD, you would take all of the harmonics in order, square them, take the square root of that total and divide by the fundamental.
In this case, because the fundamental is 60 Hz (100%), we'll use 100 amps as the number. There would be 20 amps of 3rd harmonic, 12 amps of 5th harmonic and so forth. The higher order harmonics really don't become a large number when you square them, so the lower order harmonics are significant. In this example, we end up with a number that's about 24% distortion.
If you change the fundamental current from 100 amps to 50 amps and keep the harmonic distortion values the same (the 3rd, 5th, 7th, 11th and 13th), the distortion doubles from what it was before, but the actual harmonic amperes remain the same.
It is important not to worry about the increasing harmonic percentages. At night, a lot of times, you could have a very low 60 Hz part, but the harmonic magnitudes or the actual harmonic amperes could still be the same or even a little less, but the percentages look extremely high. The actual amperes are what's causing the harmonic problems in the power system, such as the overheating.
Note that when you have harmonic current distortion in amperes, you produce harmonic voltage distortion in volts. That is why percentages can be misleading when the THD is calculated.
