Inrush for transformers connected in parallel

[bozzah]

Hello,

I'd like to know the method of calculating a combiner inrush current for transformer connected in parallel. In my case there are ten (10) 10kVA transformers connected in parallel, i.e. one 600V circuit (125A breaker) is feeding all 10 transformers (separation between transformers is about 300ft and the cable is oversized ...350kcmil due to voltage drop).

I understand the fact that during the energization stage an inrush of one transformer is about 8 to 10 times of transformer full load amps. In this case, ten transformer of the same size will be energized at the same time. What will be combining inrush current?

thanks!

 

[waross]

The simple answer is 10 times the inrush of one transformer.
But the voltage drop in the feeder will lower the inrush.
You didn't mention whether these are single phase or three phase transformers.

The poor man's estimation:
Assume that inrush is proportional to voltage.
Assume a terminal voltage at the last transformer. (#T10)
Calculate the inrush of the T10 transformer based on the assumed voltage. I would start with 400 Volts.
Calculate the voltage drop of the last cable. (#C10)
Add the voltage drop to the assumed voltage, and calculate the voltage at the T9 transformer.
Use the calculated voltage at the T9 transformer to calculate the voltage drop in the C9 cable.
Continue until a calculated supply voltage at the breaker is determined. If the calculated voltage is above or below the actual voltage, do a simple ratio and adjust the calculated values.
Normally I would just add the transformer inrush but with 3000 Ft. of cable the voltage drop may make a significant reduction in the combined inrush.

This brings up a question I will leave for others: If the X:R ratio of the cable is different then the X:R ratio of the transformer what will the effect be on the inrush?

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

 

[pwrengrds]

3-5 x FLA in my experience, though it varies a lot. Will also depend on how much load is on the secondary at the time. I don't think you will find a formula.

 

[bozzah]

waross,
it is 3ph system;