Power Factor?? How does a capacitor help?

You got it right. The saw mill I work in has two computer controlled capacitor banks. Each bank can pull in up to 5 sections of 110KVAR for a grand total of

A low power factor causes voltage drops for others on the line. Reactive power can not turn a motor and does not show up on a watt meter but it must be supplied from somewhere. Electric utilities don't operate their generators for free so if you have a poor power factor they will charge you a penalty. It is way cheaper to put capacitors on the line.

Marten

Not misguided, but perhaps incomplete.

Lightly loaded motors are inductive. Fully loaded motors less so.

The current supplied by the source is higher for the delivered power. Since the current is higher the losses are greater. THe generator is generating watts, you're using V*A, so the difference is only heating the distribution system.

Certainly (ignoring harmonics). VA=W when Pf =1.

Unless there are harmonics caused by the load (switching regulators are a problem), yes. A proper capacitor can align the voltage and current, at least at that point. Indeed you'll see capacitors on power poles to correct for Pf.

Many things surprise me.

For loads with little harmonic content, you're right on. Things get a little more complicated with switching loads.

The main reason it's "bad" is because it causes a larger current to flow then the raw wattage numbers would suggest. For example, some of those screw in mini flourescant bulbs you can get have horrible power factors. I saw one rated at 20W, yet on it it showed the following: 120VAC, 330mA. If you do the math you'll see that the power factor is about 0.5. So, in this case, to get 20W (which in a resistive load would only need about 160mA), the thing ACTUALLY draws 330mA. Now, in this case this isn't a big deal, but say everybody in your neighbourhood ONLY had bulbs (and furnaces, and air cons, and all other kinds of devices) with such a power factor? All of a sudden the utility would have to supply almost twice the amount of current to your neighbourhood. Twice the amount of current means thicker wires, more beefy power transformers, and of course, since you only pay for watts, an additional loss on generation. The adition of reactive loads to get a power factor of 1 would result in a huge cost savingsm, which is why it's done by almost all companies, since they are charged for actual VA used, not just watts. TTYL

A couple of weeks ago I flew up to the site of a project where our client had installed two banks of 5000 kVar of capacitors - part of a voltage stabilizing system on a transmission line. And this isn't particularly a large set of capacitors.

Of course for *really* large amounts of reactive power on a system, synchronous condensors are the way to go. These are rotating machines, similar to synchonous motors but with no mechanical load on them, operated with extra field current so that they supply reactive power to the system instead of consuming it. In this part of the world the electric power utility operates a few dozen of these machines, rated upwards of 150,000 kVAR.

Bill

see: Articles on Temperature and Power Control Technology in Industry and Research at:

Art

see: Articles on Temperature and Power Control Technology in Industry and Research at:

Art

I understand the utilities charge you for the current they supply NOT the current you use.

In the example below you will be paying DOUBLE of what you actually consume.

That's why / how we justify the cost of power factor correction

A+

Not true for residential/small business customers (in the US anyway). You're charged for kWH consumed. The power company has to figure out the pf stuff.

Not in the hypothetical residence. The power company meters kWH.

The power company passes off pf correction to large industrial customers (they pay for pf). The power company does the correction (or eats the loss) for the rest.

Actually, in my area of the world industrial users ARE charged for both real and reactive power. However residential users are ONLY charged for real power used. A residential user can have a load power factor of 0.1 but would only be charged for real power used. TTYL

In the computer room here, just outside an electrical room is a large box labeled "Harmonic Supressor" connected to the electrical room with a large diameter conduit. What is likely in the box to get rid of harmonics? Since it is cool to the touch I guess it doesn't resistively dissipate them.

How stupid of the utilities !!! On the other hand in the good old days residential loads were almost 100% resistive until the invasion of the electronics into everything.

Perhaps they make it up with higher $/KWH for everyone.

A+

What kind of things would make a residential user have such a bad pf of 0.1. I think a pf of 0.1 is bad and a pf of 1.0 is perfect??

repatch wrote:

I wouldn't call the utilities "stupid" over this. As you mention, in the past almost all loads were almost purely resistive, it's only recently that residential loads have become more an more reactive (usually towards the inductive side). Correcting for reactive loads of the type in homes is not trivial, in method or cost. So the utilities are kinda stuck. People would FREAK if they were charged more to get what they already get (heck, they are already freaking about the cost of electricity, personally I think power is VERY cheap, but that's just me...), so utilities are pretty much stuck at the moment. They CAN correct for it on their end but again it's a matter of cost. I for one believe it won't be very long before they'll have no choice and just impose REAL billing. TTYL

in article bmmmp9$f5n$ snipped-for-privacy@pcls4.std.com, Michael Moroney at snipped-for-privacy@world.std.spaamtrap.com wrote on 10/16/03 11:07 AM:

In all likelihood it is a low pass filter.

Bill

----------------- However the motors in a household still dominate var demands. At present and probably in the future, the cost of equipment to monitor pf may well not be worth the effort. It is probably cheaper to tie in capacitors near the transformers (not back at the source) than to modify meters on all households. The cost of residential pf is built into the cost of delivery of energy. Poor household pf is not of importance with regard to transformers and lines except at heavy load periods. In some places the cyclic nature of residential loads (which can be as severe as poor pf) is already accounted for where "time of day" billing is used but in others, it is not worth the effort at present.

For large loads, it is worth adding demand metering which, as far as I know, still is based on the peak (typically half-hour) KVA demand in a billing period is made. This peak may be due to other factors than power factor.

-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer

But why bother? When a utility has 100 000 customers, the *average* pf for residential might run something like 0.8. So the *average* cost of supplying 500kWhr includes the costs of delivering 625kVAhr (375kVARhr). For decades utilities have been going to public service commissions and justifying their rates. This has always included the costs for delivering kWhr to residential customers at less than unity pf.

The only real problem is if the *average* pf is changing (due to electronics) faster than the utility can convince the rate commission. Then the utility is in a bit of a bind for a few years until the tariffs can be updated. But its not a long term problem.

daestrom

Yep. Unity is as good as it gets. 0.1 pf means each amp of current flowing in conductors is only delivering 10% (0.1) of the energy it *could* be delivering. More losses, and larger equipment needed to supply the same energy.

Such a low pf is very unusual. Even for electronics with high harmonic content. Electric motors running with very little load on them perhaps?

If it is a result of one specific load, it might be worth it to try correcting the pf right near the load. But since residential aren't charged differently for being so far below the average home, it may not be worth it. The only savings would be reduction in I^2R losses and depending on the total current, this may not be enough to justify special correction.

daestrom

Somebody on here mentioned self contained florescent light sources as having low pfs a few weeks ago. I filed that in the back of my mind for further investigation. A couple days ago my wife gave me a florescent spotlight to replace a 40 watt bulb in an over the shoulder reading light. I must say it provides more than enough reading light. The surprise? It is rated at 5 watts! Surprise #2 - it draws 12 va! That's a pf of .41 I'm glad I'm only paying for true power! SWMBO is relamping the house with lamps such as this one.

ARM

Unloaded transformers between the source (substation) and the load(residence/commercial) will also cause lagging loads.

The Europeans are right now mandating that every device that plugs into a wall be PF corrected. This of course adds to the cost of the device and shifts the cost to the consumer. Thus the power generation transmission people don't have to worry about it.

Here in the US. It's the responsibility of the transmission/generation people to take care of it (using capacitor banks). It might change depending on how much more capacity we need. And it's a more efficient fix in the short term in that we don't need to build more generation. We simply maximize the generation we have now.

LT