Why isn't Reactive Power defined as Q = S - P ?

[lychette]

To correct for the power factor of your load you need to know its reactance and then you tune it out as near as you can, using another reactance network. When would you calculate 'Peak Power'? True, the Peak Voltage could be very relevant - but that actually corresponds to the maximum Energy (E = CV²/2) and not to the rate of energy supplied (Power). Sizing your power cables will affect the Resistance and not the Reactance and I've already been into that.
I realize that these terms are all down to usage and it is not likely that they will change. It doesn't make them correct though. "Make it Work' type Engineering is full of slightly dodgy terms and statements - from Water Analogies where they don't apply, to the expression "dB Volts". The field is littered with pitfalls because people got taught these things and pass them on. But Engineers do 'make things work' very successfully and we mostly stuff get away with approximate thinking and that magic stuff called 'experience'.

mmmm...can you give some examples of water analogies that are useful...I use them all the time...How can we stop people who use "dB Volts" from using and communicating in this way.

 

[vintageplayer]

If there were no resistance in the supply chain, your "reactive power" would be irrelevant because there would be no supply losses.

Not everything is about calculating supply losses. The reactive power Q is related to the rate of change of energy stored in your capacitors (1/2⋅CV^2) or inductors (1/2⋅LI^2). One utility of Q is that it allows you to calculate the maximum energy stored in your reactive elements during the cycle. In some contexts (e.g. reactive power control) it is also convenient to talk in terms of reactive power for controlling a voltage level.

I realize that these terms are all down to usage and it is not likely that they will change. It doesn't make them correct though. "Make it Work' type Engineering is full of slightly dodgy terms and statements

There is nothing incorrect about a reactive power. It is the maximum rate of change of energy across a capacitor or inductor. It is a valid measure of power not a "dodgy approximate".

 

[sophiecentaur]

mmmm...can you give some examples of water analogies that are useful...I use them all the time...How can we stop people who use "dB Volts" from using and communicating in this way.

Your analogies may well be valid. The problem is that 'other people' tend to be too literal in their interpretation and their memory of what you may have told them. My experience is that, here on PF (even) there are statements which are intended to be helpful, made by people who do not cross their fingers or give caveats when 'explaining' things using such analogies. The poor, unsuspecting recipient grabs it with both hands and takes the statement and treats it as gospel and passes it on - and so it goes on and on.
As for the dreaded 'dB Volts', it's probably a lost cause but people actually believe that a transformer is an amplifier, because of it.

 

[sophiecentaur]

Not everything is about calculating supply losses.

I agree that component stress is also relevant but normal circuit analysis will tell you the peak volts and current in a reactive load. I do not have experience in Power Engineering but have not come across the specification for a component in terms other than maximum working voltage and 'ripple current' etc.. How does one measure this Reactive Power quantity? If you were to do the sums and arrive at a value for the Reactive Power, I wonder how it could be applied to a circuit with more than one reactive element and aportion the component stress, without going back to square one and doing a formal analysis of the circuit.
If reactive power is a well defined quantity and has been explained in a reference then perhaps you would give a link and I can then read someone (authoritative) else's take on it. I am just quoting stuff that you can find in any circuit analysis reference that you may have handy.

 

[Wee-Lamm]

Treating scalars as if they are vectors really can't ever make sense.

Agreed. :-)

 

[Wee-Lamm]

Not everything is about calculating supply losses. The reactive power Q is related to the rate of change of energy stored in your capacitors (1/2⋅CV^2) or inductors (1/2⋅LI^2). One utility of Q is that it allows you to calculate the maximum energy stored in your reactive elements during the cycle. In some contexts (e.g. reactive power control) it is also convenient to talk in terms of reactive power for controlling a voltage level.
There is nothing incorrect about a reactive power. It is the maximum rate of change of energy across a capacitor or inductor. It is a valid measure of power not a "dodgy approximate".

Reactive power is a "limit" to the maximum rate of change as a function of design, but it is more akin to a power conditioner in that it extracts and stores power from the circuit during periods of peak supply, then releases the stored energy during periods of lowering supply. Reactive power should not cause any power loss to the measure of average or real power, but it's nature of being out of phase with reference to real power, should reduce the valleys of low power, throughout each cycle.

For reference;
An inductor drops current and passes voltage through, where a capacitor drops voltage and passes current through.

I also feel that there is a difference between a 'functional perspective that considers the math' and a 'math perspective that considers function'. With this in mind, I agree with your comment that it isn't always about measuring supply losses. Supply loss is indeed a nontrivial matter but it should not change the amount of impact a reactive load has on a circuit.

 

[vintageplayer]

I agree that component stress is also relevant but normal circuit analysis will tell you the peak volts and current in a reactive load. I do not have experience in Power Engineering but have not come across the specification for a component in terms other than maximum working voltage and 'ripple current' etc.. How does one measure this Reactive Power quantity? If you were to do the sums and arrive at a value for the Reactive Power, I wonder how it could be applied to a circuit with more than one reactive element and aportion the component stress, without going back to square one and doing a formal analysis of the circuit

Imagine you have an electrical device that is largely inductive that you need to connect to the grid. Let's say the average real power it consumes is 1MW. Guess what is going to happen if the power station only burns 1MW worth of coal? Your device is not going to run because it's not getting enough power supplied to it. The power station still needs to supply extra power to your load for the inductance, even though it gets this energy back a second later. Power stations need to know how much reactive power to supply as well as average real power. Reactive power is much nicer to talk about in this context than the voltage and current waveforms.

If reactive power is a well defined quantity and has been explained in a reference then perhaps you would give a link and I can then read someone (authoritative) else's take on it. I am just quoting stuff that you can find in any circuit analysis reference that you may have handy.

Pretty much any electrical textbook defines reactive power pretty precisely... Q = Im{VI*}

 

[sophiecentaur]

Pretty much any electrical textbook defines reactive power pretty precisely... Q = Im{VI*}

Not having a textbook available, I looked at Wiki and other sources and noticed that they all seem to rely on a diagram that implies the energy is a vector quantity. That is pretty nonensical as the basis for strict treatment of the topic. Reactive Power may be a useful number to use when describing 'good' or 'bad' loads but it seems to be based on a very dodgy first step in its derivation. Reactive components do not dissipate power so they cannot, in themselves, consume any power from the supply.
I found a very revealing paragraph http://www.allaboutcircuits.com/textbook/alternating-current/chpt-11/true-reactive-and-apparent-power/:
"We know that reactive loads such as inductors and capacitors dissipate zero power, yet the fact that they drop voltage and draw current gives the deceptive impression that they actually do dissipate power. This “phantom power” is called reactive power, and it is measured in a unit called Volt-Amps-Reactive (VAR), rather than watts".
He actually comes clean about it. What's being described is not Power - it's all in a name that's been (I would say mis-)applied to make the practicalities a bit more approachable. It's an analogy, in fact.

When you bring the idea of supplying coal to a power station you are begging a lot of questions and assuming that the term is fully justified. If you take a simple voltage source / load model and change the impedance of the load from resistive to partly reactive then the power dissipated by the load will be affected and the power supplied will be affected by the same amount. No Loss would be involved; the power delivered by the Voltage source will change by the same amount as the load power changes. So what's the difference between an ideal voltage source and a power station? An alternator winding has resistance and so will the supply cable. If a load is partly reactive then, in order to get the same Power transferred to the load, it resistance would have to be reduced (or the alternator voltage increased). That would increase the peak current values in the supply, which would increase the dissipation in the Resistive Parts of the supply circuit. The economics of supplying Electricity are crucial and supply circuit resistance is highly relevant. If the peak current increases by 5%, the lost power will be 10% more. That's not a 'Reactive Loss'; it's a Resistive Loss, brought about by the presence of reactive elements.
The increased stress on components, when PF is not unity can also be relevant but the "phantom power" is not relevant - it's the increased Voltage or Current.

 

[vintageplayer]

Not having a textbook available, I looked at Wiki and other sources and noticed that they all seem to rely on a diagram that implies the energy is a vector quantity. That is pretty nonensical as the basis for strict treatment of the topic. Reactive Power may be a useful number to use when describing 'good' or 'bad' loads but it seems to be based on a very dodgy first step in its derivation. Reactive components do not dissipate power so they cannot, in themselves, consume any power from the supply.
I found a very revealing paragraph http://www.allaboutcircuits.com/textbook/alternating-current/chpt-11/true-reactive-and-apparent-power/:
"We know that reactive loads such as inductors and capacitors dissipate zero power, yet the fact that they drop voltage and draw current gives the deceptive impression that they actually do dissipate power. This “phantom power” is called reactive power, and it is measured in a unit called Volt-Amps-Reactive (VAR), rather than watts".
He actually comes clean about it. What's being described is not Power - it's all in a name that's been (I would say mis-)applied to make the practicalities a bit more approachable. It's an analogy, in fact.

Read a reliable source?

Reactive components do not dissipate power so they cannot, in themselves, consume any power from the supply

They draw instantaneous power from the supply given by:

p(t) = v(t).i(t) = Vsinφcos(wt + 90 - φ).Icos(wt - φ) = (VI/2).sinφ.sin(2φ - 2wt)

Reactive power is defined as the maximum of this value Q = (VI/2).sinφ.

If you worked in a power station, your plant would need to be constantly running to supply and absorb this power. This means your generator will be constantly spinning, and producing a maximum of Q Watts. You could perpetually recoup this energy back from the grid (the power you send out is always returned) but you will regardless need to be outputting Watts. This is one example why reactive power is important, because it equates to extra Watts needed to be produced by a power station.

 

[Wee-Lamm]

Imagine you have an electrical device that is largely inductive that you need to connect to the grid. Let's say the average real power it consumes is 1MW. Guess what is going to happen if the power station only burns 1MW worth of coal? Your device is not going to run because it's not getting enough power supplied to it. The power station still needs to supply extra power to your load for the inductance, even though it gets this energy back a second later. Power stations need to know how much reactive power to supply as well as average real power. Reactive power is much nicer to talk about in this context than the voltage and current waveforms.

Reactive power is not part of the supply equation, but is a part of the load equation. More specifically, reactive power is a design characteristic of the endpoint device and not specifically provided by the supply. Supply is typically provided in a predetermined format, that is the voltage and current have maximum and minimum quantity thresholds, which are a nature of the design characteristics of the supply medium.

It is the device which must conform to the format of the power it can receive from the supply, then it may/will manipulate that energy to a format that suit it's own internal needs. In relating this to your scenario of a 1MW device receiving a 1MW supply, we must first acknowledge that the supply rates do fluctuate within predetermined tolerance levels. Voltage and current from the supply can fluctuate by even 10% +/- of the average rate and not have a tremendous impact on most devices, as it is expected that over time there will emerge an average supply to which the device should have been designed.

Devices that require more consistent supply characteristics, such as non-fluctuating voltage and current, can implement Reactive Power Networks which present themselves as the Power Supply 'interface' to the device. Simply, it makes more sense to have the unique endpoint device conform to the average supply presented, than to expect the supply to conform to many millions of possible unique endpoint requirements.

Edit: To adjust quote tag so my comment did not appear as part of the quote.

 

[sophiecentaur]

Read a reliable source?

Did you read the whole of that link I referenced?

They draw instantaneous power from the supply given by:

p(t) = v(t).i(t) = Vsinφcos(wt + 90 - φ).Icos(wt - φ) = (VI/2).sinφ.sin(2φ - 2wt)

And that expression is positive for half a cycle and negative for the other half - integrating to zero over a cycle. So it doesn't contribute to wasting energy. That is what I cannot get my head round.

I must try to find some source that doesn't just jump in, feet first with the 'power triangle'. There must be a lead in that justifies it better than just making assertions. Perhaps I shall have to reach for my pencil and paper.

 

[Wee-Lamm]

Not having a textbook available, I looked at Wiki and other sources and noticed that they all seem to rely on a diagram that implies the energy is a vector quantity. That is pretty nonensical as the basis for strict treatment of the topic.

Let's define a vector as "a quantity having direction as well as magnitude, especially as determining the position of one point in space relative to another. I think then; that Reactive power, requires us to view them as a vector quantity in order to understand how and where, or rather when, they fit within the device; with regard to it's power needs.

Capacitance and Induction are ways of reshaping the waveform presented at the supply, in ways that cannot be accomplished with purely resistive loads, for a simple example, converting a square wave into a sine wave. The nature of Reactive power implies that that portion of the power is somewhat time portable in that we can manipulate it to occur in a time reference from the one they were extracted from. This in turn requires us to know which point along the wave we are measuring as it relates to the mean power, what direction it is moving, and how it's qualities change in reference to it's other qualities as well as those of the original wave.

 

[vintageplayer]

And that expression is positive for half a cycle and negative for the other half - integrating to zero over a cycle. So it doesn't contribute to wasting energy. That is what I cannot get my head round.

Over the cycle it doesn't waste energy. The power station (or voltage source) always gets back what it puts out. The problem is that with a power station, this still amounts to the generator running even if you are capable of recouping all the energy back. This wears your plant and costs money because you have to pay staff. You also have maximum outputs on your generators so the more reactive power you put out, the less real power you can put out (and real power is what the power stations get paid for).

 

[jim hardy]

Reactive power is defined as the maximum of this value Q = (VI/2).sinφ.

(VrmsIrms)sinθ is way more familiar to me. I do concede that's same as (VpeakIpeak/2)sinθ

If you worked in a power station, your plant would need to be constantly running to supply and absorb this power.

true enough,, if the generator is not turning it makes no volts.

This means your generator will be constantly spinning, and producing a maximum of Q Watts.

We in the plant would say Q Volt-Amps

You could perpetually recoup this energy back from the grid (the power you send out is always returned) but you will regardless need to be outputting Watts.

We DO recoup the energy with every sub-revolution of the shaft
and we don't call them Watts because over any time interval perceptible to a human they average zero

This is one example why reactive power is important, because it equates to extra Watts needed to be produced by a power station.

That is a very strange dialect to impose on the jargon for a discussion with old power plant guys. Plants deal in watt-hours not watt-milliseconds or watt-degrees. Watts are Watts and Volt-Amps Reactive are Vars. Energy is Watt-Hours or Megawatt-Hours.Even though Q includes some Joules that do shuttle back and forth between generator and loads on a subcycle basis,
we consider reactive current to be Wattless since it won't turn the disc on a watt-hour meter.

If you want to be understood in practical circles , use terms in the way to which practical people are accustomed.

Trivia question
does reactive current cause torque pulsations in a 3 phase machine?
If not, then the prime move is oblivious to them...

 

[jim hardy]

You also have maximum outputs on your generators so the more reactive power you put out, the less real power you can put out (

Only when you run up against the generator capability curve. Stator amps or field amps...
http://www.eecs.ucf.edu/~tomwu/course/eel4205/notes/17%20Capability%20Curve.pdf

 

[Wee-Lamm]

Did you read the whole of that link I referenced?

And that expression is positive for half a cycle and negative for the other half - integrating to zero over a cycle. So it doesn't contribute to wasting energy. That is what I cannot get my head round.

I must try to find some source that doesn't just jump in, feet first with the 'power triangle'. There must be a lead in that justifies it better than just making assertions. Perhaps I shall have to reach for my pencil and paper.

Here's one perspective.
http://www.equitech.com/articles/bpng.html

 

[Wee-Lamm]

Over the cycle it doesn't waste energy. The power station (or voltage source) always gets back what it puts out. The problem is that with a power station, this still amounts to the generator running even if you are capable of recouping all the energy back. This wears your plant and costs money because you have to pay staff. You also have maximum outputs on your generators so the more reactive power you put out, the less real power you can put out (and real power is what the power stations get paid for).

The power station would not be putting out nor recouping "Reactive" power. Reactive power may indeed be present within the transmission system and also within the various components in the system, but it is only reactive within the discrete environment it exists in. Once it has passed through each discrete system and is presented at an interface, the system at the other side of the interface does not distinguish between active and reactive, it simply sees a supply.

 

[vintageplayer]

Even though Q includes some Joules that do shuttle back and forth between generator and loads on a subcycle basis,
we consider reactive current to be Wattless since it won't turn the disc on a watt-hour meter.
If you want to be understood in practical circles , use terms in the way to which practical people are accustomed.

Agree. Just emphasising that reactive power is measurable in Watts, and has real practical effects on the output of a generator (i.e. it serves some useful purpose). The more reactive power you supply, the less real power you can supply (as per the capability curve you attached). It also costs a power plant real resources, and therefore dollars, to supply reactive power. These are some of the practical reasons why you would want to know / calculate reactive power.

 

[Wee-Lamm]

Agree. Just emphasising that reactive power is measurable in Watts, and has real practical effects on the output of a generator (i.e. it serves some useful purpose). The more reactive power you supply, the less real power you can supply (as per the capability curve you attached). It also costs a power plant real resources, and therefore dollars, to supply reactive power. These are some of the practical reasons why you would want to know / calculate reactive power.

OP, post a link to validate that "Reactive power" is a measurable quantity at the supply-> interface.

I will also say that it is not measurable in Watts, which is a measure of Volt Amps.
Reactive power must be separated from apparent power, before it can be measured. VAR delineates that part that exists because of the Reactive components, it is still measured in Volt Amps but further sub-classed to differentiate it as a component of total VA.

When we measure VAR as it is segregated from apparent power, we can determine better what we expect "average power" to be, then after it has been reintegrated we should notice that we can obtain more frequent measures that are equal to "average power", which can be a benefit and especially in circuits that require wave rectification or to remove pulse and flutter, for example. These quantities may 'seem' more trivial in Hydro related concerns, but can certainly become important in Audiovisual and Data manipulation environments.

This, would seem to disagree with your notion above.
http://www.engineeringtoolbox.com/kva-reactive-d_886.html

Edit: to correct spelling.

 

[jim hardy]

Just emphasising that reactive power is measurable in Watts,

I will also say that it is not measurable in Watts, which is a measure of Volt Amps.

If in a meeting of utility folks you described reactive power as WATTS instead of VARS , eyes would roll. Thereafter you'd be ignored or asked to keep quiet.

VARS can be measured with a wattmeter by shifting phase of current 90 degrees wrt voltage. One might hand it phase A-C volts and phase B current.

 

[jim hardy]

This, would seem to disagree with your notion above.
http://www.engineeringtoolbox.com/kva-reactive-d_886.html

how straightforward and practical !

 

[sophiecentaur]

It has taken 50+ posts for me to get any proper message about Reactive Power. I now realize that it's basically about how to charge the customers a realistic price for the Energy they are using. It's a sort of weighted sum of Energy and Generating Equipment Costs. Reactive Power gives a general idea about the various stresses that poor PF subjects the supply equipment to. I find it interesting that such a simple formula gives such an aparently useful parameter for Engineers to work with.
But most of what you can read about it seems to ignore the real context and reason for the quantity being used. They tell you the formula and give you some diagrams but not the whys and wherefores. Thank you Wee-Lamm for injecting a different slant on things into the thread. (And the rest of you too!)

 

[Wee-Lamm]

It has taken 50+ posts for me to get any proper message about Reactive Power. I now realize that it's basically about how to charge the customers a realistic price for the Energy they are using. It's a sort of weighted sum of Energy and Generating Equipment Costs. Reactive Power gives a general idea about the various stresses that poor PF subjects the supply equipment to. I find it interesting that such a simple formula gives such an aparently useful parameter for Engineers to work with.
But most of what you can read about it seems to ignore the real context and reason for the quantity being used. They tell you the formula and give you some diagrams but not the whys and wherefores. Thank you Wee-Lamm for injecting a different slant on things into the thread. (And the rest of you too!)

Thank you too.

I think though, that you still see Reactive Power as something that is provided by the power supply. Reactive power is not something that can pass from one system to another and still be a measurable quantity. We may tend to imagine that hydro is sent directly from the coal plant to our fluorescent light bulb, but it is not that direct. To simply it immensely for this discussion, we must realize that there are several interfaces between the coal plant and our light bulb.

The power coming down the hydro lines may consist of a supply of 1,000 A at 700Kv. Our light bulb requires less than 1 amp at 120v. At the pole there is a step-down transform, which is a reactive device in reference to the supply lines within the delivery system domain, that filters the power into a format that my 200A-240V meter interface can cope with. My meter cannot tell if it the power it receives is reactive or not, and it really doesn't care.

The Ballast for my fluorescent lamp is also a reactive device, but the light-bulb itself cannot discern whether it has been supplied with active or reactive power, and it really doesn't care. The measure of Reactive power is only relevant within the unique system domain which caused it to be phase shifted or otherwise colored for the needs of that specific domain.

 

[sophiecentaur]

I think though, that you still see Reactive Power as something that is provided by the power supply.

No. Not me. Reactive power does represent a 'demand' on the system - otherwise no one would be bothered by it.

but the light-bulb itself cannot discern whether it has been supplied with active or reactive

Now that's a strange comment. You yourself have made the point that Reactive Power doesn't come from the supply. The light bulb, itself is merely supplied with a Voltage and it 'chooses' how much current to take. If the tubne itself is purely resistive then the power that it consumes is V.I and that's what the power station needs to provide it with. The 'demand' that results from reactive elements in the load is not an Energy Demand; it just represents an overhead that's involved in extra current or volts associated with the generator (+ all the rest of the supplystuff).
I still must say, it strikes me as really strange that a diagram is drawn which involves a Mean Power and a Maximum instantaneous VI value. The coal that's shovelled into the boiler is somehow treated as the same as the extra spec needed for the components. Little wonder that 'they' tell you just to use the formula and 'get over it', without encouraging too much throught about what it all actually represents.

My meter cannot tell if it the power it receives is reactive or not, and it really doesn't care.

If it is a Power Meter then it will only measure the in phase components of V and I. If your appliances happen to be very reactive, it will not be aware but still tell you the Energy dissipated per second.

 

[jim hardy]

If your appliances happen to be very reactive, it will not be aware but still tell you the Energy dissipated per second.

The light bulb, itself is merely supplied with a Voltage and it 'chooses' how much current to take. If the tubne itself is purely resistive then the power that it consumes is V.I and that's what the power station needs to provide it with. The 'demand' that results from reactive elements in the load is not an Energy Demand; it just represents an overhead that's involved in extra current or volts associated with the generator (+ all the rest of the supplystuff).

Draw the triangle, accept that as the old-timers deduced, reactive "power" isn't power at all it's just Volt-Amps-Reactive and is wattless.
That fits nicely with our use of RMS measurements for AC power which include averaging over some time duration of at least one cycle.

If somebody wants to study instantaneous volts, amps and power they should have at it because that's useful to understand why Vars are Wattless. That's about as far as it needs to go IMHO.
...
... But, Melville did give credit to his "sub-sub librarian" .

 

[sophiecentaur]

reactive "power" isn't power at all

There we have it: from the horse's mouth.
This has been a great, if rather inefficient way for me to get into this stuff.

 

[vintageplayer]

reactive "power" isn't power at all it's just Volt-Amps-Reactive and is wattless.

What SI fundamental units does reactive power have? What SI fundamental units does average power have?

 

[vintageplayer]

I still must say, it strikes me as really strange that a diagram is drawn which involves a Mean Power and a Maximum instantaneous VI value

Reactive Power gives a general idea about the various stresses that poor PF subjects the supply equipment to. I find it interesting that such a simple formula gives such an aparently useful parameter for Engineers to work with.

Reactive power is defined as (VI/2).sinφ. This is the amplitude of the second component in the equation:
p(t) = v(t).i(t) = (VI/2)cosφ[1 + cos(2wt)] + (VI/2).sinφ.sin(2wt)

But the above equation can also be written as:
(VI/2)cosφ + (VI/2).cos(2ωt -φ)

Why not define reactive power as the amplitude of the second component in the above equation instead? Q = (VI/2). Wouldn't this make more sense because it would tell you how much the power is fluctuating around the actual supplied real power?

 

[jim hardy]

Power is a flow of energy from a source to a load.
Power is measured by work over time.

The standard unit for power is the watt (abbreviated W) which is a joule per second.

"Reactive power" does no work because it only shuttles energy back and forth between load and source.
So the quotient work/time for reactive power averages to zero
The work done by "Reactive power" is zero. So it's not power at all. It's wattless. It requires no torque from a prime mover.

This calculation is only for cases where the force is in the direction of the velocity, and there are many cases where that is not so. Then for instantaneous power, you just multiply the product of force and velocity by the cosine of the angle between them to get the power. In the more general cases where everything varies, one often calculates the work first and then divides by the time to get the average power.

ibid

Why not define reactive power as the amplitude of the second component in the above equation instead? Q = (VI/2). Wouldn't this make more sense because it would tell you how much the power is fluctuating around the actual supplied real power?

Because VIsinθ works just fine
and we use RMS not peak or instantaneous values because that's what our meters indicate
and VARS do tell you exactly how much reactive power is many reactive volt-amps are circulating,
Our VAR meter is prominently located on the panel near the voltage regulator knobs that control VARS.

Why not define reactive power as the amplitude of the second component in the above equation instead? Q = (VI/2).

"Q = (VI/2)" ?
That's "S" per your first post. We'd use RMS and call it KVA, of course multiplied by √3 for three phase.

 

[Wee-Lamm]

No. Not me. Reactive power does represent a 'demand' on the system - otherwise no one would be bothered by it.

Now that's a strange comment. You yourself have made the point that Reactive Power doesn't come from the supply. The light bulb, itself is merely supplied with a Voltage and it 'chooses' how much current to take. If the tubne itself is purely resistive then the power that it consumes is V.I and that's what the power station needs to provide it with. The 'demand' that results from reactive elements in the load is not an Energy Demand; it just represents an overhead that's involved in extra current or volts associated with the generator (+ all the rest of the supplystuff).
I still must say, it strikes me as really strange that a diagram is drawn which involves a Mean Power and a Maximum instantaneous VI value. The coal that's shovelled into the boiler is somehow treated as the same as the extra spec needed for the components. Little wonder that 'they' tell you just to use the formula and 'get over it', without encouraging too much throught about what it all actually represents.

If it is a Power Meter then it will only measure the in phase components of V and I. If your appliances happen to be very reactive, it will not be aware but still tell you the Energy dissipated per second.

It sounds like we are 'almost' saying the same thing much of the time but, I'll pause briefly to be sure I understand if and how PF affects my position.

I think though that we are not both thinking from the same reference point where my reference is inside my device whereas you are speaking from within the distribution network. If I am right on that point, then everything within the distribution/transmission realm would be part of that device.

 

[vintageplayer]

Power is a flow of energy from a source to a load.
Power is measured by work over time.

"Reactive power" does no work because it only shuttles energy back and forth between load and source.
So the quotient work/time for reactive power averages to zero
The work done by "Reactive power" is zero. So it's not power at all. It's wattless. It requires no torque from a prime mover.
ibid

OK I understand what you're saying here. I wasn't aware that the term "Watts" was reserved only for time rate change of work and not energy.

and VARS do tell you exactly how much reactive power is many reactive volt-amps are circulating,

Isn't this reasoning circular? Reactive volt-amps are by definition VARS?

"Q = (VI/2)" ?
That's "S" per your first post.

I know it's S. If S is capable of quantifying the circulating volt-amps on top of your average power, what is the utility of defining an additional quantity, Q and calling it the reactive power?

Because VIsinθ works just fine

Sure it works fine, but couldn't P and S do everything we need? What is the real reason for working with Q?

 

[sophiecentaur]

What SI fundamental units does reactive power have? What SI fundamental units does average power have?

I'm afraid that's not a valid argument. I could ask you what are the units for Work and Torque and a dimensional argument would tell us they are the same. They are not - and the difference is very much along the lines of the difference between W and 'Reactive Power'.

 

[sophiecentaur]

my reference is inside my device

If "inside the device" was all that counted, the PF of any appliance would have no effect on the system. But the effect of PF of multiple appliances on the system is additive.
You seem to be rather preoccupied with finding a good model for 'what's really going on. I am being more pragmatic and looking at the overall effect (cost) of PF. It seems to me that PF is only of concern to the supplier, as long as out Energy Meters just measure Energy and charge us for Watts only. The actual effect of a particular PF and a particular Load will vary from place to place because the supply equipment is very much a part of the (£$) equation. If the load demand on an isolated power station were to be reduced (say all the factories in a town closed down) then the PF of the remaining houses and equipment would be pretty well irellevant.

 

[sophiecentaur]

and we use RMS not peak or instantaneous values because that's what our meters indicate

That could be real reason.

 

[jim hardy]

I know it's S. If S is capable of quantifying the circulating volt-amps on top of your average power, what is the utility of defining an additional quantity, Q and calling it the reactive power?

No, S is NOT capable of that
and maybe there's the miscommunication
what do you mean by "circulating" ? To us power guys "circulating amps" are reactive because they circulate between machines.
If you've never had control of two machines in parallel you cannot imagine the sense of wonder when tweaking one voltage regulator makes amps go up or down on both machines. The load doesn't need to accept those amps, they just circulate between the generators.S doesn't separate out of phase volt-amps from in phase volt amps
S is the hypotenuse as pointed out by Hesch in post #2

in my power plant,
P i control by how much steam i admit to Mr Turbine and my wattmeter shows me P
Q i control by how much field current i apply to Mr Generator and my varmeter shows me Q
I don't have a S meter but i do have ammeters in series with the armature windings, which, at constant voltage are an indication of S ,
and i have voltmeters between phases ,
and for a sanity check on my instruments
at constant load (constant megawatts)
...i can adjust excitation to give minimum armature amps which means there's no reactive current
... then i verify varmeter reports zero and that's my zero check on Q meter (varmeter)
...and i verify my wattmeter reports VI√3, that's my check on S .
but of course i'd have to co-ordinate that testing with system dispatcher and nearby power plants so as to not upset the grid.

That's my utility guy perspective. The lingo is well established and is the dialect that's understood in my circles.
Reactive demand on the system is set by what is connected to the system
generators supplying that reactive demand share the reactive current amongst themselves , each supplying its share,
by means of adjusting their field currents individually
raising one machines' field current will affect armature amps on all nearby machines

Changing the language will not be well received. Utilities are just that way.