/ Why AC and not DC?

This topic has been archived, and won't accept reply postings.
Al Evans on 19 Nov 2013
I have just watched the latest episode of Ripper Street, as a sub plot two rival enterprises were bidding to build a power station to supply London with electricity, one was championing AC and the other DC.
Now, I know the difference and obviously AC won world wide, but why, what advantages does it have over the DC version that some enterprises were backing and were convinced was a superior option?.
elsewhere on 19 Nov 2013
In reply to Al Evans:
Transformers - high volatge & low current allow long distance power transmission.
DC is used for some links (under the channel?) but I expect step up/down of DC is much more difficult.
Andy DB - on 19 Nov 2013
In reply to Al Evans: AC is firstly easier to generate mechanically. AC is also much better for transmitting over large distances due to the impedance loss in the transmission line.
ByEek - on 19 Nov 2013
In reply to Al Evans: According to here:

http://science.howstuffworks.com/electricity8.htm

the main advantage is because transforming the voltage from high to low and visa versa is really simple in AC land. I am sure you will remember from 3rd year (school) physics that resistance over a load (transmission cable) decreases as voltage rises hence the need to transmit power at high voltage. :-)
Wulfrunian - on 19 Nov 2013
In reply to Al Evans:

AC voltage readily convertible up or down via transformers, thereby allowing high voltage transmission over smaller cables before stepping down at the load end with relatively low losses.

Easy to convert AC to DC if required. Not so in reverse.

AC hurts less ;-)
BIgYeti86 - on 19 Nov 2013
wintertree - on 19 Nov 2013
In reply to Al Evans:

Transformers. You need high voltage from efficient transmission lines and low voltages for safe consumer use.

DC-DC transformers are coming (magneto hydrodynamic plasma devices).
elsewhere on 19 Nov 2013
In reply to Andy DB:
> AC is firstly easier to generate mechanically.

Good point. The first systems that started the industry would have no need for transformers.
needvert on 19 Nov 2013
In reply to ByEek:

School as in pre-university?

If we assume a material that obeys ohms law and ASSuME a constant temperature then the resistance is going to stay the same regardless of current or voltage (though resistive losses won't!)
NorthernGrit - on 19 Nov 2013
In reply to Andy DB:
> (In reply to Al Evans) AC is firstly easier to generate mechanically. AC is also much better for transmitting over large distances due to the impedance loss in the transmission line.

I agree with the first point but I don't think the second point is strictly true. HVDC transmission is more efficient that HVAC.
DancingOnRock - on 19 Nov 2013
In reply to needvert: agreed. High voltage, low current but same power. The lower the current the lower the voltage drop across a given resistance. Vd=IR.
MonkeyPuzzle - on 19 Nov 2013
In reply to Al Evans:

Ease of use, simplicity of equipment, no need to convert to AC for industrial applications, AC-to-DC conversion much simpler, cheaper and more efficient than DC-to-AC.

DC transmission only makes sense over distances larger than we generally have to contend with in this country. DC also has the added benefit of being able to link asynchronus networks, eg. the 60Hz European network and our own 50Hz network. It also acts as a buffer to improve network stability if the adjacent network were to suffer voltage collapse.
csw on 19 Nov 2013
In reply to Al Evans:

An interesting spinoff from the squabble between Edison and Westinghouse over domestic electricity supply, is the adoption of the electric chair as a method of execution. Edison lobbied for it quite hard - he even suggested that execution by electrocution be referred to as being "Westinghoused". His idea being that if AC power was perceived to be more lethal than DC, people would be less likely to want it in their homes.
jkarran - on 19 Nov 2013
In reply to Al Evans:

AC can be transformer coupled for voltage changes, pretty much essential for efficient transmission and safe use.

DC wiring/equipment suffers from galvanic corrosion problems.

AC is how most power is and was generated, there's no rectification step needed (nor expensive dangerous rectification equipment).

I'm sure there are more compelling reasons but those spring to mind.

jk
needvert on 19 Nov 2013
In reply to jkarran:

> ....there's no rectification step needed...

My house is prolific with AC to DC power supplies :(
Lusk - on 19 Nov 2013
In reply to Al Evans:

The wonderous machine that is the 3 phase induction motor.
Offwidth - on 19 Nov 2013
jkarran - on 19 Nov 2013
In reply to needvert:

> My house is prolific with AC to DC power supplies :(

Which makes sense, solid state converters in the 10s of Watts range are small and cheap. GiggaWatt rectifiers at the generator are rather different beasts.

It's also only in the last 30 years that predominantly-DC grid connected equipment has proliferated, before that you'd be looking at maybe a radio and bits of the TV in each household requiring DC, the rest of the connected household equipment: lights, heaters, motors is native 'AC'.
ByEek - on 19 Nov 2013
In reply to needvert:

> If we assume a material that obeys ohms law and ASSuME a constant temperature then the resistance is going to stay the same regardless of current or voltage (though resistive losses won't!)

Given that we are talking about real power transmission in the real world, why would we be assuming constant temperature or that the material might not obey ohms law? Temperature in this case is largely irrelevant.
DancingOnRock - on 19 Nov 2013
In reply to ByEek:
> (In reply to needvert)
>
> [...]
>
> Given that we are talking about real power transmission in the real world, why would we be assuming constant temperature or that the material might not obey ohms law? Temperature in this case is largely irrelevant.

Because aluminium and copper do obey ohms law and a constant temperature is assumed for comparison purposes here.
ads.ukclimbing.com
jkarran - on 19 Nov 2013
In reply to ByEek:

> Given that we are talking about real power transmission in the real world, why would we be assuming constant temperature or that the material might not obey ohms law? Temperature in this case is largely irrelevant.

I'm a bit lost following this sub-thread but I'm presuming the 'assuming constant temperature' was to ignore the effect of self heating due to the current flowing in the conductor.

Either way, the reason high voltages are used is that power transmitted is the product of current and voltage

P = I x V

You can send a lot of power at high voltage or high current or moderate voltage and current

Resistive transmission line losses are however the product of resistance (fixed by line design/materials) and the square of current

P = I^2 x R

If you double the voltage you need 1/2 the current to transmit the same power (from P=IV) but that gives you 1/4 of the resistive transmission losses (from P=I^2R).

The beauty of this becomes apparent when you look at say a 100 fold increase in voltage, resistive transmission losses are now 1/10,000th of what they were.

Doubtless teaching granny to suck eggs but possibly of interest to someone.
jk
Al Evans on 19 Nov 2013
In reply to Offwidth:
> (In reply to Al Evans)
>
> This is good:
>
> http://en.wikipedia.org/wiki/War_of_Currents

It is stunning, and was never covered in as much detail in my 3rd year physics or even the ONC I got in electrical engineering as I remember.
I was stunned to read that DC plants in the UK were still operating into the 1980's,
A Very good summation, though it possibly does still not give me the whole answer of why AC won the battle.
knthrak1982 on 19 Nov 2013
In reply to Al Evans:

Circuit breakers on HVDC are difficult to design. Basically when you break a circuit, you create an arc. An HVAC breaker will extinguish this arc when the current waveform crosses zero. This doesn't happen with dc. This makes it difficult to create a HVDC transmission grid.

For long lines, however, HVDC has advantages of conductor cost. For a given power rating and insulation level, it can be shown that dc conductors are half the cost of ac.

HVDC is also good for connecting two different transmission grids together if they use different frequencies. UK/France has a dc link.
Offwidth - on 19 Nov 2013
jkarran - on 19 Nov 2013
In reply to Al Evans:

> A Very good summation, though it possibly does still not give me the whole answer of why AC won the battle.

It really does. In one word: Transformers.

There are other benefits but it's impossible to overstate the importance of simple, comparatively cheap voltage conversion and isolation that transformers give you.

jk
climbwhenready - on 19 Nov 2013
Although AC has the advantages listed above (easy to generate, less transmission losses, more "useful" at the other end for things like industrial motors) the reason we have DC links under the channel is because we're on a different grid to europe. Although the voltage/frequency is nominally the same, the "up-down" of AC is in sync across the entire grid - in this case, across the whole UK. We're not synchronised to the western europe grid, though.

See: http://en.wikipedia.org/wiki/Wide_area_synchronous_grid for maps.
krikoman - on 19 Nov 2013
In reply to knthrak1982:
> (In reply to Al Evans)
>
> HVDC is also good for connecting two different transmission grids together if they use different frequencies. UK/France has a dc link.

French supply is 50Hz like ours
knthrak1982 on 19 Nov 2013
In reply to krikoman:
> (In reply to knthrak1982)
> [...]
>
> French supply is 50Hz like ours

Yup. climbwhenready post above explains it better than I did. Frequency is still the reason.
krikoman - on 19 Nov 2013
In reply to krikoman: In fact all of Europe is 50Hz
knthrak1982 on 19 Nov 2013
In reply to krikoman:
> (In reply to krikoman) In fact all of Europe is 50Hz

Yup. Nominally. Frequency fluctuates as loads change, generators switched on or off etc. The DC link allows us to be connected without our fluctuations affecting/being affected by their fluctuations.
krikoman - on 19 Nov 2013
In reply to knthrak1982:
> (In reply to krikoman)
> [...]
>
> Yup. climbwhenready post above explains it better than I did. Frequency is still the reason.

It's not really though is it?

It's to do with phase matching and the voltage differential.

If our voltage was higher but we still needed more energy, if you connect the two systems together with AC, power would flow out of our system not in, making things worse!!
knthrak1982 on 19 Nov 2013
In reply to krikoman:
> (In reply to knthrak1982)
> [...]
>
> If our voltage was higher but we still needed more energy, if you connect the two systems together with AC, power would flow out of our system not in, making things worse!!

Not necessarily. Tap changing transformers could address voltage differences. Phase shift transformers can be used to control power transfer across ac lines.
GrahamD - on 19 Nov 2013
In reply to krikoman:

You can connect AC systems together on a managed basis - that is how we can run many power stations on the grid and even deal with feed in.

DC is just more efficient for very high power with modern technology which is why its also used for things like off shore wind farms
stewieatb on 19 Nov 2013
In reply to GrahamD:
> DC is just more efficient for very high power with modern technology which is why its also used for things like off shore wind farms

I believe there aren't currently (pun intended) any HVDC lines from offshore wind farms to shore, at least in the UK, but there will be in the next 10 years as far offshore farms (e.g. Dogger Bank - 9GW, ~160km from shore) get built. In fact, my engineering degree project this year is 'designing' the Dogger Bank windfarm.

The reason DC is more efficient is the capacitive losses in long, high voltage AC cables - if you used AC over this length, the capacitance between the different phases would cause huge losses and a loss of coherency (the voltage signal stops looking like a sine wave). This is also why the USA doesn't have a national grid like we do - it has multiple smaller ones. For a straight long-distance transmission, HVDC is more efficient for distances greater than about 120-150km. Obviously there are practical issues (and losses) with conversion at either end, but these are usually tiny compared to the advantages of efficiency over the transmission distance.

There are under-sea HVDC lines connecting many countries in Northern Europe together. See map: http://en.wikipedia.org/wiki/High-voltage_direct_current
barnaby - on 19 Nov 2013
In reply to Al Evans: Here's a brain teaser for the AC boffins.....

Power lines obey Ohm's Law.

V=IR (voltage = current x resistance).

We accept that the temperature remain constant (ie will not affect the resistance of huge cables).

Therefore.....The higher the voltage......the higher the current.

Hang on...thats not what we wanted!! We wanted a minimal current.....to reduce energy losses along the loooooooooong power lines....

So, with a knowledge of Ohm's Law.....how is it that the high voltage used to carry our electricity around actually results in a LOW current??

knthrak1982 on 19 Nov 2013
In reply to barnaby:

See post at 1222 by jkarran.
itsThere on 19 Nov 2013
One of the fun things about HVDC is that the transformers act like a closed loop system in the event of a fault, they resists it and no/less damage is caused. They can also be switched faster than every 10ms for AC.

In reply to stewieatb: If your engineering project in on HVDC you should look into the work of one of my old lectures. (im no longer a student)

http://www.abdn.ac.uk/engineering/people/profiles/d.jovcic
barnaby - on 19 Nov 2013
In reply to knthrak1982: That may be true.....but doesnt answer the more 'basic' question I posed....!
stewieatb on 19 Nov 2013
In reply to barnaby:

It's easier to think about this in DC, but the physics work in AC too. For simplicity, let's say we have a generator of fixed power output (which we can then transform, losslessly), a long transmission line, a load of fixed power requirement which matches the generator output (with a lossless transformer before it), and a ground return line.

That's a misinterpretation of Ohm's Law, and a common one. V=IR in that case refers to the voltage drop across a component, not the 'voltage through' it (see below). Across a long transmission cable, the aim is to reduce the current, and hence the voltage drop (which by P=VI is proportional to the power loss in the cable). The aim is to have the same nominal voltage at the other end that we did at the start.

The voltage of the outbound cable's positive end above ground potential only becomes relevant when we consider the whole system - transmission line, loads and ground lines. Increasing this using transformers will not increase the current, because the load only demands so much power, and the generator can only supply so much. Hence, increasing the voltage decreases the current, which decreases the voltage drop across the lines, and hence the resistance losses in the system.
stewieatb on 19 Nov 2013
In reply to itsThere:
> In reply to stewieatb: If your engineering project in on HVDC you should look into the work of one of my old lectures. (im no longer a student)
>
> http://www.abdn.ac.uk/engineering/people/profiles/d.jovcic

It's a group project to design the whole windfarm, and somebody else is doing power transmission - but thanks!
ads.ukclimbing.com
wintertree - on 19 Nov 2013
In reply to barnaby:

> ..how is it that the high voltage used to carry our electricity around actually results in a LOW current??

The current is voltage / (resistance of transmission line + resistance of load equipment).

It is the load equipment (lights, motors etc.) that has most of the resistance, and therefore the load that determines the current through the lines, and not the lines themselves.

If the transmission lines provide a higher voltage, the load requires less current to get the same work done, and are designed accordingly. That lower current through the transmission lines means less power is wasted there as opposed to being used in the load.
barnaby - on 19 Nov 2013
In reply to Al Evans: Laudable answers all......but actually......the 'high voltage' lines are no such thing.....they have a tiny voltage end to end......the 'high voltage' is actually BETWEEN adjacent lines. The greater power delivery possible is therefore the voltage DIFFERENCE between the lines (huge) multiplied by the small current each carries.

The question is in many ways a 'trick'......a test of cognitive ability in theoretical physics.....or 'phenomenological orientation' as my master put it! Physics graduates with a first are offered 20k training bursaries to become teachers......90% of them when interviewed believe that a tennis ball flying over a net has more than one force acting on it (if spin is ignored).

They would probably puzzle over the plane on a running machine too.....but lets leave that can closed eh!
itsThere on 19 Nov 2013
In reply to barnaby: Ok so take V=IR this means voltage potential across a resistance will result in a current drawn. Constant voltage from an ideal power source gives a current drawn dependant on resistance. Lets forget about output impedance right now.

Power or P=IV or P=I^2 *R gives the power dissipated by heat from the resistance, for any current (remember we have a constant voltage).

When you put AC though a transformer you can up the voltage and reduce the current or up the current and reduce the voltage. Now we cant change the resistance but we can change the current.

The power from P=I^2 *R gives a lower power loss for lower current.
barnaby - on 19 Nov 2013
In reply to itsThere: All i was interested in was to see if anyone could explain why 'high voltage' meant low current. Youve told me what we can USE the effect for....thankyou for that egg to suck!
knthrak1982 on 19 Nov 2013
In reply to barnaby:
> (In reply to itsThere) All i was interested in was to see if anyone could explain why 'high voltage' meant low current. Youve told me what we can USE the effect for....thankyou for that egg to suck!

Your question wasn't a trick as far as I can see. And everyone has given you good answers. For a given power generated, a higher voltage means a lower current. P=IV
wintertree - on 19 Nov 2013
In reply to barnaby:

> 90% of them when interviewed believe that a tennis ball flying over a net has more than one force acting on it (if spin is ignored).

And your point is what, caller? Spin is not a force. There are however multiple forces acting on a tennis ball in flight -gravity, aerodynamic drag asymmetrically distributed over its surface, electrostatic interaction with solid objects - perhaps to weak to measure but likely to be there etc. etc.
Shearwater - on 19 Nov 2013
In reply to barnaby:
> 90% of them when interviewed believe that a tennis ball flying over a net has more than one force acting on it (if spin is ignored).

Were you discussing physicists tennis, as played in a vacuum with point masses instead of traditional balls?
stewieatb on 19 Nov 2013
In reply to Shearwater:

http://xkcd.com/669/

That said, you should see some of the hilarious approximations we make in engineering. Which we then design buildings and bridges based on.
Philip on 19 Nov 2013
In a recent Material World there was an article about DC vs AC. It seems DC is preferred for long distance and would be simpler for off-shore wind but the stumbling block was isolation. Switching high voltage DC under load is problematic whereas AC can be switched when the current passes through zero.
cuppatea on 19 Nov 2013
In reply to Al Evans:

With DC the waveform is all above the half way line.

With AC the waveform is both above and below the line.

Therefore AC uses the whole electricity pipe with no wasted space and costs less money to send to the houses and the Electicity Magnates (hehehe) can make more money.
DancingOnRock - on 19 Nov 2013
In reply to barnaby:

High voltage only means low current if the equipment (transformer) has high impedance. If you ground one end of the cable you'll certainly get high current.

Probably best not to touch them.

In your tennis ball question, why isn't the frictional air resistance considered a force? It slows the ball down.
DancingOnRock - on 19 Nov 2013
In reply to knthrak1982:
> (In reply to barnaby)
> [...]
>
> Your question wasn't a trick as far as I can see. And everyone has given you good answers. For a given power generated, a higher voltage means a lower current. P=IV

There is no power until you draw a current. The magnitude of the current depends on the resistance of the whole system.

The trick is recognising that V=I(r+R) where r is the cable resistance and R is the load impedance. V is big, r is small but R is also big. Technically as it's AC we should use z and Z but that would just confuse a lot of the audience.
Al Evans on 20 Nov 2013
In reply to DancingOnRock: Ok, simple (simplistic) question, in the Ripper Street episode the proponent of Dc had two sheep/goats in two iron cages, turned on the power supposedly equal to both cages and the one in the AC cage died, the one in the DC cage was unaffected, was this a con or is DC in some way safer than AC?
knthrak1982 on 20 Nov 2013
In reply to Al Evans:

I'm not sure if the 50Hz frequency can possibly upset the heart rhythm somehow. If so, then AC would be *marginally* more dangerous than DC.
However, what you saw was a con. Enough current will kill, AC or DC.
wintertree - on 20 Nov 2013
In reply to Al Evans:

> ... was this a con or is DC in some way safer than AC?

My understanding is this...

Electrocution can be different between the two - if you touch a source of current with, say, an arm, all the muscles in the arm contract rapidly. You typically have (big generalisation alert) two main muscles for any degree of freedom - one for "away" and one for "towards". Normally the "away" muscle wins, and so you flinch away from high voltage contact. The one - relevant to climbers! - exception is grip, where to "towards" is much stronger. If you grasp a live wire, your gripping muscles will win and you will hold on for dear death. Now with the alternating levels in AC, the muscles are not full on and you can drop it. Not so with DC where you will clench and stay clenched.

I have inadvertently tested this with 240V AC and have no interest what so ever in trying it with DC. I used to work in a lab back in 2000 that had "120V DC" screw terminals on the wall. Scary stuff.

I'm not sure that's got anything to do with the ripper street episode you mention.
knthrak1982 on 20 Nov 2013
In reply to wintertree:

Yeah, colleagues of mine who've had a jolt of both say dc is worse due to what you described.
The Ripper Street episode showed two caged animals so don't think it applies here.
krikoman - on 20 Nov 2013
In reply to Al Evans: Lightening is DC, so not safe :)

Death by electrocution is usually by burning and or suffocation depending on the current, so not the best way to go. There might be a small possibility that DC can have a delayed effect, so you may appear to be OK but die later, due to the electrolysis of some body fluids / tissues.
Mike Highbury - on 20 Nov 2013
In reply to Al Evans:
> was this a con or is DC in some way safer than AC?

He said that the DC current was not switched on. Livestock was too expensive to waste...
krikoman - on 20 Nov 2013
In reply to Mike Highbury:
> (In reply to Al Evans)
> [...]
>
> He said that the DC current was not switched on. Livestock was too expensive to waste...

It's usually very safe when it's not switched one.
cuppatea on 20 Nov 2013
In reply to Al Evans:

Were the foodthings in a Faraday Cage?
ads.ukclimbing.com
wilkie14c - on 20 Nov 2013
In reply to Al Evans:
Nothing of value here except my worthless thoughts....

1. I've always been a bit confused by the term AC/DC when describing a persons sexual orintation, as we know AC has a wave form that is equal on both sides of point zero, it swings both ways. Why then do we describe a bisexual person as AC/DC? We'd be better off naming them <if a name is needed at all> simply as an AC person as it shows they cross both sides of the line, and hetrosexual folk would then be known as DC.

2. It would have been far simpler if the band AC/DC named themselves EMF or Electro Motive Force. Naming themsleves both AC and DC implies some sort of switching or change going on but they have always sounded the same to me <on both AC and DC music devices> and havent changed at all.

3. This explaination is the greatest way ever devised to remember Ohms law:
http://electronicstheory.com/COURSES/ELECTRONICS/e101-8.htm

That is all.
Mick r - on 20 Nov 2013
In reply to wintertree: agree totally. this is the main reason we use 240v AC in homes, and not DC.

you can let go of AC. you can't with DC.

we would have a world wise shortage of electricians and DIY enthusiasts if we ran DC in the home.
Hairy Pete on 20 Nov 2013
In reply to wintertree:
> ( Now with the alternating levels in AC, the muscles are not full on and you can drop it. Not so with DC where you will clench and stay clenched.
>

Not quite true. It depends on the frequency of the AC. Very low frequency has the same effect as DC. As the frequency increase the maximum response occurs at around 50-70Hz, which, by pure coincidence happens to be the frequency of the mains. As the frequency increases further the muscles become less sensitive. At about 400Hz (I think - long time since I dabbled with this stuff), their sensitivity is once again similar to DC. Beyond 400Hz they become even less sensitive. Look at the work that D'Arsonval did - he was able to light bulbs by passing high-frequency through his body.
jkarran - on 21 Nov 2013
In reply to GrahamD:

> DC is just more efficient for very high power with modern technology which is why its also used for things like off shore wind farms

Wind turbines would be very hard to keep both maximally efficient and synchronized, rectifying the output makes sense so you can focus on the efficiency.

jk
jkarran - on 21 Nov 2013
In reply to barnaby:

> (In reply to knthrak1982) That may be true.....but doesnt answer the more 'basic' question I posed....!

Yes it does, read it again.

You're looking to transmit a fixed amount of power, that's the critical piece of information you're overlooking.

jk
jkarran - on 21 Nov 2013
In reply to barnaby:

> The question is in many ways a 'trick'......a test of cognitive ability in theoretical physics.....or 'phenomenological orientation' as my master put it! Physics graduates with a first are offered 20k training bursaries to become teachers......90% of them when interviewed believe that a tennis ball flying over a net has more than one force acting on it (if spin is ignored).

You can see why people would state that, it's easier to understand and analyse the problem if the force acting on an object is broken down either into more convenient set of vectors or into forces attributable to individual physical phenomena, gravity, air resistance, Lorentz... (why do you exclude spin?).

jk
highclimber - on 21 Nov 2013
jkarran - on 21 Nov 2013
that should I suppose be weight not gravity if we're being pedantic which I suspect we are :)
jk
mbh - on 21 Nov 2013
In reply to jkarran:
Yes.
The energy arrives at the power station at a certain rate, as you lob coal into the burner (or whatever it is you do, exactly), and a proportion of that leaves as electricity along the wires, at a lower rate determined by the efficiency of the power station. So the power injection into the grid is fixed until something at the power station changes. If the voltage across the terminals of the power station is raised, where we now include the step-up transformers as part of the power station, then the current drawn by the grid must fall.
joehobbs7 - on 21 Nov 2013
So I can't see this important issue being mentioned elsewhere, so I thought I'd chip in:

AC is most popular for transmission as it can easily be stepped up and down through the voltages, allowing high voltage transmission (and therefore low loss, to deliver a set amount of power, the higher the voltage the lower the required current and therefore losses).

For long lines though, there is not just the resistive loss to contend with, but also the inductance (http://en.wikipedia.org/wiki/Inductance for more depth). The inductance means that the timing of the peak current in the cycle lags the peak power, sapping the energy actually delivered at the end of the line. The peak current must therefore be increased to maintain the amount of power delivered, and this leads to higher losses.

In long lines, predominantly undersea cables, this inductance becomes unmanageable unless large capacitors are added to the line. As inductance is an AC only effect (voltage depends on the rate of change in current), we can avoid the inductive effect by switching to DC. There are losses switching AC-DC and back, but the transmission becomes more efficient.

joehobbs7 - on 21 Nov 2013
In reply to joehobbs7:
> The inductance means that the timing of the peak current in the cycle lags the peak power, sapping the energy actually delivered at the end of the line.

That should read:

The inductance means that the timing of the peak current in the cycle lags the peak voltage, sapping the power actually delivered at the end of the line.
jkarran - on 21 Nov 2013
In reply to joehobbs7:

> So I can't see this important issue being mentioned elsewhere, so I thought I'd chip in:

The power transmission problem is easier to understand and explain if you ignore skin effects and the non-real, reactive component of current. The result of both is basically just a bit more I^2R loss in the power lines anyway.

Very large grids exhibit some pretty strange behaviors. Despite the low frequency they can act, due to their enormous size as a distributed circuit* and one that changes it's properties as the environmental conditions change even down to birds roosting on cables!

*Google it, I've forgotten more than I ever knew about transmission lines and distributed circuits but they are interesting once you get your head around them.

jk
GrahamD - on 21 Nov 2013
In reply to jkarran:


> Wind turbines would be very hard to keep both maximally efficient and synchronized, rectifying the output makes sense so you can focus on the efficiency.

The energy will have to be converted to AC and synchronized to the net somewhere, though. That's not the reason for choosing to do that at the grid end rather than at source as far as i understand it - its all about transmission losses
jkarran - on 21 Nov 2013
In reply to GrahamD:

> The energy will have to be converted to AC and synchronized to the net somewhere, though. That's not the reason for choosing to do that at the grid end rather than at source as far as i understand it - its all about transmission losses

True but not having the synchronization requirement imposed on the turbine design/control can't hurt efficiency.

jk
itsThere on 21 Nov 2013
If there is a HVDC line and a fault occurs. The design of the DCDC transformer(i think its the transformer) prevents any damage, AC is removed from the system prevfenting a rise in voltage. You dont need any external control to turn the system off in a fault. Which would be the next transition in an AC system, so >10ms for it to react. The design of the trasformer does this, or at least i think it was the transformer that did this. Anyway HVDC is much better if a fault occurs compared to AC.
knthrak1982 on 21 Nov 2013
In reply to itsThere:

But AC circuit breakers are cheaper than ACDC or DCDC converters. In order to create a large transmission grid, you need lots of points at which to position these protective devices. DC is therefore good for long lines, not so good for grids.
wintertree - on 21 Nov 2013
In reply to jkarran:

> True but not having the synchronization requirement imposed on the turbine design/control can't hurt efficiency.

Given the highly variable nature of wind power over very short timescales it would be almost impossible to design a sane system that was grid synchronous - every time the wind dropped they'd turn from windmills into giant fans and unless you could feather the blades to reduce efficiency there would only be one wind speed at which the grid wasn't spending energy to either speed them up or slow them down...

I always assumed that there is some synchronous AC generator in the big turbines and that this goes to the grid via a DC link, but I don't actually know... Anyone?
knthrak1982 on 21 Nov 2013
In reply to wintertree:

I think that's correct, but aren't wind generators induction machines rather than synchronous?
jkarran - on 21 Nov 2013
In reply to wintertree:

Certainly on the big ones I'd assumed that there were field adjustment coils in the generator to control output voltage and variable pitch hubs (in conjunction with blade inertia) to maintain constant (or one of a few fixed) operating speed and synchronisation but it would be interesting to see how it's actually done.

The smaller ones do rectify the output then connect to the grid via an inverter.

jk
Donnie - on 21 Nov 2013
In reply to Al Evans: we do both!
ads.ukclimbing.com
itsThere on 21 Nov 2013
In reply to wintertree: Yes they feather the blades to get a set speed for the gear box, dont think any offshore turbines use a DC link yet.

Some info from a past uni project, Page 25
http://mhk.pnnl.gov/wiki/images/b/bb/Jonkman_et_al_2009.pdf
wercat on 21 Nov 2013
In reply to Mick r:

"you can let go of AC. you can't with DC.

we would have a world wise shortage of electricians and DIY enthusiasts if we ran DC in the home."


I don't think this was the case in the time before AC mains bacame the standard in the UK. It is very true that there were some rather dangerous domestic appliances still around from that time when I was a child - "Universal" mains radios that would operate from AC or DC mains at a variety of voltages and these often had a "live" chassis as they had no mains isolating transformer (as they had to work on DC). Such appliances were thus more dangerous to open up.

I know a great many people who have worked on AC mains equipment over the decades and noone thought that AC mains as less risky to work on. The old advice was to have a hand in your pocket or behind your back to avoid a shock passing from hand to hand through the thorax.

stewieatb on 21 Nov 2013
In reply to jkarran:

Most turbines use a "doubly fed induction generator", which allows the shaft speed to vary slightly around the synchronous speed. The blades are independently controlled to maximise power extracted from a given windspeed (up to a rated speed, typically 8-12m/s, beyond which a constant power is extracted). There is also a gearbox to take the hub speed (8-15RPM) up to a good generator rotor speed (~1000RPM).

The doubly fed system keeps the generator in sync with the national grid. A carefully designed combination of controlled feathering of the blades, back-torque from the generator, and (in extreme cases) braking ensures the generator is within range of its synchronous speed.

Yes, the blade inertia has a smoothing effect on the power output in gusty winds. Most onshore turbines cut out (feather blades) at ~25m/s gust speed. Offshore, where the wind is smoother and noise is less of a problem, you can increase cut out to 30m/s.
wintertree - on 21 Nov 2013
In reply to stewieatb and itsThere:

Fascinating, thanks.

This topic has been archived, and won't accept reply postings.