Are Harmonics Blowing up my Capacitors?

Is it new, and therefore under manufacturer's warranty? If so, get the supplier in 'tout de suite'!

I like to study the schematic before answering. It could be due to inrush current.

It sounds like something isn't wired right or something is defective.....That sound you described sounds like the capacitors were shorted...

I think Solar Eagle is right on. If every thing checks good it is inrush current. We leave ours online 24/7. They have a large reactor (inductor) in the connection to the 15KV and rare to loose one, if we do it is when we energize the Bank (one on each Phase) .

An open or intermittent Neutral Grounding Resistor at the utility could certainly allow transient over-voltages to occur. Here's an interesting short article on the subject:

http://www.startco.ca/library/papers/Monitoring%20NGRs.pdf

The two things which most commonly blow up capacitors are:

1. over voltage - can be spikes or resonance issues

2. Ripple current.

Also you have spoken about latching the PFC array in to an operating system with a contactor - the caps will have a residual charge on them - they could be switched into an antiphase voltage, causing a huge current spike which they would certainly not like. You have soft start on the motors, soft switching of the PFC array would also be a good idea.

Your 'wa wa' sound concerns me - can you measure the voltages and currents.

Alternately you could perhaps use three smaller PFC arrays - one for each motor, designed for the motor and each permanently connected to its motor and soft switched with the motor....

In a plant where I used to work, we had many capacitor banks for power correction, due to the large induction motor load. As we installed more VFD's, we found that our power factor dropped. We found we were blowing the capacitors. An expert that we brought in, determined that the 3rd harmonic was causing us the problem. He recommended that we go to a sponge capacitor as this was more harmonic resistant. The boss was too cheap and we just kept replacing the film capacitors. It made me wonder how much money we were saving by not using the proper capacitors.

Vic

Hi Vic,

they have soft starters (with cap banks correctly switched off during ramping), not VFDs. Harmonics are low as measured. Hope those low values include harmonics coming from neighbors. What is a "sponge capacitor"?

Sounds like a combination of ressonance and inrush current effects.

If it's a new system, vendor should debug it.

brgds

Snel

Always something good on here, an interesting post with the VFDs but I don't quite see how it rated a good answer with regards to the soft starts. In any event, what did catch my eye was the 600kvar for 2 or 3 motors. Not sure what your original PF was but breaking it down to 1 motor at 300kvar,it appears that you may be over correcting. At the very least, I would suggest checking the motor data for maximum recommended PF correction

Well, as it turns out there isn't a NGR at the utility as previously assumed. It's been mentioned that it might be resonance, but it seems to me if I remove half the kVAR's of the capacitor that will remove me from any resonant issue. The other possibility I saw in everyones posts is that it might be a shorted capacitor. Someone here mentioned that it could be a grounding/bonding issue and to verify the grounding. The cap banks frame is grounded with a 12-gauge wire; someone mentioned going to a #6 wire. Not sure if that will help. I'm still at a loss as to if it's a harmonic issue, grounding issue, or something entirely different.

Removing half the capacitance will increase the resonance frequency by 1.4x. The system is less likely to resonate at higher frequency but it will if you have some harmonics, transients, or other noise at that frequency.

One thing that will help you is that as the frequency increase, the dampening also increase which usually reduce the amplitude of the resonance. Also, harmonic amplitudes are usually smaller at higher frequencies.

Therefore, It will probably help but there aren't any guaranties.

Good luck.