Power Factor Capacitor Failures

[bmodrow]

I have little to no expertise in this area and was hoping I could receive some guidance. I have seached the forum regarding PF correction capacitors and have not came up with any solutions.

I have a customer that has some power factor correction capacitors across the feed of two air handeling units. The customer says that he recently lost some capacitors (I'm thinking due to age)and would like us to do a power quality survey. From what I am told these capacitors are are across the line at all times (wether or not the air handelers are running).

Can someone please give me some direction as to what possible problem causing areas, I should be looking for after we have collected data for a few days.

Any pointers will be greatly appreciated.

Thank you in advancne.

 

[CHDean]

With the limited data you've supplied two things jump out at me. 1.) Are there drives or other harmonic generating devices in the system and if so how bad is it?
2.) If caps are floating are you going into a lead when the air handlers aren't running and maybe causing high voltage condition. If the caps are applied specifically for the air handlers the should be switched with the motors.

 

[dede61]

In addition to what CHDEAN asks I like to ad, whats the age of these capacitors ?

 

[busbar]

Distorted local voltage and current from harmonics come to mind. Until more sophisticated PQ instruments can be obtained, comparison in readings of known-accurate true-RMS and average-responding meters may be somewhat revealing. Alternately, waveform crest factor deviating from 1.4 through ratio of peak and RMS readings may be a good initial indicator.

 

[jbartos]

Suggestion: Also, surges should not be ruled out; especially, if those capacitors are surge protection capacitors. Surges are harder to monitor. A suitable voltage recorder may be needed.

 

[bmodrow]

Thank you for the replies thus far. We do have a suitable instrument (Dranetz PP4300) for recording and monitoring. The information I received on the number of air-handelers is not correct either. The information that I now have is that ALL air-handlers run off of this feed and they are all controlled by VFDs the exact number is not known. So, the answer is yes there are other harmonic generating devices involved.

The capacitors in this installation are approximately 10 to 15 years of age. I am planning to check the values of the capacitors prior to monitoring to rule out any "bad" caps in the bank.

 

[rbulsara]

It appears that capacitros were installed when regular starters were employed. Later on starters were replaced with VFD's.

You may want to check the power factoro on line side of VFD when PFC is disconnected and AHU running. Chances are that ,depending on the VFD's , the input power factor is well above 0.9 (.94, .95 etc). The 'additional' PFC are redundant causing leading pf and over voltages etc.

I would remove the PFC's altoghether.

 

[CHDean]

PFC's and VFD's don't go together. Two options, remove the PFC's or put harmonic filtering in front of them (can be expensive)

 

[dede61]

Yep normaly 10 to 15 year old capacitors should not fail, they should last another 10 years. Harmonic filtering is the option I believe, you might take a glimps to other equipment on the same line, computer metering etc... (also the transformer) they will suffer from the harmonic distortion as well and theyre lifetime will also be significantly reduced.

regards,

Danny

 

[jaysal]

measure harmonics in the system and try to reduce it. As capacitors fail when harmonics are present.