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inrush current

    • 2 posts
    June 19, 2019 2:18 PM PDT

    A manufacturer gave the inrush current for 200 kVA transformer. Now the cable extends up to 300 meters, which can have a effect on the inrush current. If the cable parameters are known, can one calculate the revised inrush current values?

    • 156 posts
    June 20, 2019 12:12 AM PDT

    There will be no impact on inrush current. Two factors will cause to change inrush current

    1. voltage per unit at the time of energisation
    2. Load on Secondary side.

    • 75 posts
    June 21, 2019 6:20 PM PDT

    My experience is different from Hameedullah's.

    As HE said, the inrush current will depend on the "voltage per unit at the time of energization" and of course, the residual flux in the core from when in the cycle the xfm was de-energized.  There are several papers available on transmission inrush. Most agree the inrush will be at the maximum 1 out of 6 energizations. 

    Consider the energization circuit model:

    The following is from:

    4. Source impedance

    Higher source impedance relative to the transformer size limits the current that the transformer can pull from the system. The peak inrush current with significant source impedance (Westinghouse Electric Corporation, 1950) is defined as:


    I(peak) = i(0)/(1 + i(0)X)


    • i0 – peak inrush current without source impedance in per unit of the transformer rated current
    • X – source impedance in per unit on the transformer kVA base

    Other factors have less significance. The load on the transformer does not significantly change the inrush. For most typical loading conditions, the current into the transformer will interrupt at points that still leave about 70% of the peak flux on the core.

    So, yes, the cable impedance will be in series with the source impedance and will reduce inrush




     Edit to fix the mess into an equation showing the change in peak loading

     Third edit attempt to show the equation for the effect of source/cable impedance on inrush current (*^*&^%%@!*&#)


    This post was edited by Carl Coulter at June 23, 2019 7:34 AM PDT