Seems like you should do a bit of research and find out what type of transmission lines they are, what voltage is going over them and most importantly what the easement is.
We have an easement near our place. It is about 1/2 mile away and they have three transmission lines installed. The latest one was a DC line and I think it is 450K volts. (I’m not really sure of the actual voltage, but know that it is high)
When you are standing under them you can hear it hum, some people say they can feel the hair on the head move, but I have never felt that. The utilities company will tell you they are perfectly safe and not to worry and that the lines do not effect any electronics equipment. I do know the AM radio in the car won't play properly when near them, but who plays the AM radio any more?
Find out if they are permitted to expand the number of lines within the easement. Here in NH most of the power lines are being used for snowmobile trails and illegally for motorcycle, dirt bike and 4 wheelers.
All thought the utilities have an easement to cross the land and permission to cut brush they do not police the property for trespasser. This task is left up to the local police and residence to take care of. So if you see that people are using the power line area already for this type of activity it is very difficult to stop them.
If it is far enough away from where you wish to build and you are not buying the property to subdivide and sell off lots than I wouldn't worry too much.
Wayne