Green Building Forum - Power factor corection

Power factor corection

Beau — Sat, 07 Jul 2012 17:45:41 +0100

Have any of you got any experience with the above?
A local renewable energy firm has suggested using power factor correction in a small power hungry business that a friend of mine is setting up. The main power use is large electric motors, total use around 300kw when in full production.

Power factor corection

JSHarris — Sat, 07 Jul 2012 18:07:30 +0100

Posted By: BeauHave any of you got any experience with the above?
A local renewable energy firm has suggested using power factor correction in a small power hungry business that a friend of mine is setting up. The main power use is large electric motors, total use around 300kw when in full production.

I've got a feeling that the DNO will usually insist on PF correction for industrial loads like that, they certainly used to years ago, back before privatisation etc, as I remember it being discussed in part of an electrical engineering module I did back in the very early 70's when I switched careers.

IIRC industrial users have to correct the PF back to at least 0.9, and are advised to correct to 0.95 or better if they can. Correction for big inductive loads is simple enough, just large banks of capacitors that are switched in and out as the load varies.

It won't save any energy, but will make the meters read more accurately, so will give the illusion of saving energy by bringing the current and voltage better into phase with each other. My understanding is that electricity meters only had a limited ability to correct for varying power factor in the past, not sure whether the new digital ones can cope with larger PF errors OK.

Power factor corection

Ed Davies — Sat, 07 Jul 2012 18:47:09 +0100

Actually, PF correction will tend to save some energy as there will be smaller currents in the distribution network and therefore lower loses. AIUI, heavy-enough industrial users are billed for current used, as well as power used, for just this reason.

Power factor corection

JSHarris — Sat, 07 Jul 2012 19:26:15 +0100

Posted By: Ed DaviesActually, PF correction will tend to save some energy as there will be smaller currents in the distribution network and therefore lower loses. AIUI, heavy-enough industrial users are billed for current used, as well as power used, for just this reason.

If we're being pedantically precise and nit-picking, then the current remains the same whether the PF is 0.9 or unity, all that changes is the relative phase between voltage and current.

Deficiencies in the network's ability to cope with varying PF are a separate issue, and highly dependent on the local load conditions. For example, if there are two businesses on the same sub-station, one with a lagging PF and one with a leading PF, and both of the same phasor amplitude, there won't be any problems at all.

Power factor corection

Ed Davies — Sat, 07 Jul 2012 19:32:44 +0100

Posted By: JSHarrisIf we're being pedantically precise and nit-picking, then the current remains the same whether the PF is 0.9 or unity, all that changes is the relative phase between voltage and current.

Not if the real power transferred remains the same. PFs other than one will require more current.

Power factor corection

Beau — Sat, 07 Jul 2012 19:46:10 +0100

Sorry I have been wasting your time.
Apparently they have already had power correction installed taking it from .85 to .95. I confess to not really understanding but thanks for the help

Power factor corection

JSHarris — Sat, 07 Jul 2012 19:53:25 +0100

Posted By: Ed Davies
Posted By: JSHarrisIf we're being pedantically precise and nit-picking, then the current remains the same whether the PF is 0.9 or unity, all that changes is the relative phase between voltage and current.

Not if the real power transferred remains the same. PFs other than one will require more current.

We're talking electric motors in this example, a fairly simple LR load that will just shift the relative phase between current and voltage. This isn't a complex thought experiment to induce endless theoretical debate, just a crude and fairly imprecise typical industrial power scenario, where L will shift with torque loads, PF will shift along with it, and consequently the correction system will never get it to unity, just fairly close.

You can go to town doing a detailed analysis of the change in L with magnetic saturation as each motor's torque changes with load, and the consequent impact this has on PF, together with the more serious issues surrounding waveform distortion as motor core magnetisation changes non-linearly with AT, but the point I was making is sound, at least as far practical issues are concerned between the load and the meter, which is all that most consumers worry about.

Sure the DNO won't like big PF discrepancies, as it causes them all sorts of problems, but that's not the key issue here, it's going to come down to whether or not correcting to better than the DNO specified figure is worthwhile for this particular user. My guess is it may well be, as the loads are great enough that correcting to as close to unity as possible may well give savings in other areas, as I believe some of the eco ratings take PF into account for new premises.

Power factor corection

JSHarris — Sat, 07 Jul 2012 19:55:30 +0100

Posted By: BeauSorry I have been wasting your time.
Apparently they have already had power correction installed taking it from .85 to .95. I confess to not really understanding but thanks for the help" alt="" src="/forum114/extensions/Vanillacons/smilies/standard/bigsmile.gif">

0.95 sounds pretty good, not worth going better than this AFAIK, as the law of diminishing returns kicks in beyond this point.

Power factor corection

Ed Davies — Sat, 07 Jul 2012 19:58:43 +0100

Just read this afternoon/evening's PF thread on the Navitron forum. Often that forum gives good information, but sometimes you just get threads like that one.

This is a pretty good read:

http://en.wikipedia.org/wiki/Power_factor

Power factor corection

JSHarris — Sat, 07 Jul 2012 20:47:11 +0100

No argument from me with the Wiki article, pretty much as I've been saying all along. I've no intention of having anything to do with Navitron, though, and have vowed never to do business with them or view their website.

Power factor corection

Beau — Sat, 07 Jul 2012 21:24:05 +0100

Thanks for the link Ed
I will read through and try and enlighten myself.