Flying underneath powerlines is sometimes part of the buiseness - even so, avoided, if there is an suitable alternative.
But itīs not a big deal, if you are propperly trained.
As Flying Bull said, it's normally not a problem and 'some' operators do this on a regular basis due to the need to stay low.
High voltage can jump 1 cm per 10.000 Volts, this is due to the conductivity of normal air. 400 KV therefore can jump 40 cm.
To add safety to this the risk area in sweden is a radius of 2.5m in air if I remember it right.
I've done a lot of flying underneath powerlines, both crossing and following under for some hundred meters. My total time underneath powerlines is measured in hours, not seconds. Never hade any problem, despite including rainy days, snowy/dusty conditions etc.