Ah yes, transmission line (HVOTL) proximity value impacts. I have prepared many dozens of appraisal reports with the question of the impact on real estate from transmission line proximity. There's a lot of impacted sale data out there, it's not hard to find, but you need a veritable **** pot load of observations to reliably abstract the impact from the other differential noise. There are clear measurable impacts, most of the time, but the impact drops to nothing very quickly as the distance to the lines increases.
After doing so many of these studies, and being extremely diligent for multiple court appearances, I created a very large database of impacted transfers along with at least a handful of unimpacted of comparables for each, which resulted in god knows how many individual case studies. Well over 50 case studies, hundreds of sales.
Impacts generally dont get much higher than 30%, but it could happen. Most properties are not impacted more than 5% to 10%, some properties aren't impacted at all. Low value small residences in urban / densely populated areas are are generally impacted significantly less than high value,, large, high quality, rural estates by similar lines at similar distances. Depending on the impacted property type and the available comparables for reliable abstraction and isolation of the value impact, your research geography and time frame could make research lengthy. It makes sense if the appraisal problem is a lawsuit claiming total loss in value of a multi-million dollar mansion, less so for the likely negligible impact on a 2 bd / 1 bth 900 SF dinky home that hasn't seen so much as paint on a brush in 20 years.
Easiest way to find sales: follow the lines on assessor GIS parcel map viewers. They usually have aerial image base maps, so when you see a property with improvement close to the line, check if it sold in the last 5 or ten years.