AC Efficiency: Fact or Fiction?

[Guidestone]

I've heard that alternate current is more efficient than direct current. I've heard is easier to transport than dc current. Is that true? Why is that?
Also, why are three phase systems better than monophasic ones?
Thank you very much!

 

[davenn]

I've heard is easier to transport than dc current. Is that true? Why is that?

it's easier to "transport" ( send over a transmission line) because of the ease of use of transformers to step up and step down AC voltages
that can't be done with DC

 

[Guidestone]

it's easier to "transport" ( send over a transmission line) because of the ease of use of transformers to step up and step down AC voltages
that can't be done with DC

Isn't DC capable of generating magnetic flux in the core?

 

[davenn]

a transformer ONLY works with a changing magnetic field
that that ONLY happens with AC

do some googling on transformers, induction and similar things

Dave

 

[Jony130]

Isn't DC capable of generating magnetic flux in the core?

If you connect a the DC voltage across the primary winding the primary current and the flux will ramp-up in constant rate of change. So, the secondary voltage will also be induced. This means that ideal transformer work with DC voltage. But with real world transformers this will not work. Why ?? The answer is the core limitation. Because the DC voltage at primary causes the flux to ramp-up in constant rate, and this rise in flux will quickly saturate the core. And when the core is saturated the coils stop working as a inductor (loses all its inductance), now the coil work as a "primer winding resistor". So we have a short in the circuit. And Bmax for a mains transformer cores are in range of 1T ...1.6T.

 

[Averagesupernova]

This sounds like a homework question.

 

[Jeff Rosenbury]

Three phases are the minimum number of phases for constant power transfer.

In a single phase circuit, the power pulses. This is fine for many applications like small motors, but for large motors the pulsing can lead to oscillation, vibration, and other destructive problems.

 

[meBigGuy]

AC is more efficient than DC for power transmission only because it can be easily transformed to higher voltages. It is actually less efficient for the actual transmission.

Once you have high voltage, DC would be more efficient.
1. No skin effect
2. Constant power with single phase
3. reactive effects

From https://en.wikipedia.org/wiki/High-voltage_direct_current
"Depending on voltage level and construction details, HVDC transmission losses are quoted as about 3.5% per 1,000 km, which are 30 – 40% less than with AC lines, at the same voltage levels.[22] This is because direct current transfers only active power and thus causes lower losses than alternating current, which transfers both active and reactive power."

 

[rootone]

The reason why we need to step up the transmission line voltage to a very high value (over 100kV is typical), is because this means the line can deliver X watts of power, with less current flowing in the cable.
That results in more efficient transmission since less current = less energy wasted in heating up the line.
Clearly though, we can't have 100kV power supplies directly into homes, so the grid voltage is stepped down to a sub-lethal level at a local transformer close to where the power will be consumed.
As has been pointed out, you can't easily do this sort of thing with DC, whereas with AC you can.

 

[Guidestone]

This sounds like a homework question.

No homework Sir. Just killing my ignorance

 

[meBigGuy]

No homework Sir. Just killing my ignorance

Isn't DC capable of generating magnetic flux in the core?

Sometimes your questions seem to be off-the-cuff responses that have not been thought about or researched. It sometimes seems like you are not asking a well thought out technical question. It would be like me answering with "of course it is" instead of a thorough technical answer. You are capable of asking better questions than that.

 

[sophiecentaur]

Clearly though, we can't have 100kV power supplies directly into homes, so the grid voltage is stepped down to a sub-lethal level at a local transformer close to where the power will be consumed.
As has been pointed out, you can't easily do this sort of thing with DC, whereas with AC you can.

However, transformers are (and always will be) very expensive devices and it may not be too long before DC voltage changers ( step - up and down ), working at very high powers will be cheaper than Iron / copper transformers. I reckon that AC power distribution will then die a surprisingly fast death.

 

[jim hardy]

Interesting thought. I always regarded HVDC as Tinkertoy technology, plausible but impractical. But so were the automobile and computer .

It might work if we can interest enough young people in taking up careers maintaining that complex stuff.http://electrical-engineering-portal.com/8-main-disadvantages-of-hvdc-transmission

 

[dlgoff]

Interesting thought. I always regarded HVDC as Tinkertoy technology, plausible but impractical. But so were the automobile and computer .

It might work if we can interest enough young people in taking up careers maintaining that complex stuff.http://electrical-engineering-portal.com/8-main-disadvantages-of-hvdc-transmission

#7; A DC step voltage? Never thought about the possibility of getting your legs blown off.

 

[sophiecentaur]

Interesting thought. I always regarded HVDC as Tinkertoy technology, plausible but impractical.

The link from UK to France has used DC for quite a few decades and that use(d/s?) mercury arcs. Cumbersome and expensive but worth it for spreading the load over the time differences.
That link does quite a job on HVDC but, apart from the RFI comment, it's not based on probable future technology but on what's currently available. Having seen solid state move from mW to kW and having more and more complex circuits (Flawless TV displays, for a start), I have to say it's feasible to think in terms of many MW systems. Also, the network could well involve more, lower power cables.

 

[anorlunda]

owever, transformers are (and always will be) very expensive devices and it may not be too long before DC voltage changers ( step - up and down ), working at very high powers will be cheaper than Iron / copper transformers. I reckon that AC power distribution will then die a surprisingly fast death.

Congratulations Sophie on your fantastically imaginative speculation for the holidays. Even my friends who work in power electronics R&D don't have dreams as broad as that. On the other hand, if we assume that Moore's Law applies to power electronics and lasts forever, then there is no ceiling. ...

So let me add my own speculation. Suppose that thermo-electric devices were a million or billion times better than today's. Then we could move to wireless transmission of power for the last mile. A phased-array laser or maser on the pole top (or mounted on my solar/wind array in the tree top) aims at a receiver on my house and heats the receiver. The receiver(s) generate multiple DC voltages for short distance distribution within the house. Heck, that would even work for me living on my boat 200 m from shore.

Then comes the advanced version, the masers aim at each device's thermo-electric receiver directly (even mobile devices.) thus eliminating in-house wires. We would have wireless power transmission, plus complete electrical isolation eliminating ground and lightning problems.

 

[jim hardy]

So let me add my own speculation. Suppose that thermo-electric devices were a million or billion times better than today's.

EDIT Hijack Akert (sorry - I'm scrambled)
Ahhhh my old daydream.
Then we'd place thermopiles in Gulf of Mexico and Arctic Ocean , tapping off power all along the way.
Mother Nature's moving heat that direction anyhow with Gulf of Mexico edit oops, make that Gulfstream and atmosphere - watch your satellite weather this time of year you see huge sheets of clouds carrying enormous heat of vaporization from Gulf up across Tennesseee Valley then offshore at Northeast. US/Southeast Canada.
We'd help her along with her heat transport reducing Δ-temperature, maybe relieving severe weather like yesterday's .

Curious - i just can't seem to imagine decentralizing the grid. I guess i was imprinted by the big machinery.

from http://marine.rutgers.edu/cool/sat_data/?nothumbs=0&product=sst_comp&region=gulfmexico
Look how Gulf cooled between 16-20 Dec

old jim

 

[sophiecentaur]

Even my friends who work in power electronics R&D don't have dreams as broad as that. On the other hand, if we assume that Moore's Law applies to power electronics and lasts forever, then there is no ceiling. ..

I think you may be extending my argument so far that it loses credibility. (Well known arguing technique). Fact is that solid state could work over small networks now. No new technology needed to provide a home's worth of DC power. No change of conductors would be needed for lower level distribution. I'm not talking in terms of next year and the exponent in the 'Moore's Law' of Power may not be as high. But we all know that 'Wound Components" are phased out wherever they can be. . . . . .
(Sorry Jim but it won't affect either of us so there'sno need to worry too much.)

 

[jim hardy]

KVA_3phase = Vline*Iline*√3

If Earth or neutral is return, same 3 wires could carry √3 more power at same RMS voltage,
though they're insulated for √2 more.
√3 * √2 is 2.45...

i don't intend to learn those big converters.
I like lube oil and steam, wish i'd lived in days of recips and flyballs...

 

[sophiecentaur]

KVA_3phase = Vline*Iline*√3

If Earth or neutral is return, same 3 wires could carry √3 more power at same RMS voltage,
though they're insulated for √2 more.
√3 * √2 is 2.45...

i don't intend to learn those big converters.
I like lube oil and steam, wish i'd lived in days of recips and flyballs...

You have brought up a significant problem with three conductors plus a fourth, much thinner one. What volts would you put on each one? I guess the potentials wouldn't need to be arranged symmetrically; the thin conductor could carry less current and, as long as K1 is followed, there would be a range of choices. I am assuming that the converters would be smart enough to divvy up the power in an optimum way.
(Sorry about the lack of Oil and steam in my future world)

 

[anorlunda]

I think you may be extending my argument so far that it loses credibility. (Well known arguing technique). Fact is that solid state could work over small networks now. No new technology needed to provide a home's worth of DC power. No change of conductors would be needed for lower level distribution. I'm not talking in terms of next year and the exponent in the 'Moore's Law' of Power may not be as high. But we all know that 'Wound Components" are phased out wherever they can be. . . . . .
(Sorry Jim but it won't affect either of us so there'sno need to worry too much.)

I wasn't trying to ridicule your argument Sophie. I think it is a fun speculation, and a reminder that the proven ways of doing things need not be eternal.

But I thought you meant only the last single-phase parts of the distribution, not the three-phase parts.

But if there is any benefit it must be at the consumer level, not the distribution system. Simply swapping transformers for converters while using the old wires is hardly worth the trouble. The transformers are not a big cost in power distribution.

But in the homes where so many devices convert AC to DC, is where we could make savings. LED lights and electronic gadgets work best around 5V. High power devices like stoves would use higher voltages. Two sets of house wiring, 5V and 220V might be enough.

The power company would have to pick up the costs of buying all customers new appliances and gadgets as part of an AC to DC conversion project. We already have experience with that, for example when Mexico converted from 50Hz to 60Hz.

I also smile at the thought of Edison's ghost smiling from his grave. His debate with Tesla would ultimately swing back to Edison's side.

Apologies to the OP. We thououghly hijacked this thread.

 

[jim hardy]

You have brought up a significant problem with three conductors plus a fourth, much thinner one. What volts would you put on each one? I guess the potentials wouldn't need to be arranged symmetrically; the thin conductor could carry less current and, as long as K1 is followed, there would be a range of choices. I am assuming that the converters would be smart enough to divvy up the power in an optimum way.

Hmmm I'm stuck in the sixties... still thinking symmetry and not used to smart substations..
IF
one just replaced substation transformers with HVDC converters and suitable DC breakers
AND
L1 and L2 were opposite polarity and equal load
THEN
their return currents would cancel
so an existing 3 phase transmission line's neutral would only have to carry L3's return current
might one base load L1&L2 to actual demand and use L3 for the short term fluctuations?

Question for somebody closer to power systems than me. Real time sub-cycle telemetry opens new worlds for them.

Of course I'm assuming HVDC current is continuous, not pwm or something. I was never around any and just don't know how it works .
I do know one has to build into his converters protection against any modulation of load at frequency anywhere near the mechanical resonance of turbine shaft and blades , our was seven hz. Subsynchronous Resonance (SSR) is the term. That can cause a turbine to sling blades or snap a shaft and pretty much ruin your day.

(Sorry about the lack of Oil and steam in my future world)

It's still driving our pushbutton world. Does 1927 movie "Metropolis" ever come on the tube over there ? I recommend it. Could have been the inspiration for Bolero.

http://www.npr.org/templates/story/story.php?storyId=3335230]As[/PLAIN] romantic as it may seem, Ravel said the pulsing, rhythmic composition was inspired by one of the factories he had visited with his father, who was an engineer.

old jim

 

[sophiecentaur]

I have to admit that the changeover would be decidedly lumpy. Luckily, most domestic equipment is compatible. Edison had the wrong solution at the time and Tesla earned a lot of cred by forcing AC on us. But that was then and in the future things could well be different - along with more sensible use of electrical power (a more economical regime)
We already think nothing of DC-DC conversion at low power. All I'm suggesting is that the same thing that makes power supplies cheaper these days (i.e. no transformers) could well apply at higher and higher power levels. Transformers are familiar and cuddly but they will always cost what they cost today. Solid state continues to get cheaper and cheaper and more reliable. Given the trends in power generation, can we be sure that the alternator will be the main source in a hundred year's time?
The steam locomotive era finished with a bang in the UK - less than a decade from start to finish.
I would like to have a crystal ball.

 

[dlgoff]

I have to admit that the changeover would be decidedly lumpy. Luckily, most domestic equipment is compatible. Edison had the wrong solution at the time and Tesla earned a lot of cred by forcing AC on us. But that was then and in the future things could well be different - along with more sensible use of electrical power (a more economical regime)
We already think nothing of DC-DC conversion at low power. All I'm suggesting is that the same thing that makes power supplies cheaper these days (i.e. no transformers) could well apply at higher and higher power levels. Transformers are familiar and cuddly but they will always cost what they cost today. Solid state continues to get cheaper and cheaper and more reliable. Given the trends in power generation, can we be sure that the alternator will be the main source in a hundred year's time?
The steam locomotive era finished with a bang in the UK - less than a decade from start to finish.
I would like to have a crystal ball.

Yea. The tube may return.

image compliments https://en.wikipedia.org/wiki/Rectifier

 

[jim hardy]

That's got a cheaper life cycle than a transformer? Counting the education of people to keep it running ? Transformers can last a hundred years.

 

[dlgoff]

That's got a cheaper life cycle than a transformer? Counting the education of people to keep it running ? Transformers can last a hundred years.

lol Jim?

edit: you did see the . Right?

 

[sophiecentaur]

Does 1927 movie "Metropolis" ever come on the tube over there ?

The sexyist robot ever in that film.

 

[jim hardy]

lol Jim?

Well there is a natural 'Law of Diminishing Returns' .

Complexity . It confuses our tongues.

old jim

 

[Salvador]

last two threads by the OP got so hijacked that the Chinese government hacking into US files can be considered a minor inconvenience :D

as to what you guys said , first of all, simply changing all the worlds grid to some adapted DC which can run through the existing wiring is not realistic atleast not in the near term and not in a capytalist world we live in.All the equippment running in the world today would simply have to be either modified or thrown away , excep for incadescent bulbs and heater elements.I doubt that is an investment someone is willing to make for a few percent decrease in grid loss , and that's mainly the long HVAC lines across countries from power stations to cities , not the small local grids which start at the secondary side of a transformer in a local substation and hen runs to your house.

second of all I don't know why you sophie said that the DC thing could eliminate transformers, some of them yes but not all, firs of all there are many ways were rotating mechanical power has to be converted to electricity and the majority of all the devices used to do that (generators) work on AC, and quite frankly I would like to point out that the only true DC generator that is mechanical and outputs a pure DC voltage without ripple is the homopolar/faraday (disc) generator, all the others work on induction and induction means something has to alternate either way.
Also I'm no expert on HVDC but as much as I have been reading the substations still use a transformer and quite frankly every power supply even run on DC needs a transformer if it wants to isolate the supply voltage from the output voltage with galvanic isolation. and the majority of SMPS that I have seen open on my desk also have a transformer except for a few topologies usually for low power specific devices.maybe I've missed something but how else can you isolate galvanically between two circuits ? a thyristor or a transformerless converter can't do that.

 

[sophiecentaur]

Switch mode power supplies do use inductors or transformers, true but, because the frequency can be so high, the inductor doesn't need to be massive. mH rather than H.
The advantage of massive generators is due to the efficiency of big turbines - implying large alternators. Alternative systems which don't use heat engines could be nothing like as big. Small scale networks do have advantages when the control systems are up to it.
Listen, guys, I am not an Electrical Engineer but you experts in the present system could do well to consider alternatives that could result from new technology and future Energy problems. I am only flying a kite - which is one of the purposes of PF.
As for compatibility, look at the lifetime of most electronic and electrical goods. Our fridge is 18 years old and that's the oldest thing we own. Given a decade of warning, manufacturers could easily make their power supplies suitable to change from AC to DC. We have seen the transition from 405 line TV to 625 line Analogue Colour TV to Digital TV in 40 years. How many generations of computer have we been through since our first ever micro?