Do the Math.. - Page 1
Forum section: electronics / projects

author: SuzyC
author_rank: Frequent Contributor
timestamp: March 25, 2016, 10:46:46 pm
content: Does the USA know Ohm's Law? Loss in Watts= I * I * R W=(V*V)/RHas Edison proved Tesla wrong after all these years?In the news today..----------------------------An effective transmission solution requires the appropriate technology. The Grain Belt Express Clean Line will deliver approximately4,000 megawattsof 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 approximately780-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 ishow did they find a Buck Converter chipthat 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?

author: Andy Watson
author_rank: Super Contributor
timestamp: March 25, 2016, 11:25:51 pm
content: Quote from: SuzyC on March 25, 2016, 10:46:46 pmDunno! 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/QuoteI 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/

author: SuzyC
author_rank: Frequent Contributor
timestamp: March 25, 2016, 11:43:02 pm
content: 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?

author: Seekonk
author_rank: Super Contributor
timestamp: March 26, 2016, 01:18:23 am
content: 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.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 26, 2016, 01:23:45 am
content: 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_currentIn 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.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: March 26, 2016, 02:19:57 am
content: I have a feeling that HVDC is used to remove the losses that are caused by skin effect (AC resistance proportional to sqrt(frequency)).

author: SuzyC
author_rank: Frequent Contributor
timestamp: March 26, 2016, 02:27:22 am
content: 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?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 26, 2016, 02:46:21 am
content: 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.

author: SuzyC
author_rank: Frequent Contributor
timestamp: March 26, 2016, 03:10:08 am
content: 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-ohmsfor the 15x larger cable for each wire(but there are two wires, not just one)?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 26, 2016, 03:33:16 am
content: 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.

author: IanB
author_rank: Super Contributor
timestamp: March 26, 2016, 03:56:37 am
content: 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.

author: IanB
author_rank: Super Contributor
timestamp: March 26, 2016, 04:07:32 am
content: Quote from: SuzyC on March 25, 2016, 10:46:46 pmThe AC/DC converter modules look a bit like this:

author: SuzyC
author_rank: Frequent Contributor
timestamp: March 26, 2016, 04:07:44 am
content: Wow! Don't lose your cool over a few Watt-evers!IanB..isn't this anACtransformer? I seethree super HV input connectionsthat 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.

author: IanB
author_rank: Super Contributor
timestamp: March 26, 2016, 04:15:14 am
content: 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.

author: Nerull
author_rank: Frequent Contributor
timestamp: March 26, 2016, 07:15:21 am
content: 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.

author: SeanB
author_rank: Super Contributor
timestamp: March 26, 2016, 07:43:44 am
content: 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.

author: SuzyC
author_rank: Frequent Contributor
timestamp: March 26, 2016, 10:21:04 am
content: 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.

author: Kalvin
author_rank: Super Contributor
timestamp: March 26, 2016, 01:13:02 pm
content: OP's arguments reminds me of the case where some scientists proved that bumblebees cannot fly.

author: Jeroen3
author_rank: Super Contributor
timestamp: March 26, 2016, 02:15:06 pm
content: 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

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 26, 2016, 03:01:27 pm
content: 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.

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 26, 2016, 03:45:02 pm
content: 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_Intertiehttps://en.wikipedia.org/wiki/High-voltage_direct_current

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 26, 2016, 03:59:02 pm
content: 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 $$$.

author: Kalvin
author_rank: Super Contributor
timestamp: March 26, 2016, 04:15:44 pm
content: Quote from: Richard Crowley on March 26, 2016, 03:59:02 pmHey, 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.

author: rsjsouza
author_rank: Super Contributor
timestamp: March 26, 2016, 05:26:47 pm
content: 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

author: technix
author_rank: Super Contributor
timestamp: March 26, 2016, 05:48:00 pm
content: Quote from: SeanB on March 26, 2016, 07:43:44 amSCR-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

author: SeanB
author_rank: Super Contributor
timestamp: March 26, 2016, 08:22:23 pm
content: Many power stations as well have no cold start capability, many of the larger hydro plants did eventually install some cold start capability after a few large blackouts in the world. In many cases the switching stations have no cold start capability if the blackout lasts longer than the local battery backup can operate the SCADA systems, and there is air to power the switch yard. If the blackout is too long they have to bring in a generator and do a cold start to at least close the incoming breakers for local plant power.

author: technix
author_rank: Super Contributor
timestamp: March 27, 2016, 05:55:21 am
content: Quote from: SeanB on March 26, 2016, 08:22:23 pmFor the city of Shanghai the cold start is this HVDC conversion station. Three Gorges can cold start itself.

author: grochomark
author_rank: Newbie
timestamp: September 23, 2016, 04:02:40 pm
content: Just a couple of items, HV/DC is produced from AC and step up transformers and then rectified into DC for transmission (so Edison was still wrong). at the other end there is a giant inverter to produce 3 phase, 60Hz AC for distribution (similar to the pictures in this thread) see ink below...http://www.abb.com/cawp/seitp202/5685eb71f1afdf3085257ef20049e5b0.aspx

author: rstofer
author_rank: Super Contributor
timestamp: September 23, 2016, 07:00:39 pm
content: 4000 MW / 765 kV => 5200 AmpsI'm not sure what utilities use for ampacity but, for industrial use, 1000 MCM is only good to 600 Amps so we would need 9 strands on each side of the DC circuit.The resistance of bare 1000 MCM is 0.011 Ohms/1000 feet and we have 9 strands to about 0.001 Ohms/1000 feet. 750 miles would be about 8 Ohms and the loss would be 200 MW or about 5% (down and back)My guess is that they will use more than the equivalent of 9 strands of 1000 MCM. It comes down to cost of wire versus cost of MWh.Transmission losses are already in the 4% range:http://blog.schneider-electric.com/energy-management-energy-efficiency/2013/03/25/how-big-are-power-line-losses/HVDC transmission has been done for many years. There is a 2000 MW link between France and the UK that runs at 200 kV and was installed in 1986. This is really old news!https://en.wikipedia.org/wiki/List_of_HVDC_projectshttp://patelnishant5616.blogspot.com/2014/02/advantages-disadvantages-of-hvdc.html

author: rstofer
author_rank: Super Contributor
timestamp: September 23, 2016, 07:10:15 pm
content: These projects were all economically viable at the time they were built or they wouldn't have been built. The utilities have a pretty good handle on the losses of transmission for AC and DC circuits. They certainly wouldn't go to the expense of rectifying and commutating DC if they could deliver the power cheaper with AC.BTW, that 4000 MW is larger than the Palo Verde Nuclear Power Plant (3.3 GW or 3,300 MW) in Arizona.https://en.wikipedia.org/wiki/Palo_Verde_Nuclear_Generating_StationPalo Verde is the largest nuclear power plant in the US based on generation.

author: commie
author_rank: Frequent Contributor
timestamp: September 24, 2016, 09:43:03 pm
content: I haven't read all the posts but the reason ac transmission is prefered over dc is that ac will go through a transformer and so, at the power station, they step up the voltage to 11kV or so using a transformer whose output approximates an ac current source.What this means is the current in the power transmission lines is tiny and relatively constant producing very low copper losses.When the electricity arrives at the houses,etc they simply transform it from 11kV back to 240V ac(U.K.) using nothing more than a transformer. So you see, dc transmission isn't viable.

author: Monkeh
author_rank: Super Contributor
timestamp: September 24, 2016, 10:51:41 pm
content: Quote from: commie on September 24, 2016, 09:43:03 pmAt the time you posted that we were receiving around 1.3 GIGAWATT via DC transmission.

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 24, 2016, 11:11:44 pm
content: Quote from: commie on September 24, 2016, 09:43:03 pmFor "retail" local distribution and short- and medium-distance transmission that is currently true*. However, DC is provably superior for special conditions like long-distance (where the length of the transmission line approaches the AC wavelength) or where the frequency (or even phase) of the AC at the two ends are different, etc.* One can imagine a world in the future where high-power semiconductor technology will be cheaper than all that steel and iron and copper hanging around on all those power poles, etc.

author: helius
author_rank: Super Contributor
timestamp: September 24, 2016, 11:24:47 pm
content: Quote from: commie on September 24, 2016, 09:43:03 pmI'm sitting here laughing that you used "transmission" and 11kV in the same sentence. 11kV is used for local distribution, transmission uses 50kV and up. Transmission \$ \neq \$ distribution.

author: IanB
author_rank: Super Contributor
timestamp: September 25, 2016, 09:25:59 am
content: Quote from: commie on September 24, 2016, 09:43:03 pmA few things that you didn't get right about the UK electrical grid:- Power is typically generated at a higher voltage than 11 kV (for example around 25 kV)- It is stepped up for transmission across the national grid at voltages of 275 kV or 400 kV- The output of generators and transformers is a voltage source--the voltage is regulated, the current varies with power demand- The current in transmission lines is not that tiny; it could be thousands of amps- Losses in the transmission grid are "low", maybe 2 - 3%, but this is still more than 1000 MW!- The UK has several HVDC transmission links, to other grids in France, the Netherlands and Ireland- Voltages in in the UK grid are based on multiples of 11 kV:-> Transmission: 400 kV, 275 kV, 132 kV-> Distribution: 132 kV, 66 kV, 33 kV, 11 kV-> 11 kV is usually the last link in the chain before being stepped down to 240 VThe above is only a simplified summary of a large and complex system.

author: commie
author_rank: Frequent Contributor
timestamp: September 25, 2016, 02:33:52 pm
content: Jeez guys, gimme a break...If dc transmission is to be used then how exactly are you going to step up/down, oh wait, you are going to use bjt's and opamps yeah?Right, the fact that they step it up is for what reason?, to play tiddlywinks yeah?, no its to make the current (during transmission) as small as possible and why do they do that?, because power = I^2.R so making the current small i.e., stepping up the ac voltage reduces the transmission losses.

author: rsjsouza
author_rank: Super Contributor
timestamp: September 25, 2016, 02:46:00 pm
content: Back in my university days, one very knowledgeable power distribution teacher told us that the only application where HVDC transmission lines were king was in underwater links - much less overall losses when compared to AC. However the calculations at that time (mid 90's) also involved the efficiency of the inverters. This single factor may have changed since them.

author: dom0
author_rank: Super Contributor
timestamp: September 25, 2016, 03:16:42 pm
content: Quote from: commie on September 25, 2016, 02:33:52 pmHVDC is older than op amps, or transistors. One of the first viable systems was built in Nazi-Germany and later moved to the Sovjet Union. It was based on mercury arc valves.I'm surprised how many people here don't seem to understand that HVDC has been used for decades at scale, and how little it has to do with the 230 or 115 V coming from the sockets in your homes. It appears many here are not at all aware that there are power electronics beyond TO-220?

author: SeanB
author_rank: Super Contributor
timestamp: September 25, 2016, 03:34:09 pm
content: Your typical power line runs at full load at around 1kA per conductor, and a typical voltage drop at that load is around 100V per kilometer. Want to increase power capability you either go up in voltage, or you add extra conductors to the existing, making a bundle that can carry a higher current, but with the individual cores being separated with spacers so they can be cooled with natural convection, to keep the temperature rise at full load within the sag allowance. You can run the conductors at a much higher current, but the heating and expansion means the conductors might get low enough to arc over tolocal vegetation, which has caused at least one major US blackout from the cascading trips after the one heavily loaded line tripped out after flashing over to a tree at full load.The main advantage of HVDC is that the power transferred is simply V times I, not like AC where the voltage is limited to 70.7% of the peak ( RMS voltage versus the peak half cycle voltage) and thus the power transferred is 70% of the capacity, all else being the same.

author: Monkeh
author_rank: Super Contributor
timestamp: September 25, 2016, 03:38:07 pm
content: Quote from: commie on September 25, 2016, 02:33:52 pm'is to be used'? HVDC transmission IS used. In multi-gigawatt scale all over the world.There is an active 270kVDC 2GW link between the UK and France.There is an active 250kVDC 250MW link between Scotland and Northern Ireland.There is an active 450kVDC 1GW link between the UK and the Netherlands.There is an active ±200kVDC 200MW link between the UK and Ireland.And to put all those to shame, there is an active 800kVDC 8GW link in China. Coming online next year, a 1.1MVDC (yes, MEGAVOLT) 10GW link in China. That one will be 3,333km long.Obviously DC transmission isn't viable, that's why China's spending more money than you can comprehend building a 10GW DC transmission system.And no, you don't use "bjt's and opamps", you use massive fucking thyristors.Here's some thyristor arrays from a ±350kVDC 1.2GW link in New Zealand:

author: SeanB
author_rank: Super Contributor
timestamp: September 25, 2016, 03:52:35 pm
content: Building each one of those stacks must take the entire month's supply from the manufacturer of those high voltage SCR units. They build them very conservatively, and test them very thoroughly, so that you have a very reliable block, and you always keep a spare locally so you can quickly swap in a good spare if one of them starts to show degradation.

author: dannyf
author_rank: Super Contributor
timestamp: September 25, 2016, 04:12:25 pm
content: Quoteyou can benefit from reading a little bit more and talking a lot less.QuoteHaving read that, i'm pretty sure that time machines were operational before the 1800s.

author: commie
author_rank: Frequent Contributor
timestamp: September 25, 2016, 04:26:21 pm
content: still nobody has answered my question as to how you go about raising and lowering dc electricity with 90% efficiency?.Remember chaps we need to go from 240Vdc to 1000000Vdc and back again, what do have in mind?Also, I need a vacuum cleaner that works of dc, anyone?

author: Monkeh
author_rank: Super Contributor
timestamp: September 25, 2016, 04:30:54 pm
content: Quote from: commie on September 25, 2016, 04:26:21 pmDo, we don't. And you should take a look at the efficiency figures on modern switch mode supplies some time.ps. 90% is hilariously low for distribution, let alone transmission.QuoteYou are once again confusing consumption, distribution, and transmission..

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 25, 2016, 04:31:14 pm
content: Wow, the level of ignorance around here is sometimes astounding. HVDC transmission of utility electrical power was developed back before any of us were born (1930s) in Sweden and Germany. There are scores of HVDC transmission lines, both land and undersea all over the planet. There is a 3.1GW line not far from where I live now (Portland, Oregon) which links to where I used to live (Los Angeles, California).I am equally astounded at people who repeatedly flaunt their gross ignorance in public and never bother to do the most trivial research. Apparently they enjoy looking foolish in public.You could start here:https://en.wikipedia.org/wiki/High-voltage_direct_current

author: technix
author_rank: Super Contributor
timestamp: September 25, 2016, 04:32:26 pm
content: Quote from: Monkeh on September 25, 2016, 03:38:07 pmThat 1.1MVDC link runs from Three Gorges to Shanghai, the city I live in. It is in the news for quite a while already. HVDC is much, MUCH more efficient than AC. (For those that is worried about black starting, Shanghai's entire grid can be started using the HVDC conversion station since it have PWM communication and GTO switching support.)

author: rstofer
author_rank: Super Contributor
timestamp: September 25, 2016, 04:36:25 pm
content: Quote from: commie on September 25, 2016, 02:33:52 pmIn Reply 31 I posted a link to a document listing DOZENS of operating HVDC links with more coming on line this year and next. This stuff has been around since 1965 when a HVDC link was built in New Zealand. So, about 50 years or so!Old news...

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 25, 2016, 04:39:27 pm
content: Remember that back before we had high-power semiconductors (or high-power firebottles, either) there was the "War of the Currents" back when they were scheming how to distribute electricity as a utility.Thomas Edison was pushing for DC distribution, but that was completely impractical on a large scale back in the 19th century. OTOH, George Westinghouse was a proponent of AC distribution which allowed economical high-voltage transmission, and easy conversion to lower voltage for distribution and "retail delivery". Clearly the Westinghouse plan was the practical one and AC distribution is essentially universal around this planet.Ref:https://en.wikipedia.org/wiki/War_of_CurrentsBut we are in the 21st century now, and high-power semiconductors are practical for large-scale and widespread utility applications. As I said in a previous post here, it is not difficult to imagine a future where we could see DC distribution in the "last mile" and the end of those big "pole pig" transformers at the top of wooden power utility poles.

author: commie
author_rank: Frequent Contributor
timestamp: September 25, 2016, 04:51:13 pm
content: Quote from: Monkeh on September 25, 2016, 04:30:54 pmStill waiting for a reply on how we step up/down d.c. voltages up to 1000000V??Quote from: Monkeh on September 25, 2016, 04:30:54 pmLast time I checked power transformers, both for transmission and distribution were about 90% efficient.Quote from: Monkeh on September 25, 2016, 04:30:54 pmI don't think so, but we will talk about how we convert 1000000V to 240Va.c. a little later but first we need the d.c. equivalent of an a.c. transformer?

author: rstofer
author_rank: Super Contributor
timestamp: September 25, 2016, 04:57:10 pm
content: Quote from: commie on September 25, 2016, 04:26:21 pmYou don't change the DC voltage, you change the AC voltage before it is rectified.What's hard about that? A hydro plant probably generates at 12 kV (I'm just guessing) because higher levels would cause massive dielectric issues at the generator. It absolutely does not generate 500 kV directly. The generator output voltage is stepped up to 500 kV by ordinary transformers. No big deal. Just a step-down transformer in reverse (more or less).Now, the 500 kV AC is rectified and, voila', we have 500 kVDC. Send it on its way.At the receiving end, there needs to be a commutating setup to convert it back to AC. Still not a big deal. It's been done since '65. Read the list of projects I posted earlier. This HVDC stuff is over 50 years old and pretty well understood.Those windmills are going to generate fairly low voltages and it will need to be stepped up quite a bit to get into the 500 kV range. No big deal. In fact, I wonder if the windmills generate DC. Considering variations in wind speed, I would think synchronizing to 60 Hz might be a challenge.Now we can keep the level at 500 kV AC while we transmit it around the countryside and then step it down to some intermediate voltages like 115 kV. Finally, the 115 kV will get stepped down to 12 kV at local substations and distributed to neighborhoods where small transformers will step it down to utilitization levels.Where I worked, we bought our power straight off the 115 kV lines and stepped it down ourselves. Then we distributed the 12 kV to about 20 unit substations where we dropped it to 277/480V 3 phase. We used the 480V for motor loads, the 277 for lighting loads and stepped the 480V down to 120/208V 3 phase for receptacle loads. Pretty common stuff except for buying power at transmission levels.In New York City, the utility was still distributing DC to end users up until 2007:http://cityroom.blogs.nytimes.com/2007/11/14/off-goes-the-power-current-started-by-thomas-edison/?_r=0OT but interesting: New York City still uses 25 Hz for their Amtrak system. It is generated at a hydro site as well as being converted from 60 Hz:https://en.wikipedia.org/wiki/Amtrak%27s_25_Hz_traction_power_system

author: rstofer
author_rank: Super Contributor
timestamp: September 25, 2016, 05:05:54 pm
content: Quote from: Richard Crowley on September 25, 2016, 04:39:27 pmI doubt that DC will ever be useful in the home or workplace. Induction motors are just far superior to DC motors and it really isn't practical to do DC->AC conversion for every single motor.The ordinary wall clock is dependent on the accuracy of the 60 Hz line. Sure, we could convert to GPS clocks but that's a stretch for a microwave. Yes, we could network all the appliances and have them all use NTP to set their clocks but that's going to be a while.Voltage level conversion of one kind or another will still be required. Transmission and distribution voltages will always be much higher than utilization voltages. Substituting commutators for transformers will still leave something on the poles (unless the utility takes the opportunity to underground everything).

author: dom0
author_rank: Super Contributor
timestamp: September 25, 2016, 05:05:54 pm
content: Quote from: Richard Crowley on September 25, 2016, 04:39:27 pmWell technically most of Europe never had those in the first place.

author: SeanB
author_rank: Super Contributor
timestamp: September 25, 2016, 05:06:50 pm
content: Conversion is easy, you convert it to a chopped DC and use a transformer, then rectify it again and send down the line. At high power you tend to use the good old fashioned SCR stacks, using a set of 6 banks ( 3 phases in star, 3 in delta to get the 30 degree phase angle so you reduce ripple), each with their own commutation stack. Then you simply use a large LC filter on the output and send it out as low ripple DC. Incoming again you use the SCR banks and the same transformers and LC tanks, and the same transformers with delta and Y windings. That way you can have a bidirectional power transfer, with the one side acting as either a synchronous rectifier or a power switching stage, using identical stages each side.Efficiency can easily go over 90% at full load, helped as well by the line losses being lower as there is no inductive effects and skin current with DC, plus the extra 30% more power transmission available on the same line over AC, along with only needing a single conductor with earth return, or just a pair of conductors.And in any case, your typical vacuum cleaner, which uses an universal motor, will quite happily run on either AC or DC.

author: IanB
author_rank: Super Contributor
timestamp: September 25, 2016, 05:09:09 pm
content: Quote from: commie on September 25, 2016, 04:51:13 pmIf a 240 MVA power transformer were only 90% efficient it would have heating losses of 24 MW. Don't you think it might get a little bit hot?

author: dom0
author_rank: Super Contributor
timestamp: September 25, 2016, 05:10:34 pm
content: MW-range transformers go beyond 99 % efficiency...

author: commie
author_rank: Frequent Contributor
timestamp: September 25, 2016, 05:33:22 pm
content: Quote from: rstofer on September 25, 2016, 04:57:10 pmSo you generate 12kv ac then rectify it and send it on its way, could you be a bit more technical how you turn it back to manageable Vac with 90% efficiency that a transformer does?, and lastly, why rectify it to average dc what advantages have you achieved?Why not leave the electricity ac?

author: commie
author_rank: Frequent Contributor
timestamp: September 25, 2016, 05:35:22 pm
content: Quote from: dom0 on September 25, 2016, 05:10:34 pmBig sorry, I'm 9% out.

author: IanB
author_rank: Super Contributor
timestamp: September 25, 2016, 05:36:05 pm
content: Quote from: commie on September 25, 2016, 05:33:22 pmPlease do a little bit of research before posting. It will make you look a bit less ignorant and you will also benefit by learning something.

author: commie
author_rank: Frequent Contributor
timestamp: September 25, 2016, 05:44:18 pm
content: Quote from: IanB on September 25, 2016, 05:36:05 pmCome now, don't get upset, I want to hear about how you change 1000000Vdc back into ac. you can't use a transformer so what do you use?, I'm so intrigued to know.

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 25, 2016, 05:52:23 pm
content: Where did you get this 1000000V number you are tossing around? Did you pull it out of your ..um.. hat?If you spent as much time reading on the interweb as you do posting, you might learn what actual practical voltages are used for both AC and DC transmission and distribution. And you will see a history of how conversion between AC and DC (in both directions) has evolved over more than 80 years.

author: commie
author_rank: Frequent Contributor
timestamp: September 25, 2016, 05:59:23 pm
content: Quote from: Richard Crowley on September 25, 2016, 05:52:23 pmHere.....Quote from: Monkeh on September 25, 2016, 03:38:07 pm

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 25, 2016, 06:06:21 pm
content: So what did you learn when you read this reference?https://en.wikipedia.org/wiki/Ultra-high-voltage_electricity_transmission_in_ChinaI was interested to learn that the HVDC line near me (The Pacific DC Intertie) was originally designed with a 1.3MV line to Hoover Dam on the Colorado River.Note that all those high-voltage devices (both vacuum-tube and solid-state) are assembled from stacks of lower-voltage components. They simply string enough of them in series to handle whatever voltage is required. Not nearly as mysterious as you seem to think it is.

author: rstofer
author_rank: Super Contributor
timestamp: September 25, 2016, 06:10:49 pm
content: Quote from: commie on September 25, 2016, 05:44:18 pmThe exact same way a UPS system works. You commutate the DC voltage with a bunch of SCRs (really BIG ones, and not SCRs exactly) and convert it to AC. As I have said before, this is old news, the technology is at least 50 years old. Yes, the particular devices have evolved but the techniques, not so much!This is an explanation:https://en.wikipedia.org/wiki/HVDC_converterNotice that converters are inherently bidirectional. They convert AC to DC or DC to AC.Just think about the average UPS. There are 3 blocks. First we convert AC to DC, next we store DC in a battery and discharge it when the AC fails, and, finally, we convert the DC back to AC. There are probably millions of these systems around. Some small, some large, but they all do the same thing. You can't store AC and you can't power a PC with DC.Even variable speed drives do the same thing. They take in some line voltage (and phases), convert it to DC and then control the conversion frequency back to AC. Thus they can control the speed of an induction motor. On the way by, they can convert from single phase input to 3 phase output. It's done every day!

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 25, 2016, 06:23:37 pm
content: Note that more and more appliances and tools for consumer use (which nominally operate on AC mains power) actually use DC motors internally, even converting the mains power to DC and using poly-phase drivers for the motors. Clearly, they wouldn't go to this expense and complexity if there weren't some benefit. And, of course, we have seen switch-mode power-supplies (SMPS) now for decades in all manner of consumer and industrial goods. These days old-school "heavy-iron" power supplies with mains-frequency transformers are getting rare. Much to the dismay of the "golden ears" audiophools.To see some practical examples of poly-phase DC motors in consumer appliances, recommend this series of videos from The Post Apocalyptic Inventor...https://www.youtube.com/channel/UCDbWmfrwmzn1ZsGgrYRUxoA/search?query=motor

author: commie
author_rank: Frequent Contributor
timestamp: September 25, 2016, 06:26:13 pm
content: Quote from: Richard Crowley on September 25, 2016, 06:06:21 pmWhy transmit electrical power using dc when ac is alot more manageable?I'm still waiting to hear about the dc receiving end, and how to change 1MVdc back into usable ac?Stacks you say, what are these?.. diodes.. transistors what....?We have already established that transformers are 99% efficient at ac, these 'stacks' you talk about, efficiency converting 1MV dc into 117Vac sinusoids??, what about chopping byproduct harmonics etc and I can go on....

author: dom0
author_rank: Super Contributor
timestamp: September 25, 2016, 06:28:48 pm
content: Richard: Although it is hard to refute that the massive influx of extremely low cost SMPS cause strong pollution of the radio spectrum. Not to speak of things like powerline communications...QuoteWell technically you can and in larger installations this isn't at all unusual. It's just that the small voltages used in smaller UPSes (24 V or even 12 V) are impractical.QuoteSCR-like devices.QuoteThe output AC is for transmission, ie. >100 kV AC.QuoteFilters.

author: dom0
author_rank: Super Contributor
timestamp: September 25, 2016, 06:29:49 pm
content: In case you haven't noticed almost all HVDC projects are point-to-point (sometimes with a "tap" station or two), while AC transmission are mesh-like with a great many stations interconnected. Think a little about it and you might just get behind why that is.

author: IanB
author_rank: Super Contributor
timestamp: September 25, 2016, 06:38:32 pm
content: Anyone get the feeling we are being trolled?

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 25, 2016, 06:42:47 pm
content: Quote from: commie on September 25, 2016, 06:26:13 pmYou are starting with a false assumption.AC is NOT "alot more manageable" for very long distances because you start turning the transmission line into a transmitting antenna and "broadcasting" the power out into the air where it is completely wasted.AC is NOT "alot more manageable" for cases where you have high capacitance lines like under-sea where you would lose a large fraction of the power in capacitive loading.AC is NOT "alot more manageable" for situations where you have different frequency (or phase) at the two ends of the path. There is a very short ("back-to-back") DC intertie in South America between Brazil and Argentina because Argentina uses 50Hz power and Brazil uses 60Hz. And even in places where the same frequency is used, if the interconnection is BETWEEN REGIONS that are not phase-locked, DC completely eliminates the phase conversion problem. For example the Pacific DC Intertie near me interconnects between the Northwest grid and the Southwest grid. While they are both 60Hz, they are not phase-aligned because they are not interconnected with AC.QuotePerhaps you missed the part where it explained that the converters are INHERENTLY BIDIRECTIONAL. Exactly the same devices that convert AC to DC are used for converting DC back into AC.Quotehttps://en.wikipedia.org/wiki/Thyristorhttps://en.wikipedia.org/wiki/Thyristor#HVDC_electricity_transmissionQuotePerhaps you missed the part about using poly-phase conversion (6-pole is quite common) to reduce harmonics. Sooner or later people will grow weary of spoon-feeding you information that you should have looked up for yourself before coming here.

author: Pjotr
author_rank: Frequent Contributor
timestamp: September 25, 2016, 06:48:33 pm
content: Quote from: Richard Crowley on September 25, 2016, 06:23:37 pmHello there, I am not an audiophool (whatever that me be). And my ears have no opinion on those PSU's. But those modern ironless wall warts have the annoying habit to last only for 2 years or so. Some a bit longer but alas. Those old-school "heavy-iron" power supplies appear to last forever.

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 25, 2016, 07:01:30 pm
content: Quote from: Pjotr on September 25, 2016, 06:48:33 pmLandfills are clogged with examples of cheap mains-frequency power supplies that lasted only a short while.And, conversely, there are thousands (millions?) of examples of properly-designed and maintained SMPS that are still going strong for several decades."Old-school" mains-frequency power supplies tended to be conservatively designed back in the "good old days". While we see everywhere cheap knock-off products from certain parts of the planet with minimalist designs and thrown together with floor-sweeping components. No surprise that we perceive a higher failure rate.

author: rstofer
author_rank: Super Contributor
timestamp: September 25, 2016, 07:09:48 pm
content: Quote from: commie on September 25, 2016, 06:26:13 pmDo your own homework!You have already been given links to many documents that explain exactly how these systems work and Google has another million references or so.It is being done, it has been done for more than 50 years and the fact that you don't understand it doesn't make it less viable. You just need to get with the program and read the documentation.With the 3 phase converter, harmonics were a problem. With the 12 phase converter, harmonics begin at the 11th and 13th and are considerably diminished. So the industry seems to be settled on the 6 phase inverter with the significant harmonics at the 5th and 7th.The electrical grid tends to smooth things out. Particularly the motor loads.This has been done for decades and it seems to work! And it must be cost effective or it wouldn't be done at all.

author: Zero999
author_rank: Super Contributor
timestamp: September 25, 2016, 07:50:31 pm
content: Quote from: SeanB on September 25, 2016, 05:06:50 pmI've always wondered about electrolysis when using earth as a return for DC? How do they stop the anode becoming corroded and the emission of gasses and caustic chemicals such as NaOH?QuoteLots of mains appliances nowadays can work off DC because they have switched mode power supplies and even devices containing induction motors can work fine off DC because they already have an inverter for speed control.

author: Mr.B
author_rank: Unknown
timestamp: September 25, 2016, 09:07:28 pm
content: Quote from: Monkeh on September 25, 2016, 03:38:07 pmI have stood in that very hall many years ago.It has an eerie feeling ofshearsheer power...EDIT: My Typo

author: IanB
author_rank: Super Contributor
timestamp: September 26, 2016, 01:41:21 am
content: Quote from: Mr.B on September 25, 2016, 09:07:28 pmI guess one could say it iscutting edgetechnology?

author: Mr.B
author_rank: Unknown
timestamp: September 26, 2016, 01:45:04 am
content: Quote from: IanB on September 26, 2016, 01:41:21 amBugger... Thinking slower than typing rate.

author: JS
author_rank: Frequent Contributor
timestamp: September 26, 2016, 04:29:15 am
content: Quote from: Hero999 on September 25, 2016, 07:50:31 pmGood luck with your hand held drill. They are universal motors and those do work on DC, but the speed control is done with a triac, once it's triggered you'll need to unplug it to turn it off. Will need to change those for a PWM speed control and light dimmers.About your other concern, there are many things that will suffer from DC corrosion, any contact will have troubles in the long run, probably more attention about contact material will need to be taken. Earth shouldn't have much persistent current anyway and we could use 2 phaseOk, polarities, so the reference to earth is balanced and again, no persistent or predominant current should flow to it.JS