Cap Bank - Acoustic noise

[dpc]

We have a client with a noisy pole-mounted, 12.47 kV cap bank. They went so far as to replace all three single-phase cans, and tightened all connections, but the noise remains.

My first thought was harmonic currents, but they are not aware of any significant non-linear loads in the vicinity. It's mostly residential and light-commercial. The noise is apparently significant and it has been noisy for some time with a couple of call-ins from passersby regarding the noise. They have measured the current with a 15 kV current probe on the overhead line and found the currents balanced and correct for the cap bank size. They do not have a means of measuring the harmonic spectrum or waveform in this location.

Any thoughts on a noise source?

Thanks,

dpc

David Castor

http://www.cvoes.com

 

[rbulsara]

Could they analyze noise frequency spectrum and see if matches electrical harmonics?. I am not an acoustic expert either, but a wild guess:

Could the physical characteristic of the pole and arm structure be a factor or a solution?, if it's natural frequency resonates with some harmonics. But that would result in vibrations, generally. But beefing up the base support or vibration isolators could be tried. I would think, involving an acoustic consultant could help, if there is no direct electrical solution.



Rafiq Bulsara

http://www.srengineersct.com

 

[wareagle]

dpc
When you say "couple of call-ins from passersby regarding the noise" are you referring to reception on a car radio?

 

[alehman]

Audible noise from capacitors is not unknown -

http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=296267

Are you sure it's the capcitor(s)? Are there any PT's or other ancillary equipment that could be making the noise?

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[dpc]

Noise is audible noise, not RFI. Nothing else on the pole. When cutouts for the caps are opened, noise goes away.

Harmonic current makes some sense, but there does not seem to be an obvious source.

David Castor

http://www.cvoes.com

 

[redfurry]

You can have resonance when the ratio of the capacitance of the bank to the system impedance matches the square of a given harmonic

see:

http://ecmweb.com/powerquality/electric_power_factor_correction_3/

This can happen even when there are no major sources of harmonic load on the system.

If you know what the fault level is on the system, you might be able to calculate this. Alternatively, if you can supply the capacitor bank through another circuit (with a different impedance) you could test and see if it makes a difference.

 

[bacon4life]

Per one of our other engineers about a problem we had a few years ago:

Yes, we had problem with 900 kVAR banks. As best as we could figure out it looked like the size of the enclosure and/or the core may have reached a natural frequency and vibrated so much that we received phone calls from those around them. You could also feel the pole and sidewalk vibrate. Weird stuff.

 

[ghostbuster7]

Last week,measured harmonics on a utility 27 kv. cap bank located in a double 80 mva. tansformer station.It is primarily residential and light industrial loads.With the cap bank turned on the 5th and 7th current harmonics increased by approx. 10 db.There was also an increase in the voltage harmonics by approx. 5 db.Hope this may be of some use in your situation.

 

[dpc]

Thanks for the feedback. I believe this is a 600 kVAR bank at 12.47 kV.

I'm waiting for some field feedback regarding any non-linear loads nearby. Unfortunately, they don't have a simple way to measure harmonic distortion at this location - no PTs or CTs.



David Castor

http://www.cvoes.com