High voltage line do not affect the radio signals what they will affect is the electronics in your plane and/ or helicopter.
It is true that all power line produce 'EMF' and the higher the voltage the greater the EMF. Now in high voltage tower the line could carry upto 500,000 volts and the magnetic field it produces would probably be in excess of 200-300ft ( Getting weaker as it goes further ). Try and stay at least that far away from them, because it could mess up the circuits in the receiver or ESC.
When you pass a metal object through a magnetic field it produces voltage in that object.

What Could interfere with your signal are wireless transmission tower or other wireless devises in the area. They will transmit at very high wattages and definetly interfere with your signal. Sometimes if the feild is in between office buildings (they could be upto 15 miles apart) they may have wireless line-of-sight antennas which will also interfere with the signal and you won't even know what happened.

These are the joys of living in the city.