how long do power factor capacitors last??

I have a service that has some really old capacitors on the 480V line
and some that are not so old, probably 10-15 yrs. They are different
manufaturers but identical application. The newer ones have failed and
are shorting out. The old ones are hanging on just fine.
Does anyone know what average lifespan of this type a capacitor in an
industrial environment such as a mine could be expected. This of
course would be with normal use and no huge transients etc.
Chris
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| I have a service that has some really old capacitors on the 480V line | and some that are not so old, probably 10-15 yrs. They are different | manufaturers but identical application. The newer ones have failed and | are shorting out. The old ones are hanging on just fine. | Does anyone know what average lifespan of this type a capacitor in an | industrial environment such as a mine could be expected. This of | course would be with normal use and no huge transients etc.
It depends on how cheaply they were made. Now days, things are getting cheaper and cheaper, and I don't mean price.
Reply to
phil-news-nospam
Early failure of capacitors can be blamed on harmonics.
Hopefully the caps were made after ~1978, or you may have some polychloringated biphenyl remediation in your future..
?s falke
Reply to
s falke
Indeed.
In a well-designed modern installation, particularly in a situation where non-linear loads are present or likely to be installed, it has been considered good practice to insert series inductance to tune a cap bank to about 4.7 times the fundamental frequency. This provides a means by which to keep harmonic currents through the capacitors within reason.
However, in situations with poor maintenance, as the capacitors fail the network's resonant frequency rises, and when it lands on the 5th or 7th harmonic the capacitors are exposed to currents and voltages well in excess of rating. Catastrophic failure often results.
Moral - Do not run a tuned bank if it has ANY dead caps!
Reply to
BFoelsch
Two "behaviors" must be carefully weighed though. Capacitor-fuse operations can spoil your energy/power budget, but capacitor-can deflagration can spoil your workmen's-comp rates.
?s falke.
Reply to
s falke
Of course.
I don't think I've ever seen a catastrophic failure of a low-voltage cap bank, but have seen quite a few MV ones in flames. I will agree that the new caps, made up of groups of "beer cans" don't seem to last as long as the old welded units with bushings.
Your comment reminds me of a situation I had about 15 years ago. A large, well known electrical equipment manufacturer had provided all the electrical work for a municipal water plant. Mostly no problem, but there were never-ending issues with the LV cap banks "blowing up." All the engineers and big brass worked on the problem (over the phone, of course) for months. Finally, I was hired to actually travel to the site and see what was going on.
Now, of course, each engineer specified tests I was to do to prove that his theory was right. I was sent with oscilloscopes, chart recorders, line disturbance monitors, you name it. Obviously, this was a very complicated and subtle problem, or else the brass would have found it.
I didn't take any of the gear out of the car. Upon arriving at the site, I was shown the "blown up" cap banks. I was expecting to see soot on the ceiling, or evidence of a blast, but absolutely nothing was visible. Careful examination showed that the failed banks were missing the discharge resistors. Further examination found the burned-up resistors laying in the bottom of the bank. Super careful examination showed the tiniest spot of melted copper on the capacitors where the missing resistors should have been.
The problem was that the cap bank manufacturer had provided the wrong discharge resistors. The supplied resistors were sized for 240 V, not the 480 that was used at the site. The 2 watt carbon resistors would overheat and crack. They would start to arc across the crack, and slowly burn up the carbon resistance material. After a while the carbon was all gone, and you had a phase-to-phase arc across the line. This would of course make a great big bang and blow the bank fuses. Occasionally the arc would go to ground, a nd trip the ground fault on the service entrance, which was an electronic breaker which had all the settings as shipped (set to minimum).
It was great when we had a meeting with the brass to discuss this "major" issue. Everyone was rooting for his pet theory, but they were all wrong.
Sometimes, too much analysis leads to paralysis!
Reply to
BFoelsch
in article snipped-for-privacy@giganews.com, BFoelsch at snipped-for-privacy@comcast.ditch.this.net wrote on 8/9/04 2:56 PM:
I presume that the discharge resistors you are talking about are there to remove any residual charge when the capacitors are disconnected from the line. Is that correct?
Bill
Reply to
Repeating Rifle
Yes, per NEC 460-6.
Thanks in advance.
Reply to
BFoelsch
Re: dischange resistors.
How are these sized? IOW: is there a requirement that a fully charged cap will drop to, say, 10% within a certain time.
Otherwise, why not ship low voltage caps with say, .5 meg, resistors. They could take 600 volts and draw less than a Watt.
EMWTK
Reply to
John Gilmer
From the 2002 NEC:
460.6 Discharge of Stored Energy. Capacitors shall be provided with a means of discharging stored energy. (A) Time of Discharge. The residual voltage of a capacitor shall be reduced to 50 volts, nominal, or less within 1 minute after the capacitor is disconnected from the source of supply. (B) Means of Discharge. The discharge circuit shall be either permanently connected to the terminals of the capacitor or capacitor bank or provided with automatic means of connecting it to the terminals of the capacitor bank on removal of voltage from the line. Manual means of switching or connecting the discharge circuit shall not be used.
Ben Miller
Reply to
Ben Miller
Using a capacitor to propagate moderate to high power is nonsense, unless savings due to power factor correction far outweighs the maintenance costs of the banks and the reliability problem.
Dave M.
Reply to
Dave M.
in article 4118bc05$0$ snipped-for-privacy@dingus.crosslink.net, John Gilmer at snipped-for-privacy@crosslink.net wrote on 8/10/04 5:01 AM:
Energy storage capacitors are often shipped with a shorting wire.
Bill
Reply to
Repeating Rifle
We were building a 400 Hz inverter for the SR71 and the output was tuned with capacitors. These capacitors had to work at 71 degrees C and 100,000 feet altitude. We were running a bank of Metalized Mylar capacitors. The test were run in a back lab where we had the equipment to produce the vacuum of 100,000 feet altitude and it was, luckily break time. When we heard loud explosions coming from that lab. Luckily no one was in the lab. When we went back into it we found that the caps had exploded and sent shrapnel all over the place. When the shrapnel from the first cap in the bank (There were about 15 caps in it) hit the next one it went off and so on down the line.
Yes Caps can go off quite violently.
Reply to
bushbadee
Altogether different service demands and capacitor design. Bang is still possible
Reply to
Don Kelly
as a reliance electric service engineer, i was urging a group of steel company electricians to always close the cabinet door when energising the cabinet on early solid state inverters. as we closed the door and closed the 'big switch' one went off and christened the cabinet insides. it was a great demonstration! sammmmm
Reply to
sammmm
Would 10 years in a non harmonic industrial environment be reasonable? 600V 100KVAR is typical ratings
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Chrisd
Reply to
.
i've not seen any caps that were bad. (in ordinary P-F correction use) i've applied and also checked quite a few. i understand that there are fuses internally in many. we had some caps in 180 Hz tripler setups go but they were in steel ingot induction furnaces and were water-cooled. if the P-F gets too close to a leading situation the current will go haywire. i'm not engineer enough to get deep in the theory. good luck, sam
Reply to
sammmm
I have some in a plant that I think suffered some large voltage swings in the supply line when a neutral opened and teh ground wasnt good enough to take care of it. They are 100Kvar 600V ABB units that eventually failed, some quite violently.
I just assumed that they had been damaged by poor power quality and replaced them. There are no harmonics anywhere in the system.
Now I have the same type of ABB units except they are 480V 60 Kvar failing in another plant. I do not know the power history of that installation and thats why I wonder if these are a short lived solution (10 yrs) or is there a better way to go. The ABB ones have many small cells inside and the replacements Commonweath Sprague have 10Kvar cans inside. Hopefully they will last longer.
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Chrisd
Reply to
.
Our enviromental crew decided it was time to change the ones in our 20 year old UPS, never had a problem with them since installation. A few went bad right after installation. They decided to do a complete change out based on age. For a period of about a year we had 1 or 2 caps fail per month then level off after that to no failures for a year. Prior to the change out there had been no failures beyound the first year of the installation. Think they should have left them alone.
Reply to
Jimmie
In a benign environment power capacitors should last 20 years. This can be shortened by many things including heat, dielectric degradation, physical damage, etc.
Can you tell me about the different types of caps (such as name/type and model) and describe the nature of their failure? I would avoid self-healing capacitors, especially in an industrial environment. They can fail on overvoltage transients that break down a section of the dielectric but fail to vaproize the corresponding section of the metal layer (i.e. the capacitor fails to self-heal). The result is an internal arc which burns away the dielectric and releases gasses which burn. I understand acetylene can be formed and will shoot out any hole or rupture in the can in a gout of flame. I have seen failed dry self-healing caps which, by appearance, would support this.
Sometimes capacitors are applied and everything works out fine. On the other hand, if you continue experiencing problems, you may be wise to speak to a consultant. They can examine your specific applications to see if there are any concerns that need to be addressed. They can recommend capacitor types and ratings, reactor types, control requirements, the whole works. Sorry for the cop out, but there can be so much involved with capacitors.
j
Reply to
operator jay

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