Combined Transformer Inrush

[mayanees]

I'm coordinating an electrical system where a 1600 amp breaker controls six (6) K-rated dry-type transformers.
To simplify the problem, I'll say they are all 150 kVA.

The full-load amperage on a 150 kVA xf at 480 Volts ~ 180 amps. The inrush for a K-13 transformer can be as much as 15X.
Using a range of inrush from 8-15X, that could equate to (8*180*6) 8640 amps - (12*180*6) = 12960 amps.
The 1600-amp panelboard breaker has a maximum Instantaneous setting of 6X, or 9600 amps.
The transformers are separated physically by probably 15-20 feet of cable, as they are located on different floors.
I'm trying to dispositon this one way or the other - that the breaker trips on inrush or it doesn't, but I can't find any supporting data showing the cumulative effect of multi-transformer energization.
In a perfect world, I would set the instantaneous to accommodate the inrush at the 15X level (provided the system had the capacity), but my breaker wont allow it.

Can anyone comment on inrush for multiple transformers?
I think the physical layout of the building, with separation between devices, may prevent them all from inrushing together, but I can't find any data on that.

Thanks for any comments,.
John M

 

[waross]

I would start by calculating the limiting effect of the cable impedances. Another option is to calculate the minimum length of cable required to reduce the inrush to an acceptable level and then specify a minimum length of cable for each transformer. This technique has been used to reduce the available fault current to acceptable levels for lower rated switches and breakers.
You may find also that the impedance of the cable feeding your breaker adds useful impedance to limit the transformer inrushes. Consider also whether the impedance of the transformer feeding the breaker will limit the inrush somewhat.
The bottom line is that when the simple calculations yield an unacceptable result, a rigorous solution based on all the system parameters may reassure you that the breaker will withstand the actual inrush. You may fudge the system parameters by adding cheap impedance in the form of additional cable length.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mrcheese]

First of all get the actual inrush data from the transformer manufacture. While instantaneous inrush of a K13 transformer can be higher that 15X (especially on the 0 crossing) most transformers when actually tested are in the 7-9x range (in my experience)

Ohm's it not just a good idea, Its the law.