Should I be more concerned with 100A of 5th harmonic or 80% THD?  Can I increase my linear load to cancel my harmonics?

Should I be more concerned with, say, 100 amps of harmonic current or 80% distortion?

Current distortion at 80% sounds pretty terrible. But you have to take into consideration such things like having a light load on a big system or is that percentage a large percentage of your system capacity. You have to know what you’re asking in regard to harmonic current versus harmonic distortion. It’s like comparing apples and oranges. Current distortion in terms of amperes and current distortion in terms of percentages is definitely different.

For example, if I had 100 amps of harmonic current and my 60 Hz current was 200 amps, then 80% distortion on 200 amps would be 160 amps. If we compare that to 100 amps, then obviously 80% distortion would be worse. But, if my 60 Hz current was 100 amps, then the 80% distortion is 80 amps and so 100 amps would be worse. It really depends on which one is more actual amps of harmonic current—that is the key because that current is what’s causing the voltage distortion and doing the damage to your system in terms of overheating. So, when there is voltage distortion, the actual amps are what’s more important than the percent THD on the current.

Say we’re comparing just a single harmonic, for example, the 5th, to the percent distortion. Then it’s really more important to understand and calculate the percent THD. The percent THD equals the square root of the sum of the squares of all the harmonics. For example, h2 squared (the 2nd harmonic) plus h3 squared plus h4 squared… divided by h1 or the fundamental. So, if we had, for example, 80% THD—if the fundamental current was 100 amps, then the square root of the sum of the squares of this would be 80, giving us 80% distortion. So, by this example, 80 amps is important to understand. Now what happens if you change the distortion level by changing the fundamental? For example, if the load becomes very light overnight, the 60 Hz load becomes light and this number is now 50 amps—now there is a problem. The 60 Hz current is even less than the fundamental current so the percent distortion is going to be even bigger—160%. This situation doesn’t usually happen on the phase conductors but it certainly can happen on the neutral conductors. 

Percentages can be very misleading. Don’t overly concentrate on the percentages because the amps of current are what causes the harmonic issues and the voltage distortion and heating associated with them.