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Power transmission

01/12/2009 8:55 PM

I hope this question doesn't sound too silly. Is it possible to transmit power digitally? We can transmit sounds by converting analog (sine waves) to digital and back to analog again over great distances. The analog signal is a very small voltage. A power source is a huge analog sine wave that is transmitted via wires and stepped-down to smaller voltages. Why wouldn't digital work ?

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#1

Re: Power transmission

01/12/2009 10:25 PM

The short answer is that there is no advantage to doing so, and lots of disadvantages.

The long answer is below.

Why is analog information digitized?

Converting low-level analog information to digital to send it a long distance has the advantage of higher fidelity - better signal-to-noise ratio, and error correction. The downside is that you need a transmission line to do it, because the trade-off is the faster digital speed required. If you don't understand why digital requires more bandwidth, see the explanation at bottom, if you need it.

How is the electrical grid different than an analog signal?

Electrical power is transmitted to do things like light lights, heat and air condition homes and businesses, run electrical appliances and power industrial processes. These all require raw, brute power. You could clearly sample the high tension line and come up with a digital bit stream to describe the potential of the line every ten microseconds, say. And you could pipe that information anywhere in the world, instantaneously these days. In fact, I'm sure that sort of thing is done by the instrumentation within a power plant. But sending the information on what the power waveform looks like is not the same thing as sending the power. Anymore than having a picture of your big brother who is in the Army is going to dissuade the neighborhood bully from pounding you. Sometimes there is no substitute for the real thing!

Why digital requires higher speed transmission:

Digital requires higher speed because you basically sample the analog waveform at a certain rate and send a digital word that describes the analog value during each sample period. For instance the electrical power grid works at 50 or 60 Hz. The period is therefore 20 or 16.7 milliseconds. If you wanted an accurate digital representation of the waveform, you might decide that sampling the potential at every degree is sufficient. There are 360 degrees, so the sample time is now 20 or 16.7 milliseconds divided by 360, or 55.55 or 46.3 us, respectively. In order to get good digital transmission, you want each one to zero or zero to one transmission to be nice and sharp. That is, ideally the individual bits look like nice clean rectangular waveforms. In practice, you can get away with waveform rise and fall times about one-tenth the actual waveform duration, so now you need rise and fall times on the order of 5 microseconds. The frequency spectrum associated with the rise or fall time can be estimated as

f (Hz ) = 1/(pi *tr) or 1/(pi *tf),

where,

f (Hz ) is frequency in Hz,

tr and tf are rise and fall times, respectively.

With 5 us, the frequency is calculated to be 64 kHz. That has a wavelength of 4.7 km. If your transmission line is longer than about one tenth of that, your transmission line needs to have a controlled impedance and be matched at source and load.

You can do that for information, and you could do a number of things to reduce the effective bandwidth by clever coding, but there is no way you are going to transmit power at 64 kHz. So even if there were a benefit, which there isn't, there is no way to do it efficiently - the losses would kill you.

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#2

Re: Power transmission

01/13/2009 2:50 AM

Digital is good for transmitting information rather than power as you can manipulate th 1s and 0s to encode lots of information.
Power is hopefully continuously on (eg in a '1' state) and the biggest concern is efficiency of transmission, which is why we use high voltge AC lines. (high volts=lower current, and it's the current which gives you the losses)
Turning the power on and off rapidly to transmit 1s and 0s, or modulating it in some other way wouldn't improve efficience it would make it worse.
There are various systems which transmit information on the powerlines along with the power, but they are beset by problems and generally a bad idea.

Del

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#3

Re: Power transmission

01/13/2009 11:36 PM

Power is already digital, light off = 0 Light on = 1

But seriously all the steps needed to make a power signal from analog to digital and back again would incur a lot of extra losses and the whole step up and step down transformers are based on AC.

Like previous posters said digital is great for data transmission and control but not for energy transfer

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#4

Re: Power transmission

01/14/2009 1:05 AM

Hello

Analog to digital conversion is just a matter of sampling, the sampling rate is the main concern. with higher sampling rate you earn better sound.

but at sound transmission the amplitudes are very low. transmitting low amplitude signals long distance in analog format will diminish the signal to noise levels due to wire resistance.

but power transmission is made via high voltage levels. See the calculations below:

the amplitude to be transfered: V and the current is I

wire resistance: R

Power loss: I*I*R

now say voltage to be transfered is ten times greater: 10V and current is ten times smaller say I/10

New power loss is : (I/10)*(I/10)*R = I*I*R/100

When we increase the voltage ten times and decrease the current ten times the power loss decreases 100 times.

so power signals are not transformed to digital siganls (since to be able to transmit the same power an unreachable sample rate will be neded) but stepped up tranformed to higher voltage levels without loss on total energy, and minimized the wire losses, then stepped down to usage levels near cities.

but sound signals are digitized since higher sample rates are achievable on smal electronics, and amplitudes are already very low. transmitting the sound on analog ways will completely make it lost.

another point: while transmitting the sound into digital format we lose the third and fifith etc.. i.e. all odd harmonics, mostly just the mane sine wave is sampled. but power transmission does not prefer it, the elimination of an harmonic causes elimination of a part of total power. we prefer the user elminates the undesired parts.

total power is sum of the square of all harmonics.

a sample power transmission line.

Generator 30KV->Step-up transformer150kV-->step-up transformer 300kV-->transmision line-->step-down transformer 150kV--> step down transfomer-->distribution line-->step down transformer 4kV-->Step down transfomer 380 or 220 V-->End user

one last point, musicians generally does not like digital music equipment and they say the sound is a bit "weak". this means that a professional audience always understands the difference between analog and digital sound due to the sampling rate losses.

hope I could be helpfull.

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#5

Re: Power transmission

01/14/2009 1:41 AM

Firstly "digital transmission" does not refer to the mode of transmission, but rather to the data (information) encoding scheme. All transmission modes are essentially, by its nature, analogue. So, quite simply, because "digital transmission" refers to communication and not to power transmission, it is really not possible to do.

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#6

Re: Power transmission

01/14/2009 3:56 AM

There is another small problem -

As you know the power normally transmitted in DC form (preferable) ar AC form with 50 or 60Hz frequency -

The digital (square wave) is a mix of infinite number of sine waves (remember fouriers transform?)

Now Let the wire carry all the sine waves - visualise what happens

- The resistance of wire is same for all (approx for the time being)

- The inductance is also same.

The reluctance () of the wire = R+ωL will be dependant on the frequency and you will land up in more reluctance for higher frequencies and vice versa.

Corona and other types of line losses also depend on frequency.

In the end the signal will have some mysterious shape.

However the above assumption are not same (simply because of skin effect) so more losses and heating up of the conductor.

The capacitor banks (pf corrections) are tuned to the operating frequency for AC lines, now it will be in out of tune for all other frequency.

And mind the power of the first few harmonics are large enough so the all above are very significant.

Lastly why should I nicely generate sinusoid power and change it to sq wave ?

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#7

Re: Power transmission

01/14/2009 5:18 AM

Ron,

After over 35 years in the electrical game I agree with the posts responding to you, but the damnedest thing appeared in a publication of some sort I came across a short while back which mentioned "Digital Power Transmission Lines" as a future technology but no further detail or useable references. So your question is not silly and we may hear about it in the "Journo press", but I can find no value in the concept of using squarewaves for long transmission (yet they are used in strange places such as powering Phase Module Gate Drivers (100volts 25khz square wave to be precise)).

High frequencies require smaller transformers but lose out over distance due to capacitive losses phase to phase and phase to ground. Couple this with higher XL as frequency increases then all the advantages of the AC HV distribution system go out the window.

Maybe what the Journos are misquoting is "digitally controlled" grid networks where smart meters "negotiate" with the Supply Authority for the best energy deal. That technology is feasible and apparently being rolled out. Useage data is transmitted by either fibre laid up in the power cables or by superimposed frequencies on the base power frequency. This (ripple frequency) is old and proven technology and a form of it has been in use for decades to control switchable loads for controlled tarriff metering.

There are drawbacks to superimposed frequencies though, such as noise from fluoro ballasts, destruction of switchmode power supplies etc.

Hope that helps.

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#8

Re: Power transmission

01/14/2009 9:30 AM

I think your mixing terms from two fields of engineering, analog and digital describe information theory and communications. While power transmission is a separate field.

DC voltage is the equivalent of digital, such as the address or data bus in a processor, each line is on or off. AC voltage is the equivalent of analog.

However, "Digital" information is transmitted through an analog sine wave. One example is Frequency Modulation (Analog) and Frequency Shift Key (digital) use very similar circuit theory. The difference is that FM in infinitely variable and FSK (or by phase, PSK) would use discrete shifts. This would be similar to digital TV, satellite radio, etc transmissions.

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#9

Re: Power transmission

01/19/2009 2:13 AM

This is a terribly strained analogy, but let me give it a whirl.

For the purposes of this discussion, let's say I own a company that makes cakes. There is a 10 mile long vacuum tube (like the one at the bank drive-through window) between my office and my bakery. I can take a cake in my office and analyze it and determine the recipe for it and write down the recipe and put the paper in the tube and send it to my employee in the plant and he can use that information along with some raw materials on site to recreate that cake to a tee. However, if I put that cake in the tube and tried to send it to him, he would get the cake, but its structure would be seriously degraded and that would negatively affect the cakes marketability. This is analogous to taking an analog signal (the cake) and converting it to digital (the recipe) and sending it long distances with little or no degradation to be recreated at the destination (another cake - a copy of the first using different raw materials).

On the other hand, if my bakery needed raw material (cake batter), it would do precious little good for me to write the recipe for it and send it to them in the tube. They need the raw material - not the blueprints. So I could hook up a pump to my batter box (snorfle) and pump the stuff through the tube to the bakery and it would be just as good coming out as going in. Much unlike doing the same with the cake would result in. This is analogous to delivering power.

So if my guy gets the recipe in one tube and the batter in another tube, it beats the hell out of me stuffing cakes in vacuum tubes and expecting to maintain a sufficient level of revenue to pay for vacuum tube maintenance.

If this is the best analogy I could come up with, maybe I should just go to bed.

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