I have ridden under high voltage lines alot, both as a kid and now as an adult, where they are part of our fairly modest trail system at the barn. I had no problem with them at all as a kid, but as an adult find them a little creepy but have had no actual problems with them either. I figure they are unavoidable in trail riding as often they provide the only right of way through some areas.
The only situations I have heard of there being stray electricity, which I have never experienced myself, is in rainy weather. Also I understand that the amount of voltage through the power lines varies and if it’s heavy, you can hear them crackling and popping and fizzing overhead. If you have to cross a powerline, it’s said to be safer to cross near the towers rather than under the middle of the cables, but honestly around here people rack up hours and hours on hydro right of ways and no problem. People hike on them, and our local powerline is also a bike route.
All the accounts of potentially dangerous stray voltage that I’ve read about have come not from big powerlines, but from infrastructure breakdown in urban areas,
Go out and enjoy!