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General AC versus DC question

  1. Jun 28, 2009 #1
    Im not formally trained in electrical engineering but have recently been forced in the last 6 months or so to get up to speed on many elec eng power type subjects. One of which is AC versus DC. I understand the history/science of it and why AC eventually won out. However, one story I encountered some months ago was the large electrical technology company ABB being awarded the contract to build a large DC line from mainland Europe to the UK. After all of my reading this story struck me as odd (based on what little I knew).

    Anyone have any insight/reasons why a large project like this would be designed in DC and not AC? Also, are there in fact cases where a DC system is more applicable/appropriate than an AC system, on a large scale anyway? One reason I thought of was that since its electricity being generated on one grid and being sent to another, packaging it as DC would be akin to just wrapping up a 'product' and shipping it off and then not having to worry about as much from the supplier's side. But Im not sure.
    This is a massive mult million euro project so Im sure there exists a good reason. Just curious to know how it might tie in with the AC versus DC argument.
  2. jcsd
  3. Jun 28, 2009 #2


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    The main reason AC won out over DC is because at the time, it was much easier and less expensive to convert high voltages (necessary for long distance transmission since low currents/high voltages are less lossy than high currents/low voltages) to low voltages (necessary because 1) it's safe for people and 2) it's easier and less expensive to design equipment to use lower voltages) for AC--since you need only an efficient, reliable , simple and relatively inexpensive transformer. Today, however, it's fairly easy and inexpensive to build efficient DC-DC step-down and step-up converters and there are several HVDC transmission lines in use today.
  4. Jun 28, 2009 #3
    Ah okay. This makes perfect sense. Im not sure why it never occured to me that the necessary conversion devices were not developed at the time of AC being chosen as the main distribution current. I guess now with these devices being well developed a HVDC system has as much chance as a HVAC system to be put into use, depending on the circumstances.
    Thanks for the answer.
  5. Jun 28, 2009 #4


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    Oh, they had them, even back them. One of the earliest ones was the motor-generator (called a mo-gen or just mg for short) which, as the name implies, was simply a DC motor driving a DC generator. But these were expensive, noisy, inefficient and unreliable since they were mechanical and had moving parts which could wear out or break.
  6. Jun 28, 2009 #5


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    DC links are still commonly used for long distance lines.
    It makes the cable slightly cheaper, since you only have to design for the constant voltage rather than the peak, but the main advantage is that you don't have to have the two grids in sync. If you have an AC line the swedish and UK systems would have to be in phase - with a DC line it doesn't matter. The existing links between the UK and France are DC because of this.
  7. Jul 4, 2009 #6
    It is very to interesting to read these things. Really it is a news for me that d.c. can also be stepped up or down. Many high school level physics text books still say that the one of the differences between ac and dc is that the former can be stepped up or down while the latter can not be. Will you please brief me what is the principle behind this new techniqe of stepping up or down dc?
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