Will riding horses under high tension power lines make my horse scared?

Will the show grounds have a warmup area which is near or under the power lines? If so, then make plans to arrive early enough so that you can take your mare there and work with her until she relaxes and accepts the sound. The power lines make noise because they give off magnetic energy- and what your mare is responding to is the vibration that produces, rather than the noise itself. If you get to the show well before your cl, you should be able to work your mare near the lines. I would call the show secretary and ask where the warmup area is in relation to the powerlines. That way, if you know it's close, you can make your plans accordingly.