Do harmonic and 60 Hz load currents change at the same rate?

Your percent distortion can increase overnight to as much as 50%, 60% and 70%. But, during the day, your percent distortion may be at 30% or 20%. Which is worse? It goes back to the amperes of harmonic current. The percentages of load changes do not change linearly.

For example, a typical power system with a utility source, a transformer, bus and a variable frequency drive (VFD) feeds a motor. There may also be some other across-the-line motors and lights. What generally happens is the load profile overnight might be low but then during the day, it goes up and maybe comes back down again overnight. It has a cyclic pattern for that 24-hour period. 

But what happens is the 60 Hz current also follows that cyclic pattern, whereas the harmonics for the VFDs and other such loads do not follow that linear progression. As more load is added, the percent changes one to one. What happens with VFDs is they are very heavily front-end loaded with harmonics. 

Even with a very light load, the harmonics can be very high. The percent THD is based on the square root of the sum of all the harmonics squared divided by the fundamental value. Because the fundamental changes linearly and the harmonics do not, the percent THD overnight and in times of light load can become a much higher number. But just because the percent distortion is high, doesn't mean that the overall actual harmonic amperes are higher overnight or during offload times. It is important to realize percent distortion versus actual amperes.