Transformer Inrush

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robeward

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Doing a small coordination study. Info from the transformer manufacturer is the typical x12 inrush for .1 sec and x25 inrush for .01 seconds.

I believe in the x12 inrush.

I understand the x25 is possible but my question is how likely will it be a factor when coordinating the instantaneous regions of a LV breaker?

we are looking at a L-frame SqD breaker with 5.3 micrologic trip unit. The bottom of the instantaneous curve is right on .01 seconds. Should the instantaneous pickup be slid out past the x25 mark? There is not instantaneous delay for the breaker and it can't be cut off.
 
The 25x is mostly theoretical, because it requires a slew of "just so" circumstances to coincide, such as the point in the sine wave at which the circuit connects, residual magnetism in the core, etc. The duration is typically sub cycle, so most breaker trips will not have time to react to it.
 
Thanks for the answer. I really don't like adjusting the instantaneous setting of the breaker to clear that point.

from what i have found reading it appears this only occurs as a "hot-load pickup" where short lapses in power occur on loaded transformers. haven't really found any technical documents on the subject and definitely can't find anything pointing to extreme adjustment of settings to prevent nuisance tripping.
 
I had a instance with a 75KVA trans at 480v. Kept tripping the main ahead of the panelboard that fed the tranny. Main was 3200A and the panelboard was rated at 600A.
After a long conversation with GE the supplier of the tranny. It was determined that the special totally enclosed tranny that was speced had inside a 225 KVA winding set. The inrush current was so great it threw the main to the building. We had to coordinate and reset the upstream trip settings.

GE told us this was a internal spec secret!:rant::lol:
 
thats a good point. supposedly my customer has spoken with the manufacture of the xfmr and those were the numbers given but i also note that these are also industry accepted approximations which leads me to believe they are not the actual.

seems like actual inrush data should appear on a dataplate with the other info. after all every reference out there from any manufacture of protective devices warns against inrush when selecting protection devices.
 
robeward said:
thats a good point. supposedly my customer has spoken with the manufacture of the xfmr and those were the numbers given but i also note that these are also industry accepted approximations which leads me to believe they are not the actual.

seems like actual inrush data should appear on a dataplate with the other info. after all every reference out there from any manufacture of protective devices warns against inrush when selecting protection devices.
Click to expand...


In my case the info was withheld. The only documentation that I finally got was a hand configured table from the GE engineer. GE's defense was that if the info was published customers could use this tranny in place for a larger unit since it actually was inside.
 
Jraef said:
The 25x is mostly theoretical, because it requires a slew of "just so" circumstances to coincide, such as the point in the sine wave at which the circuit connects, residual magnetism in the core, etc. The duration is typically sub cycle, so most breaker trips will not have time to react to it.
Click to expand...

I agree that a lot of conditions have to occur simultaneously for the inrush to hit the maximum; however this is a 2nd harmonic current which actually occurs at a slight delay from time 0+. The inrush will decay rapidly within the first few cycles, but then will subside slowly; the lower the resistance the more slowly, and could take seconds.

As far as the instantaneous setting; yes, I would set it outside the 25X mark, especially since the manufacturer supplied the number.
 
another question on same topic for learning purposes.

are inrushes higher on low voltage or high voltage side? so if the inrushes you were told by manufacturer were for a step down transformer and you used a step up transformer how much difference would there be?
 
robeward said:
another question on same topic for learning purposes.

are inrushes higher on low voltage or high voltage side? so if the inrushes you were told by manufacturer were for a step down transformer and you used a step up transformer how much difference would there be?
Click to expand...
The inrush is not determined by high versus low voltage but by inner versus outer winding in the core.
The more closely coupled the energized winding is to the core the higher the inrush compared to the working current for that winding.
For a transformer used in the design direction the inrush may be 10x normal while for a transformer used in the backwards direction it may be 25x, as shown in the manufacturer data posted earlier.
 
robeward said:
another question on same topic for learning purposes.

are inrushes higher on low voltage or high voltage side? so if the inrushes you were told by manufacturer were for a step down transformer and you used a step up transformer how much difference would there be?
Click to expand...

Here is a thread on reverse fed xfm.

http://forums.mikeholt.com/showthread.php?t=164311&page=2, post 16

And here is the GE paper.

(edit to add)
And yes, I would set up the CB coordination such that the xfm inrush would not trip on worst case - cause worst case happens. And then you will look like a - well you choose the word.:thumbsdown:

ice
 

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thanks for the paper IceWorm. at least my instincts are correct. would struggle to produce the math to prove it but at least on the right track.

i failed to mention earlier that this is a reverse fed transformer so that is likely contributing to our situation also.

i also need to gather more info on the ATS that is being used.
 
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