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not_important
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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.
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ShadowWarrior4444
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| 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?
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not_important
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| 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?
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Google UHVDC and you should get several useful hits, including papers from a conference and field tests.
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MagicJigPipe
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See! Edison didn't completely loose the Current War. His brainchild (DC Transmission) is making a comeback long after his death!
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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ShadowWarrior4444
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| 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]
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MagicJigPipe
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Isn't AC transmitted in multiple kilovolts, as well?
[Edited on 5-13-2008 by MagicJigPipe]
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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not_important
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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]
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MagicJigPipe
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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.
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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not_important
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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.
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Nixie
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You can build pure DC transformers with no conversion to AC, but you need superconductors.
http://nobelprize.org/nobelfoundation/symposia/physics/ncs-2...
\"Good is a product of the ethical and spiritual artistry of individuals; it cannot be mass-produced.\" --Aldous Huxley
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ShadowWarrior4444
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| 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.
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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]
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12AX7
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| 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
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ShadowWarrior4444
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| 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.
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-jeffB
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| 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: | | 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. |
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.
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12AX7
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| Quote: | Originally posted by ShadowWarrior4444
| 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. |
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: |
| 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
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ShadowWarrior4444
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| 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]
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MagicJigPipe
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| Quote: |
they fly up in helicopters and sit on the line.
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That does sound INCREDIBLY dangerous even if the wires weren't live. That would take much skill and even then...
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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12AX7
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| 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: | | 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. |
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
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ShadowWarrior4444
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| 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. |
*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.
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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]
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12AX7
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| 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
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ShadowWarrior4444
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| Quote: | | 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. |
"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: | 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. |
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]
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ShadowWarrior4444
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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*)
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Nixie
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What a horrid interface wikipatents has. And where's the PDF?
Much better: http://www.google.com/patents?id=bKErAAAAEBAJ&dq=4378587
\"Good is a product of the ethical and spiritual artistry of individuals; it cannot be mass-produced.\" --Aldous Huxley
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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.
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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ShadowWarrior4444
Hazard to Others
Posts: 226
Registered: 25-4-2008
Member Is Offline
Mood: Sunlight on a pure white wall.
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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.)
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