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#13402 09/02/02 10:34 PM
Joined: Apr 2002
Posts: 2,527
Bjarney Offline OP
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).]

#13403 09/03/02 07:42 AM
Joined: Aug 2001
Posts: 7,520
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 09:12 AM
Joined: May 2002
Posts: 381
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 01:19 PM
Joined: Dec 2001
Posts: 2,495
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 07:46 PM
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 09/03/02 11:44 PM
Joined: Apr 2002
Posts: 2,527
Bjarney Offline OP
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 02:54 PM
Joined: Aug 2001
Posts: 7,520
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 04:52 AM
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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