Capacitor Bank

[Be_O_Be]

I'm new to posting here so please bear with me.
I'm dealing with a capacitor bank that serves a freezer warehouse approx. 100k sq.ft. When operating the PF is perfect. The cap bank fuses originally installed were 30amp and would occasionally pop on a single phase. Over time these fuses have been increased to 50amps, can't get a clear answer on who made this decision but this has resulted in (2) different trips by the fire dept. over a two year period. .The over sized fuses are overheating and arcing. 50 amp fuses are not the solution.
I will be going back to the original configuration, 30 amp fuses.
The customer & I both understand the value of the cap bank but the question that I can't answer clearly to the customer is "what causes the fuse to trip".

 

[zbang]

Is it the always same fuse, two, or do all three blow at random (one at a time)? If you have a marginal or intermittent section, it should only blow the fuses at either end of it and leave the third fuse alone. That would help isolate which section needs examination. What voltage? Can you put a current recorder on all three phases? There may be some significant unbalance. (Another little bird is whispering "wacky harmonics", but I can't think about that right now.)

 

[don_resqcapt19]

That appears to be excessive heat damage caused by a poor connect at the top fuse clip.

 

[Besoeker]

That appears to be excessive heat damage caused by a poor connect at the top fuse clip.

Could be. Surprising that has melted the ceramic all the way to the bottom and even the end cap on the capacitor terminal at the bottom.

 

[mbrooke]

Im thinking you have a faulty capacitor. That looks like a serious overload or heat. Either that something is loose and getting very hot. I would just replace the center capacitor and associated fuse holders.


Is this happening to only the center capacitor or the others as well.

 

[vicdog]

I had some fuse clips like that and the bolts were too long. The end of the bolt hit the fuse and prevented a good seat in the clip. Fixed it before power up.

 

[templdl]

Could be. Surprising that has melted the ceramic all the way to the bottom and even the end cap on the capacitor terminal at the bottom.

Yes, I would agree with you guys that it's a live parts issue specifically the upper part.
No telling what damage has been done to the clips because of the heat and should be replaced.

 

[Be_O_Be]

The ceramic on the outside of the fuse is in pieces within the cabinet. The fuse holders are tight to the fuse & the wire tightens down on the holder when in place. When these fuses were up sized to 50 amp this is when the heating and arcing started. This time it was arcing through the fuse for nearly one hour before I arrived, dressed down and disengaged the bank.
There is a charge in the fuse that when deployed pushes a pin from the top of the fuse.This pin pushes the cable on top and the cable moves a micro switch to the NO position, taking that capacitor out of line.
With the 50 amp fuses there isn't a big enough issue to discharge the pin and clear the fuse.
This worked with 30 amp fuses, all three phases have had to be replaced once each individually over five years.
I am also leaning towards a harmonics issue.

Thanks for the replies.

 

[petersonra]

have you talked with the manufacturer of the capacitor bank about this?

A fuse that gets this hot ought to trip. I wonder if the issue is the kind of fuse being used, or maybe a bad lot of them.

Maybe something went wrong and a bunch of 30A fuses tripped and whoever fixed it could not figure out what went wrong so he just upped the fuses to keep them from blowing.

Do the capacitors have an AC voltage rating on them? Does the voltage rating exceed the voltage they are seeing?

 

[templdl]

First off replace all of the live part of the fuse holder. Once they have gotten as hot as it appears the live parts are no longer capable of maintain tension on the fuse Farrel's and well get hotter causing the fuse to fail. The fuse itself should fail and clear and not the fuse holder.

 

[Jraef]

First off replace all of the live part of the fuse holder. Once they have gotten as hot as it appears the live parts are no longer capable of maintain tension on the fuse Farrel's and well get hotter causing the fuse to fail. The fuse itself should fail and clear and not the fuse holder.

I totally agree. The most likely scenario to me would be that your capacitor burned through, the first 30A fuse failed as it should have, it was replaced by another, it of course failed because the cap was still bad. Third one failed again, now your fuse clip tension is compromised. Upping to 50A fuses allowed more current to flow through the burned through cap, but the weak fuse holder over heated the fuse ferule and melted the fuse case before the fuse element could clear it.

 

[Besoeker]

The ceramic on the outside of the fuse is in pieces within the cabinet. The fuse holders are tight to the fuse & the wire tightens down on the holder when in place. When these fuses were up sized to 50 amp this is when the heating and arcing started. This time it was arcing through the fuse for nearly one hour before I arrived, dressed down and disengaged the bank.
There is a charge in the fuse that when deployed pushes a pin from the top of the fuse.This pin pushes the cable on top and the cable moves a micro switch to the NO position, taking that capacitor out of line.
With the 50 amp fuses there isn't a big enough issue to discharge the pin and clear the fuse.
This worked with 30 amp fuses, all three phases have had to be replaced once each individually over five years.
I am also leaning towards a harmonics issue.

Thanks for the replies.

A couple of thoughts.

The entire fuse got extremely hot for it to fail in the manner that it did. I think the source of heat that discoloured the terminals came frome the fuse, not vice versa. I have seen this sort of damage on maybe two or three occasions all from the same cause.
Fuses can take sustained limited overloads (as opposed to fault current) for very long times. I took the curve of a 50A fuse and it can sustain 80A for about two hours. In that time, it will get extremely hot and that's what I think may have happened here.

That raises the why question of course. A problem with that particular capacitor would seem to be the most likely. Past fuse failures might have been with power on transients. I'd be inclined to rule out harmonics at this stage - the fuse on the left is pristine and it would need a unique set of circumstances for that to have such a catastrophic effect on one leg. Not impossible, I guess.

And a couple or three questions.

Have the capacitors been tested for insulation and capacitance value?
Has the current been measured?
Why was the arcing allowed to continue for an hour. Seems odd.

 

[Be_O_Be]

"And a couple or three questions.

Have the capacitors been tested for insulation and capacitance value?
No but they will be.

Has the current been measured?
Not since this was installed. Values should have dropped due to motor & lighting upgrades.

Why was the arcing allowed to continue for an hour. Seems odd."
Maintenance man calls head of maintenance at home, head of main. calls fire dept. then myself, fire dept. calls power company, everyone waits. I arrive before power company and I have the pleasure of shutting this down.
Is there a better way, absolutely.

 

[GoldDigger]

Not since this was installed. Values should have dropped due to motor & lighting upgrades.

The current from the service may have dropped, and the need for (calculated size of) the capacitor bank may have decreased as a result of motor upgrades, but the current in the capacitors should not have changed. If it has, it is the result of changes within the capacitor bank or changes in the service voltage within the allowed range.
The idea is to know how that current compares to the size of the fuses, as per the original design.

PS: I see you say that the PF is perfect during operation. That tells me that either the bank was originally on the small side or the changes made to the motors increased the efficiency without reducing the reactive load total.