Potential Transformer w/ leakage capacitor in need of a TF

Discussion in 'Homework Help' started by non-EE needs help, Mar 13, 2012.

  1. non-EE needs help

    Thread Starter New Member

    Mar 13, 2012
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    0
    Hello,

    I'm sorry to say I do need a lot of help on this HW problem.

    "The equivalent circuit of a 200kV:67V potential transformer is illustrated in Figure P1. Compute the transfer function (ratio of the secondary voltage phasor to primary voltage phasor) of this PT in the frequency range 10 Hz to 1200 Hz. The indicated parameters in the figure are: Lm=20,000 H, r1=24,000 ohms, L1=640 H, r2=0.003 ohms , L2=0.00007 H, C=2.5 nF." (equivalent circuit attached, the CN version is the primary side is a series RL in parallel with another L, than the Xfrm and capacitor, and the secondary side is another series RL)

    What I think I want to do is replace the capacitor with two shunt capacitors, but the additional inductor in the way is making it messy. After this step....I need to create a equation for voltage across the inductor.

    The secondary side is a lot easier, assuming I know my capacitor, it should just be a series RLC.

    Thoughts? I guess I kind of see the big picture, I'm just getting scared about how to do each step.
     
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  2. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    Your model has no labels for the primary and secondary of the internal transformer, so I assume you are to treat it as ideal (infinite primary and secondary inductance, unity coefficient of coupling).

    Have you been given worked examples of similar problems? What method was used to solve them; mesh analysis, perhaps?
     
  3. non-EE needs help

    Thread Starter New Member

    Mar 13, 2012
    2
    0
    Unfortunately, the only example I have doesn't have the second inductor (one in parallel with my Xfrm). I've taught myself quite a bit of mesh/nodal analysis, so I wanted to start there and figure out my current going through my capacitor. I know I need to convert it to a shunt capacitor, but my big uncertainty is how to apply any knowledge of the xfrm. So in this case, I clearly don't have a power sources (Vin) as I've known them. Its just an open circuit. So that has my confused....
     
  4. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    You will need to assume a source Vin; that should be easy to deal with.

    The ideality of the internal transformer will cause difficulty, but it's not impossible.

    Is the worked example you have from a textbook? If so, can you take a picture of it and post it as an attachment? That would help me see what assumptions and methods your text expects.

    Without seeing what your text does, I would use the mesh method. I would add a Vin at the left of your schematic and a load Rload to the right end. I would label the internal transformer with symbols for the primary and secondary; something like Lp and Ls.

    Post a picture of an example from the text if you can; that would help me help you.
     
  5. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    Have you had any luck with this?

    Using the mesh method, I got a result that makes sense.

    If you'll add Vin and RL as I suggested, you shouldn't have any trouble writing the 4 mesh equations.
     
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