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XFMR inrush

XFMR inrush

XFMR inrush

(OP)
I have two questions!  
1.  Does inrush vary depending on the loading of the XFMR?  I have a 75kVA XFMR that is lightly loaded.  
2.  How do I determine the inrush of this XFMR say if the load is loaded 50%?

RE: XFMR inrush

Load can damp the inrush (shorten the duration), but doesn't affect the peak magnitude.

RE: XFMR inrush

Hi JBUDA54,
 
When a transformer is energized with a load on its secondary, the primary voltage will be slight lower than that if there was no load applied, as the voltage drop across the system series resistance is caused by the inrush+load currents.
 
This voltage reduction will make the magnetig flux in the transformer winding+core slight smaller, so the peak of the inrush current will be slight smaller than that without load.

I would say as davidbeach said, "load can dump the inrush" but, for practical purpose, it can be considerared as a secondary effect.

Best regards,

Herivelto Bronzeado
 

RE: XFMR inrush

The inrush experienced will depend on where on the voltage waveform the switching device contacts close and the residual magnatism in the core of the transformer at the time of enerigization.  IEEE recommends using 12 times full load for inrush values when performing coordination studies (Buff Book) but usually 8 times is adequate for coordination on industrial power systems.

I just performer inrush recordings on a 1MVA transformer using an RPM power monitor and in 10 closings the highest recorded inrush was 7 times full load and the lowest 3 times. This was on an unloaded transformer. The test was repeated with 70% load connected and in 5 closings the inrush did not exceed 5 times.  The average duration of the inrush events was 107ms.

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