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Registered Member #158
Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282
I know this thread is a bit old but since it was brought up again I thought I'd take a stab at the above question. Like EVR said a transmission network is actually pretty complicated. Although a line is bidirectional and not uniderectional per-say, when your dealing with AC there are bad things that happen when you try to take several hundred megawatts going in one direction and lets say, turn it around to going the other direction in like 1-cycle. Although I dont have all the technical reasons on hand, that is one of the issues that happened in the north eastern U.S. and into canada a couple years ago. Due to a cascading transmission like failue, other interconnecting transmission lines which were pushing power one way had to turn around and go the opposite way. This occurance will cause all sorts of protective relays on lines and generators to open. Hence a large part of the north east U.S. went out black. Obviously there were alot of other factors at play in that particular scenario, but that 'turn-around surge' really caused some problems. I dont know if thats what EVR was getting at but thats about all I can think because otherwise most transmission lines are bidirectional. I am however not sure what the difference with a DC line would be in the above scenario?
Registered Member #321
Joined: Wed Mar 15 2006, 03:33AM
Location:
Posts: 14
Sulaiman wrote ...
OK, but what makes a line (or part of the system) uni-directional? AFAIK wires, transformers, capacitors etc. don't know about directionl, voltage, current, power, phase etc. etc. meters don't care.
..... So what makes a high voltage ac line/part of a national grid uni-directional?
in our power systems analysis text book the way they calculate power flow was using a [ABCD] constants (it's essentially the same as for microwave frequency equipment) one of the results that was also weird the first time the prof persented at lecture, was that with AC lines it becomes possible for the voltage at the receiving end to be higher than at the sending end depending on how much current is drawn by the recieving load. the equations are a little messy and i'm in no mood to plagiarize the book right now
Registered Member #158
Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282
...with AC lines it becomes possible for the voltage at the receiving end to be higher than at the sending end depending on how much current is drawn by the recieving load.
I beleive what you are refering to is the Ferranti Rise. I cannot remember formulas off hand but it has to do with the Mvar loading on the line, and if it is very lightly loaded or open ended, the line will act like a 'shunt capacitor' and the voltage will rise. The highest voltage is seen at the open end of the line. On transmission lines shunt caps are used for var support and help boost the voltage. With an excessive amount of var support and not very much load, the voltage will be high.
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AC, DC, and Power Transmission
