Power Factor Corr Cap Making Hissing Noise

[VTer]

We just installed a 500kVAR 60Hz 480V 3phase delta connected power factor correction bank on our 480V switchgear. The 500kVAR capacitor unit is made up of (5) 100kVAR units tied to a common bus in one enclosure. Each 100kVAR unit contains multiple electrolytic capacitors.
Upon energization of the capacitors we noticed a distinct hissing sound coming from the capacitors. We had a meter on the circuit and electrically everything looked good. The currents were where they were supposed to be and very balanced. The voltage was fine and there were no excessive harmonic voltages or currents. Vthd was around 1.8% and Ithd was around 14%. We left the caps energized for about 30 min and the hissing sounds went down but was still noticeable. The cap bank is currently off because I am not sure if this hissing noise is a characteristic of recently energized caps or is this a sign of some issue within the capacitor bank?


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[ScottyUK]

Electrolytics in a PFC bank? Really? I can't comment on north amercian practice but that would be unusual over here.

I'm surprised they aren't self-healing polypropylene film types if it is a new-ish installation or oil-paper if an old-ish one. Is there any possiblity that the hissing is vibration causing caps to move relative to each other? A pressure-being-released type noise is almost certainly bad news.

 

[davidbeach]

I'm pretty sure they're not electrolytic capacitors. If they are, then leaving it shut off is a very good idea.

 

[VTer]

Gentlemen, I apologize I just double check and they are indeed polypropylene film types. Please disregard the electrolytic part

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[VTer]

BTW, the hissing did not sound like it could be associated with any sort of vibration.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[magoo2]

Just curious - why 5 cans and 500 kvar for a 3 phase installation? I would have used a 300 kvar bank with 3 of the cans or a 600 kvar if you had another.

You do have low THD but the cap bank sets up a tuned frequency with whatever inductance there is. I'd be tempted to try it with 300 kvar and see what you get in terms of hissing and THD.

 

[VTer]

The 500kVAR was based on the kVAR demand for the system. The unit is made up of 5 individual 100kVAR Units on a common bus. Each rectangular 100kVAR housing contains numerous cylindrical metal can capacitors with individual discharge resistors. Each individual 100kVAR housing is separately fused, so we can remove fuses to go down in steps of 100kVAR.

The 500kVAR cap was also sized to avoid any sort of resonance at the common harmonic frequencies present in the system. Based on the calculations, the resonant frequency was at around 8.5 harmonic for the 500kVAR unit. There was no evidence of any resonance when the cap was energized and there was no presence of the 8th harmonic.

We could remove 100kVAR units from the circuit and try to identify where the hissing is coming from, but I wanted to check with manufacturer and with you gentlemen if you experienced this before and what the potential causes for the hissing sound could be. SO far I have not heard anything from the manufacturer.


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[magoo2]

I've worked with a large number of filter banks which are usually designed to avoid the 5th and 7th harmonic typical of variable speed drives and 6 pulse converters, but I've never heard of a situation where the capacitor bank is unsymmetrical. If you calculate the resonant frequency with your arrangement, you'll get 2 different values.

If you're tuned to the 8.5 harmonic, sounds like you're expecting lots of 9th. I'll let others chime it, but I think it's quite unusually to have two cans across two phases and one across the other.

I still can't explain the hiss but your arrangement has got me puzzled.

 

[davidbeach]

I got the impression that it was 5 x 100kVAr 3-phase units, not single-phase units.

 

[VTer]

There are five individual 100kVAR three phase delta connected units on common bus fused individually to combine for 500kVAR three phase 480V rated 60Hz unit.

I think the hissing could be from clearings due to self healing properties after some initial investigation but the electrical characteristics are within acceptable tolerances so I am not sure what could be causing the noise. Is hissing noise a indication of clearings?

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[waross]

The hissing may be benign, but please forgive me for wondering if you will post back something along the lines of;
"Well, after the explosion, we discovered..........."
In your position I would be trying to isolate and remove from service the hissing caps.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[VTer]

Waross,

The caps are off. The bank was only energized for a total of 30 minutes with the hissing sound continuous throughout that duration. It seemed that the hissing was coming from more than just one housing. As I posted above, all the electrical data shows no obvious issues as all the parameters are within the tolerances. I think this is a manufacturing problem and the caps are experiencing clearings that are causing the hissing sound. What is not clear to me is why the caps are clearing and why was this issue not discovered during manufacturing testing?

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[LionelHutz]

Have you measured the capacitors since turning the bank off. If a capacitor was self-clearing faults that much then I would expect you to find the capacitance is below the lower tolerance of the capacitor. Film capacitors typically have a -0% tolerance on their capacitance.

 

[VTer]

Lionel, we did measure every single cell and found that they are all within the acceptable manufacturing tolerances. The caps were only energized for about 30 minutes so I am not sure how much capacitance would be lost in that time. I am thinking about a 1000VDC megger test to check for any discrepancy between the five different housing?

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla