You can Google "living under high voltage power lines" and get a bunch of hits.
I lived near
them when I lived in SoCal and rode under
them frequently --- which my horses did not like - especially when it was cold out and vibration sound increased.
I attributed their dislike to the noise bothering their ears; which are a lot more sensitive than mine and, I didn't like the sound.
Would I want my horses living 24/7/365 days under those high tension wires?
No I would not, but that's just me. That means I would never consider buying a farm that had high tension wires running thru any portion where livestock lived or any sort of feed was being grown - human or otherwise. Again, that's just me
My thought is to read credible articles on the Net, talk to the horse owners that already have their horses in that pasture. Ask the credible ones that actually pay attention to their horse's environment, as opposed to the one who just "threw" their horse out there and might see it once or twice a month; how would they know anything?
Then make your decision