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Nixie
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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.
\"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 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.
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12AX7
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| 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
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not_important
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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
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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
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ShadowWarrior4444
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*huggles not_important for the information and references*
| Quote: | | 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). |
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: | | 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. |
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.]
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not_important
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| 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]
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12AX7
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| 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: | | 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
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ShadowWarrior4444
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| Quote: | Originally posted by 12AX7
| 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? |
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: | 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. |
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]
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12AX7
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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
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not_important
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| 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. |
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. |
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: | | 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? |
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.
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