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From: JosephKK on 16 Feb 2010 22:19
On Sun, 14 Feb 2010 21:53:51 -0600, "Tim Williams" <tmoranwms(a)charter.net> wrote:

>"Hammy" <spam(a)spam.com> wrote in message
>news:ggahn590bbl2lh3ku34a07qitr83ffhlur(a)4ax.com...
>> I'd say that's filed on under costly and difficult to implement.
>> Are you suggesting that all current pole transformers be replaced with
>> SMPS's like a buck or sepic?
>
>Exactly!
>
>I would guess polyphase forward converters more likely at that power level,
>but exactly what's used isn't a big deal.
>
>Not at all difficult, we have the technology. It would just be really,
>really expensive.
>
>The true expense might not be that big a deal, since pole pigs have their
>own cost. But they last for 30 or 50 years or more, and asking to replace
>all of them when they're younger than half their design life (on average),
>that essentially doubles the cost already.
>
>Tim

I have to agree there would be massive objection at the suburban residential
level. If you convert the pole rigs (or vault/pad transformers) too massive
inductors to provide 120/240 60 Hz single phase you might get it to go.

Not only that those pole pigs have turns ratios of 10:1, 20:1 and even 50:1,
i am not knowledgeable enough to evaluate the impact of that. Nor do i know
much about how solid state systems are made for 12 kV and up.
From: Paul Keinanen on 17 Feb 2010 03:18
On Tue, 16 Feb 2010 22:18:33 -0600, "Tim Williams"
<tmoranwms(a)charter.net> wrote:

>"JosephKK" <quiettechblue(a)yahoo.com> wrote in message
>news:5gnmn5lbbc5b3l9bct9i8tdola0uq6t0hr(a)4ax.com...
>> Nor do i know much about how solid state systems are made for 12 kV and
>> up.
>
>Do they even? Seems to me there's kind of a hole in the middle... VFDs are
>easy enough to build (easy being a relative term) from watts up to ~1MW. I
>know they make SCRs rated for 2kV or so, and even more amps, so you can get
>a few solid megawatts from just a few of the things.

Going from a single semiconductor to two or more semiconductors in
series seems to be a big threshold.

In EU and other countries using the IEC low voltage definition (< 1 kV
AC, <1.5 kVDC) using higher voltages will increase costs and reduce
availability of qualified persons. Thus, one tries to remain in the LV
area. For instance, wind turbines in the MW class often use the
standard 690 V voltage.


>For HVDC, they make monster cascodes of them, able to switch ~1MV and some
>kA's with moderate efficiency (the voltage drop is large over an entire
>stack, so effiency tops out in the 90-95% range IIRC -- an awful lot of heat
>when you're switching gigawatts, but still not terrible overall).
>
>I have no idea if anyone makes anything inbetween.

There is no real demand.

>Megawatt motors are
>often supplied by AC in the 10s of kV range, but I don't know if anyone
>makes a VFD-like supply using the same voltage internally (without passing
>it through a transformer to make ~1kV instead).

Since big (1000+ MW) nuclear power station generators operate around
10 kV, I do not understand, why a <100 MW motor would require anything
higher than 10 kV.

From: Przemek Klosowski on 21 Feb 2010 21:31
On Sun, 14 Feb 2010 17:30:48 -0600, Tim Williams wrote:

> Not too bad. The technology exists today to replace our infrastructure
> with DC -- same voltages for transmission efficiency reasons,

Remember that peak voltage of an AC transmission line is 1.414 higher than
the nominal RMS value. Normally this doesn't matter: you just space the
conductors a little farther on the HV pylon---but when the cable needs to
be buried or at the sea bed it needs to be much more expensive, so buried
or undersea transmission lines almost always are DC.
From: Paul Keinanen on 22 Feb 2010 01:30
On Mon, 22 Feb 2010 02:31:36 +0000 (UTC), Przemek Klosowski
<przemek(a)tux.dot.org> wrote:

>On Sun, 14 Feb 2010 17:30:48 -0600, Tim Williams wrote:
>
>> Not too bad. The technology exists today to replace our infrastructure
>> with DC -- same voltages for transmission efficiency reasons,
>
>Remember that peak voltage of an AC transmission line is 1.414 higher than
>the nominal RMS value. Normally this doesn't matter: you just space the
>conductors a little farther on the HV pylon---but when the cable needs to
>be buried or at the sea bed it needs to be much more expensive, so buried
>or undersea transmission lines almost always are DC.

A buried or undersea cable has a much higher phase-to-phase and
phase-to-ground capacitance than an overhead line. These capacitances
must be charged and discharged every half cycle, consuming a lot of
reactive power and hence current. With a very long cable, all the
current carrying capacity goes to this (dis)charging and no current
can be delivered to the load.

In principle this could be compensated with a compensation station
every few kilometers, but for reliability and serviceability, an
undersea compensation station is not a good idea.

On a DC link, the cable capacitance is not an issue, it is charged
once when the power is applied and finally discharged into the load,
when the source is disconnected perhaps within a few months.

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