Wind turbines get bigger and bigger

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Short answer is : No it can't ... and the different matter involves first chopping-up the DC to make AC

MichaelG.

Whole bunch of pros and cons for AC and DC. Which is best depends on the circumstances.

It's more efficient to generate AC than DC and AC can be easily transformed up and down to suit the customer. This is important because energy losses are much lower in high voltage systems. It pays big time to increase volts to transfer energy over distance and drop it down to much safer (but wasteful) levels so people can use it without frying themselves. Powering your telly off 33,000Vac would be quite exciting.

AC has several disadvantages; electronics, electroplating, and many other processes need DC. Before inverters almost all high power variable speed motor (trains and lifts etc) had to be DC because it could be controlled. Most seriously, the current reversals in an AC cable are resisted by the wire, causing energy to leak and the flow to be impeded. The effect is small at 50Hz but it causes serious losses over long distances. One way of reducing the effect is to space the cables further apart, which is best done by hanging the wires off pylons. Another plus for pylons is that it's only necessary to insulate the cables at suspension points and air helps keep the wires cool. A buried cable brings the conductors very close together so their impedance rises, they have to be very well insulated throughout, and they're difficult to keep cool, ie less power can be transferred. On the other hand they're protected from the weather and they don't fall on people.

AC Cable losses become serious in overhead lines at about 500 miles. Underground AC cables become inefficient over much smaller distances, say 10 miles. Beyond those distances it rapidly becomes more efficient to convert the AC to extremely high voltage DC. DC current flows steadily in one direction and there are no reversals to impede the flow. It's not easy though: high voltage AC/DC conversions require techniques far more expensive than a simple robust transformer. For that reason you tend to find high voltage DC transmission used only for very long overland runs and for underwater lines such as the power feed between England and France.

Dave

The high voltage transmission cables (HVTC) are all DC. One can transmit power better for any given voltage using DC because the voltage remains constant, as well as the current. To transmit the same power by AC single phase for a start the peak voltage will be 1.414 times that of the DC and the current would need to peak higher by a similar amount. This is to counteract the times when the voltage is zero for the AC.

Consequently, the cable insulation for 400MV DC equivalent for a single phase AC would need to withstand 565 MV. Not an inconsiderable requirement. Would need to work out I^2 R losses, but should be equivalent, although the heat generated within a much more heavily insulated cable would downrate that cable. AC grid power is normally transmitted via 3 phase, but the same basic principles apply. Our Natinal grid, like others, uses uninsulated conductors - not very good for underground or underwater cables! - to keep losses due to temperature rise to a minimum.

Further problems would arise with synchronising the frequency between several different grids.

Edited: SOD started keying earlier than me - or does it more quickly!

Edited By not done it yet on 06/04/2018 18:42:10

Thanks, SillyoldDuffer and not done it yet - I can understand that, although my knowledge of electricity is still abysmal. My late father, who spent his working lifetime in the electricity supply industry, must turn in his grave when I try to express opinions on the subject!

Edited By Mike on 06/04/2018 18:41:38

With subsidy rates tumbling, it is no-longer about the subsidies. If you are stupid enough to rely on subsidies, you are basing a business plan on the word of a politician. Politician's words are worth the water they are written on. If there is a subsidy it needs to be the icing on the cake, with a sound business underneath. Subsidies can be withdrawn at a moment's notice - as they were for solar plants in the UK. The withdrawal of subsidies has not stopped solar farms, because they are economically viable without subsidies. There is a community funded one up the road from me, started after subsidy withdrawal, and most local residents are now getting cheaper power because of it, and the farm is still making money.

If it's all about making money, where is the harm in it. It's the way capitalism incentivises people. But relying on subsidies is the best way of destroying your business because it puts your business in the hands of those who have nothing to lose when the political wind changes.

Regards

Richard

Richard, I disagree with you. You are writting how things *should* work, unfortunately they are working entirely different and we are no longer living in capitalism.

Subsidies are extremely important for businesses now because they are one of last remaining lifelines for them.

So lets beging with our Western banking sector.

Do you really believe that it would still exist in current form without *subsidies* from taxpayer following Lehmann Brothers collapse?

Now look on all these European projects. There are subsidies, EU funds everywhere. Any green, less green and not green at all but politically well connected entity will get EU subsidies for some esoteric green projects.

The same holds true with entities promising innovation. There are countless subsidies there, owners and managers are getting rich enough to buy mansions, luxurious cars and porn star girlfriends but hardly any innovations are coming out of that. One time I wanted to buy an item from engineering company which is thriving on subsidies. Purchasing procedure was so complex that I have realized that I have incurred more cost on this company than any possible profit they could make by selling me ordered item. I have asked manager over the phone why it is so and he was surprisingly honest. I was advised that they are living off EU subsidies, they may not sell anything at all as long as they are keeping up with production of documents as required by Brussels and that it *is* their business model. I have asked "what then?" Answer was "Then we will be rich so we can close down or apply for grants in another project".

Doing business in an honest way is long time gone. Now we are left with fraud, ripoffs, corruption, subsidies etc. This is last show in town before lights go off.

Capitalism no longer exists.

Martin

Edited By Martin Dowing on 06/04/2018 22:25:11

My thanks to Dave for explaining this in simple terms. You have cleared up quite a few points which I did not understand. There is much talk here about building "Super Pylons" to transmit power from one place to another,and of course,no-one wants them in their back yard [or even their area ] Everyone wants them underground,but the E S B says that this would not work due to too high losses and heat problems. I understand now what they mean.

The only time I ever came across D C was many years ago when I worked on a 4 High Rolling Mill which was driven by a huge D C motor,supplied by a Mercury Arc Rectifier. It looked like something out of Dr Who,or Star Track ! These days the only place I see it used is for operating the tram system in Dublin [750 V D C ]

I never knew about the problem of AC reversing in the lines,never stopped to think about it,but of course it must,it does it all the time,by it,s very nature. Sometimes one can,t see the wood for the trees.

Thank you again for taking the trouble to explain