Why is my harmonic distortion higher at night than during the day?

Here's a question I've gotten many times: Why is my harmonic distortion higher at night than during the day? 

Now the question to why is my distortion higher at night than during the day really comes down to this? What are we talking about? Are we talking about percent THD or are we talking about the actual harmonic current and the answer almost a hundred percent of the time I would say is percent THD. Let me show you what I'm talking about. Let's talk about why my harmonic percent THD would be higher overnight. 

Let's take and graph my harmonic current. And now this is in actual amps from midnight. So at 12:00 a.m. to noon, 12:00 PM to midnight again, 12:00 AM. Let's say overnight, I have some and then again in a typical commercial building. I've got some harmonic load from my air conditioners, perhaps on and off. And then maybe during the day the lights come on and my background harmonics are higher in terms of actual amps. During the evening, they go back down to what they were previously in the morning. So what are my loads going to be? I have HVAC loads with variable frequency drives on them. I have LED or fluorescent lights, and I have some computers, so all that during the middle of the day is what we're talking about, where the loads come on. Now let's take a look at what the actual amps of 60 Hertz current would be. 

Now, of course, you have a similar load maybe during the evening because you have your air conditioners and other things that are not on drives. Then during the day, you might have perhaps some other loads that are,again 60 hertz. And maybe it comes back down. So, if your 60 hertz load looks like that, then you can see that the percentage of the comparison between night and day would be about the same. Your THD, which is your I harmonics, essentially divided by your I 60 hertz would be about the same. But, here's the fact of reality. Really during the day, it doesn't look like that in the red line, usually overnight, your 60 hertz load might be pretty light. Then, during the day it can become very high and then overnight it becomes very low. Again and again, this is for a typical system, even with harmonic lows in the background. Now, when you take this number divided by this number, we end up with high THD overnight, high harmonics over I 60 Hertz is very much higher overnight than during the day. So if I now plot my THD value, it goes like this and it goes back up. So, the interesting part of this is I THD. 

This is I 60, and let's ignore the red one for now, because that's not very typical that the 60 Hertz doesn't change dramatically over the day. The black here is the I harmonics. So again, because it's proportional and it's a percentage, the actual THD value. Let's say your THD value overnight is 70%. And, let's say your UMD value during the day is 25%. You're going to say, well my THD value is way higher overnight than during the day. But, if you look at it, the actual amps of harmonic current – the black line here are worse during the day. 

My concern is not about percentages and this is where again I find that probably most of the confusion is with harmonics in terms of harmonic. Current specifically is that people get caught up in the fact that the percentages go up when in fact they should be looking at the actual amps of harmonic current to see what the effect is because the actual amps are what will get you in trouble. Because those currents provide distortion on the voltage as well. So, the magnitude of the current is what's important, not the percent THD.