Power factor correction unit totally blown up
Power factor correction unit totally blown up
(OP)
Hi all,
I have a bit of an unusual one here, and I was wondering if anybody has come across something like it (or maybe a PFC expert could point me in the right direction).
I recently took a look at a power factor correction unit that had pretty much totally blown up. All of the capacitors have failed (either burst open or are short circuited). Some of the resistors on the contactors are burnt. In many places there are burn marks on the busbar - very strange for a 415V system where the busbar is spaced at around 40mm apart.
Some other relevant facts:
I have a bit of an unusual one here, and I was wondering if anybody has come across something like it (or maybe a PFC expert could point me in the right direction).
I recently took a look at a power factor correction unit that had pretty much totally blown up. All of the capacitors have failed (either burst open or are short circuited). Some of the resistors on the contactors are burnt. In many places there are burn marks on the busbar - very strange for a 415V system where the busbar is spaced at around 40mm apart.
Some other relevant facts:
- It wasn't a lightning strike - it was during the middle of the day and clear weather
- The harmonics on this site are practically zero - probably the only source of harmonics is the fluoro lights in the offices. Because of the remoteness of the site and the lack of competent electricians, the management stipulated that the entire plant be run on star/delta and autotransformer starters. All they have is electric motors.
- No other equipment was damaged at the time
- The PFC unit was second hand. The last owner used it for around 2 months and then it sat unused for 5 years
- The ambient temperature at the time was not excessive for this area (maybe 30 degrees celsius)






RE: Power factor correction unit totally blown up
As some of the capacitors have burst open, I wouldn't be surprised if a plasma was formed by the escaping gasses which caused the burn marks that you saw.
I take it that there was blocking inductors in this unit?
RE: Power factor correction unit totally blown up
Without a small mopunt of leakage current to maintain it, capacitors over time lose their film of oxide that covers the surface of the positive plates and acts as an insulator. When it's gone and you energize it, you get a dead short across the plate layers. After sitting unpowered for a year or more, they must be put though a procedure called "reforming" before being fully enegized. If you did not know to do that, then that is the likely cause of your failure. I have seen them literally explode on VFDs that people have bought on eBay for cheap, not knowing that they had been in storage for years.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
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RE: Power factor correction unit totally blown up
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Power factor correction unit totally blown up
RE: Power factor correction unit totally blown up
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Power factor correction unit totally blown up
Unfortunately, I think I wrote the PM manual for that project and told them about capacitor reforming if they even have the system off line and in storage. Oh well, it wont likely hurt anything.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Power factor correction unit totally blown up
Ausphil
RE: Power factor correction unit totally blown up
jraef - PFCC are film type and not electrolytic.
RE: Power factor correction unit totally blown up
•The harmonics on this site are practically zero - probably the only source of harmonics is the fluoro lights in the offices. Because of the remoteness of the site and the lack of competent electricians, the management stipulated that the entire plant be run on star/delta and autotransformer starters. All they have is electric motors.
So, that tends to push me in the other direction if they're at a remote site (so likely few other harmonic generators nearby or even on the same circuit), and they don't have many harmonics of their own.
Interesting thread...and I'm at a loss.
RE: Power factor correction unit totally blown up
If the system is delta is it running ungrounded? Is the PFC system expecting to see a grounded wye system but was exposed to full phase-phase voltage across wye connected capcitors during a ground fault?
If the system is ungrounded, could ferroresonance come into play?
RE: Power factor correction unit totally blown up
Thanks for the comments.
I will check out the harmonics on the plant, but I would bet my first born child that it is almost zero. The entire plant only consists of electric motors, and they only have star/delta and autotransformer starters. The plant was fully running at the time the board tripped and power went out, so I'm fairly sure we can eliminate the starters. As for harmonics from neighbours, well, there aren't any neighbours. They are the only customer on that feeder (the other customer went bankrupt about 6 months ago).
The PFC unit is connected on the busbars of the incoming supply to the plant, doing correction for the whole installation. Pretty standard sort of stuff.
RE: Power factor correction unit totally blown up
Have you calculated the expected resonant frequency of the bank for each different switched combination of capacitance?
You can measure for harmonics on the system which might give some indications of what could be the problem but it won't show you what the capacitors were actually subjected to. It doesn't take much to overload the capacitors if the cap bank and power system were tuned to the right resonant frequency.
You may want to look at a de-tuned replacement bank if you can't figure out a likely cause of the failure. At the very least, look at higher voltage rated capacitors de-rated for the system voltage.
RE: Power factor correction unit totally blown up
For all the caps to burst, They must have all been online. Is this a reasonable assumption?
I suspect an over voltage on the feeder. A switching surge or other event that sent a high voltage transient down the feeder.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Power factor correction unit totally blown up
RE: Power factor correction unit totally blown up
A further explanation:
On a long HV feeder, the voltage drop may be predominantly reactive. The source voltage may be high to compensate for the line voltage drop.
As the capacitors reduce the reactive component of the current the reactive component of the voltage drop decreases.
If the capacitor bank is poorly configured, the bank may supply VARs to the source. With the reactive current going towards the source the reactive component of the voltage drop becomes a voltage rise. In extreme cases the voltage at the load may exceed the supply voltage.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Power factor correction unit totally blown up
RE: Power factor correction unit totally blown up
RE: Power factor correction unit totally blown up
RE: Power factor correction unit totally blown up
Thanks for the many comments. I think we have identified the source of the problem. Well, maybe the source of the problem, maybe a symptom.
When we took a close look at the busbars we could see that there was tracking near the standoffs. The tracking was unusual though - the burn marks only appeared to be on the busbar, not down the standoffs and across the back of the board to the other side. In other words, it was as if there were sparks occurring between the busbars.
Now, we aren't sure why this was happening but it definitely has been happening. Whole chunks of copper are missing out of these busbars and it looks like it has been happening for some time.
RE: Power factor correction unit totally blown up
RE: Power factor correction unit totally blown up
There had been a water damage to a plant and the insurance company didn't want to pay. Instead they said that they should send a company to clean up everything and then pay for any damage that could be attributed to the water. When that damage occured a few months later, they said it wasn't caused by water but by overvoltage. Which is rather ridiculous.
I was hired to do an investigation and found this: http://www.gke.org/pub/files/Copper%20bar%20with%2...
This is where it started. A low energy arc had started inside a hollow stand-off insulator where water had stayed after the cleaning. The inner surface had carbonized and then the arc started. It was a new 500 V system with insulated neutral. So currents were kept low. But, of course, when ionized air propagated into the panel - the real arcing started.
Is this something similar to what you saw?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.