Distortion on the Power Line

Nurse! Thorazine! Aisle 6. Man down!

You’ve gotten some good answers here, but the distortion problem you’re having could just be the very nature of the electrical distribution system itself. The electrical grid in the US is a very dynamic system. Utilities are routinely seeking to balance loads and efficiently deliver power, and some electric utilities are also under state-mandated reliability improvement programs. So the circuit you are on today may not be the circuit you are on tomorrow. Even your supplying substation may change over time.

That businesses and factories have reduced operations or even closed entirely must have had a huge impact on electric demand and use. At the same time, residential electric use must be on the increase as so many people are confined to their homes, and often working from there, too. All of those factors combined make it likely that your electric utility has made changes to its distribution system. Whether that’s the cause of your distortion is a separate question. It might be a good idea to take the suggestions of @jea48, which might help narrow down the root cause of the distortion and is good advice in any event.

I’m just trying to be helpful here. Noise and distortion are two different things. That why you have the expression for Signal/Noise + Distortion. This all fits in by the way with the discussion regarding the dodgy subject, “What is the signal and why is it subject to vibration, RF interference and physical characteristics of the conductor?“ We’ll get there. Well, maybe we won’t, who the hell knows?

Typically a solar power system would have very low voltage THD unloaded, but the potential for higher current THD loaded. They are measured and rated for current THD.  The output impedance should be a lot lower than the output impedance of a shared transformers unless the system is rather large. To that end, the grid should be dominant but no guarantees. It is certainly something to consider.