Path: utzoo!attcan!uunet!amara!khai
From: khai@amara.uucp (Sao Khai Mong)
Newsgroups: sci.electronics
Subject: Re: Free power from 'whispering wires' ??
Message-ID: 
Date: 12 Jul 88 12:58:37 GMT
References: <3677@tekgvs.GVS.TEK.COM>
Sender: khai@amara.UUCP
Organization: Applied Dynamics International, Inc.
Lines: 77
In-reply-to: jans@tekgvs.GVS.TEK.COM's message of 11 Jul 88 17:25:25 GMT


In article <3677@tekgvs.GVS.TEK.COM> jans@tekgvs.GVS.TEK.COM (Jan Steinman) writes:
>
>   <<4. What percentage of the high-voltage lines that one sees in the countryside 
>   are AC and what percentage are DC?>>
>   
>
>   I don't think this is correct.  I've seen pictures of huge (I mean 40-60 feet 
>   tall) thyristors, which are supposedly used for line synching high voltage DC 
>   transmission lines to a local grid.  I believe the article mentioned that DC 
>   transmission was efficient enough to more than overcome the conversion losses.
>

No, almost all lines are indeed AC; there are only about ten or so DC
links in the USA.  Seeing pictures does not imply their widespread
use.  Look at HT (high tension) lines across the country.  You are
sure to see only three or six wires (ignore the differently-sized
protective ground wire) and that is almost a dead giveaway that it is
an AC line.  (Three phase xmission) HVDC transmission is quite a new
idea compared to AC and there has not been enough time for it to be in
widespread use.  Perhaps because you live in Oregon, you may have had
biased opinion: There is indeed at least two HVDC lines from Oregon or
Washington to Southern California.  It's called the Pacific intertie.

Cons against DC

1.  Those thyristors and associated control circuitry are very expensive
and in most cases not worth the extra money. 

2.  Conversion to DC produce a lot of power line harmonics which require
still more expensive filters to remove them.

3.  HVDC switches are still infant technology.  It is very hard to stop
hundreds of amps flowing at xxx000's voltage.  AC switches are easier
because they use the zero crossing to extinguish the arc.  As a result
there are no HVDC networks; they are exclusively used for point to point
transmission and reconversion back to AC.

4.  HVDC transformation to higher and lower DC voltages is not feasible
at the moment.  Another factor against networks.

5.  For unknown reasons, environmentalists oppose DC lines more than
AC lines;  hence it is more difficult to get right-of-way's to build
DC lines.

6.  Residential power distribution will always be AC for my lifetime.

Some Pros for DC.

1.  Good for connecting networks of different frequencies.  Sometimes
they even have DC transformers back-to-back without any lines.

2.  You don't have to worry about power system stability and 
power transmission limits.  In fact, power system stability can be
enhanced by the use of DC with appropriate controls.  Power system stability
is the reason for those whole area blackouts, like the one in
New York in 1965, etc.

3.  More efficient when you talk only about transmission line losses.

4.  Capability to operate using only one wire in emergencies.  (Frowned
upon by environmentalists, sailors who have to navigate over these
cables., etc)

5.  Underwater transmission  (E.g. under the English channel) because
DC at steady state does not care about the high capacitance of underwater
cables. 

6.  Great thesis topic for budding PhDs.

This is by no means complete.  Remember, Thomas Edison lost out to
Westinghouse, Tesla, et al, and would still lose today.
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