Noise

..."The cause of this sound is alternating voltage and currents that make a transformer's steel core extend and contract. The result is the familiar vibrating hum. This phenomenon is scientifically referred to as magnetostriction."...

http://www.acoustiguard.com/blog/419-transformer-noise.html

So check the tightness of your PT lamination bolts, may have stretched due to higher nearby heat load from higher load operation, now at lower load, bolts might not be so tight. Also check current transformers for iron core noise

Try viewing the panel box under load with an infrared camera or viewer.See if there are any junctions heating up.Many possibilities for the noise,including a bad CT,PT,a junction termination breaking down.Do not ignore this issue,it could potentially be very dangerous

Current creates a magnetic field. AC current creates a vibrating magnetic field which generates a vibrating force on other current carrying conductors and ferrous metals. The force is stronger at close proximity. Anything loose will allow this vibrating force to generate vibration (sound). If the length and mass of the vibrating object cause resonance at the power frequencies, production of sound will be particularly efficient. Try restraining your busbars at different points if you think they might be resonating at the power frequencies. Just a thought.

A Corona discharge is usually accompanied by an ozone odor,sort of like a bleach odor.This is a very dangerous sign,and remedial action must be taken immediately. A corona "buzz" sounds like an arc welder when the arc is too long. If you ever hear it,you will always know the sound and the smell.

Does he know the difference?

Some corona has a crackling overtone on the base ‘buzz’ frequency....

Corona would depend on voltage and presence of moisture. At increased load, the voltage is likely to be a bit lower and moisture also is likely to evaporate due to heat produced by current.

Hence, it seems more likely cause is inadequate busbar supports. Was the noise there right from the day of installation??

Corona is inoized air, not water/moisture.

I am aware of it. Thanks any way.

What I meant by moisture is that in presence of humidity in air, possibility of corona discharge increases. I have seen this in 400kV switchyards just after a rain.

shubh raatri

And Yes, indeed, air does contain moisture, my apologies. The ozone is burned air which contains minute particles of moisture, however any heat built up will not evaporate the moisture from a cable. It will make matter worse and increase ozone.

thanks for your reply

its a vibration 'buzz' sound.

I say laminations, if no loose mounting bolts. Without information on bus supports, etc, it will be hard to differentiate types of mechanical noise

Henry98,

Thanks for posting the feedback.

Suppose the vibration is from busbars. Appreciate your confirmation.

One of the main objectives in these fora is learning. The feedback from the OP always is valuable.

I nudge the breakers a bit and see which one/s buzz. That sometimes does it.. but this is a whole other level.. food for thought.