Do the Math..

[SuzyC]

Does the USA know Ohm's Law?    Loss in Watts= I * I * R     W=(V*V)/R

Has Edison proved Tesla wrong after all these years?

In the news today..
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An effective transmission solution requires the appropriate technology. The Grain Belt Express Clean Line will deliver approximately 4,000 megawatts of low-cost wind power from western Kansas to Missouri, Illinois, Indiana and neighboring states that have strong demand for clean, reliable energy. The clean energy will be transported via an approximately 780-mile overhead, direct current (DC) transmission line.

DC is the most efficient and cost effective technology to move large amounts of power over long distances, due to its lower electricity losses and smaller footprint than comparable alternating current (AC) lines.
------------------------------

But what really astounds me is how did  they find a Buck Converter chip that can handle this kinda power at the other end of the line!

Highest HV DC transmission lines operate at 765KV DC.

What is the DC resistance of this overhead (not a superconductor) 780-mile x 2 wires transmission line having this length?

And the Power Loss is....._______________Watts?

What would the Magnetic Field created be (in comparison with the Earth's) in the nearby?

[Andy Watson]

But what really astounds me is how did  they find a Buck Converter chip that can handle this kinda power at the other end of the line!

Dunno! but the panels labelled ICT (Inter connects?) in the link below are usually DC links - because it avoids the necessity to synchronise the AC frequency between countries. So the technology is possible.

http://www.gridwatch.templar.co.uk/

What is the DC resistance of this overhead (not a superconductor) 780-mile x 2 wires with the length?

I guess that it's not that different from the resistance for AC - but without the capacitive and inductive elements.. Presumably they are pushing the DC voltage as high as possible to make this thing viable?

 
edit: there's a French one too: http://www.gridwatch.templar.co.uk/france/

[SuzyC]

My Dear Watson: Believe it or not, any wires, even a pair of wires strung over 780 miles, has inductance.

But no one yet has done the math!

Watts the problem, Watson?

Does the magnetic field created cause each of the two wires to attract each other to create arc-over or short circuits or do they repel each other?

[Seekonk]

The wiring is limited by the peak volts whether AC or DC.  Just think about how much area under the curve is wasted with AC.  You can get more power with DC over the same infrastructure.

[CatalinaWOW]

This Wikipedia article gives a reasonable summary of how it is done, and even mentions an existing line of comparable power and double the length.

https://en.wikipedia.org/wiki/High-voltage_direct_current

 In that article they claim losses for this type of line in the 3% neighborhood for a line of this length.  The widespread use of this type of line would imply that number is right.

Doing the math with this number then says losses are in the 12 megawatt range.  Big number, but it is spread over 760 miles.  Since the current in a 765KV line would be in the 5000 amp range, it implies the resistance must be quite low.  R=P/I/I~160 ohms.  This implies wires on the upper end of the range quoted for such lines (750 square mm).  The numbers all rough out within reason.

[skipjackrc4]

I have a feeling that HVDC is used to remove the losses that are caused by skin effect (AC resistance proportional to sqrt(frequency)).

[SuzyC]

CatalinaWow,


Wow!, you are 20 miles too short!

Ok, but still: 160-ohms/(760*5280*12)=3.227 micro-ohms/in

.000003227-ohms/in ..that's really low resistance per inch.

And also this means for 4000 megawatts, approx. 83-watts of power dissipated per inch.

That means the cable(even if was square-shaped with an area of 750 mm^2) would have a width/height of only 27.386 mm..what material would yield 3.227 u-ohms per inch with these tiny dimensions?

Why does this seem counter-intuitive to me..the diameter of the battery cables in my car are thicker than that, but the maximum current capacity for a battery cable is not rated or able to handle 4000 amps continuously..or am I wrong?
 
I have seen battery cables melt their plastic insulation and catch on fire in seconds when short-circuited. Can a car-battery deliver 4000-amps?

[CatalinaWOW]

I haven't gone back to bulk resistivity to check that math, but these resistances are in line with what is reported in the data sheets for aluminum cable with a steel core of the type used in power line construction.  In the US traditional units are still widely used for this and resistances are usually reported in ohms per 1000 feet, with wires often given in circular mils area.  The largest cables used for these large lines are about one and a half million circular mils.  I haven't seen tables that go up to that size, but at 100,000 circular mils the resistances are something like 0.16 ohms/1000 feet.   Rough numbers that is 0.08 ohms/mile for the 15 times larger cable.  So the numbers are not wildly wrong.

Yes it is a low resistance, but it is really big wire, and since it is DC, it all participates.

[SuzyC]

Wow! Are you doing the math?

You say 160 ohms?

((780 miles * 5280 ft )/1000-ft)*.16-ohms=659-ohms
(780 miles *5280*.08-ohms)/1000-ft=329.5-ohms for the 15x larger cable for each wire(but there are two wires, not just one)?

[CatalinaWOW]

These numbers are within a small factor.  If you read the Wiki article on transmission lines it mentions that they are configured in a variety of ways, including multi-conductor configurations.  I am sure that the people designing these lines do go through the numbers in detail, more than someone sticking his nose in over the internet.  They have options, such as multiple conductors, commissioning an even larger conductor  or going to an even higher voltage.  There are other areas of wiggle room.  Maybe the line will be designed for only 3550 MW and the PR people rounded that up to 4000 MW.  Maybe they expect peak power transfer to occur during cold weather when resistance is down.

The numbers show that they are not attempting to violate laws of physics or even simple electrical calculation laws.  The fact that they are getting this project financed indicates that the costs associated with making the conductors large enough are acceptable.  The fact that technology to convert high voltage DC to locally distributable AC current for 70+ years trumps the fact that Mouser and Digikey don't stock 765kV modules.

If you want to take issue with their design you will have to go into a very great deal of detail, identifying construction costs, technical risks and a whole list of other things about the complete project.  They will do it in such detail that things like worst case temperature of the line on the temperature coefficient of resistivity, and weather erosion of the wires over the projected life of the line are taken into account.  Typically doing this takes a fairly good size team years to complete, and the decisions between one technology and another may be based on differences of only a couple of decimal places.

[IanB]

Also remember that the stress on the insulators in a transmission line is the potential difference between the cable and ground. If you have a DC transmission line you can have bipolar conductors at +X volts and -X volts relative to ground at 0 V, giving an effective transmission voltage of 2X volts with only X volts across each insulator. Doubling the voltage for a given power allows you to halve the current, when means the resistance losses are reduced by four. This is one of the fundamental reasons that a two conductor DC line is competitive with a three conductor, three phase AC line for a given duty.

[IanB]

But what really astounds me is how did  they find a Buck Converter chip that can handle this kinda power at the other end of the line!

The AC/DC converter modules look a bit like this:

[SuzyC]

Wow!  Don't lose your cool over a few Watt-evers!

IanB..isn't this an AC transformer? I see three super HV input connections that would correspond to multi-phase AC.

 How do they commutate approx. 765KV or maybe even 1.2MV DC to get 120VAC to power my toaster?

I would just like to know, what would be a proper(optimized) diameter of the wire to be used to achieve approx 3% loss for 4000MegaW carrying DC?

And what would be the magnitude of the magnetic force?

Here's the thing. Birds migrate and sense the compass magnetic fields to determine their path. They might get lost.
People are affected by magnetic field exposure. High (DC generated) magnetic fields have been proven to cause the human brain to show pronounced changes in brain functions, including inhibition, demonstrable changes in memory, and other changes affecting emotional and rational behavior.

I am worried, people sometimes lose their moral compass.

[IanB]

Well, magnetic fields are proportional to the current and independent of the voltage. So any power line of any voltage is carrying roughly the same current, whether it be 7 kV or 700 kV, and therefore generates the about the same magnetic field.

[Nerull]

It's a good thing they have someone incapable of stringing together a coherent thought to fix their moral compass for them, demanding that other people do calculations for them. I mean, if you did the math, it might not say what you want it to say, right? You can always claim other people are doing it wrong, but a true crank never risks proving themselves wrong.

[SeanB]

Commutation is easy, just get 10 000 1kV SCR units, each with an optical triggering input, and stack them in series. Then use a really big semiconductor laser to provide the 100W or so of power required to drive those 10k fibre ends all at once, or simply split the load over a few smaller lasers and include some protection so a failed laser does not cause a cascade failure ( really bad at this power level, really not nice to be in the same building as bad), and duplicate 11 more times to get a three phase switch and commutation switch.  12 switches because you want to reduce the switching losses, and doubling the size of the transformer and the extra iron and copper is a very small part of the build cost to get a single pair of SCR and commutating SCR per transformer tap. Just need some small 10MV capacitors, some small resistors ( only a few meters long, so small) and a sodding big transformer, then some low voltage ( only 150kV rated) capacitors and inductors on the input/output side for filtering.

Incidentally most HVDC lines are a single bundle on the pylon, typically here a rope of 6 large aluminium sheathed cables that are held suspended by a very long glass insulator pair. They have regular spacers along the line so that there is little whipping in the wind, and a set of dampers each side of the suspension point ( those things that look like dumbbells sticking out of the line) so that the cable does not build up energy from resonance.

The return path is from a massive buried earthing mat at each station, so consider that as well in the line loss, it has to be massive so that you do not get a massive ground potential difference coming off the mat, and large enough that it couples to the local ground with low resistance. Requires regular watering in dry conditions or you start having large losses.  Here there are 2 HVDC lines, one 200kV positive and the other 200kV negative, but they often run in reduced power mode ( failed line, blown up by war, no technicians to maintain the one end) so use the earth return a lot.

Also consider HVDC lines are inherently bi directional, they use the same stations at each end, as synchronous rectification is lower loss than a diode stack, and allows quicker turn off on high voltage trips, you can turn off before the end of the cycle by force commutating the active stacks much faster than the mechanical switches can start moving to do a physical breaking of the line, complete with the high voltage arcing that might take a second or two to break the current.

[SuzyC]

Thanks Wilfred and SeanB, I read the .pdf and it seems to somehow show me how a much smaller Ausssi-Tasmania transmission line can work.

Every day or so someone on this forum has a question about their LM317  1.2-15V 1.2A bench power supply.

Forgive me, I am ever more fascinated by startling news of somewhat bigger switching power supplies.

Is it wrong to think big on this forum or mention math?

When I read in the news that somehow a D.C. transmission line is going to reliably and efficiently and safely carry 4000 MegaWatts of power over 780 miles, resist lightning strikes and surges..well then, I really would like to know how in the world can this be done.

I still have trouble wondering how they manage to parallel or series thousands of wind turbines, what way do they use to convert their outputs to synchronize them and rectify their individual outputs to create about a million volts DC at the Oklahoma end and then manage to transmit this power over 780 miles and convert it back to 60-Hz smooth AC at the other ends in distant US states and then even use this power to synchronize our wall clocks.

And what about short-circuits or lightning surges and switching source and destination distribution nodes on or off? Even the Tasmania-Aussie line has gone down.

To me, the real heroes of this world are the unmentioned few engineers that make it possible to have hot showers that work, toilets that flush, sewers that work,  clean water at the turn of the tap, lights that stay on 24-7 so that we have reliable power in every house, so even someone like me can make toast in the morning and play with my LM317 PS at night.

[Kalvin]

OP's arguments reminds me of the case where some scientists proved that bumblebees cannot fly.

[Jeroen3]

Wind turbines use asynchronous generators. There is no synchronisation involved, this happens automatically.
If the wind drops a bit, the process reverses to keep it in sync for when the wind picks up again, making a huge fan.

If you know the parties involved you can find a lot of sources.
http://new.abb.com/docs/default-source/ewea-doc/hvdc-light.pdf

[CatalinaWOW]

The tenor of your original post suggested that someone did the math incorrectly.  Saying something like-"I don't understand how such high power systems work.  Can anyone fill in this or that blank?" would have received a different response.

Of course, a few seconds of googling would have gotten much of the information.  At this point in time your best approach to getting information about how a system of almost exactly the same size works would be to do some googling specifically on the Brazilian system mentioned in the Wiki link I referenced.  Even if it requires using google translate to get stuff written in Portuguese, although I suspect there is lots of English material in this system.

Your follow up question on whether such systems are ethically correct is more complex.  If you are going to have 300 million people getting hot showers, things have to be done on a very large scale, which will have impacts.  The only fundamental solution to such problems is to have far fewer people.  Which raises other ethical questions.

[Richard Crowley]

I used to live at the south end of the Pacific DC Intertie and now I live at the north end. They convert three phase 60 Hz AC at 230 to 500 kV to ±500 kV DC (1000 kV pole-to-pole) at 6200 Amps (by my calculation).

The northern converter station is at Celilo, just over the hill from The Dalles where there is a large Google server farm right on the Columbia River.  The southern end is the converter at Sylmar right on Interstate highway #5 and just south of the giant above-ground pipes that bring water into the Los Angeles basin.  The converter stations (Celilo and Sylmar) are reversable. So we send hydro power from Oregon down to LA in the summer to keep them cool, and they send power up to us in the winter to keep us warm.

The earth ground connections are fascinating. The northern one is 1067 cast-iron electrodes buried in a 2 mile circle under a rice field. The Sylmar grounding system is a line of 24 silicon-iron alloy electrodes submerged in the Pacific Ocean at Will Rogers State Beach suspended in concrete enclosures about one meter above the ocean floor. Connection between the converter stations and the ground arrays uses conventional overhead wires on towers. So the conductors are carrying thousands of amps at essentially zero voltage.

High power DC transmission was developed back in the 1930s in Sweden and Germany. I suspect they did all the calculations correctly as there are scores of these systems current running around the planet and utilities and governments have billions invested in the technology.

https://en.wikipedia.org/wiki/Pacific_DC_Intertie
https://en.wikipedia.org/wiki/High-voltage_direct_current

[Richard Crowley]

We have been producing reliable 7x24x365 power from several big-hydro dams on the Columbia River for decades. (And selling surplus to Los Angeles). But try driving up the Columbia Gorge on a random day. You will see hundreds of windmills and only 10% of them are ever turning. Seems like a colossal waste of billions of $$$.

[Kalvin]

We have been producing reliable 7x24x365 power from several big-hydro dams on the Columbia River for decades. (And selling surplus to Los Angeles). But try driving up the Columbia Gorge on a random day. You will see hundreds of windmills and only 10% of them are ever turning. Seems like a colossal waste of billions of $$$.

Hey, it is free energy, don't complain. Within next thousand years it has paid back. Maybe a bit sooner, like within two hundred years, with a good wind. Anyway, it seems that only 10% of the windmills are actually hooked to the grid in order to get the energy to make the turbines turn.

[rsjsouza]

Years ago I visited the Itaipú dam in Brazil and the HVDC converter station. Details are in an old post of mine.
https://www.eevblog.com/forum/chat/largest-value-passive-components-in-existance/msg433185/#msg433185

[technix]

Commutation is easy, just get 10 000 1kV SCR units, each with an optical triggering input, and stack them in series. Then use a really big semiconductor laser to provide the 100W or so of power required to drive those 10k fibre ends all at once, or simply split the load over a few smaller lasers and include some protection so a failed laser does not cause a cascade failure ( really bad at this power level, really not nice to be in the same building as bad), and duplicate 11 more times to get a three phase switch and commutation switch.  12 switches because you want to reduce the switching losses, and doubling the size of the transformer and the extra iron and copper is a very small part of the build cost to get a single pair of SCR and commutating SCR per transformer tap. Just need some small 10MV capacitors, some small resistors ( only a few meters long, so small) and a sodding big transformer, then some low voltage ( only 150kV rated) capacitors and inductors on the input/output side for filtering.

Incidentally most HVDC lines are a single bundle on the pylon, typically here a rope of 6 large aluminium sheathed cables that are held suspended by a very long glass insulator pair. They have regular spacers along the line so that there is little whipping in the wind, and a set of dampers each side of the suspension point ( those things that look like dumbbells sticking out of the line) so that the cable does not build up energy from resonance.

The return path is from a massive buried earthing mat at each station, so consider that as well in the line loss, it has to be massive so that you do not get a massive ground potential difference coming off the mat, and large enough that it couples to the local ground with low resistance. Requires regular watering in dry conditions or you start having large losses.  Here there are 2 HVDC lines, one 200kV positive and the other 200kV negative, but they often run in reduced power mode ( failed line, blown up by war, no technicians to maintain the one end) so use the earth return a lot.

Also consider HVDC lines are inherently bi directional, they use the same stations at each end, as synchronous rectification is lower loss than a diode stack, and allows quicker turn off on high voltage trips, you can turn off before the end of the cycle by force commutating the active stacks much faster than the mechanical switches can start moving to do a physical breaking of the line, complete with the high voltage arcing that might take a second or two to break the current.

SCR-based conversion halls cannot cold start a grid after a total blackout.

In Shanghai Minhang HVDC Conversion Station, eastern terminus of the HVDC line from the Three Gorges Dam, one of the several converters uses optically-fired IGBT instead of SCR. This allows this station being used as the sole cold start source for the entire city, as the old one, Nanshi Power Plant, a coal-fired plant sitting right next to the city center, was decommissioned (the building ts still there as it is a historical site, being the first power plant of China) before 2010 Shanghai Expo. Should Shanghai suffer from a total blackout, this HVDC conversion station bootstraps the city grid, and pulls Qingshan Nuclear Power Plant (the second most powerful power source of the city after this HVDC conversion plant, backed by the dams) online