[ekaggata]

There are earlier topics but little or nothing in depth I could find.

High Voltage DC (HVDC) offers the ability to transfer electrical energy over thousands of kilometers with realistically lower power loss.

This, of course is NOT a direct solution for a liquid fuel crisis such as we currently have. It may not even be an indirect one.

But, to me, it is another very clear part of the overall solution we have for shifting to a renewable (maybe plus nuclear, but not necessarily) electricity generation, with all the advantages that confers re: climate, pollution and fossil fuel depletion.

I was reminded of this by The Watt Podcast 78. Certainly an interesting discussion from someone who's working directly in this field.

He speaks of the currently-being-installed Xiangjiaba-Shanghai UHVDC transmission project which is apparently about 2000km in length at a 6GW rating to take hydropower from Western China to the Eastern seaboard.
He talks more about the DESERTEC project, which has been mentioned on this website before, and there is a pdf available on the Watt Podcast notes to the show.

I think the point about the technology that impresses me the most is that it is already in use, even heavily in use, although the massive usage of Saharan sunlight is still merely an idea. He claims that a very large part of recent grid installation is HVDC, certainly in any case he quotes both Europe and China in this regard.

There are perhaps two distance scales on which this technology might be important:

First is that it can certainly allow large scale wind and solar arrays sited very far from population centres to be viable. This is a plausible short term scenario, but it can only partly offset the problem of intermittency which these sources suffer from.

Second, and this was not really discussed on the podcast, is the very largest scale possible implementation, not just North-Africa to Europe but across timezones. In theory this might take away the "availability" issue of solar (i.e. night time). I must admit that's a bit of a stretch, but to solve the extraordinary problems we have right now, you have to think big!


(PS To put my cards on the table, I am a strong fission advocate, so this to me is not the only game in town when it comes to the grid...)

[small_steps]

While the line losses for an HVDC line are smaller than traditional AC lines, HVDC is not a game changer in that respect. so we go from 92% to 95% efficiency for transmission from point A to point B (with conversion). Not a big deal in that respect. What is done, and is important, and mostly missed in the podcast for instance, is that there is no more reactive voltage drop in the cables (mostly capacitive for underground, and mostly inductive for overhead lines.) This effect (reactance) is what largely limits the power flow between ends of a line (at least for overhead lines). What appears to be happening is that they (ABB) are becoming more confident in the ability to series and parallel these switches.

+/-400kV line, how many series switches are needed in the converter if switches are rated at 4.5kV each?

That is what makes HVDC work, the podcast moderator had some odd fascination with the cables however.

We'll still have utilization issues (intermittent of source), which is the real deal breaker of RE. So you make the investment of resources and capital in RE, and the investment in the transmission, but you might be using 1/3 of the time. How does the economics work? How do the economics work relative to xxx?
Global Grid?

HVDC may be more efficient than conventional AC lines, but over distances that are meaningful, the efficiencies -> 0. Unless we somehow come to using superconducting lines of some sort, but that is adding a whole layer of wishes upon wishes.

[ekaggata]

Interesting post small_steps - I appreciate it. I'm off to bed now, I'll take a closer look tomorrow.

[TheDude]

Stu Staniford wrote a piece on a global solar grid tied together with HVDC: Powering Civilization to 2050. May be of interest.

[Newfie]

While I'm more than a little rusty on my power transmission theory there are some practical issues.

First, a couple of months ago Scientific American did a cover piece on how to save America by doing big, BIG, solar in the American SW. Then transmit it to the NE via HVDC, then store the energy underground in Natural Gas chambers, let it out at night.

By contrast read the North American Energy Reliability Corporation (NERC) 2008 reliability report. They have identified several major corridors where we need to reinvest in our transmission line infrastructure. Can't do it because of NIMBY. And these are relatively short lines of existing technology. You should read this report in any case. Scary stuff if you can parse out the technocratic BS.

Second I actually worked on a 3-phase to DC to 1-phase power system about 25 years ago. These are NOT simple transformation processes. Quite a bit more infrastructure is required to make it work, lots more than a simple transformer.

So I am skeptical. Still I need to read up on the technology more.

[tsakach]

There are non-technical issues at the heart of problem in the US power grid that need to be addressed before tinkering around with the technology.

In the view of Casazza and many other experts, the key error in the new rules was to view electricity as a commodity rather than as an essential service. Commodities can be shipped from point A through line B to point C, but power shifts affect the entire single machine system. As a result, increased long distance trading of electric power would create dangerous levels of congestion on transmission lines where controllers did not expect them and could not deal with them.
The system was never designed to handle long-distance wheeling,” notes Loren Toole, a transmission-system analyst at Los Alamos National Laboratory.

What's wrong with the electric grid?

[Starvid]

Soooo what?

HVDC has been around in one form or another for 50 years. Transmitting power has never been the problem we are facing now, nor has generating the power been.

We can for all practical means generate as much power as we could ever need, transmit it where ever we need it and do this for as long as we feel like.

That doesn't answer how people are going to be driving their cars in 10 or 20 or 30 years.

[small_steps]

That doesn't answer how people are going to be driving their cars in 10 or 20 or 30 years.

Much less than we do now. I think anyone who understands PO and what our current lifestyles exist as comes up with the simple understanding that our current ability of mobility is unparalleled, and for the next few generations (at least) will not and cannot be matched. And that we currently take many things for granted, but our children will not be able to do the same. Prepare them to utilize the best of their abilities, and hope for the best. That is what mankind had done in the past, and while briefly forgotten during the age of abundance, will be common forevermore.

[pedalling_faster]

in order to handle HVDC you need very specialized power supply components, e.g. diodes with Peak Inverse Voltage of 1000 volts.

but, it creates jobs for high voltage power design engineers, which keeps us off the street. some of the most fun design projects i've had were HVDC supplies for travelling wave tubes, radar, etc.

i don't think there's anything to pick holes in. the technology works. i don't have a comprehensive overview; i just had the opportunity to work on some of the equipment for a few years.

[Newfie]

in order to handle HVDC you need very specialized power supply components, e.g. diodes with Peak Inverse Voltage of 1000 volts.
But, it creates jobs for high voltage power design engineers, which keeps us off the street. Some of the most fun design projects i've had were HVDC supplies for travelling wave tubes, radar, etc. I don't think there's anything to pick holes in. The technology works. i don't have a comprehensive overview; i just had the opportunity to work on some of the equipment for a few years.

PF,
The little I know says that there are specific challanges to HVDC.

I just glanced through the Wiki posting on that subject and will get back to it in a week or two. I'm curious.
HVDC
Since we are dealing with 50+kV systems, 1,000PIV is not much. You must use many in series.

600 to 750 VDC systems are pretty common for electric railroads and subways while other systems like Amtrak use single phase over power on the order of 13kV. Interestingly 25Hz is standard with much new work being 25Hz and some 17Hz in Europe.

I'll talk to my traction power buddies about this. My sense is that DC is a PITA for a lot of reasons. Perhaps it makes sense for specialized applications.

[Niagara]

Starvid, something I've been meaning to ask for some time now.

Peak oil is not an energy crisis. It is a liquid fuel crisis.

What kind of bullshit is that? Our nearly 7 billion inhabitants on the earth are a result of a drawdown of petroleum; millions of years of solar energy stored under the ground. Once that energy store is gone, it's 'dieoff' season.

Not an energy crisis? So if we don't have an energy problem, why can't we simply make liquid fuel to replace oil?

We could electrolyze seawater to make all the hydrogen we need to power out vehicles, heat our homes, make fertilizer from atmospheric nitrogen etc...

So there's no problem, right? I mean if there's no energy crisis, what stops the party from continuing on for centuries?

You come out with some good posts, but lose the sig line. You lose credibility.

[Dezakin]

Starvid, something I've been meaning to ask for some time now.

Peak oil is not an energy crisis. It is a liquid fuel crisis.

What kind of bullshit is that? Our nearly 7 billion inhabitants on the earth are a result of a drawdown of petroleum; millions of years of solar energy stored under the ground. Once that energy store is gone, it's 'dieoff' season.
Not an energy crisis? So if we don't have an energy problem, why can't we simply make liquid fuel to replace oil?

We can. It costs money and takes decades to build the infrastructure of course, which is why life is gonna suck for a while.

We could electrolyze seawater to make all the hydrogen we need to power out vehicles, heat our homes, make fertilizer from atmospheric nitrogen etc...
So there's no problem, right? I mean if there's no energy crisis, what stops the party from continuing on for centuries?

Nothing. The problem we face is a short term infrastructure problem. We have ample energy, and demonstrated capacity to utilize it (see nuclear fission)

[ekaggata]

Sorry I haven't found time to keep up with this thread...
Here are two pictures I took from Asplund's presentation at the DESERTEC conference. You can find his presentation along with lots of others at the site. This presentation was given on the afternoon of the 23rd April.


What I like about the first picture is that it illustrates in a very concrete fashion how much in use this technology already is. Lots more detail is of course fleshed out in the rest of the presentation, I recommend downloading it and all the rest. Most of the material is about Solar Thermal technology as far as I can tell. There are some very beautiful pictures, kind of renewables porn

[ekaggata]

While the line losses for an HVDC line are smaller than traditional AC lines, HVDC is not a game changer in that respect. so we go from 92% to 95% efficiency for transmission from point A to point B (with conversion). Not a big deal in that respect.

I find it difficult to understand what you're saying here, when you look at the graph I posted above. Over 1000km distance scales it seems to make feasible what is totally infeasible with AC. This seems to line up with the real life installations he's talking about. Have I misread the information? How is it "not a game changer"?
(By the way I have a Masters in Physics, so in theory I should understand the technical explanations, but actually my knowledge of AC circuit theory is patchy at best, so go slowly ... )

[small_steps]

Over 1000km distance scales it seems to make feasible what is totally infeasible with AC. This seems to line up with the real life installations he's talking about. Have I misread the information? How is it "not a game changer"?

The graph shows the ability to transmit power vs distance. For DC lines, the only limiting factor is the IR (Current Resistance) drop. So as a line increases in length, the voltage that one sees on one end vs the other slowly varies in amplitude (given constant current through the line). For the AC line, there is an additional voltage drop, and that is due to the creation of the magnetic field(s) along the line from the current(s) in the line. Therefore, the impedance takes a form of R + j X, where R is again the resistance, and X represents the voltage drop accounting for the magnetic field (think Faraday's Law). So the voltage drop from one end to the other takes the form of I (R + j X_L). Now in HVAC lines, the "X_L" is much greater than "R", so the current (and power flow) is limited by the reactance (the j X_L portion of the impedance).

However, this can be compensated for by inserting a series capacitor (or capacitors) in the HVAC line. What this does is to add negative reactance (-j X_C) to the overall line impedance so that the overall line impedance takes a form of R + j X_L - j X_C. However, the ABB graphic neglects the addition of the series capacitor in the line, and then shows the cheap AC line vs the expensive DC line. Apples to oranges if you will.

This applies to overhead lines, underground lines are dominated by capacitive reactance, so compensation of the line is nearly impossible. This is where I see HVDC being the better choice, and you can see that in the installations (shown in your figure) of the undersea lines in northern Europe. HVDC lines are also used to connect areas which have different frequencies (50 / 60Hz) etc, which also accounts for a number of other installations in the figure.

Hopefully, I've given enough detail, and in a fairly straightforward explanation. Let me know if you'd like a different explanation, or more or less detail.

[ekaggata]

The graph shows the ability to transmit power vs distance. For DC lines, the only limiting factor is the IR (Current Resistance) drop. So as a line increases in length, the voltage that one sees on one end vs the other slowly varies in amplitude (given constant current through the line). For the AC line, there is an additional voltage drop, and that is due to the creation of the magnetic field(s) along the line from the current(s) in the line. Therefore, the impedance takes a form of R + j X, where R is again the resistance, and X represents the voltage drop accounting for the magnetic field (think Faraday's Law). So the voltage drop from one end to the other takes the form of I (R + j X_L). Now in HVAC lines, the "X_L" is much greater than "R", so the current (and power flow) is limited by the reactance (the j X_L portion of the impedance).

So far, so clear. Thanks for the explanation.

However, this can be compensated for by inserting a series capacitor (or capacitors) in the HVAC line. What this does is to add negative reactance (-j X_C) to the overall line impedance so that the overall line impedance takes a form of R + j X_L - j X_C. However, the ABB graphic neglects the addition of the series capacitor in the line, and then shows the cheap AC line vs the expensive DC line. Apples to oranges if you will.

Also very clear - in and of itself. What I don't understand, though, is why in that case we don't talk about HVAC over long distances. Is there some technical difficulty with having these capacitors, or is it only a matter of expense? In which case apples to apples ought to be easy to construct, no?

The impression I got was that HVDC is the only realistic option for 1000km+ installations. Would HVAC over 1000km be possible? Has it been done?

Are there stability issues? (From Wikipedia HDVC page: "A generator connected to a long AC transmission line may become unstable and fall out of synchronization with a distant AC power system.")

This applies to overhead lines, underground lines are dominated by capacitive reactance, so compensation of the line is nearly impossible. This is where I see HVDC being the better choice, and you can see that in the installations (shown in your figure) of the undersea lines in northern Europe. HVDC lines are also used to connect areas which have different frequencies (50 / 60Hz) etc, which also accounts for a number of other installations in the figure.

This may be an unimportant technical detail, but I'm curious - why do undersea cables have a higher capacitance?

Thanks.

[small_steps]

Also very clear - in and of itself. What I don't understand, though, is why in that case we don't talk about HVAC over long distances. Is there some technical difficulty with having these capacitors, or is it only a matter of expense? In which case apples to apples ought to be easy to construct, no?
The impression I got was that HVDC is the only realistic option for 1000km+ installations. Would HVAC over 1000km be possible? Has it been done?
Are there stability issues? (From Wikipedia HDVC page: "A generator connected to a long AC transmission line may become unstable and fall out of synchronization with a distant AC power system.")

While one can compensate for the series inductance, there is also a shunt (parallel) reactance that is created by the charging and discharging of the capacitance (to ground - and other phases) of the line. (Ever hear of loading coils in telephone lines? - same effect - trying to compensate for the capacitance of the line - more below). But for high current lines, we are already trying to compensate for inductance, and inductors of these current ratings, would be by any measure, impractical.

I would say that the only practical method to transmit electrical power over significant distance is HVDC, because of these issues we have been discussing, not efficiency.

For AC systems, the ability to transmit power between two places is largely the relative phase angle between the locations, not the amplitude of the voltages, as well as the reactance between the locations. However, for DC systems, itis simply the relative voltage between the locations, as well as the resistance of the line.

Another manner to think about transmission lines is as a shaft between the generator and load. If the shaft is long, there is less likelyhood of both ends of the shaft to be nearly at the same angle.

This may be an unimportant technical detail, but I'm curious - why do undersea cables have a higher capacitance?

Capacitance is proportional to area, and inversely proportional to distance between potential. C~ A/d. Now undersea cables have only a fairly thin insulating layer between the current carrying conductor (at high voltage), while the overhead lines have significant distance between the conductor and ground (and much less capacitance).

Hope this helps clarify the issues....

[Newfie]

Small steps,

Thanks for going over that, good refresher.

I have ignored HVDC for a long time and came across it once again in a recent Scientific American article which did nothing to explain the advantages.

The downside of HVDC is the expense of the converter station at each end. No? Higher voltage means less loss so, transformers were a cheap way to step up AC voltage. Back then the conversion methodology was mechanical. So that explains why we use AC.

Now solid state conversion makes HVDC more affordable. What I found interesting is the supposed increase in power density. By the articles I read we can move much more power on a given power line profile. That is a big issue due to the NIMBY factor.

The NERC (North American Electric Reliability Corporation) has identified several corridors that need to be strengthened and note the NIMBY factor. However I did not see where they identified HVDC as a potential solution. Or am I missing something?

Sounds like ABB might be a decent investment choice?

[small_steps]

The downside of HVDC is the expense of the converter station at each end. No? Higher voltage means less loss so, transformers were a cheap way to step up AC voltage. Back then the conversion methodology was mechanical. So that explains why we use AC.

Now solid state conversion makes HVDC more affordable. What I found interesting is the supposed increase in power density. By the articles I read we can move much more power on a given power line profile. That is a big issue due to the NIMBY factor.

The NERC (North American Electric Reliability Corporation) has identified several corridors that need to be strengthened and note the NIMBY factor. However I did not see where they identified HVDC as a potential solution. Or am I missing something?

Sounds like ABB might be a decent investment choice?

I don't have the numbers at my easy disposal, but much of the cost of transmission lines is due to litigation. The cost of the lines, the purchase of the right of way, and the man hours and equipment tends to be something on the order of 15% of the overall cost. Would be interesting if the litigation cost could be mitigated and the more capable more expensive HVDC lines were installed. What would the overall cost become?

Likely to be an effect of relative unfamiliarity of HVDC equipment and capabilities on NERC's part. Not sure however. Could be that the corridors are relatively short, and cost benefit does not favor HVDC, but I would have imagined that they would have commented on that if that were the case.

[ekaggata]

Thanks again to small_steps from me I hope you're not too tired of all the questions!

Sounds like ABB might be a decent investment choice?

It does indeed look like a superb company, doesn't it? And, glancing over its numbers, even with its recent run-up in price, I don't think it would be considered expensive by normal standards (e.g. P/E). When it comes to investment, I am a neophyte, but this one is tempting!
Here's someone probably talking their book, but all the same...