ECN Forum Forums Technical Discussion Electrical Theory and Applications Sizing Power factor caps?

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.

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..

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).]

"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.

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).]

Dnkldorf
I hope you won't let this die out. I want to hear the rest of the story.

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...

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.

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).

"'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