Why are harmonics intermittent (not always there at certain times, days, etc.)?

One of the questions we get is why do my harmonics seem to be intermittent? 

One of the things I would comment on if I talk about the intermittency or the irregularity of harmonics is that it’s not so much that the harmonics themselves in terms of the RMS values are changing much. It's usually a percentage that's changing. Let me just show you. For example, on a variable frequency drive, this happens to be an 18 pulse drive. Let's look at a graph once and take a look at this curve or this table. What’s interesting about this to me is that if you look at all harmonics, meaning all the RMS values of harmonics added together, it's 8.2 amps at 30 Hertz, which is half loading of the drive. When you look at full load of the drive, it goes up to 10.8, which is not really a proportional change. So, you would think half load, half the harmonics, but in fact, most of the harmonics show up at a light load, and then it tails off as you go up in load. 

If you look at the fundamental, the load at 30 Hertz, because of the cubic function of the effect of drives and the energy savings that's associated with it changes dramatically from the fundamental being 30.8 amps up to 182 amps. So, when you take a look at that and you compare the fundamental, not changing or changing significantly, and the harmonics not changing a lot, then if I show you on a graphic here of what could happen on your power system. If you plot let's say the value of your harmonics and maybe it changes a little bit in terms of RMS, and during the day, it goes up a little bit, and maybe goes back down a little bit overnight. If this is right in the middle of the day at noon, and then this is 12:00 a.m., and this is 12:00 a.m. here. Basically, what happens is the actual RMS value of the harmonics doesn't change a lot. Now let's take a look at the 60 Hertz value. The 60 Hertz value on the other hand might do something like this, and it might change dramatically during the day. And, then it might go down overnight and kind of mimic something that's going on there. So, if I plot my percent, this is my 60 Hertz value. 

If I plot my percent, THD then what's going happen is my percent, THD is going be much higher overnight and much lower during the day. So this is percent THD because percent THD Is basically taking the square root of all the harmonics times a hundred percent divided by H one, which is the fundamental. So, if the fundamental where the 60 Hertz part goes up, then the percent distortion goes down for a relatively continuous value of harmonics in terms of RMS. 

So, if we take a look at what's going on with the actual differences in harmonics, it's not so much that the harmonics are intermittent are changing, but the percent THD can change dramatically throughout the day. If you watch voltage distortion, by the way, it will trend with the amount of harmonic currents. So, the percentage of actual amps or harmonic, or the actual value harmonic amps will change with the percent THD on the voltage because impedes of the system is generally the same. The percent current distortion will change dramatically, depending on how much 60 Hertz current you have in the background. And, it's almost always going to change dramatically the opposite of what you might think. So, higher percentage overnight, lower percentage during the day, if you have a relatively constant RMS value of harmonic.