Damaged PFCs

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ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
I have a small industrial customer that is seasonal in nature. About three years ago we installed a 100 kvar PFC for plant wide correction. In the same time period we also replaced existing PFC capacitors on individual motors. A 50 kvar, a 40kvar, and a 35kvar, the four are the same brand and type, internally fused.

Early this year we installed some new equipment, built and designed by others, that contained 2 - 20hp vfds and 2 - 5hp vfds. along with associated control and other misc starters. We also added 2 - 3hp vfds of our own. System is 480/277, 3- 300 kva transformers and under 1000 Hp connected.

While the plant is shut down for the winter we are checking motors with a megger and visually inspecting all capacitors for problems. We found the four listed above had failed. All the other PFC in units in the plant are of different design and are unharmed. The ones damaged have multiple small individual capacitors while the others have three larger units. Management will have to decide if we step up to a PFC that can handle harmonics, if that is the problem. My question is why did the new units fail while the others were not noticeably damaged?
 
ptonsparky said:
I have a small industrial customer that is seasonal in nature. About three years ago we installed a 100 kvar PFC for plant wide correction. In the same time period we also replaced existing PFC capacitors on individual motors. A 50 kvar, a 40kvar, and a 35kvar, the four are the same brand and type, internally fused.
Click to expand...
Did you install 1-100 kvar along with the 50, 40 and 35 kvar? Did you install these at the main?


ptonsparky said:
While the plant is shut down for the winter we are checking motors with a megger and visually inspecting all capacitors for problems. We found the four listed above had failed. All the other PFC in units in the plant are of different design and are unharmed. The ones damaged have multiple small individual capacitors while the others have three larger units. Management will have to decide if we step up to a PFC that can handle harmonics, if that is the problem. My question is why did the new units fail while the others were not noticeably damaged?
Click to expand...
What size were the other capacitors that did not fail? With respect to the VSD's where were they located?
 
bob said:
Did you install 1-100 kvar along with the 50, 40 and 35 kvar? Did you install these at the main?

The smaller ones were replacements to individual motor PFC and are at the motors approx 100' from MCCs. The 100kvar is feed from a fused disconnect at one of the MCCs and is manually controlled.

What size were the other capacitors that did not fail? With respect to the VSD's where were they located?
Click to expand...

Looking at notes from the help there is a 40 kvar at a motor location, and a 26 kvar within 10' of freq drives. I believe there are 1 or 2 more wtihin a few feet of the vfd's and some more remote. No PFC is connected to any of the motors with drives.

We looked at billing history today. The 100 Kvar was purposely taken off line for remodel in January but did not function again at startup in May. It was functional in Dec 05. Internal fuses were not blown in any of them.

edit: Don't know how I got part of the reply where it is...
 
Jraef said:
Could be the caps themselves. Some cheaper designs do not use polyhydric (alcohol based) electrolytes, which means that if the outside temperatures get cold enough, they freeze. If power is put to them at any time when frozen, they short.

Read the end of this article for more information on electrolytes.

http://www.faradnet.com/deeley/chapt_05.htm
Click to expand...

They weren't cheaper when I had to pay for them but that means nothing. I will research temperature limitations now. Thanks
 
Sounds Like Harmonics

Sounds Like Harmonics

You may want to further investigate the Total Harmonic Distortion at these locations. Adding the VFD's likely added siginifcant vTHD, which can interact with the caps at the higher harmonic frequencies. The PFC local to the units are more likely to be subjected to higher THD the the main PFC on the service, which could help explain the failure of the smaller caps. You will need to know the spectrum breakdown and magnitudes to make sure the PFC you are using is suitable for the application
 
ptonsparky said:
Looking at notes from the help there is a 40 kvar at a motor location, and a 26 kvar within 10' of freq drives. I believe there are 1 or 2 more wtihin a few feet of the vfd's and some more remote. No PFC is connected to any of the motors with drives.

We looked at billing history today. The 100 Kvar was purposely taken off line for remodel in January but did not function again at startup in May. It was functional in Dec 05. Internal fuses were not blown in any of them.

edit: Don't know how I got part of the reply where it is...
Click to expand...
The caps you list seem to me to be larger that what is usually associated
with motors. I believe you are having harmonic problem. These large caps present a lower impedance an thus attract the harmonic currents. The
only way you will solve this problem is to use a capacitor unit that is tuned to the specific harmonic frequency.
 
bob said:
The caps you list seem to me to be larger that what is usually associated
with motors. I believe you are having harmonic problem. These large caps present a lower impedance an thus attract the harmonic currents. The
only way you will solve this problem is to use a capacitor unit that is tuned to the specific harmonic frequency.
Click to expand...

These are good sized motors. 75hp on up to 200. The 50 Kvar was furthest away and the 100 kvar the closest. I knew that we could have some problems but am suprised that it was limited to just one Mfg specific model.
 
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