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Thread: VFD MODELING FOR SKM POWER TOOLS

  1. #1
    Join Date
    Jul 2017
    Location
    Saint Louis
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    15

    VFD MODELING FOR SKM POWER TOOLS

    Hey Everyone,

    Hope you are all doing well. I have questions pertaining to VFD modeling in SKM power tools and I don't really have a lot of resources on this subject so I was hoping someone could steer me in the right direction. My project involves multiple motors over 100 HP with VFD's and when I ran my arc flash evaluation the program put all of my VFD's in By-Pass Mode, which consequently upped my short circuit contribution by at least 10kA. Has anyone who has used SKM power tools dealt with this? Also, if anyone knows of where I can find good examples of power systems studies that required VFD modeling in SKM power tools that would be greatly appreciated.

    Best Regards,
    Engineer in training

  2. #2
    Join Date
    Mar 2013
    Location
    Rutland, VT, USA
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    260
    Why not ask the real experts - SKM? If you have a maintenance contract in place they should be able to provide technical assistance.

    That being said, I guess I don't understand your question. The results are what would be expected as if the VFDs are non-regen and you put it in bypass, then VFD effectively removed from the circuit and motors will contribute fault current back to the bus. Is 10kA giving you a problem?

    You might want to consider how realistic it is for all (100%) of the VFDs are bypassed at the same time. Perhaps more realistically it would be one, maybe 2.

  3. #3
    Join Date
    Jul 2006
    Location
    San Francisco, CA, USA
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    6,780
    If your VFDs have bypass, then it's a valid study. If not, then you clicked on something that told SKM that they have it. It's been a while now, but if I recall correctly their VFD model may come with bypass selected as the default and if you don't have it, you must un-click something.
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  4. #4
    Join Date
    Jan 2016
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    PA
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    Quote Originally Posted by Jraef View Post
    If your VFDs have bypass, then it's a valid study. If not, then you clicked on something that told SKM that they have it. It's been a while now, but if I recall correctly their VFD model may come with bypass selected as the default and if you don't have it, you must un-click something.
    spot on
    http://www.skm.com/faq19.html

    The VFD will also affect the short circuit study results based on the contribution values from the "VFD page 2" section. If the by-pass switch is checked, PTW will by-pass the VFD and only the impedance of the cable will be considered.

    if you do have bypasses they should be included to get a worst case scenario
    you may want to look at the load side '100% let through' and 'regen' parameters also
    you need info on the vfd's



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    Last edited by Ingenieur; 09-10-17 at 11:03 AM.



  5. #5
    Join Date
    Jul 2017
    Location
    Saint Louis
    Posts
    15
    Hey Everyone,

    Thank you for your responses. After reading your comments and discussing with the electrical engineer on the project, I'm understanding how the VFD's effect the system a little better. I know if the VFD has a bypass switch then the fault current contributions from the line and load side (motor contribution) is considered to simply bypass the VFD and is unaffected and usually represents the worst case for fault analysis. I talked with the electrical engineer and he said something similar to what "wbdvt" said, "how realistic is it for all the VFD's to be in bypass mode?" I have allowed my skm model to put the highest HP motor in bypass mode, which I think is a more realistic situation.

    Best Regards,
    Engineer in training

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