Having never been to a major hydroelectric dam and been in the room where the cables are, I can't exactly tell you. Most likely it's an effect similar to what high tension electric transmission wires have (from the really big power poles that cross the countryside and connect cities). The electrical voltages along those lines are so high that sparks happen, which cause tiny regions of ionization in the air, which then expand because they are so hot into tiny explosions - and the huge number of little shockwaves (thunderclaps, really) that these explosions generate are what you hear as the buzzing sound.

The hum of the wires is caused by the electric fields.The electric fields are actually strong enough near high voltage wires to ionize the surrounding air. The hum is actually a continuous drone of discrete pops from molecules ionizing!