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Short circuit bracing of DC bus

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    Short circuit bracing of DC bus

    Assume a set up where multiple rectifiers which are non-active (aka diode banks, not IGBTs) feed a large DC bus. Several inverters are also connected to the DC bus. Inverters run induction motors. This setup is commonly known as "Common DC". Rectifiers outputs and inverters inputs can be isolated from DC bus via DC disconnect switches. Inverter inputs are fused.

    My question is: what determines the short circuit bracing required for the DC bus? In my opinion, inverters' capacitors are only the contributing factor, and source/motors can be ignored.

    thanks
    "Because it's there!"
    George Mallory

    #2
    Do the rectifiers have any filter capacitors associated with them?

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      #3
      Originally posted by synchro View Post
      Do the rectifiers have any filter capacitors associated with them?
      There are no capacitors connected to rectifiers which is pretty standard. In modular applications like this where rectifiers and inverters are separated capacitors are always installed on inverters.
      "Because it's there!"
      George Mallory

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        #4
        The following is a comment from EasyPower about rectifier fault current:

        "Diode rectifiers can have DC terminal fault current close to the AC side fault current."

        http://help.easypower.com/ezp/10.0/C..._DC_System.htm

        I don't know how accurate that statement is, but it indicates that rectifier fault current should not be ignored in most cases.

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          #5
          Are you using DC fuses on the feeds off of the bus to the individual inverters? If not, why not?

          The DC rated fuses (Bussman JKS) we use in our common DC bus inverter applications are rated for 65kA fault duty, so we use 65kA as the bracing value for our DC bus bars. Some of the larger fuses (like Bussman 170M for applications over 600A) can go up to 100kA, but we don't brace the internal DC bus to that level because the rectifiers are listed at 65kA on the input side anyway, and at 100kA AFC on the AC input you must use a current limiting fuse on the anyway.
          __________________________________________________ ____________________________
          Many people are shocked when they discover I am not a good electrician...

          I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

          Comment


            #6
            Originally posted by synchro View Post
            The following is a comment from EasyPower about rectifier fault current:

            "Diode rectifiers can have DC terminal fault current close to the AC side fault current."

            http://help.easypower.com/ezp/10.0/C..._DC_System.htm

            I don't know how accurate that statement is, but it indicates that rectifier fault current should not be ignored in most cases.
            I am a little confused; then why the DC fault current is only 14kA compared to 48.6kA at the input of the rectifier?? Seems like rectifier is only contributing 9kA to the DC bus...
            "Because it's there!"
            George Mallory

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