Capacitor Bank failure mode?

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chris kennedy said:
What type of monitor are we viewing here? (looks pretty cool)
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It is a Dranetz-BMI PowerXplorer PX5 it is very cool. :cool:

MedCombo.jpg


Now if only I had a little bit of a clue how to operate it or interpret the data it provides. :mad::confused:
 
Looks ok to me. The harmonics might be considered approaching marginal but they are below 5%. The flicker is probably ok as well. You have about 250 kvar of reactive power and 200 kvar of capacitiors.
 
You are very smart to be cautious here Bob. I have seen PF correction caps EXPLODE!! Not little poofs either. Major shrapnel damage all around. Good luck.
 
My question is what is the harmonic profile?
Can your device produce a harmonic histogram? This would tell us what harmonics are actually in the system.

Once we know what harmonics exist and the available short circuit current, it becomes possible to determine if resonance may be a problem with your 200kVAR cap bank. The typical formula we use is: Short Circuit kVA divided by Capacitor kVAR = resonant frequency.
 
Also, your meter shows the total power factor (TPF) and displacement power factor (DPF). The capacitors can correct the DPF.

The kvar is = sqrt[ kVA^2 * (1 - pf^2) ].

In your case: kvar = sqrt[ 423.3^2 * (1 - 0.796^2) ] = 256.2 kvar

oh yeah, and see what Jim just said about a resonance check.
 
iaov said:
You are very smart to be cautious here Bob. I have seen PF correction caps EXPLODE!! Not little poofs either. Major shrapnel damage all around. Good luck.
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Would a slow ramp-up of voltage with a 3ph autotransformer help?
 
LarryFine said:
Would a slow ramp-up of voltage with a 3ph autotransformer help?
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At the point of resonance, the voltage across the capacitor rises towards infinity causing the capacitor to fail. It is a frequency thing not an applied voltage issue.
 
jim dungar said:
At the point of resonance, the voltage across the capacitor rises towards infinity causing the capacitor to fail. It is a frequency thing not an applied voltage issue.
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But wouldn't we be delivering less energy to the resonant circuit?
 
jim dungar said:
My question is what is the harmonic profile?
Can your device produce a harmonic histogram? This would tell us what harmonics are actually in the system.
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I think so.

I plan on leaving the meter in place for about a week. At that point I will download the file from the meter into the manufacturers software and I believe it can generate almost any kind of report imaginable.


Unfortunately the man who really knew how to use this meter has left the company.
 
jim dungar said:
At the point of resonance, the voltage across the capacitor rises towards infinity causing the capacitor to fail. It is a frequency thing not an applied voltage issue.
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The voltage would rise towards infinity?

I thought this would be a parallel resonant circuit? In that case the impedance would be lowest at the resonate frequency and the currents would rise.


Here is an interesting read on this subject:
http://www.elec.uow.edu.au/iepqrc/files/technote2.pdf
 
ELA said:
The voltage would rise towards infinity?

I thought this would be a parallel resonant circuit? In that case the impedance would be lowest at the resonate frequency and the currents would rise.


Here is an interesting read on this subject:
http://www.elec.uow.edu.au/iepqrc/files/technote2.pdf
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The impedence of a parallel resonant circuit is very high. There is still a very large current that circulates between the cap & the inductor (the impedence of each individual device is still low). That high impedence combined with the large current flow is what causes the voltage to rise.

There is always some damping (the voltage never really goes to infinity) , so starting with a lower voltage would help at least a little.
 
Thanks Steve,
I had my series and parallel resonant circuits reversed :roll:
 
ELA said:
Thanks Steve,
I had my series and parallel resonant circuits reversed :roll:
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No problem - its pretty easy to get those mixed up.

Bob:

That does seem like they have a lot of reactive power. Is that all from the compressors? I'm supprised they are running so much at this time of year. (I don't know about Mass, but it was a whopping 36 degress here last night.)
 
iwire said:
I think so.

I plan on leaving the meter in place for about a week. At that point I will download the file from the meter into the manufacturers software and I believe it can generate almost any kind of report imaginable.


Unfortunately the man who really knew how to use this meter has left the company.
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Bob, just for giggles, would you switch off the capacitors for a while and record that data as well so you have comparative values? I think people here would also be interested. That may answer some questions. The harmonics are surprisingly high. Where would they originate from in a relatively benign system?

That the fault was unrelated to the equipment in place makes perfect sense. I had a feeling....but then I discovered it was only gas:D
 
weressl said:
Bob, just for giggles, would you switch off the capacitors for a while and record that data as well so you have comparative values?
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Did he ever have them on? I took it that the wires were disconnected from the breaker and Bob was checking things out and getting some suggestions as to what to look for/check prior to hooking it back up.
 
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