"characterize"? In terms of interpreting measurements or in terms of "predicting by simulation"?

Basically, all THD come from just 3 sources:

• switching (loads on and off). These are short-lived and of lesser interest in most cases.
• very low-frequency harmonics (lower than the grid frequency), stemming from "wave group switching" (in effect "consuming" complete half waves or full waves in a resistive load). These can be annoying if they result in illumination flicker due to a periodically varying effective grid voltage.
• the "usual" harmonics - caused by all sorts of "rectification side-effects". Rectification of all sorts is the most common cause of harmonics. PFC (Power Factor Control) attempts to reduce these harmonics.

Hope this helps - or sparks further discussion.

thanks for your answers to you both

Merry Christmas