Sciencemadness Discussion Board - He

He

Nixie - 30-12-2007 at 02:56

Helium is running out. I just saw on TV

Maya - 30-12-2007 at 02:58

has been for awhile since I think they get it from natural gas deposits

Nixie - 30-12-2007 at 03:12

Guess we better hurry up with ITER

Of course, who knows what'll happen with it now that Congress has cut out all US investment into it...

MagicJigPipe - 1-1-2008 at 13:01

I wonder how pure the He is that's sold at Wal-Mart? If it's good enough I'll just go buy a few of those tanks.

The_Davster - 1-1-2008 at 13:45

The He intended for use in balloon filling is diluted with oxygen so that people who inhale He to make their voice funny do not suffocate themselves.

[Edited on 1-1-2008 by The_Davster]

Helium

MadHatter - 1-1-2008 at 17:38

Primarily from natural gas wells in Texas although there are some wells in Poland. If fusion
ever makes it to the electrical grid we should be able to "synthesize" helium so-to-speak.
Price is probably going through the roof !

JustMe - 1-1-2008 at 17:49

Not good for a key branch of radiology!!!

http://www.airproducts.com/medical/uk/hospital/products/mri/...

http://www.metrowestdailynews.com/archive/x471274468

Without liquid helium, all that technology is just a very expensive paperweight.

[Edited on by JustMe]

microcosmicus - 1-1-2008 at 20:50

Of course, helium does get made naturally as a radioactive decay
product. Given that there is a fair amount of low-level radioactivity
in most rocks (I remember once being in an accelerator facility and
having the fellow point a detector at the walls to demonstrate the
background radiation from the concrete in the walls) here is a
question for a geologist: How much helium is generated annually
by the low-level radioactive decay in rocks?

As far as synthetic helium, you could make do with a conventional
fission reactor --- make some isotopes which undergo alpha decay
and collect the helium. For instance, whacking boron-10 with a
neutron makes two helium-4 nuclei as well as a tritium, which subsequently
becomes a helium-3 nucleus. Since 19% of boron is the right isotope and
borax is in plentiful supply, I suppose this might be one way of trying
to cope with a helium crunch.

Waffles - 2-1-2008 at 09:59

I have an answer to both of your questions. Many many times more helium is generated by simple radioactive decay every year than human industry could use. On the other hand, how much of that helium is generated in the Earth's crust or mantle below 1 or 2 miles down (a reasonable mine depth)? Probably about 99.9999999999%.

As far as synthetic helium, helium-3 is used quite commonly in ultra-low-temp research- the problem is that it is of such low natural abundance (parts per million) for an already rare gas that it is not possible to separate a reasonable amount from the naturally occurring. So it is in fact made artificially, by neutron bombardment of Li, N, and B, if memory serves. Despite the relatively large demand (basically every university in the US, certainly, as well as many companies and government projects) the price is somewhere around US$5,000,000 per kilogram

. In other words, synthetic production involving nuclear transmutation is mindblowingly expensive, even for a product with a large demand.

Quote:

Originally posted by microcosmicus
Of course, helium does get made naturally as a radioactive decay
product. Given that there is a fair amount of low-level radioactivity
in most rocks (I remember once being in an accelerator facility and
having the fellow point a detector at the walls to demonstrate the
background radiation from the concrete in the walls) here is a
question for a geologist: How much helium is generated annually
by the low-level radioactive decay in rocks?

As far as synthetic helium, you could make do with a conventional
fission reactor --- make some isotopes which undergo alpha decay
and collect the helium. For instance, whacking boron-10 with a
neutron makes two helium-4 nuclei as well as a tritium, which subsequently
becomes a helium-3 nucleus. Since 19% of boron is the right isotope and
borax is in plentiful supply, I suppose this might be one way of trying
to cope with a helium crunch.

MagicJigPipe - 2-1-2008 at 14:52

We could just set off a bunch of hydrogen bombs. That would be the most asthetically pleasing way

Surely I'm not the only one that thinks a nuclear bomb going off is nothing short of majestic? It just looks so.... awe inspiring... Albeit destructive and environmentally detrimental. But it's just amazing. Especially fission induced fusion bombs.

[Edited on 2-1-2008 by MagicJigPipe]

[Edited on 2-1-2008 by MagicJigPipe]

rwmohawk1small.JPG - 12kB

chemkid - 2-1-2008 at 16:14

beautiful

Maya - 2-1-2008 at 16:19

MJP,

I knew you were twisted....

look , the easiest way to extract He is from NG. extracting from air will be somewhat more cumbersome

Nixie - 2-1-2008 at 19:30

This poster is on my room's wall (it was the only fusion bomb poster the poster place had):

<img src="http://imagecache2.allposters.com/images/pic/CFJ/2412~Hydrogen-Bomb-Posters.jpg">

[Edited on 2-1-2008 by Nixie]

JohnWW - 3-1-2008 at 01:10

Quote:

Originally posted by iamthewafflerAs far as synthetic helium, helium-3 is used quite commonly in ultra-low-temp research- the problem is that it is of such low natural abundance (parts per million) for an already rare gas that it is not possible to separate a reasonable amount from the naturally occurring. So it is in fact made artificially, by neutron bombardment of Li, N, and B, if memory serves. Despite the relatively large demand (basically every university in the US, certainly, as well as many companies and government projects) the price is somewhere around US$5,000,000 per kilogram

. In other words, synthetic production involving nuclear transmutation is mindblowingly expensive, even for a product with a large demand.

That is why one of the purposes of NASA wanting to return to the Moon is to mine its surface rocks and soil for the large amounts of He-3 that have been adsorbed fom the solar wind. It is likely to be cheaper than synthesizing He3 on Earth, or separating it from He in the atmosphere.

He-4 is formed largely in Earth's crust as alpha particles from the decay of U-238 and its daughter decay products. A small amount is from even longer-lived rare earth and actinide element isotopes such as Th-232. U and Th and rare-earth elements all occur in granite, especially in the lower depths (pegmatite) of granite masses, and in sedimentary and metamorphic rocks derived from granite. It is in such sedimentary rocks that the natural gas deposits, containing He, of Texas and Poland occur.

MagicJigPipe - 12-5-2008 at 10:09

Quote:

Originally posted by The_Davster
The He intended for use in balloon filling is diluted with oxygen so that people who inhale He to make their voice funny do not suffocate themselves.

This is wrong in at least one case and it's likely that most others do not have oxygen and/or nitrogen in them (that is usually reserved for deep water SCUBA applications, eg heliox and heliair). The balloon kits that are sold at Wal-Mart are 99.99% helium (I just called the company and it also says so on their MSDS). I'm sure this is because customers don't want to waste money, weight and cylinder space by filling 20% of it with oxygen. Also, this would surely decrease bouyancy quite a bit.

Now that I know this I'm going to buy a couple for small inert gas uses in the lab. They even have a conveinient nossle and valve for semi-controlled release rates. It's no regulator but it should suffice for many uses. I can't believe people waste He on balloons!

DrP - 13-5-2008 at 03:42

Quote:

Originally posted by JustMe
Not good for a key branch of radiology!!!

http://www.airproducts.com/medical/uk/hospital/products/mri/...

http://www.metrowestdailynews.com/archive/x471274468

Without liquid helium, all that technology is just a very expensive paperweight.

[Edited on by JustMe]

So can't they use Nitrogen or some other gas - Or is it not cold enough to get the superconductivity they can get with liquid He temperatures?

[Edited on 13-5-2008 by DrP]

Pulverulescent - 13-5-2008 at 04:18

Quote:

Originally posted by MagicJigPipe
Especially fission induced fusion bombs.

'Didn't know there was any other kind?

P

ScienceGeek - 13-5-2008 at 08:36

Quote:

Originally posted by DrP

Quote:

So can't they use Nitrogen or some other gas - Or is it not cold enough to get the superconductivity they can get with liquid He temperatures?

[Edited on 13-5-2008 by DrP]

I asked the HNMR operator at the local University that same question, and he said that it is simply more expensive to use materials that are superconductive at higher temperatures, as opposed to use liquid Helium.

-jeffB - 13-5-2008 at 08:53

Quote:

Originally posted by DrP
So can't they use Nitrogen or some other gas - Or is it not cold enough to get the superconductivity they can get with liquid He temperatures?

No, it's not. Even for the experimentation our group does with "high-temperature" superconducting sensor coils for MRI, we chill them near liquid-helium temperatures, because even high-Tc superconductors have better characteristics at lower temps.

There are research groups trying to develop MRI magnets that use high-Tc superconductors, but it's still a long way from commercialization.

There are other ways to reach the required low temperatures, but none are practical at present for a volume as large as an MRI magnet. Some clinical systems do have closed-cycle helium cooling, but most as I understand it do not -- they just vent evaporated helium into the air.

If helium gets much more expensive, or if it starts to be rationed, clinics will move to install reclamation systems. At present, it's not seen as offering a good return on investment. Collecting, compressing and reprocessing liquid helium is a pretty big undertaking.

(Apparently, at one point, our hospital actually ran a tube underground from their MRI facility to a room in the physics building. In that 20x20-foot room, they put a huge bladder, which would collect helium until it filled most of the room. At that point, they'd recompress the helium for reprocessing. Eventually, they stopped bothering.)

MagicJigPipe - 13-5-2008 at 09:15

Quote:

Originally posted by Pulverulescent

Quote:

Originally posted by MagicJigPipe
Especially fission induced fusion bombs.

'Didn't know there was any other kind?

P

Well, yeah but I didn't want to just say "fusion bomb" as it is possible to build a purely fusion bomb. Maybe a fusion bomb was created that we just don't know about

12AX7 - 13-5-2008 at 09:57

Quote:

Originally posted by ScienceGeek

Quote:

Originally posted by DrP
So can't they use Nitrogen or some other gas - Or is it not cold enough to get the superconductivity they can get with liquid He temperatures?

Specifically, critical field varies with temperature, so even high-Tc superconductors needs to be cooled substantially before they can carry any appreciable current (current flow means magnetic field generated at the surface of the superconductor). Relatively warm type I superconductors (which are more robust in mechanical and conductive characteristics) are used for this reason.

Tim

ShadowWarrior4444 - 13-5-2008 at 12:51

Quote:

Originally posted by 12AX7

Specifically, critical field varies with temperature, so even high-Tc superconductors needs to be cooled substantially before they can carry any appreciable current (current flow means magnetic field generated at the surface of the superconductor). Relatively warm type I superconductors (which are more robust in mechanical and conductive characteristics) are used for this reason.

Tim

A useful book on High Temperature Superconductivity for those interested: Introduction to High-temperature Superconductivity By Thomas P. Sheahen

I would also like to mention that superconducting power cables are being installed in my area as I write this. (New York City area) In the core is a liquid nitrogen pipeline, surrounded by a high-temp superconductor, most likely YBCO. They may use mercury-based ones for the larger margin of error, though--163 Kelvin, as opposed to 92 for YBCO.

Hospitals may not want to use high-tc superconductors due to the fact that most of them are ceramics, and therefore difficult to wind into coils. (Accidentally breaking a ceramic coil would also mean replacing the entire device, unless one had a particularly clever in situ sol-gel repair system, such as the ones used to make fiberoptic chemical sensors.)

MagicJigPipe - 13-5-2008 at 14:00

Seems like it would use more energy to produce the LNO2 than would be saved by using the superconductor.

ShadowWarrior4444 - 13-5-2008 at 14:19

Quote:

Originally posted by MagicJigPipe
Seems like it would use more energy to produce the LNO2 than would be saved by using the superconductor.

For transferring power into NYC/LI from the upstate hydroelectrics? This is one of the highest population density areas in the US. The money saved from zero resistive losses far outweighs LN2 production. Also, it is likely that they simply recycle the LN2 at either end, never allowing it to become gaseous, thereby avoiding the need to fractionally distill from air.

It will probably be configured so that there are two or more wires transferring LN2 in opposite directions and being cooled at both ends, possibly by Stirling coolers for the efficiency.

MagicJigPipe - 13-5-2008 at 17:51

I just never thought the loss was so much as to justify something like that. I suppose it must be a great distance. Damn, I just can't imagine the energy it must take to keep that much mass at such a low temperature. Really though, if this is so cost effective, you would think every electrical company with the same population/electricity demand density dynamics would employ this method. I mean, that's what the US is all about, saving money to make money. I'm not saying I don't believe it, I'm simply saying I'm not so sure that it would save that much energy.

Imagine how massive the wires must be (total).

[Edited on 5-13-2008 by MagicJigPipe]

not_important - 13-5-2008 at 18:36

Ultra-high voltage DC lines are tough competition for long haul applications. Loses of 2% over a 2000 km 12 GW line can be had for no more cost than existing lower voltage DC and less than AC lines, both at 3 to 5 times the loss of the UHVDC.

Superconductive transmission lines make sense within dense environments, especially when trying to reuse existing utility tunnels and corridors. But even in suburban densities the UHVDC has the advantage, it requires no more right-of-way than existing transmission lines, and in some cases less. There's been work done on retrofitting existing lines, adding the UHVDC while maintaining the AC lines. After completion the AC line would be segmented and feed using special converter stations from the UHVDC, providing conventional access to smaller consumers/utilities on the route while still delivering more power to urban hubs.

ShadowWarrior4444 - 13-5-2008 at 19:08

Quote:

Originally posted by not_important
Ultra-high voltage DC lines are tough competition for long haul applications. Loses of 2% over a 2000 km 12 GW line can be had for no more cost than existing lower voltage DC and less than AC lines, both at 3 to 5 times the loss of the UHVDC.

Superconductive transmission lines make sense within dense environments, especially when trying to reuse existing utility tunnels and corridors. But even in suburban densities the UHVDC has the advantage, it requires no more right-of-way than existing transmission lines, and in some cases less. There's been work done on retrofitting existing lines, adding the UHVDC while maintaining the AC lines. After completion the AC line would be segmented and feed using special converter stations from the UHVDC, providing conventional access to smaller consumers/utilities on the route while still delivering more power to urban hubs.

Have the difficulties controlling corona discharge been resolved in HVDC lines? Would you happen to know what insulation is used?

not_important - 13-5-2008 at 19:41

Quote:

Originally posted by ShadowWarrior4444
Have the difficulties controlling corona discharge been resolved in HVDC lines? Would you happen to know what insulation is used?

Google UHVDC and you should get several useful hits, including papers from a conference and field tests.

MagicJigPipe - 13-5-2008 at 20:08

See! Edison didn't completely loose the Current War. His brainchild (DC Transmission) is making a comeback long after his death!

ShadowWarrior4444 - 13-5-2008 at 21:04

Quote:

Originally posted by not_important
Google UHVDC and you should get several useful hits, including papers from a conference and field tests.

Hmmm, the Beijing conference report seems to indicate that they are still using silicone rubber as the insulation of choice for UHVDC transmission lines.

I must say, having worked with plasmas and HV cables in the past I particularly dislike HVDC as a mass power transmission system--kilovolts can do very annoying things when operating conditions are not flawless. They also still need to use those pesky static inverter plants, within which a great deal of power is lost.

I personally am waiting for the development of spin-aligned metallic hydrogen cables, room temperature superconductivity will be fun. In addition, it may be possible to overlay data transmission onto a superconducting power cable, which shouldn’t really be attempted with HVDC, unless you like plasma tweeters.

P.S. Sorry for the thread hijacking so: Metastable solid Helium as a power source will be fun too!

[Edited on 5-14-2008 by ShadowWarrior4444]

MagicJigPipe - 13-5-2008 at 21:35

Isn't AC transmitted in multiple kilovolts, as well?

[Edited on 5-13-2008 by MagicJigPipe]

not_important - 13-5-2008 at 21:50

Yes, but for a given cross-section line inductance limits the current sooner than with DC. The peaks of AC also push it into breakover sooner; for a voltage V as DC or RMS AC (same power) the AC peak is 1,414 x V, meaning 850 kVAC is hitting peaks of 1,200 kV while 850 kVDC has peaks of 850 kVDC.

[Edited on 14-5-2008 by not_important]

MagicJigPipe - 14-5-2008 at 17:41

Another thing I don't understand is how DC can be generated directly by conventional means. I always thought that AC's sine wave was caused by the rotation of a magnet's north and south poles inside a coil of wire at ~60Hz. How could anything but AC be generated this way?

I used to know quite a bit about electronics but for some reason I never studied this type of thing much.

not_important - 14-5-2008 at 17:52

The generation is still conventional AC, which is rectified for the 800+ kV DC for transmission. Even with the AC to DC to AC conversion for longer transmission lines the loses are less than with AC transmission.

Note that some superconductive transmission line schemes are also based on DC, again because of the additional reactive loses from AC.

Nixie - 14-5-2008 at 18:22

You can build pure DC transformers with no conversion to AC, but you need superconductors.
http://nobelprize.org/nobelfoundation/symposia/physics/ncs-2...

ShadowWarrior4444 - 14-5-2008 at 19:41

Quote:

Originally posted by not_important
The generation is still conventional AC, which is rectified for the 800+ kV DC for transmission. Even with the AC to DC to AC conversion for longer transmission lines the loses are less than with AC transmission.

Note that some superconductive transmission line schemes are also based on DC, again because of the additional reactive loses from AC.

I can forgive using DC in a superconductor--the main reason to use AC in a conductor is the decrease in resistive losses.

It should also be noted that even though HVAC has higher peak voltages than DC, AC does not have the tendency to develop a capacitance with its own insulator. This capacitance, however small, can cause catastrophic flash-overs in a UHVDC line. Though, this is not my *main* problem with UHVDC--that would be the logistics of the requisite static inverter plants; they’re noisy, inefficient, and bring woe upon the staff technicians. They do look pretty, though--very futuristic.

P.S. I would also like to see the official live-line repair techniques for UHVDC. Taking the worker up to 850kv would not be recommended. Perhaps an integrated conductor faraday cage suit, conductive grid along the outside of a full body silicone suit. This would at least protect the worker, though it may still cause problems.

[Edited on 5-14-2008 by ShadowWarrior4444]

12AX7 - 14-5-2008 at 21:46

Quote:

Originally posted by ShadowWarrior4444
I can forgive using DC in a superconductor--the main reason to use AC in a conductor is...

...Because Tesla noticed it can be stepped up and down with transformers. We've been "stuck" with AC ever since.

Quote:

AC does not have the tendency to develop a capacitance with its own insulator.

Excuse me? Abuse of terminology I think.

Quote:

This capacitance, however small, can cause catastrophic flash-overs in a UHVDC line.

Do you mean the line capacitance storing energy? AC lines do that too. And they have the reactors (big fat inductors sitting in the substation yard aside the transformers) to cancel it, so the lines carry mostly real current.

Quote:

Though, this is not my *main* problem with UHVDC--that would be the logistics of the requisite static inverter plants; they’re noisy, inefficient, and bring woe upon the staff technicians. They do look pretty, though--very futuristic.

I don't know much about the efficiency, but I'd just as soon assume they're on the same order as other inverter and transmission technologies -- SCRs and IGBTs have a couple volts drop out of a couple kilovolts per device, so you can expect losses on the order of roughly 0.1% for the silicon -- essentially negligible. This will be swamped by the greater losses of transformers and reactive components, which might constitute a few percent (2-10% is typical for transformers, but I'd hesitate to guess as much as 10% for transformers of this scale -- after all, we're talking tens of megawatts of loss to dissipate!).

I wouldn't think they'd be very noisy, but then again maybe not. I found a ref which states inverter transformers are ran at higher field strength, which generates more magnetostriction in the core = more buzz. I also found a ref that states buildings should be considered for noise abatement, so it's not like they aren't considering it.

Quote:

P.S. I would also like to see the official live-line repair techniques for UHVDC. Taking the worker up to 850kv would not be recommended. Perhaps an integrated conductor faraday cage suit

Well, how is that any different from how they do HV AC maintainance already? So your hair stands on end, so what, they're probably required to get a hair cut anyway.

Tim

ShadowWarrior4444 - 14-5-2008 at 22:44

Quote:

Do you mean the line capacitance storing energy? AC lines do that too.

All of my comments regarding the capacitance of an HVDC line are taken directly from the Beijing conference reports. AC lines cannot accumulate charge on the insulator because AC passes through a capacitor--capacitors in an AC line act as resistors. (I am not speaking of inductance.) In a DC line, the insulating dielectric will become charged, should this exceed the maximum tolerance for the insulator, dielectric breakdown will occur causing a flash-over on the line. Though, to their credit, the engineers in Beijing stated that they had reduced this occurrence to 0.5 events per year.

Quote:

And they have the reactors (big fat inductors sitting in the substation yard aside the transformers) to cancel it, so the lines carry mostly real current.

Reactors are used in AC lines to limit fault current, and in DC lines to remove any residual oscillation and filter out radiofrequencies.

Quote:

Well, how is that any different from how they do HV AC maintainance already? So your hair stands on end, so what, they're probably required to get a hair cut anyway.

14kv maximum on AC transmission lines is very much different from 850kv DC-- static charge may acumulate on the worker, resulting in some unpleasentness should he come too close to ground. Effectively, 850kv has alot more 'jumping' power than 14kv. This is another dificulty encountered when constructing inverter plants.

As a side note: I am not, despite appearances, a Tesla fanboi--I simply would like *most* of the kinks in UHVDC to be worked out before going on an infrastructure upgrade spree. Electrodynamics is a much more mature science than electrostatics; and at HVDC, electrostatic phenomina come into play quite heavily.

-jeffB - 15-5-2008 at 06:54

Quote:

Originally posted by ShadowWarrior4444
All of my comments regarding the capacitance of an HVDC line are taken directly from the Beijing conference reports. AC lines cannot accumulate charge on the insulator because AC passes through a capacitor--capacitors in an AC line act as resistors. (I am not speaking of inductance.) In a DC line, the insulating dielectric will become charged, should this exceed the maximum tolerance for the insulator, dielectric breakdown will occur causing a flash-over on the line. Though, to their credit, the engineers in Beijing stated that they had reduced this occurrence to 0.5 events per year.

But the insulating dielectric in an AC line gets charged, too, it's just that it gets charged in alternating directions 60 times per second. They must be talking about some sort of longer-time-scale polarization of the dielectric, but I don't know what the mechanism or consequences would be.

Quote:

Okay, I can see that. 765KV AC would have comparable ability to "reach out and touch someone", but the notion of keeping the charge when you pull away from the line is important. On the other hand, the capacitance of an isolated human body is pretty tiny, so the amount of charge you'd actually keep from the DC line seems like it would be smallish.

12AX7 - 15-5-2008 at 09:28

Quote:

Originally posted by ShadowWarrior4444

Quote:

Do you mean the line capacitance storing energy? AC lines do that too.

AC lines are not coupled through capacitors, that would waste energy. They are straight from transformer to transformer, with only reactive components alongside (mainly reactors to tune the line).

Quote:

In a DC line, the insulating dielectric will become charged, should this exceed the maximum tolerance for the insulator, dielectric breakdown will occur causing a flash-over on the line. Though, to their credit, the engineers in Beijing stated that they had reduced this occurrence to 0.5 events per year.

So what are you trying to say, creepage is worse over insulators?

Quote:

Well, how is that any different from how they do HV AC maintainance already? So your hair stands on end, so what, they're probably required to get a hair cut anyway.

14kv maximum on AC transmission lines is very much different from 850kv DC

Where did I say 14kV? That stuff is almost trivially worked on, with poles or insulated buckets. I was referring to lines of the same magnitude: they fly up in helicopters and sit on the line. Matter of fact, it would be even safer than working on AC lines, because the heli and worker need only charge up once, not 120 times per second. (There would still be a continuous current, because the heli has corona discharge points, so the charge rod would just spark less often than on an AC line.)

Tim

ShadowWarrior4444 - 15-5-2008 at 13:49

Quote:

AC lines are not coupled through capacitors, that would waste energy. They are straight from transformer to transformer, with only reactive components alongside (mainly reactors to tune the line).

I never said they were, I have been talking about the capacitance of the *insulator.* The silicone insulator acts as a capacitor's dielectric. AC does not accumulate a charge on that dielectric, DC does.

Quote:

Where did I say 14kV? That stuff is almost trivially worked on, with poles or insulated buckets. I was referring to lines of the same magnitude: they fly up in helicopters and sit on the line. Matter of fact, it would be even safer than working on AC lines, because the heli and worker need only charge up once, not 120 times per second. (There would still be a continuous current, because the heli has corona discharge points, so the charge rod would just spark less often than on an AC line.)

You can't possibly be serious about bringing a heli up to 850kv DC; it would act like the collector on a Van De Graff generator, not to mention the massive corona and resultant ozone generation would ensure that heli's maintenance costs skyrocket.

[Edited on 5-15-2008 by ShadowWarrior4444]

MagicJigPipe - 15-5-2008 at 14:16

Quote:

they fly up in helicopters and sit on the line.

That does sound INCREDIBLY dangerous even if the wires weren't live. That would take much skill and even then...

12AX7 - 15-5-2008 at 14:25

Quote:

Originally posted by ShadowWarrior4444
I never said they were, I have been talking about the capacitance of the *insulator.* The silicone insulator acts as a capacitor's dielectric. AC does not accumulate a charge on that dielectric, DC does.

Oh, well the insulators don't have much capacitance, maybe a few hundred picofarads I would guess depending on internal structure. Due to the capacitance acting as a reactive voltage divider, the AC potential will be evenly distributed along it; due to leakage (noticably depending on surface condition), DC will distribute along the insulator as well. Creepage is a problem on any high voltage insulator, this is hardly a new problem.

Quote:

Hey, it works for AC. You think AC doesn't also give corona? I'd bet the blades look like a circle of UV radiation when observing this aerobatic act through a UV camera (they have UV cameras to detect corona leakage, as a matter of fact... looks like fun stuff). The heli is only connected for a minute while the lineman moves on or off, so the total amount of loss, corona, ozone (do you even know how dilute that ozone actually is? Especially with the thousands of CFM of turbulent air whipping past the blades?) is negligible. AC would be more hazardous to the avionics than DC, as it's AC for one, and the sparking is continuous until a direct connection is made. But they do just fine, so you need not worry.

Tim

ShadowWarrior4444 - 15-5-2008 at 15:01

Quote:

*bursts into tears!* AC will not charge a heli like it was the sphere on top of a van de graff generator. (Whereas DC will make it into a floating ball of happy little sparks.)

Unless they plan to do something tricky, that is. This is quite a bit of speculation, hence why I wanted to see the *official* UHVDC maintenance procedures. They must be outlined somewhere, either in conference reports or service logs from the prototype system.

Quote:

Oh, well the insulators don't have much capacitance, maybe a few hundred picofarads I would guess depending on internal structure. Due to the capacitance acting as a reactive voltage divider, the AC potential will be evenly distributed along it; due to leakage (noticeably depending on surface condition), DC will distribute along the insulator as well. Creepage is a problem on any high voltage insulator, this is hardly a new problem.

The seriousness of a dielectric breakdown in an HVDC system is quite heavy. One breakdown can cause the vaporization of the surrounding insulation, leaving the wire unprotected. This is not so much a concern about creepage and tracking damage as it is about sudden and catastrophic faults.

(Note: 150 picofarad[.00015 microfarad] at 850kv=50 joules, which is already entering the 'range of enjoyability.')

[Note 2: This whole bit isn't off-topic at all, really! "He" could mean alot of things.]

[Edited on 5-15-2008 by ShadowWarrior4444]

12AX7 - 15-5-2008 at 19:22

Quote:

Originally posted by ShadowWarrior4444
*bursts into tears!* AC will not charge a heli like it was the sphere on top of a van de graff generator. (Whereas DC will make it into a floating ball of happy little sparks.)

Except it DOES. One hundred and twenty times every second! (And at a higher peak voltage, as has already been mentioned earlier in this thread.)

I'm surprised you aren't comparing the AC-charged heli to a pointy-topped Tesla coil. Tesla coils and VdG's are both high voltage devices, and they both produce corona discharges and arcing when the electric fields around them become strong enough to ionize the surrounding air. Voltage is voltage and the rate doesn't really matter, not under 1MHz. Lines are no different. These concerns are for the power companies to consider, which since HV AC appears to be successful, they shouldn't be too concerned about.

Quote:

The seriousness of a dielectric breakdown in an HVDC system is quite heavy. One breakdown can cause the vaporization of the surrounding insulation, leaving the wire unprotected.

The wire is already unprotected. If they're using the same strategy as over-land HV AC lines, they're bare aluminum with a steel core. There isn't any insulation cheaper than raw distance! (These installations must be awesome to watch in the rain. Just imagine the rain attracted and pushed away by electrostatic force!) Undersea cables obviously aren't so lucky.

Quote:

This is not so much a concern about creepage and tracking damage as it is about sudden and catastrophic faults.

(Note: 150 picofarad[.00015 microfarad] at 850kv=50 joules, which is already entering the 'range of enjoyability.')

Yup. Lots of "enjoyables" when you get into the megavolt range.

For extra credit (show your work), calculate the bulk capacitivity (capacitance per unit length) of a line (assume in free space), then the amount of energy stored in, say, a hundred feet of that line. (You might also calculate how long it takes to discharge, assuming velocity factor = 1, and from that, the average current during that discharge, and thus the approximate impedance of the line.) Finally, calculate the energy stored in the same capacitance at the waveform peak of today's HV AC lines, and how much reactive power is spent charging those lines 120 times per second.

Tim

ShadowWarrior4444 - 15-5-2008 at 20:38

Quote:

"Although the results are still limited, considering the urgent need of the coming UHVDC projects, recommendations have been given for the suitable shed profiles for silicone rubber insulators of a large diameter and being installed at vertical position [7]"
[7] W.M. Ma, B. Luo, Z.Y. Su, Z.P. Dang, Z.C. Guan, X.D. Liang, U. Åström, D. Wu, E.Y. Long,
H.G. Sun, ”Preliminary recommendations on the suitable shed profile for HVDC station
insulators with silicone rubber housing”

"The silicone rubber insulators with a shorter creepage distance than that of porcelain insulators in HVDC station have operated satisfactorily. The use of silicone rubber insulators has also contributed to the achievement of the low pollution flashover rate."
"Operational experience published in literatures on DC line insulators, e.g. [2-3], confirmed the fact that silicone rubber insulators can perform well with a shorter creepage distance than that of porcelain insulators. Flashovers caused by pollution are not any more the major contributing factors to the failures of the outdoor insulation."

[2] S.S. Low, G.R. Elder, “Experience dictates future insulator requirements” IEEE
Transactions on Electrical Insulation, Vol. EI-16, No. 3, June 1981, pp. 263-266
[3] X.D. Liang, S.W. Wang, Z.Y. Su, “Experience with composite HVDC and HVAC
insulators in China: from design to operation” Proceeding of 2003 World conference on
Insulators, Arresters, and Bushings, pp15-22

Quote:

AC, regardless of frequency, has a different interaction with a capacitor than DC does. The helicopter, a van de graff sphere, and a tesla toroid are all capacitors. When the van de graff generator is turned off, it retains its charge. If you run HVAC through a capacitor, like a tesla toroid, it will be capacitively coupled out. This was the basis for tesla's wireless energy transmission system.

[Edited on 5-15-2008 by ShadowWarrior4444]

ShadowWarrior4444 - 15-5-2008 at 21:17

Ancillary Idea: How feasible would it be to transmit power at very low frequency (1Hz) then return it to 60Hz without using an inverter? Perhaps a device such as
http://www.wikipatents.com/4378587.html would be effective. If the power was transferred at UHV, it should have the same benefits of UHVDC--at low frequency the parasitic inductance will be quite low, and the insulators will not accumulate a charge. A static inverter plant will also not be required.

Perhaps if the power were transferred as a polyphase system, it would be easier to return to 60Hz, as well.

(Obligatory notice of Berne Convention attachment, if it does. *quizzical look*)

Nixie - 15-5-2008 at 21:48

What a horrid interface wikipatents has. And where's the PDF?
Much better: http://www.google.com/patents?id=bKErAAAAEBAJ&dq=4378587

MagicJigPipe - 15-5-2008 at 21:58

Uhhh... The interface doesn't look that bad to me. I found the .pdf file in less than 2 seconds. I mean, it's right there. You just have to register like freepatents.com. No big deal.

ShadowWarrior4444 - 15-5-2008 at 22:11

Quote:

Originally posted by Nixie
What a horrid interface wikipatents has. And where's the PDF?
Much better: http://www.google.com/patents?id=bKErAAAAEBAJ&dq=4378587

I prefer google patents to most other patent sites as well, their UI is vastly superior. Thank you for the link.

It *does* seem that using a mechanical interface would be much more efficient than an inverter--it would be akin to the flywheel devices currently used by power companies to store energy. (I seem to recall electrical energy to mechanical energy conversions to be some of the most efficient possible.)

Nixie - 15-5-2008 at 22:17

ShadowWarrior4444, you seem to be knowledgeable about things electric, so perhaps you could help me with my X-ray machine power supply, seeing as to how 12AX7 is ignoring my PMs.

ShadowWarrior4444 - 15-5-2008 at 22:25

Quote:

Originally posted by Nixie
ShadowWarrior4444, you seem to be knowledgeable about things electric, so perhaps you could help me with my X-ray machine power supply, seeing as to how 12AX7 is ignoring my PMs.

I may just be able to! I've had some experience with compact medical x-ray machines:

The newer ones use a linac--an electron gun at one end of a vacuum tube and a tungsten target at the other. 100kv is sent through the electron gun, and as the high energy electrons hit the tungsten target, the bremsstrahlung (breaking radiation) is given off as x-rays.

12AX7 - 15-5-2008 at 23:35

Quote:

Originally posted by ShadowWarrior4444
"The silicone rubber insulators with a shorter creepage distance than that of porcelain insulators in HVDC station have operated satisfactorily. The use of silicone rubber insulators has also contributed to the achievement of the low pollution flashover rate."
"Operational experience published in literatures on DC line insulators, e.g. [2-3], confirmed the fact that silicone rubber insulators can perform well with a shorter creepage distance than that of porcelain insulators. Flashovers caused by pollution are not any more the major contributing factors to the failures of the outdoor insulation."

Yeah, so they're talking about the insulators, I guess silicone coated porcelain? Sounds like a good idea. If that flashed over, it would leave carbon soot and SiO2 ash, which is potentially better or worse than otherwise. The wire is already bare, it's not going to be any worse for wear except for a black mark (and probably a generous crater from the arc, depending on amperage).

Quote:

AC, regardless of frequency, has a different interaction with a capacitor than DC does.

This is a false statement: any practical implementation of DC is merely AC of relatively low frequencies (perhaps nanohertz for long term HV DC lines). True DC is the limit as frequency goes to zero, with some nonzeroing phase (since any amplitude of sin(pi * n), n = integers, is still zero). So to say there is an interaction regardless of frequency, then state that there is a difference *at a frequency*, is a direct contradiction.

I already stated the practical ways in which voltage is distributed along an insulator.

Quote:

The helicopter, a van de graff sphere, and a tesla toroid are all capacitors. When the van de graff generator is turned off, it retains its charge. If you run HVAC through a capacitor, like a tesla toroid, it will be capacitively coupled out. This was the basis for tesla's wireless energy transmission system.

So? And when you run HV AC through a helicopter, it will capacitively couple some energy out as well. And when you charge a helicopter with HV DC, it too will retain a charge (in as much as the corona bleeders on the airframe allow). At 60Hz, we aren't talking much capacitive transmission -- the lines themselves make far better antennae than a puny helicopter. I mean, the wavelength is some 5000 kilometers, it takes a lot of length to make any practical electromagnetic radiation.

Did you try any of those estimations I suggested? Now that I've said that, I'm getting curious about the exact numbers myself.

Tim

not_important - 16-5-2008 at 01:01

Anytie you get above a handfull of kV, things start getting tricky regardless if it's AC or DC. While Tim pushes detailed technical points, I will just comment that HVDC is not new, and is in use in many locations. The Quebec - New England transmission line is nearly 1500 km long, operates at +-450 kV, and is rated at 2 GW. The reasons DC is chosen over AC is not because it kewl, but because it is lower cost to install and operate.

DC transmission is most useful for long haul use, where besides greater efficiency and lower cost, it has the additional benefit of not having to worry about synchronisation of the power networks at either end; grid stability issues generally are less.

Tossing out some material from several papers on the subject:

Quote:

The field and corona effects of transmission lines largely favor d.c. transmission over a.c. transmission. For a given power transfer requiring extra high voltage transmission, the d.c. transmission line will have a smaller tower profile than the equivalent a.c. tower carrying the same level of power. This can also lead to less width of right-of-way for the d.c. transmission option. Due to the space charge formed around the conductors, an HVDC system may have about half the loss per unit length of a high voltage AC system carrying the same amount of power.

Long undersea cables have a high capacitance. While this has minimal effect for DC transmission, the current required to charge and discharge the capacitance of the cable causes additional I2R power losses when the cable is carrying AC. In addition, AC power is lost to dielectric losses.

Moreover, modern HVDC systems are designed to operate unmanned. This feature
is particularly important in situations or countries where skilled people are few, and these few people can operate several HVDC links from one central location.

Maintenance of HVDC systems is comparable to those of high voltage AC systems. The high voltage equipment in converter stations is comparable to the corresponding equipment in AC substations, and maintenance can be executed in the same way. Maintenance will focus on: AC and DC filters, smoothing reactors, wall bushings, valve-cooling equipment, thyristor valves. In all the above, adequate training and support is provided by the supplier during the installation, commissioning and initial operation period.

Normal routine maintenance is recommended to be one week per year. The newer systems can even go for two years before requiring maintenance. In fact in a bipolar system, one pole at a time is stopped during the time required for the maintenance, and the other pole can normally continue to operate and depending on the in-built overload capacity it can take a part of the load of the pole under maintenance.

Some of these aspects are:
• No limits in transmitted distance. This is valid for both OH lines and sea or underground cables.
• Very fast control of power flow, which implies stability improvements, not only for the HVDC link but also for the surrounding AC system.
• Direction of power flow can be changed very quickly (bi-directionality).
• An HVDC link does not increase the short-circuit power in the connecting point. This means that it will not be necessary to change the circuit breakers in the existing network.
• HVDC can carry more power for a given size of conductor
• The need for ROW (Right Of Way) is much smaller for HVDC than for HVAC, for the same transmitted power. The environmental impact is smaller with HVDC.
• VSC technology allows controlling active and reactive power independently without any needs for extra compensating equipment.
• VSC technology gives a good opportunity to alternative energy sources to be economically and technically efficient.
• HVDC transmissions have a high availability and reliability rate, shown by more than 30 years of operation

http://www.areva-td.com/scripts/solutions/publigen/content/t...

http://www.hvdc.ca/pdf_misc/dcsum.pdf

http://www.worldbank.org/html/fpd/em/transmission/technology...

http://www.rmst.co.il/HVDC_Proven_Technology.pdf

ShadowWarrior4444 - 16-5-2008 at 01:58

*huggles not_important for the information and references*

Quote:

Silicone coated porcelain would be far too expensive to implement effectively--they were speaking about silicone vs porcelain, the results being that silicone was found to be more effective. And... destroyed insulation, especially in the inverter plant, or anywhere close to ground wouldn’t cause just *one* arc.

Quote:

This is a false statement: any practical implementation of DC is merely AC of relatively low frequencies (perhaps nanohertz for long term HV DC lines).

AC power must alternate, not simply waiver slightly. If the polarity of that electricity never reverses, it is not alternating current.

Quote:

Yes, at the low freq of 60Hz, there isn't much capacitive coupling, however due to the alternating nature of the current, whatever is not capacitively coupled out of the heli is neutralized by the change in polarity. This is why a capacitor will act as a resistor in an AC circuit; since there is no polarity reversal in a DC circuit, the capacitor charges. Infact, the lower the frequency, the more resistive a capacitor is, the reverse being true of an inductor. Hence why inductors called radiofrequency chokes can be used to filter out hi-freq interference.

With regard to not_important's information, it appears that they have thought of a maintenance plan involving the deactivation of whichever pole is to be repaired. This is as I suspected--bringing a worker or chopper up to 850Kv would be too hazardous, requiring more elaborate grounding protocols.

I'm still rooting for the spin-aligned metallic hydrogen, though.

Ancillary: Are there any opinions on my conception of a low-frequency (1Hz), UHV transmission system? [It may have been lost in the torrent.]

not_important - 16-5-2008 at 03:02

Quote:

Originally posted by ShadowWarrior4444
Ancillary Idea: How feasible would it be to transmit power at very low frequency (1Hz) then return it to 60Hz without using an inverter? Perhaps a device such as
http://www.wikipatents.com/4378587.html would be effective.

They issued a patent in 1983 for a dynamotor?

Quote:

If the power was transferred at UHV, it should have the same benefits of UHVDC--at low frequency the parasitic inductance will be quite low, and the insulators will not accumulate a charge. A static inverter plant will also not be required.

Perhaps if the power were transferred as a polyphase system, it would be easier to return to 60Hz, as well.

(Obligatory notice of Berne Convention attachment, if it does. *quizzical look*)

Even ULF AC has the problem of peak vs RMS voltage, you need additional clearance for that 1,4 X peak voltage.

While inductive losses will drop, you'll still have losses from charging and discharging the capacitance in the system.

Note that step-up/down transformers are going to be much larger than those for normal mains frequency.

I think you'd have to model the system, or at least the transmission line, to see how it would act with the lower frequency. Even at 1 Hz a power line that's hundreds of km long is a transmission line with reactive components.

I don't have hard number now, but I believe that static inverters are more efficient than motor-generator pairs; plus they do not have moving parts to deal with. The motor-generator could be more desirable if couple with storing energy in rotating mass as an integrated unit.

[Edited on 16-5-2008 by not_important]

12AX7 - 16-5-2008 at 10:26

Quote:

Originally posted by ShadowWarrior4444
Silicone coated porcelain would be far too expensive to implement effectively--they were speaking about silicone vs porcelain, the results being that silicone was found to be more effective.

I don't think so, the insulator could be dipped or sprayed with silicone. There's got to be a strong insulating support somewhere in there, it's not exactly feasable to anchor the wire into the tower with a big bar of steel and slather that with silicone, eh?

Quote:

This is a false statement: any practical implementation of DC is merely AC of relatively low frequencies (perhaps nanohertz for long term HV DC lines).

AC power must alternate, not simply waiver slightly. If the polarity of that electricity never reverses, it is not alternating current.

LOL, what is this "waiver" business? I have plenty of experience in electronics and let me tell you, 60Hz is one lazy waiver! Here's some scale: your average jellybean transistor reacts in a tenth of a microsecond. Anything that happens over tens of miliseconds (line frequency is a good 20,000 times slower than your average transistor) is essentially "DC".

A strict definition of DC (i.e., lim F --> 0) is impractical. Hence DC is a viewpoint. A high frequency circuit may operate at, say, 20GHz and hence consider variations on the order of 10MHz and below "DC". An AC-coupled audio amplifier might consider 20Hz as AC (since it's at the end of the audio band, and thus a signal of interest) and below 5 or 10Hz as DC (discarding it, since it's an AC amplifier). A DC-coupled amplifier may not have any preference for calling some band AC or DC (because it's all important signal), but a practical point for the signal to become AC might be considered when the amplifier's gain starts rolling off.

Quote:

Ancillary: Are there any opinions on my conception of a low-frequency (1Hz), UHV transmission system? [It may have been lost in the torrent.]

Horribly impractical. The transformers would be 3600 times larger for the same power level and power loss.

Tim

ShadowWarrior4444 - 16-5-2008 at 12:06

Quote:

Originally posted by 12AX7

Quote:

The silicone rubber insulators even with shorter creepage distance than that of porcelain insulators have been proven to be a good alternative in many stations. Today, silicone rubber housing is used for all equipments in HVDC stations. Even for station post insulators, composite types with silicone rubber sheds are available and under evaluation.

The use of hydrophobic coatings and booster sheds on porcelain insulators have been proven, both in operation and laboratory tests, to be good alternatives for strengthen the insulation. Although, conventionally, these alternatives have been considered as only remedy methods for pollution flashovers, when come to the severely polluted conditions, they are competitive alternatives to indoor DC yard.

Insulators with resistive glaze have superior pollution performance under AC voltage and even under DC voltage in laboratory pollution tests. However, such insulators available on market failed to pass the 1000 horse salt-fog test under DC voltage.
D. Wu, Z. Su, “The correction factor between DC and AC pollution levels: review and
proposal”, 10th ISH, August 25-29, 1997, Montreal, Canada.

To wit: Porcelain and Silicone are both competing insulators, if you were to use porcelain, you would not be using silicone on the same thing--the porcelain insulates from HVDC all by itself; therefore using both on the same device would waste money.

Quote:

I will say it again, if the current never reverses polarity, it is DC. Calling an alternating signal "DC" is a horrible thing to do--it will lead to fried diodes very quickly. (As well as anything else polarity dependant.) 20GHz electronics do not treat lower frequencies as "DC," they treat them as low-frequency AC, and usually install high-pass filters to attenuate those frequencies.

While you can make the argument (as you seem to be doing) that in a given time frame, an AC line would have current traveling in only one direction, this would be a highly impractical definition for DC. Perhaps if the circuit only existed during that one time frame? But as the polarity *will* eventually reverse, it is not a DC signal.

To not_important:

Quote:

Note that step-up/down transformers are going to be much larger than those for normal mains frequency.

Would it be practical to split the incoming AC with voltage dividers, then send it to motor/generator pairs. After conversion to mains frequency, it could *then* be fed into transformers. Or perhaps run a series of motors directly off the transmission line.

Quote:

I don't have hard number now, but I believe that static inverters are more efficient than motor-generator pairs; plus they do not have moving parts to deal with. The motor-generator could be more desirable if couple with storing energy in rotating mass as an integrated unit.

This was a thought I had when looking at the design of the frequency converter, especially since most power companies already have extensive existing flywheel storage facilities. Granted, some use superconducting rings or gas pressure storage.

Are there any other efficient frequency converters?

[Edited on 5-17-2008 by ShadowWarrior4444]

12AX7 - 16-5-2008 at 14:18

Motors operate on the same principles as transformers, therefore a 1Hz motor is also 3600 times larger than a 60Hz unit of the same power and loss.

Tim

not_important - 16-5-2008 at 22:06

Quote:

Originally posted by ShadowWarrior4444...
To not_important:

Quote:

Note that step-up/down transformers are going to be much larger than those for normal mains frequency.

Well, first off you need to step up the voltage at the sending/generating end, generators just don't crank out power at 100s of kV potential. A divider is no help there.

And what do you mean by "voltage dividers"? Resistive ones waste power. Capacitive dividers? I suggest you do the calculations for what is needed to work with several thousand amps at 1 Hz; those are going to be really big caps. Inductive dividers are effectively transformers.

As already noted, the lower the frequency the larger and magnetic machine gets. The voltage and power levels are non-trivial, SFAIK large generators and motors work with voltages from a few kV to maybe 15 kV. That means you're looking at a string of 30 or so motors, each of which must be securely mounted but electrically isolated (remember the high v side one will see peaks of 600 to 1200 kV above ground).

Quote:

The flywheel storage systems that I know of deliver 10 to 100 of KW for a few seconds, giving time for a backup power generator to come on-line. Several utilities have asked for proposals for flywheel system capable of delivering a few MW for a few 10s of seconds - maybe a minute. This is far less than the 100s of megawatts to several GW of a large power transmission system. If power companies are using flywheel storage, I'm betting it's part of a UPS system to keep the plant up if the grid goes down, avoiding a black start.

I did some searching and found details on several existing, being built, or in planning, HVDC systems. From those it looks as the total loss of the AC=>DC=>AC conversion runs 1,5 to 2 percent. Good electric motors and generators run around 95% efficient, so the dynamotor combination would be expected to run 90% efficient or so. You're dealing with friction, copper, and iron losses; I'm not sure how the lower frequency would affect the last two.