capacitor's

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elect36

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1400amp service 3-phase and power comp. says power factor at this plant isn't rite and is requireing a capacitor at service. But this plant doesn't have a switchgear, their main disc. means comes from a 800 amp dis. and a 600 amp dis. which is fine because it'll fall under the six switch rule, but what are some options on how to install a cap. on this type service and still be code. Install it before disc. or after or can one capacitor be used for both disc.? :eek:
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Re: capacitor's

One way to handle this kind of problem is to add power factor capacitors to your largest motors so they are switched on and off with the motors.

These can be wired in downstream of the motor starter. they can generally be purchased with fuseblocks so all you have to do is mount, wire them up, and plug in the fuses.

<edited>
It seems in your case they are suggesting a big capacitor be added to the incoming line. If you do this, keep in mind the current requirements to get the PF back above where they want it should be relatively light, so you can probably install it downstream of either disconnect.

[ August 12, 2005, 09:39 AM: Message edited by: petersonra ]
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Re: capacitor's

I've not done the design for this type of configuration before. But I think you can also add a third disconnect, and have it supply only the capacitor bank. The problem is sizing it for a load that might vary during the day. So you might want to look into capacitor systems that automatically switch on and off, as the need dictates.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Re: capacitor's

Originally posted by charlie b:
I've not done the design for this type of configuration before. But I think you can also add a third disconnect, and have it supply only the capacitor bank. The problem is sizing it for a load that might vary during the day. So you might want to look into capacitor systems that automatically switch on and off, as the need dictates.
why would you care if you had too much? you won't get penalized for that, and it helps the rest of the network.

i still think the real answer is to put PFC caps on large motors, rather than a single PF bank on the main line.
 

kiloamp7

Senior Member
Re: capacitor's

I dont know the details, but I think you can get into some strange overvoltage problems occasionally if you "overcorrect" the power factor.

I've always felt that instead of bulk capacitance connected to the bus, put the capacitance at the offending load, typically the induction motors.

Switch the capacitance on & off with the motor & re-size the overload relay "heaters" for the lower current draw.
 

69boss302

Senior Member
Re: capacitor's

We installed two PF correction capacitor banks on our system, we did have a switchgear. You should want to be after the disconnect.

Our banks automatically adjust for load on the switchgear. Yes you need to be very concerned about over correction. If you go to large you can cause overloading without even seeing it on any meters. You could end up going over the unity power factor and you will start to try and carry reactive load from the POCO.

The best way to install individual capacitors for pf correction is as close to the load as possible, because you can loose the effectiveness due to the reactance of the cables themselves.
 

bphgravity

Senior Member
Location
Florida
Re: capacitor's

Perhaps synchronous motors can be used to replace some of the induction motors or just added to the mix to help correct the PF.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Re: capacitor's

Originally posted by 69boss302:
If you go to large you can cause overloading without even seeing it on any meters. You could end up going over the unity power factor and you will start to try and carry reactive load from the POCO.

The best way to install individual capacitors for pf correction is as close to the load as possible, because you can loose the effectiveness due to the reactance of the cables themselves.
I agree with your second point, as for placement, but the reactance of the cable idea is just not an issue. That reactance is there regardless of where you put the capacitors, and has the same net effect on the overall system.

As for your first point, it is indeed possible to have too much capacitance, BUT, the only time it would affect the current draw on your service in an upward direction is at times when your load is low, so the chances of it overloading anything is very low.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Re: capacitor's

Originally posted by bphgravity:
Perhaps synchronous motors can be used to replace some of the induction motors or just added to the mix to help correct the PF.
motors run off vfds generally run at very near unity PF. sometimes it is actually less expensive to add a vfd than PFC caps, especially considering some of the other benefits you can get from a VFD.
 

rattus

Senior Member
Re: capacitor's

Do we all agree that ideally the caps should be installed close to the motors? Otherwise, in a long run between the caps and motors, one could have significant reactive currents and resistive losses.
 

69boss302

Senior Member
Re: capacitor's

one could have significant reactive currents and resistive losses.
I agree rattus, but apparently Bob doesn't
but the reactance of the cable idea is just not an issue
As for your first point, it is indeed possible to have too much capacitance, BUT, the only time it would affect the current draw on your service in an upward direction is at times when your load is low, so the chances of it overloading anything is very low.
I beg to differ with you, but most plant's I've been in and around have this problem almost every weekend and especially holiday's. I suffer this from air compressors and chill water systems all the time. They start shutting things down for weekend and the equipment is designed to handle large loads. Air pressure start's fluctuating, compressor starts cycling, and chillers and piping will practically freeze up, and that's in Southern Texas. No load is more common than you may think, and yes it all depends on the plant.

[ August 14, 2005, 09:00 AM: Message edited by: 69boss302 ]
 

elect36

Member
Re: capacitor's

their 2 biggest motor's are a 200 hp and a 125 hp motor that they want feed from the 800 amp disc. they want to come out of the 800 and set two 400 amp disc. for each motor and that's all on the 800 only.so those being the biggest motors and the farthest one being about 70' away we could probably set the cap. before both 400 amp disc. POCO. has had problems with this plants other building as well. They say power factor over there isn't right either and a cap. never got installed over there. that building keep messing up the POCO. trans. and blowing their fuses and i believe they said they ended up oversizing their equip. in the long run.
 

69boss302

Senior Member
Re: capacitor's

elect36:

Are these cap's being installed just to correct for the motors? If so they should be tied into the motor starter so they are only on line when the motor is on line. You can tap just after the disconnect, but most of the time if it's just a cap for the motor, you put it as close to the motor as possible and come of the motor leads.

Have there been any calculations done to size the capacitor's properly for the motors? Again too much can be worse than none at all sometimes. If they are concerned about the entire power to the facility, it is possible to get power factor correction capacitor banks that adjust to the demand. These are usually a large cost though. We installed them because we get a credit for our pf if we stay above .9 up to a .95 pf.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Re: capacitor's

Considering the fact that he has now stated most of his problem is coming from a couple of fairly large motors, we all seem to agree the best bet is to wire in some PFC caps in parallel with those motors so they switch on only when those motors come on.

It is indeed possible to get some I^2R losses if you install the PFC caps a long distance from the actual loads, but that is a different issue. If the only thing the plant cares about is correcting their PF problem so the utility gets off their case, a central cap bank may make sense. I think it is a less then ideal solution, but in most cases, it's not going to overload anything. I suppose you could come up with some scenario where it would, but it is unlikely.

It would make some sense to get the PFC bank as clsoe to the load as possible when they do install it, since it probably adds no cost and will reduce I^2R losses somewhat, but this kind of thing will more likely be dictated by available space rather than the few bucks a week of electricity you might save.
 

69boss302

Senior Member
Re: capacitor's

Yes Bob now that he has stated that is his concern I agree with you.

And I get concerned about the oversized capacitor's because from the sounds of it he just has a company that says the POCO wants our PF corrected, put in these capacitors that we have sitting in our store room. Yes a little to big, and the overload doesn't happen. And as much as I hate crunching numbers and formulas, you need to figure out what you need and then at least come close.

Putting the capacitors far away from the load, again the loses are probably nothing, but they can amount to something if you don't give it any consideration and just start arbitrarily doing things. It's the complacency that it leads to that is the problem.

Edit: I believe on the Mike Holt free stuff there is a pf capacitor corection calculator.

http://www.mikeholt.com/documents/calculations/formulas/capacitorsize.xls

[ August 14, 2005, 10:00 AM: Message edited by: 69boss302 ]
 
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