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#13402 - 09/02/02 11:34 PM DC Line  
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
Three-phase AC power is fairly universal all over this planet, but occasionally there are deviants.

A picture of a DC-power line, rated at ±500kV 3100 amperes.

At its ends there are converters to 230kV 3ø for "local" use.

[This message has been edited by Bjarney (edited 09-02-2002).]

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#13403 - 09/03/02 08:42 AM Re: DC Line  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England
There is still an under-Channel DC link between Britain and France, to our mutual benefit. I believe that DC was chosen originally simply to avoid the problems of trying to synchronize the AC grids of our two countries.

#13404 - 09/03/02 10:12 AM Re: DC Line  
Hutch  Offline
Joined: May 2002
Posts: 381
South Oxfordshire, UK
Likewise the Cohora Bassa Dam on the Zambezi River in northern Mozambique uses 533 kV DC lines to transmit power to the industrial heartland of South Africa over 800 miles away.

#13405 - 09/03/02 02:19 PM Re: DC Line  
Texas_Ranger  Offline
Joined: Dec 2001
Posts: 2,399
Vienna, Austria
The same in Austria. We get power from the Czech Republic, but our PoCos didn't trust the frequency of the czech power. Now power is created as Ac, rectified, goes througha 500V DC line to Austria and is there converted back to AC

#13406 - 09/03/02 08:46 PM Re: DC Line  
Mike_Riverside  Offline
Joined: Aug 2002
Posts: 33
is there an advantage of making AC converting to DC for transmition, then converting back to AC? Why is this done, do they still do this?

#13407 - 09/04/02 12:44 AM Re: DC Line  
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
About AC>>DC>>AC, it’s interesting to see postings from different parts of the world. has a lot of readers! In the US and Canada there are a number around the continent, and typically the use of earlier installations were for cost-effective power transfer of bulk power over long lines. One form of economy can be a north<>south system, where the north end uses more power during the winter, and the south region needs more power in the summer. It’s all pure economics. If it weren’t cost effective for both ends, it would be a poor investment.

More recently, there are a few back-to-back stations where there in no DC line, just a comparatively short + and – bus between two converters in one building. The idea is that, given increasing price savings with advancing electronics, is that the two AC sides of the station do not have to be synchronized, and can be large, independent systems. In some areas, the two AC systems are even at different frequencies. Also, power can be “sent” either direction with a minor change in electrical-operating characteristics.

Typically the stations are rated at hundreds or thousands of megawatts, and hundreds of kV, but experiments are underway with HVDC “Lite” where the voltage may be 25kV, and at tens to a few megawatts capacity.

#13408 - 09/04/02 03:54 PM Re: DC Line  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England
The original reason behind the England-France D.C. link was to reduce the need to provide higher generating capacities in each of our countries.

France is one hour ahead of us, plus they tend to eat and work different hours anyway, so our peak-demand times don't coincide.

#13409 - 09/05/02 05:52 AM Re: DC Line  
Trainwire  Offline
Joined: Mar 2002
Posts: 360
The latest and greatest railroad locomotives do the same thing. Generate ac, with 6000 hp diesesels, turn it to dc, then turn it back into ac again. the exception is that when they turn it back into ac again, they vary both the voltage and the frequency to the motors, for speed control and wheel slip control. They can now for intence, hold a train on a hill with just the motors, and no brakes, without the motors burning up. plc's are a wonderful thing.

So the inverter technology is advancing at a rapid rate.


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