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Burnt capacitor problem


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#1 luclandry

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Posted 20 January 2006 - 02:02 PM

Hello everyone,

I am new to this forum. I would like some help on determining the cause of a capacitor that burnt out in one of our buildings.

The capacitor was installed less then 10 years ago in the electrical room of the building by the local power compagny when they removed one of the meters (probably the KVA meter). It is a 30 KVAR, 28.9 Amps, 600 Volts, 3 phase from Nokia company distributed by Gentec. At the front end of the capacitor we have a 3 phase switch fused at 30 amps. The non-linear loads in the building are the transformers, a few small motors for ventilation situated on the roof, fluorescent lighting, some household electronics, and a 5 hp motor for the elevator. The 600 V, 3 phase motor and its soft starter controller are situated in a small room on the roof of the building. The building is 4 stories high. The hole building is heated using electricity. Also, we have an emergency generator that can supply the building.

Last week we had some problems with our elevator. It turned out that the small emergency battery of the controller, in the soft starter, needed to be changed. When we were trying to solve the elevator problem, we decided to look at the capacitor bank and we found out that the 3 fuses for the switch were burnt. We replaced the fuses and at some time during the day one of the fuse was burned again. The fuse had melted and the capacitor was smoking. We shut off the switch and called the electrician witch confirmed that the capacitor was burned out.

Since there is no alarm to indicate when the fuses burn out the first time we don't really know how long it was since they were burned out. Was it a week or months?

Here are my questions:
1. Was it necessary to have a capacitor bank in this building? Does the power factor really needs to be corrected?

2. What would be the cause of the blown out capacitor bank? Such capacitors usually can last from 20 to 50 years. Is it normal that it burn out after 8 to 9 years of operation? Is it common to have manufacturer's defects?

3. Do you think that the equipment of the elevator had any influence on the capacitor blowing out?

4. Is it possible that the capacitors burnt out because of harmonics?


Thanks for your help,

Luc
in Lac-Etchemin, Canada

#2 jraef

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Posted 21 January 2006 - 12:03 AM

1) Necessity is a tough call. If you were at any time being penalized for having a poor power factor, then yes. If your loads are only as you state them, then I can't see you paying a big PF penalty, you just don't have much in the way of inductive loads. Sometimes people put in capacitors in the misguided belief that they save energy, which is not really true, especially in a case like yours. More likely, since you mention that the electic utility installed it, they were trying to use it to boost or balance the voltage. That is more of a cheat than a fix, it doesn't solve the problem but it makes the meter read correctly. Most utilities will deny that they do it, but I see it done all over, especially in rural areas.

2) Lots of things can blow out capacitors. Lightning strikes are the worst culprit. Not necessarily a direct hit, but in the general area where enough energy could have traveled downstream.

Freezing is a big one. If your caps are in an area that is exposed to outdoor temperatures, the electrolyte inside can freeze and swell when the power is off, such as after a power failure during a winter storm ( I hear that has been known to happen in Canada on occasion). Then when power is restored, the electrolyte is frozen and the cap film shorts the instant power is applied to it.

Over excitation, possibly from having the capacitor on all the time even when some of the motors are not running (although that is usually not a problem for the capacitors).

Age is a factor as well. Where did you hear 20 - 50 years life expectancy from a capacitor? More like 5 - 10.

Also, resonances in the circuit, i.e. interactions with other capacitors or capacitive coupling of long wire runs.

3) Not likely. The biggest problem with capacitors and soft starters is when the caps are connected downstream of the starters, because the rapid switching of the SCRs causes a resonance in the caps, and the cap charging current looks like a short circuit to the SCRs. It sounds as if this capacitor was upstream of the soft starter, so probably not.

4) Possible, but not probable IMHO.
"He's not dead, he's just pinin' for the fjords!"

#3 marke

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Posted 21 January 2006 - 01:03 AM

Hello luclandry

Welcome to the forum.

jraef has already given you the answers that you need, so there is not a great deal that I can add.
Basically, you do not need power factor correction unless you are paying some form of power factor or KVA penalty. If you pay for KWHrs only, power factor correction will do nothing for you.
A poor displacement power factor is caused by inductive loads such as motor and inductively ballasted lighting. The capacitance should be controlled in accordance to the inductive load and should not be permanently connected. If you are paying a power factor penalty, and you leave the capacitance on line all the time, then you will have a poor power factor from the capacitor when the load on the building is low. Power factor is normally controlled with the inductive loads, i.e. the appropriate amount of correction is switched on with the inductive load, this is known a static correction, or bulk corrected where a controller is used to monitor the power factor and switch in the appropriate amount of capacitance to bring the power factor close to unity.

The life of the capacitors is very dependant on the quality of the capacitors, the stability of the supply, the harmonics on the supply and the ambient conditions. In some cases, the capacitors can last less than 6 months!
If the voltage is very unstable and is subject to many surges, you may need to increase the voltage rating of the capacitors.
Most modern capacitors are self healing. This is achieved by using a metalised film such that if a fault develops, the metalisation vapourises and the problem is effectively disconnected. The value of the capacitance slowly reduces over time as cells become damaged.
If there is a significant harmonic voltage, (caused by electronic circuits and discharge lighting), the capacitors will carry a high harmonic current. This will over heat the capacitors and cause early failure. The use of detuning reactors in series with the capacitors will reduce the capacitive current and extend their life.
When you fit power factor correction capacitors, measure the current through the capacitor and if it is higher than it should be, you have a harmonic current problem and should fit detuning reactors.
I always recommend regular checks of the current drawn by the capacitors as this is a good indication of their health. There are many systems in service where the capacitors have failed and no one has noticed.

Best regards,

#4 luclandry

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Posted 21 January 2006 - 08:23 PM

Thanks jraef and marke for your answers.

The 20 to 50 life expectancy is from some information that I got in an engineering book, but I am no expert in the matter. If they normally only last 5 to 10 years, I will make sure that it is
necessary before re-installing another. It comes quite expensive.

Jraef pardon my ignorance but what does IMHO mean?

I was told that when the local power compagny first installed the capacitor, at the same time they removed one of the meters that was in the electrical room. This upgrade was made in the same month that we upgraded our elevator gear. We installed a new motor and the soft starter. This is why I thought that the elevator equipment had an influence on the capacitor. My guest is that they removed the KVA meter. At the present time there is no meter that measures the kVA and
our electrical bill only shows the KWHrs.

Do the fluorescent ballasts really have an effect on the power factor? This is an old building and the ballast probably have lower power factor than the ballast sold today. Would the low power factor from the fluorecent ballast be enough to need a power factor correction for the whole building?

I have contacted the power compagny and they will come and check out the situation. If they suggest the replacement of the capacitor what would be the best solution? Any suggestion?

Thanks again,

Luc

#5 marke

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Posted 21 January 2006 - 09:00 PM

Hi Luc

If your electrical bill is based purely on KWHrs, there is no advantage to you for having the capacitors unless the bill is based on KWHrs on the condition that the capacitor is fitted. I would tend to leave the fuses failed and fitted and just ignore the fact that the capacitor is not working.
Capacitors can create a number of problems with resonance and in effect they tend to magnify any surges on the supply unles you have a very strong supply.
The addition of power factor correction is primarly to reduce the reactive current flowing in the supply. This reduces the copper losses in the supply, but does nothing for the consumer as a rule.
If the capacitor is permanently connected, the reactive current will be capacitive when the motors are not running and this is just as bad as an inductive reactavie current in the supply.

Older flourescent fittings use inductive ballasts and these certainly do degrade the power factor. Some ballasts are corrected as standard.

Best regards,

#6 jraef

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Posted 21 January 2006 - 11:34 PM

IMHO is short for In My Humble Opinion.

Sorry about using an acronym that is not universally understood, I just got lazy.
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