Answer / muthukumar

Because of high voltage the air molecules surrounding the HT
line are ionised.As the HT line carries AC current the
alternating magnetic field produced will cause the ionised
molecules to vibrate resulting in humming noise.

You can also consider the case that there will not be any
humming noise in case of HVDC transmission.

Answer / aruna devi

due to corona loss

Answer / kathirvel.r

Huming sound occured in HT AC Transmission only,Not in Dc
Transmission.Because, Corona Effect occured onlly for
AC.May i correct.

Answer / ilononurb

this is what we called Corona Effect. this is due to water
content of the air that sorroud in the transmission lines
that carrying high voltage ionization is taking place. So
humming sound is audible in the system.

Answer / subhash tp

The humming sound is coming from transformer
at the time of working is due the magnetic expansion
and de-expansion at the ends of transformer cores.
At the time of working the transfomer core ends are
expanding when supply voltage occures its positive half cycle
and the transformer ends are de-expanding at the time of supply voltage occures its negative half cycle .This expansion &de-expansion is making humming sound,and also this wii continue during the transformer is working.

Answer / oarhe ize

Because of the HEAT loss along the line.

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