Brownout is not THD.
Some reading material for you. Yes, my formal education does include electronics engineering.
"IEEE Standard 519-2014(opens in a new tab) advises on the permissible harmonic distortion limit for electronic equipment. As per the article, computers and allied equipment like programmable controllers typically require AC sources that have no more than a 5% harmonic voltage distortion factor,
with the largest single harmonic being no more than 3% of the fundamental voltage."
https://generatorbible.com/blog/what-is-thd/