THE PHYSICS OF... POWER

[FionaZuppa]

Anyone brave enough to summarize the content of this thread???????:angel:

anyone but me would be good.

in summary, post #1 describes a classical impedance mismatch problem. mismatched impedance between a src and load may result in wasted power. all energy is conserved. no free ride for amps anywhere.

is that a good summary?

 

[GoldDigger]

anyone but me would be good.

in summary, post #1 describes a classical impedance mismatch problem. mismatched impedance between a src and load results in wasted power. all energy is conserved. no free ride for amps anywhere.

is that a good summary?

Nope. Since we are not dealing with transmission lines the issue of impedance matching is a colossal red herring.

In the early days of electricity "scientists" told Edison that the generator impedance had to match the load impedance for efficient power transfer. Which would have resulted in a lot of melted generators. Fortunately Edison was not convinced. :angel:

The scientists were used to trying to get the absolute maximum power (instantaneous) out of sets of chemical batteries with no regard for overall energy efficiency. A bad model to use for commercial power transmission.

 

[wwhitney]

Anyone brave enough to summarize the content of this thread?

2 / pi != 1

Cheers, Wayne

 

[Carultch]

2 / pi != 1

Cheers, Wayne

Since when does 2 divided by pi factorial equal 1?

 

[FionaZuppa]

Nope. Since we are not dealing with transmission lines the issue of impedance matching is a colossal red herring.

In the early days of electricity "scientists" told Edison that the generator impedance had to match the load impedance for efficient power transfer. Which would have resulted in a lot of melted generators. Fortunately Edison was not convinced. :angel:

The scientists were used to trying to get the absolute maximum power (instantaneous) out of sets of chemical batteries with no regard for overall energy efficiency. A bad model to use for commercial power transmission.

no, it is. if it wasnt then the correction to Z mismatch would not be a cap (X_C) when load had a X_L component. if it wasnt then poco would not be doing cap bank switching at times when feed lines become very inductive due to dynamic customer loads.

 

[wwhitney]

Since when does 2 divided by pi factorial equal 1?

!= means not equal. So 2/pi does not equal 1.

Cheers, Wayne

 

[GoldDigger]

no, it is. if it wasnt then the correction to Z mismatch would not be a cap (X_C) when load had a X_L component. if it wasnt then poco would not be doing cap bank switching at times when feed lines become very inductive due to dynamic customer loads.

The reason utilities use PFC banks and switch them is to minimize I²R losses in their transmission system by reducing the reactive current the network has to carry, not to match source and load impedance.
If the caps were for impedance matching they could just put them all at the generation facility instead of distributing them all over their network, close to the large loads.
A transmission line's characteristic impedance is not a factor in AC distribution where the line lengths are small compared to 1/4 wavelength at 60 Hz.
If the load were right at the generator, with minimal resistive losses in between there would not be as great a demand for PFC. (There is still also a limit to the amount of VAR that a given generating facility can produce, but that is not based on "impedance matching" either.)

You appear to be cheerfully throwing around a buzzword you do not understand.

Correcting line PF to 1 is not generally called impedance matching even though the goal of PF=1 can be reached by using capacitive reactance to balance (match?) inductive reactance.

 

[FionaZuppa]

The reason utilities use PFC banks and switch them is to minimize I²R losses in their transmission system by reducing the reactive current the network has to carry

Correcting line PF to 1 is not generally called impedance matching even though the goal of PF=1 can be reached by using capacitive reactance to balance (match?) inductive reactance.

i think i do agree with you on this

the tuning is a form of "impedance matching", the effects are very similar to mismatch in RF transmission when there is a mismatch in Z (standing wave). power transmission is really no diff except you are attempting to tune out reactance, commonly known as PF correction. the reactance we are discussing is in fact an "inhibitor" to real power delivery, you need to push more amps to get the required real power delivered, etc.

Theory
Impedance is the opposition by a system to the flow of energy from a source. For constant signals, this impedance can also be constant. For varying signals, it usually changes with frequency. The energy involved can be electrical, mechanical, acoustic, magnetic, or thermal. The concept of electrical impedance is perhaps the most commonly known. Electrical impedance, like electrical resistance, is measured in ohms. In general, impedance has a complex value; this means that loads generally have a resistance component (symbol: R) which forms the real part of Z and a reactance component (symbol: X) which forms the imaginary part of Z.

In simple cases (such as low-frequency or direct-current power transmission) the reactance may be negligible or zero; the impedance can be considered a pure resistance, expressed as a real number.



Power factor correction
Power factor correction devices are intended to cancel the reactive and nonlinear characteristics of a load at the end of a power line. This causes the load seen by the power line to be purely resistive. For a given true power required by a load this minimizes the true current supplied through the power lines, and minimizes power wasted in the resistance of those power lines. For example, a maximum power point tracker is used to extract the maximum power from a solar panel and efficiently transfer it to batteries, the power grid or other loads. The maximum power theorem applies to its "upstream" connection to the solar panel, so it emulates a load resistance equal to the solar panel source resistance. However, the maximum power theorem does not apply to its "downstream" connection. That connection is an impedance bridging connection; it emulates a high-voltage, low-resistance source to maximize efficiency.

On the power grid the overall load is usually inductive. Consequently, power factor correction is most commonly achieved with banks of capacitors. It is only necessary for correction to be achieved at one single frequency, the frequency of the supply. Complex networks are only required when a band of frequencies must be matched and this is the reason why simple capacitors are all that is usually required for power factor correction.

 

[GoldDigger]

i think i do agree with you on this

the tuning is a form of "impedance matching", the effects are very similar to mismatch in RF transmission when there is a mismatch in Z (standing wave). power transmission is really no diff except you are attempting to tune out reactance, commonly known as PF correction. the reactance we are discussing is in fact an "inhibitor" to real power delivery, you need to push more amps to get the required real power delivered, etc.

There you go off the deep end again. There are no standing waves on the power distribution network at scales below backbones and interties. And I²R losses are totally unrelated to transmission line impedance of the power grid. When you have standing waves on a transmission line the source may not even have to push extra power because of it, but the voltages on the transmission line can be destructive and a source designed to feed the characteristic impedance of that transmission line (typically very low, like 50-100 ohms) will not be efficient driving a different apparent load.


BTW, FZ. it is normal to give credit to the source of "authoritative" quotes like the ones you are using.
Even Ingineur does that.
Glad to see that you are getting good information about the practical consequences of reactive loads in the power network.

 

[Sahib]

Speaking to Sahib:
The energy coupled between primary and secondary of a transformer is very clearly being moved magnetically.
But if you place a resistive load on the secondary of a transformer, the primary current will be very nearly in phase with the primary voltage, with only a small reactive component for transformer magnetization.

-Jon

Consider this. An electromagnet lifting a load. Prior to commencement of lifting, there is an amount of
reactive and active energy consumed by it. During lifting process, there is a decrease in reactive energy taken by it due to shrinkage of magnetic field between it and load. But the reduced part of reactive energy can not just disappear due to energy conservation law. It should be converted to work of lifting load. This may be practically checked with KW and KVAR meters connected to the electromagnet. What do you think?

 

[GoldDigger]

Consider this. An electromagnet lifting a load. Prior to commencement of lifting, there is an amount of
reactive and active energy consumed by it. During lifting process, there is a decrease in reactive energy taken by it due to shrinkage of magnetic field between it and load. But the reduced part of reactive energy can not just disappear due to energy conservation law. It should be converted to work oflifting load. This may be practically checked with KW and KVAR meters connected to the electromagnet. What do you think?

The "reactive energy" is not something that just sits there. It must be pumped by voltage from the AC supply.
When you shrink the magnetic field the same applied voltage no longer pumps as large a magnetic field but the inductance goes UP because the magnetic circuit is now completed through ferrous metal.
If I kept the magnetic circuit the same but reduced the applied voltage, would you try to say that I have somehow extracted energy from the magnetic field? No I have just reduced the magnitude of the energy I am pumping back and forth.
The "reactive energy" cannot supply any energy to another load nor can it do any work.
When the magnetic field is doing work (attracting the armature) work is being done through the magnetic field all right, but that work is reflected in resistive current through the electromagnet, not a conversion of the existing "magnetic energy".

If you use permanent magnets, work can be done by the magnetic field with no externally applied power because the overall energy contained in the magnetic field is lower after the pieces come together. That is not what happens with electromagnets and "reactive energy."

 

[Sahib]

The "reactive energy" is not something that just sits there. It must be pumped by voltage from the AC supply.
When you shrink the magnetic field the same applied voltage no longer pumps as large a magnetic field but the inductance goes UP because the magnetic circuit is now completed through ferrous metal.
If I kept the magnetic circuit the same but reduced the applied voltage, would you try to say that I have somehow extracted energy from the magnetic field? No I have just reduced the magnitude of the energy I am pumping back and forth.
The "reactive energy" cannot supply any energy to another load nor can it do any work.
When the magnetic field is doing work (attracting the armature) work is being done through the magnetic field all right, but that work is reflected in resistive current through the electromagnet, not a conversion of the existing "magnetic energy".

If you use permanent magnets, work can be done by the magnetic field with no externally applied power because the overall energy contained in the magnetic field is lower after the pieces come together. That is not what happens with electromagnets and "reactive energy."

My last post is actually an invitation for verification of conversion of reactive energy to active energy. Please put up the results of the verification if possible to you.

 

[GoldDigger]

My last post is actually an invitation for verification of conversion of reactive energy to active energy. Please put up the results of the verification if possible to you.

I cannot verify the conversion of reactive energy to active energy because it does not happen, OK?
Nor can I demonstrate with a practical example how it does not happen, since your description is so confused. It is impossible to state clearly what would constitute verification to you.

 

[FionaZuppa]

There you go off the deep end again. There are no standing waves on the power distribution network at scales below backbones and interties. And I²R losses are totally unrelated to transmission line impedance of the power grid. When you have standing waves on a transmission line the source may not even have to push extra power because of it, but the voltages on the transmission line can be destructive and a source designed to feed the characteristic impedance of that transmission line (typically very low, like 50-100 ohms) will not be efficient driving a different apparent load.

Glad to see that you are getting good information about the practical consequences of reactive loads in the power network.

i didnt say there were standing waves, now did i. i said the inhibitor P_Q is like how a standing wave is an inhibitor. the practical power network is Z, some of that is R and some of it is an inhibitor, you call it reactive power. in both cases you want to correct the problem by removing the inhibitor.

 

[GoldDigger]

i didnt say there were standing waves, now did i. i said the inhibitor P_Q is like how a standing wave is an inhibitor. the practical power network is Z, some of that is R and some of it is an inhibitor, you call it reactive power. in both cases you want to correct the problem by removing the inhibitor.

And I stated (with some explanation) that standing waves are not necessarily an inhibitor to power transfer, and that your analogy is therefore flawed. Reactive power is always a source of energy loss, but I would not go so far as to call it an inhibitor of energy transfer. The energy gets transferred just fine, you just lose a little bit more of it than you would with a better PF.

In the case of standing waves and mismatches between transmission line impedance (design impedance) and actual impedance it really makes it harder for the source to feed the designed power output of the source into the line in the first place (unless you redesign the source). That, to me, is inhibition.

 

[Sahib]

I cannot verify the conversion of reactive energy to active energy because it does not happen, OK?
Nor can I demonstrate with a practical example how it does not happen, since your description is so confused. It is impossible to state clearly what would constitute verification to you.

Again, during lifting the power factor continuously rises. There are two possibilities for increase in power factor: reactive power fixed and active power increases or reactive power decreases and active power increases.In present case only the latter possibility, due to shrinkage of magnetic field between electromagnet and the load as the latter moves up. This can be easily verified with meters. What is confusing in this?

 

[GoldDigger]

Again, during lifting the power factor continuously rises. There are two possibilities for increase in power factor: reactive power fixed and active power increases or reactive power decreases and active power increases.In present case only the latter possibility, due to shrinkage of magnetic field between electromagnet and the load as the latter moves up. This can be easily verified with meters. What is confusing in this?

If A decreases and B increases at the same time, that by itself provides no evidence at all for the assertion that A has been transformed into B.

 

[Sahib]

If A decreases and B increases at the same time, that by itself provides no evidence at all for the assertion that A has been transformed into B.

Well, take into account energy conservation law and review your above reasoning please.

 

[winnie]

Consider this. An electromagnet lifting a load. Prior to commencement of lifting, there is an amount of
reactive and active energy consumed by it. During lifting process, there is a decrease in reactive energy taken by it due to shrinkage of magnetic field between it and load. But the reduced part of reactive energy can not just disappear due to energy conservation law. It should be converted to work of lifting load. This may be practically checked with KW and KVAR meters connected to the electromagnet. What do you think?

I replied to this point already.

The energy stored in a magnetic field is real energy which can be used to do work.

An real world practical example of the sort of energy conversion you describe is seen in a switched reluctance motor or switched reluctance generator, where magnetic circuits change reluctance due to motion. Electrical power can be converted to mechanical power via the magnetic intermediary, or the reverse, all by appropriate timing of when energy is dumped into the magnetic field.

This is actual energy in a magnetic field being converted to actual electrical or mechanical energy.

As I have been harping on, 'Reactive power' involves actual energy shutting back and forth with no net energy delivered to the load. This is actual energy which could be converted to real power. But there is not much actual energy here, it is a small amount of actual energy shuttling back and forth.

A 1W load consumes 1 J per second.
A 60Hz 1VA pure reactive load involves something on the order of 0.003J shuttling back and forth.

Over any long time period, the total actual energy shuttling back and forth with reactive power is very small compared to the actual energy delivered by real power to the load.

Over any long time period, the total _real_ energy lost (energy used as current in phase with voltage, thus real power) in the wires used to shuttle that energy back and forth will be much larger than the energy being shuttled.

By conservation laws, the initial 'start-up' of a reactive load involves real power', and similarly any change in the reactive power being consumed by a load will involve real power being absorbed or returned to the system. But this power term will be very small indeed unless we are looking at a system where the magnetic circuit changes at the AC line frequency.

-Jon

 

[Sahib]

I replied to this point already.

The energy stored in a magnetic field is real energy which can be used to do work.

An real world practical example of the sort of energy conversion you describe is seen in a switched reluctance motor or switched reluctance generator, where magnetic circuits change reluctance due to motion. Electrical power can be converted to mechanical power via the magnetic intermediary, or the reverse, all by appropriate timing of when energy is dumped into the magnetic field.

This is actual energy in a magnetic field being converted to actual electrical or mechanical energy.

As I have been harping on, 'Reactive power' involves actual energy shutting back and forth with no net energy delivered to the load. This is actual energy which coupe be converted to real power. But there is not much actual energy here, it is a small amount of actual energy shuttling back and forth.

A 1W load consumes 1 J per second.
A 60Hz 1VA pure reactive load involves something on the order of 0.003J shuttling back and forth.

Over any long time period, the total actual energy shuttling back and forth with reactive power is very small compared to the actual energy delivered by real power to the load.

Over any long time period, the total _real_ energy lost (energy used as current in phase with voltage, thus real power) in the wires used to shuttle that energy back and forth will be much larger than the energy being shuttled.

By conservation laws, the initial 'start-up' of a reactive load involves real power', and similarly any change in the reactive power being consumed by a load will involve real power being absorbed or returned to the system. But this power term will be very small indeed unless we are looking at a system where the magnetic circuit changes at the AC line frequency.

-Jon

It is important to keep distinction between active reactive and apparent power. That is why they have
Watt, VAR and VA units respectively to avoid confusion in dealing with them.