At 01:01 PM 8/15/2006, Gary Schafer wrote:
>They use DC to prevent skin effect loss at AC.
also to make it possible to stabilize the system. It's easier to control
the power flow with DC... one end is a constant current source (or sink)
and the other is a constant voltage load(or source).
With AC, you have all sorts of phase shifting needed to control the power
flow. The management of active and reactive power flow in a large
interconnected system is a very big challenge (and, in fact, was
responsible for that big blackout a few years ago...). The relative phase
(and frequency) of the two systems that are interconnected determines the
power flow between the systems (higher frequency feeds power to lower
frequency, as I recall).
On short transmission lines, this isn't a huge problem.. you change the
source or sink reactance and the power flow changes fairly quickly. But on
a long (>100km) line, there's a fair amount of energy stored in the line
and you can get transients that take hours to die out. There's a story
about the Pacific Intertie (which goes from Grand Coulee dam to Southern
California) where they put a parallel (60Hz) line to double the capacity,
and it was impossible to stabilize the power flow between source and sink,
as well as the distribution of the power between the two lines. So, they
implemented a 500kV DC line that terminates in the Sylmar converter station
at the north end of the San Fernando Valley.
DC links are also used to interconnect systems that aren't inherently
synchronized. Since there's no reactive power to worry about with DC, you
don't care about the relative phase or frequency. There's several examples
of this. Itaipu Dam in Iguazu uses a DC link to move power between the
Paraguay side and the Brazil side (one of which is 50Hz and the other
60Hz). There's a short link in North Dakota also.
>73
>Gary K4FMX
>
> > -----Original Message-----
> > From: towertalk-bounces@contesting.com [mailto:towertalk-
> > bounces@contesting.com] On Behalf Of ve4xt@mts.net
> > Sent: Tuesday, August 15, 2006 2:21 PM
> > To: Alan NV8A; towertalk@contesting.com
> > Cc: Craig Clark
> > Subject: Re: [TowerTalk] High tension lines
> >
> > An interesting tidbit of info: in Manitoba, transmission lines from
> > generating stations to major centres are so long, they run them at DC,
> > because at 60 Hz they'd be... resonant.
> >
> > 73, kelly
> > ve4xt
> >
> >
> > > From: Alan NV8A <nv8a@att.net>
> > > Date: 2006/08/15 Tue PM 12:39:34 CDT
> > > To: towertalk@contesting.com
> > > CC: Craig Clark <jcclark@wildblue.net>
> > > Subject: Re: [TowerTalk] High tension lines
> > >
> > > Perhaps somewhere in my collection of clippings I still have the article
> > > I found many years ago in the reputable UK _Wireless World_
> > publication
> > > (which has since changed its name, I think). I don't recall the author's
> > > name, but he argued that the confusing findings in studies of possible
> > > correlation between proximity to HV lines and the incidence of cancer
> > > was due to the failure to distinguish between 50Hz transmission
> > systems
> > > and those using 60Hz. Somewhere between those frequencies, he
> > argued,
> > > was some crucial turning point in the effect on the human body.
> > >
> > > Alan NV8A
> > >
> > >
> > > On 08/15/06 06:30 am Craig Clark wrote:
> > >
> > > > With all due respect, there has not been a correlation of cancer to
> > > > high tension lines. In 1979, Brodeur wrote "The Zapping of America"
> > > > where he tried to correlate cancers to proximity to high tension
> > > > lines and electrical substations. He followed up with several other
> > > > long screeds in the New Yorker Magazine on the same subject. All
> > were
> > > > long on emotion but short on science.
> > > >
> > > > At Ham Radio Magazine, we supported quite a bit of research on RF
> > > > radiation and cancers working with some of the best in the field of
> > > > radio and epidemiology. As I remember, this was the area Overbeck
> > > > was most concerned about due to his activity on the VHF/UHF
> > > > bands. What we found was that non-ionizing radiation was unlikely to
> > > > cause any form of cancer. This is what you have around power lines
> > > > and HF amateur radio stations.
> > > >
> > > > I know nothing about the medical reasons of susceptibility to
> > > > cancer. I do know that Brodeur was manipulating facts to prove his
> > point.
> > > >
> > > > Finally, I personally would not locate my ham station anywhere near a
> > > > high tension line.
> > > _______________________________________________
> > >
> > >
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