ECN Forum
Posted By: Dnkldorf Sizing Power factor caps? - 02/23/06 12:49 AM
How would size these if the PF changes from .62-.78 depending upon the load?

The highest reading I took during a 3hr test was .84. Would you calculate it at the highest reading or average it out?


Dnk...
Posted By: Bob Re: Sizing Power factor caps? - 02/23/06 03:42 AM
Is the customer being bill for a KVA demand?
You would size the caps based on the max demand and PF. Suggest you contact the utiltiy and get the max demand from them.If the PF is 0.84 I would try to improve it to
0.98 or something close depending on the size of the caps rrequired.
Posted By: Radar Re: Sizing Power factor caps? - 02/23/06 05:34 PM
Dnk - Rather than calculating based on min, max or average, I think I'd try to figure out what the nominal PF is, I mean what is the prevailing PF reading most of the time. It might be dificult to ascertain, but it seems to me that if you correct for that level, then the amount of time of over correction and under correction will be minimized, and the amount of time of 'just right' correction will be maximized.

Radar
Posted By: Bob Re: Sizing Power factor caps? - 02/23/06 05:57 PM
Radar
Thats one way of doing it. However if you are being billed for the max KVA demand that method will cost you money.
Posted By: Radar Re: Sizing Power factor caps? - 02/23/06 08:36 PM
OK - I can see that. That being the case, should we figure out what the PF is while at max KVA demand and correct for that?

Radar
Posted By: Bob Re: Sizing Power factor caps? - 02/24/06 12:38 AM
Radar
That is correct. The utility can tell you what the KVA demand and PF is. You can then caculate the required capacitors to correct the PF to say 0.98 or 0.99.
Posted By: Radar Re: Sizing Power factor caps? - 02/24/06 01:14 AM
OK Bob - let me ask another question here (I feel like we're leaving Dnk outta this). Your first question was whether the customer is being billed for KVA demand. What if they weren't, would we then be back to what I said earlier about figuring out the PF that takes the largest amount of time and correct for that, recognizing we'd be over correcting or under correcting at other times? Hopefully we'd be covering a major portion of the day with close to 1 correction.

Or would we still figure the hishest KVA demand and correct for that? Or would we simply forget the whole thing altogether if the poco is not billing for KVA demand?

Radar
Posted By: Dnkldorf Re: Sizing Power factor caps? - 02/24/06 02:06 AM
Radar, you're not leaving me out of anything....

I am reading everything you guys say, and learning. I have more input, but I am waiting to hear responses to kinda formulate my next post...

I agree to what Bob is saying, however my readings were taken off a subpanel, @ 250' away from the service. Somehow I think I need to contact the power company, and do a whole plant survey with readings from the service to each subpanel, draw a line diagram and possibly correct in different areas rather than a bulk correction at the service entrance....

The sub panel is a 400A 480v panel. The customer is a huge machine shop.

What is your guys take on this idea?

Dnk..
Posted By: Bob Re: Sizing Power factor caps? - 02/24/06 03:39 AM
If the customer is being billed on a KW demand then there is no need to install the caps. They won't reduce the demand charge.

Dnk
If the main is only a 400 amp I doubt that there is a KVA demand. Probably a KW demand.
If there is a KVA demand charge, I think installing the caps at the main will be a cheaper installation. How did you measure the PF at the panels?
Posted By: Dnkldorf Re: Sizing Power factor caps? - 02/24/06 11:34 AM
Bob, service is 1200A 480V. That was just a subpanel reading.

Would correcting PF reduce line losses?

Fluke 434B (3) phase analyzer..

Dnk..

[This message has been edited by Dnkldorf (edited 02-24-2006).]
Posted By: WFO Re: Sizing Power factor caps? - 02/24/06 12:21 PM
Quote:
"Would correcting PF reduce line losses?"

It depends where you put them in relation to the load. The KVA between the cap and the load it's correcting remains the same. The KVA up to that point is what drops.

For example, installing caps at the substation will correct the PF to the substation (and therefore, line losses) but will not contribute to reducing line losses between the sub and your plant.
Posted By: Dnkldorf Re: Sizing Power factor caps? - 02/24/06 12:34 PM
WFO, that's what I thought.

So in theory, If I was to correct at the subpanel, this would reduce losses from the main to the subpanel?

And If I correct at the main, this would reduce losses from the substation to the main.

Is that why you try to correct as close to the load as possible, if installation costs permit such a design?

Dnk..
Posted By: Radar Re: Sizing Power factor caps? - 02/24/06 03:38 PM
Hey - A little dim light just went on.

Realizing this would not be too practical, but in theory only, and discounting the lighting system for the moment, would it be a cool idea to figure the PF & KVA for each individual piece of equipment, and install correction right at the equipment so the correction switches on and off with the equipment? The overall system PF correction at any given moment would be determined by which equipment was actually running, and having the correction at the load would minimize line losses in the wiring system.

This leads to the obvious question of why don't equipment manufacturer install PF correction right on their equipment as a standard feature?

Too much 'pie in the sky"? Just theoretical wondering.

Radar


[This message has been edited by Radar (edited 02-24-2006).]
Posted By: Bob Re: Sizing Power factor caps? - 02/24/06 04:13 PM
"Would correcting PF reduce line losses?"
Yes. Example: If you have a large motor with a 0.80 pf and you correct pf to say 0.98 you will have a reduction in the current flow and thus decrease line losses up to the motor.

"So in theory, If I was to correct at the subpanel, this would reduce losses from the main to the subpanel?
Yes

"And If I correct at the main, this would reduce losses from the substation to the main."
Yes

"Is that why you try to correct as close to the load as possible, if installation costs permit such a design?"
Yes. The capacitor size is dependent on the size of the motor. You need to contact mfg for a suggested size.

"This leads to the obvious question of why don't equipment manufacturer install PF correction right on their equipment as a standard feature?"
Cost. All costomers are not charged for a low PF and thus no need to correct it.
Posted By: WFO Re: Sizing Power factor caps? - 02/25/06 01:41 PM
Quote:
"Is that why you try to correct as close to the load as possible, if installation costs permit such a design?"

Yes. The motor still has reactive power (Var) requirements, which the capacitor is now providing. The wire between the two still carries this reactive power.
By the same token, if you don't connect the capacitor where it comes on and off with the load, you'll have the opposite problem. When the motor goes down, if the cap stays on, you now have just as bad a PF, except now it's leading instead of lagging.
In terms of your bill, this probably isn't that much of a concern since you usually get penalized for your PF at peak demand.
However, even at no load the caps would be hot and you would have some issues.

But remember that there is a law of diminishing returns to apply here. There is a point where the cost of additional capacitors is not justified.
In other words, correcting to 95% PF might save you tons of money, but correcting to 98% may be twice as expensive and only return slightly better savings.
If billed on a PF penalty, you would need to know what the POCO's limits are.

[This message has been edited by WFO (edited 02-25-2006).]
Posted By: Bob Re: Sizing Power factor caps? - 02/26/06 04:26 AM
Dnkldorf
I hope you won't let this die out. I want to hear the rest of the story.
Posted By: Dnkldorf Re: Sizing Power factor caps? - 02/26/06 12:01 PM
I'll try not too Bob..

I have some investagating to do it seems, and some research on my end, however, if anyone wants to throw more info out there, I'm willing to "suck" it all up.

I'm heading back there early this week, I'll post my findings if you guys are interested...it will take awhile to trend data though.....


Dnk...
Posted By: RODALCO Re: Sizing Power factor caps? - 02/27/06 10:10 AM
By improving the PF you will reduce the line currents and hence losses in the mains or submains.
You can also get away with a smaller size cable if PF correction is permanently installed, because of the lower currents drawn.
If the utilty doesn't charge for kVA's and has no thermal MDI's fitted, you can leave it without caps. although more copperlosses in mains.

As what WFO sais he is correct that normally you aim for a PF of 0.95, and not 0.98, that will be twice as expensive and will take a long time to pay back for the extra costs.

You certainly don't want to get unity PF with Caps because the risk of resonance. This can cause unstable mains voltages which may be a multiple of the nominal voltage and can cause a lot of damage.
Posted By: WFO Re: Sizing Power factor caps? - 02/27/06 12:19 PM
Quote:
"You certainly don't want to get unity PF with Caps because the risk of resonance."

Expand on this if you will. I sorta, kinda, almost half-way understand resonance (but not really).
Posted By: Bob Re: Sizing Power factor caps? - 02/27/06 03:51 PM
"'As what WFO said he is correct that n you aim for a PF of 0.95, and not 0.98, that will be twice as expensive and will take a long time to pay back for the extra costs."

I am in total total disagreement with this statement. First of all you don't know what it will take to improve the PF from .95 to .98. and secondly you don't know the penalty charge per KVA. I've done a number of these and return on the investment is as little as 1 year to 3 years. That's not too shabby
Posted By: Dnkldorf Re: Sizing Power factor caps? - 02/27/06 04:06 PM
A few answers that I got so far from the utility company:

1. PF is 70.3
2. Billing demand is 255KW

Penalty is not so straight forward though, they factor in a couple formulas and then which ever is highest, that is the cost.

The POCO don't make understanding charges very easy, is that for a reason?

The guy was using words like "ratchet minumum", whatever that is...

Dnk..

[This message has been edited by Dnkldorf (edited 02-27-2006).]
Posted By: JBD Re: Sizing Power factor caps? - 02/27/06 06:57 PM
Utility demand charges and power factor penalitites are not normally directly related.

A demand charge is a fee the utlity charges for having to provide equipment that will meet your peak KVA loading (even if they bill you per KW). A ratchet clause says they get to charge you at your highest demand rate for the next 12 months even if you never hit that level again. When calculating pay back remember it may take many months before your billed rate "ratchets down". I have at least 1 customer that will stop production in order to not exceed a specific demand level.

Power factor penalties are very utility dependent. Some will even have a larger penalty for exceeding "unity" then they do for being below it. I target my calculations at .95, this allows me to usually fall between .92 and .98 by the time I am done.

In my immediate area there is no major utility that imposes a PF penalty. They used to have incentives to correct PF but those ended almost 10yrs ago. now almost all PF is done to improve the customer's system performance only.
Posted By: Bob Re: Sizing Power factor caps? - 02/27/06 07:09 PM
If the customer is being billed 255kw then there will be no reduction in the bill by improving the PF. The demand is probably not large enough to cause a PF penalty.
Posted By: Radar Re: Sizing Power factor caps? - 02/27/06 08:28 PM
Dnk - I'm presuming you mean the numerical value for PF is 0.703?? Check out these numbers to see if I'm on tract here.

KW = 255
PF = .703, which means the phase angle is 45.33°.
KVA = 363
KVAR = 258 (which is more than the KW)
Quote
The demand is probably not large enough to cause a PF penalty.
I tend to disagree. Although the utility may not charge for low PF, if they do, I'd say this one qualifies.

Radar
Posted By: Bob Re: Sizing Power factor caps? - 02/27/06 08:56 PM
Radar
You make a good point. Most utilities that have a power factor penalty apply it at about 1000 kva. Loads smaller just do not cause a problem. There may be some utilities that may have a lower demand requirement.
Posted By: Radar Re: Sizing Power factor caps? - 02/27/06 09:18 PM
Makes sense Bob. Even though the system Dnk is dealing with stinks (PF wise), it may be too small to incur a PF surcharge. But what if the customer is being billed for KVA demand instead? Seems like a good candidate for corrective measures in that case. Otherwise, not really worth fooling with?

Radar
Posted By: Dnkldorf Re: Sizing Power factor caps? - 02/27/06 09:35 PM
Guys, I'm not quite sure how they bill yet, but, the guy at the POCO did a quick calculation. He stated if I corrected to .9, this would shave 35KW of the billing demand.

All charges, distribution, generation,transmission, and transition are based on billing demand...

This would appear to be a 13% reduction in costs..however small though..

This formula they use appears to be the most difficult part to understand so far..

This is a good beginner exercise for me..start small, and go bigger...

One other note, the past month has been slow here. They are in the midst of a product redesign again. I think I need a whole year of data to really be effective. So the PF could actually be lower, and the demand could actually be higher.

(anyone understand Exolon/Peco rate formulas?)

Dnk..
Posted By: Radar Re: Sizing Power factor caps? - 02/27/06 10:55 PM
I gotta go catch a train, but I have time to throw a few numbers in here. at .9 PF:

PF = .9
Phase Angel = 25.84°
KW = 255 (as Dnk said)
KVA = 283
KVAR = 123

Some improvement indeed.

Radar
Posted By: WFO Re: Sizing Power factor caps? - 02/28/06 02:36 AM
Quote:
"(anyone understand Exolon/Peco rate formulas?)"

No, but I work for a Utility and it gripes me to no end when one of our employees can't take the time to make an explanation understandable.

You have every right in the world to get a decent explanation of your billing. If you get a some nimrod who's only looking for quiting time, request to speak to someone else. Every Utility is just like any other company, you get good employees and bad ones. From reading these forums, I get the distinct impression that most customers get the bad ones.

DON'T TAKE IT! You are entitled to an adequate and CLEAR explanation.

(....deep breath....calm blue ocean..... ........ohmmmmmmmmmmmmmmmmmm...........)

OK. Rant over.

As for the remark that PF correction would lower your Kw demand, I think everyone here is in agreement that PF correction does not change your Kw. So I can only assume that;

1. You are billed on Kva rather than Kw, or

2. Your total bill incorporates a PF penalty that is addressed as a percentage of the Kw and added to it.

IN either case (warming up for another rant here), there is no excuse what-so-ever that the Utility cannot give you an adequate explanation of the way you are billed.
Posted By: Dnkldorf Re: Sizing Power factor caps? - 02/28/06 03:24 AM
WFO, from what I got from him, it is #2...

The thing that erks me the most is how on their web site they publish their tariffs rates. They call one charge one thing, but call it something else on the bill. I guess they leave it up to you to figure out what is what..

And there is no mention of "ratcheting minumums" any where that I can find on the site under the HT tariff. Why can't they make it cut and dry?

Anyway back to the original topic..

I'll be down tommorrow, and take some quick measurements, let you know what I find..

Radar, you catch that train?

Dnk...
Posted By: Radar Re: Sizing Power factor caps? - 02/28/06 03:18 PM
That I did. Currently I'm lucky enough to commute by train, which is pretty nice except for every once in awhile when something happens. But I'm wandering off topic again.

Radar
Posted By: Dnkldorf Re: Sizing Power factor caps? - 03/02/06 09:56 PM
Update...

POCO does charge a PF penalty on KW usage. They have minumums that must be met, yet they are hidden, and was a real bugger getting this info from them:

1-185KW must maintain a PF of .80 or better
185-2500KW .90 or better

And this is both on GS(commercial) services as well as HT(high voltage transmission) services.

So, then..I was able to retrieve bills that show the PF of the place ranges from .69 to .76 from 12 months ago.

Would you attempt to size PF caps from the .69 to say a .90. That would push the PF from .76 to .97, wouldn't it?

What would happen if you corrected the .69, to say .93, and then when the loads changed, it forced the .76 either at unity or slightly above?

Dnk..
Posted By: Bob Re: Sizing Power factor caps? - 03/02/06 10:20 PM
"So, then..I was able to retrieve bills that show the PF of the place ranges from .69 to .76 from 12 months ago.
Would you attempt to size PF caps from the .69 to say a .90. That would push the PF from .76 to .97, wouldn't it?"

You need to maintain a PF of 0.90 or better.
Use the kw load at 0.69 and correct to 0.90 or better. Check the load at 0.76 and using the capacitors installed, see what the caps correct the PF at that load.


[This message has been edited by Bob (edited 03-02-2006).]
Posted By: Dnkldorf Re: Sizing Power factor caps? - 03/19/06 10:02 PM
Ok here I go again.

I think I got it but need more help..

Using the following givens;
255Kw load
PF .7
V=4160
60Hz

I get the following results:
To correct PF to unity
39mf caps
current in the caps 62a

To .9 PF
21mf caps
current 33a in caps

To .8 PF
11mf caps
current 16 in caps

This should be right, but I need the help in going backwards. If I correct from .7 to .8 and use a 11 mf cap, how do I calculate the change when the load goes from .7 to .75?

I got something wrong in my sheet and can't figure this one out...

I plan on using switching units, but still want to know how to figure it out...

Thanks..

Dnk....
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