If in fact a walnut it may or may not still have a leader, depending upon its age or size. Regardless, if it still has a leader and you prune it, you will just be hastening its move to a naturally decurrent form.

Nonetheless, presuming in fact a walnut, you cannot continue to prune this tree to keep it, say, 15-20 feet tall. You are the inheritor of an all-too-common bad planting decision, and you will have to choose whether you want to enjoy many fun years of paying certified arborists to come out and prune this tree off the power lines. It is very common to see walnuts35-45 feet tall in subdivisions and if it likes the soil then it will grow up and out and give you lots of leaves for your compost pile, to boot.

Dan

Let's get one thing straight. YOU cannot do any trimming on ANY tree that is that near powerlines. Never. That might be one of the more common methods of homeowner tree trimming injury or death. That and hoisting a running chain saw up a ladder.

Picture would definately help here, however DO NOT do any pruning near power lines, contact a certified arborist, and not just a common tree hacker.
Its possible he could direct it through pruning away from the power lines and you wouldn't have anything to worry about for another 10 - 15 years. But without pictures I couldn't tell you or sure.