Why does a humming sound occur in high voltage transmission lines?

This humming sound is coming due to ionization (breakdown of air into charged particles) of air around the transmission conductor. This effect is called as Corona effect, and it is considered as power loss.

Whenever the voltage passes through a conductor exceeds a certain value the air molecules around the conductor got ionized. the ions moving in the atmosphere, they take energy from the supply so a hissing noise will occur.. this effect is called the corona effect.

The HT lines cause humming because of the corona effect. when the voltage of transmission lines increases too much to produce an electric field of the magnitude of 10^7 v/m.That is more than the electric field required to break the intermolecular bond of the air molecules. Hence the air particle around the lines get ionized and start moving freely. and during this movement, several collisions occur resulting in a humming sound.

Electric fields around a transmission line conductor wire can become concentrated enough to create an electric discharge. 
A crackling or humming sound is generated by small electrical discharges, often caused by water drops that collect on the conductor when it rains or when there is sufficient moisture in the air.

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