Hi.

During my GCSEs, I was taught two things about the high voltage cables, the sort that go accross pylons (the big metal sort).

Science taught me that they used AC because it lost less energy whilst traveling the long distances involved. However, Electronics (which regularly lied to us) taught us that they used AC because it meant that if you touched a cable (how you do this i'm not sure...) the chance of it being at full voltage is small, and there is an equally small chance that it will be a 0v, which would cause you no harm.

I was never sure about the reason given by electronics, as at 50Hz, the time it was around 0v was incredibly small, and would go to full voltage in an also very small time, so you would probably still get electrocuted.

So, I am asking who was right, Science or Electronics? I wouldn't be supprised if Electronics was just plain wrong.

Thankyou

Tim

 

I do not know where you got your teachings from, but they are weird. Your "science" teaching is not really valid, as heat loss should ideally be about the same for the same amount of (average) power transferred.
Your "engineering" teaching is true, but I would call it an advantageous by-product of AC voltage rather than the reason for it.
AC power is transmitted, IMO, simply because power generation is mainly carried out by turbines, which provide AC power.

 

And the fact that it is much easier to get the high voltage / low current that makes pylons safer (i hope i got it the correct way round) with the use of transformers.

 

to be fair DC is the best form of voltage for transmitting (very) high power.

AC was chosen since it is easily generated (using syncronous generators), easily stepped up to high voltage

high voltage is prefered since the cable losses will be dominated by V^2/R instead of I^2*R
thus as far as power is concerned less is lossed in heating the cables


and to easily step-down to various levels say 230V for domestic and then in a house to say 24V for electronics

The comment abt AC being safer is 1/2 true, if you have ever touched a AC line it is ok (still not nice), but DC is, well dont do it. I have been shocked by AC, 230V alot and I am careful but not as worried abt DC I have been shocked by DC once (28V) and never again. In DC systems you tend to have alot of DC-link caps, you short them and you could have thousands of amps flowing through you, it only takes 20mA to your brain/heart to kill you.

IF you have a DC shock you will tense up and if it is around a cable you wont let go.
AC has points of zero voltage so it is more pulsing and more chance to pull teh cable out.

either way dont do it

 

Ok, this thread is verging on the ridiculous.
Edison would've approved. What's the basis for your claim?

It's the same equation, the only thing that dominates is Ohm's law. Reduce the current flowing through the transmission lines or increase the cross-sectional area of the lines to minimize power losses. A transformer to step up voltage on one end and down on the other costs far less than increasing the wire gauge for long distance power transmission.

A few statements that shouldn't go unchallenged. Science vs. electronics is a false dichotomy. Electronics is a subset of physics, itself a subset of science. What may appear as "lies" should probably be chalked up to misunderstanding before placing the blame on an abstract entity.

 

ah, no, sorry. By 'Science' I mean the subject that we taught in school. (my into paragraph said it was about stuff during my GCSEs, although non-UK people may not know whatthey are). So when I said 'science taught me blah', i meant that i learnt 'blah' in my science lessons. And my comment that electronics regularly lied to us, meant that our electronics teachers lied to us, rather than the subject lying to us. The reason that I phrased it like I did, was because out teachers only taught us what they had been told to teach us,so it wasn't really them that lied to us, bu the subject that was lying. If that makes any sense.

Anyway, back to things that matter.

It really makes that much difference? I didn't know that.

So basically, power (in this instance) is transmitted using AC because that it was is created by the generators, and it is easier to step up AC to high voltages. You also say that what our science teachers taught us was wrong, because DC is more efficient? But you would say that what our electronics treachers taught us about less chance of being seriously electrocuted, is true, but not the reason for power being transmitted as AC?

Thanks alot

Tim

 

Generally teachers in schools know next to nothing about Electronics, there may be the odd exception - but it will be rare!. So you need to take anything they tell you with a 'pinch of salt'.

AC and DC generators are both pretty well as easy to build, the main advantage of DC is that you more often need DC power than AC power. Using DC mains prevents the requirement for rectifiers in electrical equipment - however, it's far simpler to convert AC to DC, than DC to AC.

The AC/DC debate has been raging for years, the obvious time was back when electrical distribution first started.

AC won, hands down, simply because of it's ease of voltage conversion.

Ohms law tells us that V=IxR, so for a given piece of cable R is constant and the voltage drop across the cable (V) is dependent on the current (I) passing through it.

When distributing electricity over long distances the cable costs become huge, this can obviously be helped by using thinner (cheaper) cable, but then the voltage drop over the cable becomes a problem. By increasing the voltage you can send more power down the same cable, because the current (and consequently the voltage drop) is lower.

This is only practical with AC, as transformers simply, cheaply, and reliably perform the required conversions.

It's this AC power distribution which makes the National Grid possible, the supporters of DC power suggested vast multiples of small power stations, pretty well one on every street corner!.

As for the relative dangers of AC and DC, this was also publically argued, with one side actually killing cows in the street to prove how dangerous the other sides system was. The climax was the introduction of the Electric Chair in the USA, which (if I remember correctly?) was designed by Edison, using Westinghouse DC generators.

From my point of view I consider DC more dangerous, as you already suggested DC tends to make you hold on, whereas AC tends to make you 'spasm' away. However, either can quite happily kill you, so you should always take great care.

One other 'risk' with DC is the possibility of it being stored when the power is turned off (charged capacitors), you can't really get that with AC.

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ah, cheers Nigel, that was what I wanted to know.

 

I suggest you do some reasurch, there is a high power DC-link connecting Europe to the UK. One of the ppl I work with designed and built it.

DC is preferec for (very) high power transfer over distance based upon skin effect. You transfer teh power by AC at that power and the efficiency plummets

In teh UK we have a very well organised grid of power stations and sub-station so there is no real lengh of just pure AC power.

In teh US down California and also in Norway where there is long stretches of AC power to trasmit they have alot of problems.

Also Edison proposed lots of small DC power station everywhere (There was alot of logic behind it), To state Edison without his justification (AC is the best over short distances for high-power) is like stating the 45min claim but forget to mention that it is for field ordanace.

The proble round Tesla/Edison time was there was no way of generating DC at very high voltages thus voltage drop though conductors were high, unlike AC where it could be generated and transformed to high voltages and thus reducing the voltage drop when power was drawn.

There was a big proposal abt 5years ago bringing back Edisons original idea abt local small generators (but using AC instead) since the power levels being trasmitted is from a few very big stations is starting to cause problems

 

which does what? I'm guessing the link is between england and france - and I can't believe the french let us share their power? (no offence intended to any frech people...)

 

It does exactly that. The link is between Eng and France (via the chunnel) but all of europe can share power and we were the final link.

We are talking abt alot of power being transmitted and DC was teh only way.

I cannot remember how long it has been up and running, but last summer was teh first time that we sent power to europe (they had major heat wave and lots-o-aircon was on). up until then the whole of the UK was in a power deficit to europe.

We cannot even susstain ourselves power-wise. But to be fair we do have the JET and it does take a fair few thousand of amps to fire it up

 

moving away from the orginal topic i know, but this is quite interesting.

err, what?

 

lol

JET = Joint European Toroid.
It is (at the moment) the world highest power fusion reactor testbed in the world. It is based in Oxford, it was from this reasurch that the minimum size of a fusion reactor that produces more energy than it takes to run was calculated (the JET is abt 50% too small for sustain power generation).

They have managed 30sec of contious fusion but as I stated the heat produced is not enough to maintain the reaction. To start the whole process up they flood the toroid with Deuterium and tritium. They then pass through it 3.2MA to heat up the gas mixture to transform it into a plasma and also to reach fusion tempuratures

 

Well Styx, I took your suggestion and did some research. My apologies for jumping to conclusions. It is a case of technology progressing to the point where HVDC transmission has become a feasible alternative to AC. For those interested, here's some background info on the topic of HVDC power transmission:

http://www.worldbank.org/html/fpd/em...nology_abb.pdf

http://encyclopedia.thefreedictionary.com/HVDC

The first article in particular was an eye-opener for me, especially the cost vs. distance (p.6,7) comparisons between AC and DC. Both articles mention that HDVC is preferable for long distance (>700 km) transmission
I'm a little doubtful of the "skin effect" argument though, the effect increases with increasing frequency and is pretty much negligible under audio frequencies. Of course, "negligible" could be "significant" at Grid power levels. This statement from the second article makes more sense:

The map at the end of the first article shows your cross-channel (2000 MW) installation. The Chinese appear to have the largest HVDC system, at a whopping 3400 MW!

 

I can verify this with many teachers. I one time asked my electronics teacher about zener diodes, he just babbled on about something irrelevent and continued marking the exams. Hes been teaching for 30 years !!!
And yes it is easier to transmit power through AC simply because you can convert the volatges back down to 230/120 volts. It simply provided the most economical way to distribute power...AC won hands down .

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