Why are we charged for watts?

[Jraef]

And this is kind of what I am not liking.

People that have good power factors are paying for those that don't. Thus, there is only a motivation to reduce watts, even at the expense of power factor.

Not necessarily. There are different forms of utility tariffs depending on what type of customer / consumer you are. If you are a residential consumer, they don't bother with separate PF charges because a) it only counts in the aggregate effects, so the cost of metering all the residential users would not be worth the effort, and b) as Zog says, they are burying the cost in the bill in other ways. If you are an industrial or large commercial user however (usually defined as a 3 phase service) then you do most likely get a kVAR meter and thus a separate charge for having poor power factor.

If we were charged for VA's, we would be motivated to reduce those, which would be a much more fair system, and more effective at attaining the goal of total energy efficiency.

But....that may be bad for CFL sales....

Hmmm... If you are motivated to reduce VA, you are motivated to reduce Watts, it's virtually impossible to increase only your reactive power consumption, i.e. "wasting power factor".

D'oh, serves me right for going to lunch and posting when I got back, not noticing the new responses. Never mind, everything I just said here has been covered.

 

[charlie b]

Another thing: Unloaded motors . . . aren't actually doing any work until they are moving pistons.

I will take issue with this claim. We are confusing our languages here. Even if we don't recognize the fact, we all speak at least two different languages. One is what I like to call "conversational English." The other is the language of our profession.

An unloaded motor is, in fact, doing some small amount of "useful work," using that phrase in the language of our profession. It takes some amount of "useful work (i.e., watts)" to get the motor spinning, and some amount of "useful work" to keep it spinning. Now switching to "conversational English," an unloaded motor will not move any pistons, or cut any wood, or push any water, or roll any conveyor, or lift any weights, or do anything that the owner of the factory might call "useful." It won't generate any revenue, or ehlp get any product out the door.

 

[mivey]

I will take issue with this claim. We are confusing our languages here.

Agreed. We should be saying lightly-loaded motors.

 

[zog]

If there is any resistance, there will be losses due to heating, and the generator will have to continue to supply energy to make up for those losses. But if the load is purely capacitive or purely inductive, then the generator will have to provide an initial amount of energy, to start the "energy exchange" (that I mention earlier) going. Thereafter, ignoring friction, as you suggest, the generator and the (purely capacitive or purely inductive) load will exchange energy with no additional push needed from the generator's prime mover. The weakness in this statement, of course, is that the wires that connect the generator to the load will have some resistance. That brings us back to my first sentence in this post.

I used to use a superball in class to demonstrate this, takes some energy for me to lift the superball to a height but after I release the ball it will bounce to nearly the same height, of course it won't bounce forever due to "losses". But in a perfect world, the superball would bounce to the same height forever, just transfering energy.

This simple demonstration made a lot of poeple understand reactive power for the first time.

 

[mivey]

I used to use a superball in class to demonstrate this, takes some energy for me to lift the superball to a height but after I release the ball it will bounce to nearly the same height, of course it won't bounce forever due to "losses". But in a perfect world, the superball would bounce to the same height forever, just transfering energy.

This simple demonstration made a lot of poeple understand reactive power for the first time.

Sweet! I'm going to plagiarize this for my next class. Thanks.

 

[ControlFreak]

I will take issue with this claim. We are confusing our languages here. Even if we don't recognize the fact, we all speak at least two different languages. One is what I like to call "conversational English." The other is the language of our profession.

An unloaded motor is, in fact, doing some small amount of "useful work," using that phrase in the language of our profession. It takes some amount of "useful work (i.e., watts)" to get the motor spinning, and some amount of "useful work" to keep it spinning. Now switching to "conversational English," an unloaded motor will not move any pistons, or cut any wood, or push any water, or roll any conveyor, or lift any weights, or do anything that the owner of the factory might call "useful." It won't generate any revenue, or ehlp get any product out the door.

Well, I agree with you. I could just as well say "lightly loaded motors." I know they aren't running with zero watts, but it's certainly very low, while the KVARs remain fairly consistent.

 

[gar]

101129-1758 EST

K8MHZ:

To say that a purely capacitive load is the same as no load at all makes no sense to me.

It provides a reactive load that requires current flow in the source system, but provides virtual no dissipative load, and none for an ideal capacitor. This example was to try to force you to see the reactive losses in the system separate form those resulting from any resistive loads.

Now, if you threw in an inductor and made a tank circuit, I would concede that this would be very close to an open circuit as far as how much energy is needed from the generator.

I do not believe I implied that a capacitor was an open circuit. I was assuming an ideal capacitor and that would dissipate no power, and I wanted current flow. By dissipate I am saying the the electrical energy is transformed into some form of energy that is not electrical. Mostly this would be heat or mechanical work. Also could be radiated electromagnetic waves.

In my feeble mind, a purely resistive, a purely capacitive and a purely inductive load all will require some torque on the shaft of the generator to supply.

Once reactive energy is introduced (charge the system), and for ideal capacitors and inductors in the system there is no loss, and therefore no new energy needs to be added, and no added torque to the ideal alternator. Only when resistance (loss) is added is torque required.

An ideal parallel resonant circuit at resonance looks like an open circuit. An ideal series resonant circuit at resonance looks like a short circuit.

.

 

[BJ Conner]

Here's a good PP presentation from SquareD. IT the beer/foam/mug analogy.
KVA doesn't represent power just capacity. If you keep in mind that Mugs (required KVA of the system) are very expensive then the need to keep power factor down makes more sense.
If your buying beer thats half foam you have to buy twice as many to get the same beer.


http://www.squared.com/us/applications/industrial.nsf/LookupFiles/PowerFactorCorrectioninTodaysElectricalNetworks.pdf/$file/PowerFactorCorrectioninTodaysElectricalNetworks.pdf

 

[zog]

Oh no, not the beer and foam theory.

 

[SAC]

Here is an example showing how VA billing might work...

For example, consider the theoretical case given by gar of 0 PF (purely reactive load). Lets say the load is 240VA (1 amp at 240v). Let's say the total equivalent impedance of the POCO infrastructure is 10 ohm at 240v. That means the generation required to deliver this power is 1A * 10 Ohm = 10w. So we are charging the customer based on 240VA, but for 10w of generation.

Now consider a purely resistive load, PF = 1, with the same 240VA. That is drawing the same 1 amp, with the same 10w dissipated in the POCO infrastructure. But now the load is actually using 240w. So the generation capacity to supply it is 250w. Using VA, they would be charging this customer based on the same 240VA, but they are actually using 250w of generation capacity.

So the 0 PF customer is being charged 24 times what the actual generation required is (240VA vs 10w), and the 1 PF customer is being charged .96 times the what the actual generation is (240VA vs 250w). The 0 PF customer is being charged about 25x what the 1 PF customer is being charged, based on the actual generation capacity that is required!

On the other hand, with billing based on watts, the 1 PF customer is charged infinite times more than the 0 PF customer! (0 PF customer pays nothing)

Of course it isn't quite this simple (for example, this example ignores the infrastructure cost of providing the capacity required in either case) - but it illustrates some of the principals that are at work.

 

[mivey]

Here is an example showing how VA billing might work....

For the demand portion of the bill, VA billing would make sense for some of the cost components. The pf penalty is one means to recover the difference in a VA demand charge and a watt demand charge for some of the cost components.

But the OP was talking about energy and it would not make sense for the net energy portion of the costs:

...
With the proliferation of CFLs and all the discussion about the poor power factor it seems to me that the POCO is not getting paid for all of it's energy
....
Maybe I am missing out on something, but if I was the POCO I would charge for Volt Amp Hours, not Watt hours.

 

[zog]

Sweet! I'm going to plagiarize this for my next class. Thanks.

I have a slide I can send you too if you like. Won't let me attach it here.

 

[kwired]

We posted at the same time.

OK, here is how I see it. Let's say you have a load with a poor power factor. The watt meter will only record the watts you are using. The poor power factor presents impedance along with additional line losses which, in turn, requires more energy at the generation point than if the power factor was good.

I think that charging for VA's and recording watts would be a better way to monitor and conserve energy use.

Plus it would likely cut down on a bunch of paperwork.

Methinks the system we use today was created before it became the norm for people to monitor energy use with the intent of cutting it down. But that still doesn't explain to me why metering is done in watts, not VA. Maybe it's a public service regulation issue.

Not true, I am afraid. Mivey said it one way, at the bottom of post 11. I'll put it another way, using several examples. Reactive power is an exchange of energy between (1) The magnetic field of a utility generator, and (2) The magnetic field of a motor load. It can also be an exchange of energy between (1) The magnetic field of the secondary windings of a transformer, and (2) The magnetic field of a motor load. It can also be an exchange of energy between (1) The magnetic field of a utility generator, and (2) The electric field of a capacitor bank.

In all cases, it is an exchange of energy. Energy flows in, and then energy flows out. In the interim, it does not do any "useful work." But if it were not present, the motor would not have a magnetic field to drive its rotor in a circular motion. The fact that energy moves in and out causes the total current in the transmission line to be higher, and therefore there are additional loses. That is something the utility would prefer to minimize, which is why they sometimes give credits or penalties to large customers, based on the power factor of the facility.

Power factor costs the POCO in distribution equipment more so than in energy costs. Larger conductors, transformers, etc. or corrective equipment.

 

[mivey]

I have a slide I can send you too if you like. Won't let me attach it here.

Thanks. I already made one for some upcoming classes.

 

[mivey]

Power factor costs the POCO in distribution equipment more so than in energy costs. Larger conductors, transformers, etc. or corrective equipment.

Correct. When all is said and done, there are three cost components in the electric rate:
#1) Base costs. These are costs incurred whether or not anything is ever used. Examples would be a meter, the cost to render a bill, etc.

#2) Demand costs. These are costs that are determined by how fast energy is pulled off the system. They determine things like source & equipment size, etc.

#3) Energy costs. These are costs that are based on how much energy is consumed. Fuel is a good example.

Unfortunately, we do not always have the exact data or the cost efficient means to bill these cost components exactly as they are incurred by each customer. Plus, there are savings through economies of scale that we must also pass along to the end user: one big happy family so to speak.

We usually try to group customers with similar usage into the same rate group so we can do a fair job of modeling the group as a whole. If enough customers (or a smaller group with a significant amount of usage) can show they can modify their load to make them different enough from the normal rate group, and reduce costs, we will design a rate to compensate them for their efforts.