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Homework Help: Underground cables

  1. Jun 5, 2010 #1
    Hello,

    I just got a small question.

    If a 50Hz grid line is carried underground for a distance of 10km. And it is also known that the capacitance is 400nF per phase per km. What is the total reactive power generated in the 10km length?

    Then I could say that we have a total of 400nF * 10 = 4000nF per phase.

    [tex] Power = \frac{V^2}{X_c} = \frac{400k^2}{\frac{1}{2 * \pi * 50 * 4000nF}} = 127MVAr. [/tex]

    1. However, this is the power per phase isn't it? From my lecture notes it is the total reactive power however I can't figure out why.

    2. Also, I'm trying to find the charging current in each phase of the line.

    According to my lecturer power in each phase is the power I got earlier divided by 3, but the same capacitance is used if the formula I^2Xc is used.

    Please help, this is really confusing me!
     
    Last edited: Jun 5, 2010
  2. jcsd
  3. Jun 5, 2010 #2
    Anyone please?
     
  4. Jun 7, 2010 #3

    berkeman

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    Staff: Mentor

    What are you using for the "voltage"? The voltage that is listed for a 3-phase line does not show up across each wire pair...
     
  5. Jun 7, 2010 #4
    Sorry, my mistake. The question says it's a "400kV grid line"
     
  6. Jun 7, 2010 #5

    berkeman

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    Staff: Mentor

    So for a 400kV 3-phase transmission line, what is the voltage difference between each pair? See if the answer makes more sense now.
     
  7. Jun 7, 2010 #6
    I'm not sure I follow. I think I misunderstood the question to be honest. When you're told that a cable is a 400kV cable, what's the meaning of that exactly? I know that when it comes to 3 phase systems for example, when you're told that you have a 415V supply, that means that the line to line voltage is 415. However I'm unsure about this.
     
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