Do all harmonics come from the loads or do some come from the utility source?

We've talked a lot about where harmonics come from. But do all harmonics come from the loads or do utilities and other sources out there produce harmonics.

We talk about harmonic currents coming from loads like VFDs or from computer power supplies or anything electronic basically pushing the current out of the system up through the busway up through the transformers, back out onto the utility system. Because current flows in a loop it leaves the loads, goes all the way out, and comes back to the load. In in fact, harmonic currents are generated by the loads, but can we have sources of harmonic that come from the utility side? The answer is yes, in two ways. One is if we have harmonics coming from your neighbor or if we have voltage distortion coming from the source. It's possible but I'm going to draw it out for you and show you what I'm thinking.

Let's draw a simple power system here and we have again utility transformer maybe this is the point of common coupling. Generally speaking, we could have maybe another transformer and downstream maybe some load. Let's call it a VFD. Then we say this harmonic current comes from the load. Then it circulates back up on the system. It goes out and it comes back to itself again.

Current flows in a loop, so it goes out and comes back. Depending on where it goes out on the system it could go all the way back. If we had, say for example a harmonic filter here maybe some of the current goes through the filter and circulates back to the load, but current flows out of the loads goes back to the source and it kind of flows in that path.

Where else could harmonic current come from? Well maybe your neighbor over here has a transformer with some VFDs. Those currents could in fact come out of their source and come down into your system. But why would they want to? The reason they might want to be, is because the source impedance should be low. The current should want to flow back up through the system but in fact the current might come down this way. For example, if you did have a harmonic filter or more traditionally, a standard capacitor that may be tuned or detuned, this looks like an attractive path. We could then have harmonic currents leaving your neighbor, coming into your harmonic filter or into your capacitor, and then flowing back to themselves. Always remember current must flow in a loop.

Let's take another example. A second example might be that we have a transform for here. Could we in fact generate harmonic voltage distortion? Most of the time we say currents come from the loads and we have that current flow back up through the system. But if we don't have loads down here. And we have distorted voltage up here, we're going to draw a current down through it. For example, a motor that's cross the line and that's proportional to that voltage would be an example. The current would in fact maybe look and could be out of phase. It could look distorted. You would have voltage distortion causing the current distortion. It's a more unusual situation this way. Current would flow this way toward the load from the source that is distorted. But ultimately the reason that the voltage source is distorted is because of harmonics somewhere in the system unless that generator itself is creating harmonics on its own.

A third example would be to have a UPS here. Let's put in a small UPS that creates a square wave. Square wave voltage has fundamental voltage but then it has a lot of the other frequencies associated with it. For the same reason loads downstream will draw a current that may look proportional to that voltage distortion.

So, do all harmonic currents come from the load? Most harmonic currents come from loads. They could be your load or the neighbor's load.

Does harmonic voltage come from you by the currents that are getting distorted on the system? Yes, but you can also have sources, generators and specifically small UPS's that cause voltage distortion that would then translate down into the system. But most times we think about harmonic currents coming from the loads flowing back up through the system.