Inrush current calculation
Inrush current calculation
(OP)
Hi All,
I am sure that there might have been discussions about inrush current but I did not find any information relevant to my question.
I am trying to justify an information received from a manufacturer about inrush current calculation on a 1 MVA 13.8 kv transformer Dry type transformer. It talks a value of 6 to 8 times of full load current. The transformer is Delta/ Star Neutral grounded
As you know traditionally the concept of fuse selection for primary of transformer is to use 12X I(FL) for .1 sec and 25X I(FL) for .01 sec.
So how do I agree to the above value of 6 to 8 times inrush.
After talking to the manufacturer, I found out that while calculating the value of Inrush current theoritcally they
assume certain value of residual flux, which might lead the transformer in saturation for a short period of time and increase the inrush current.
Where in these calculations can we consider the impact of "Instand of Closing". Will that increase the value of inrush current and how does residual flux increase the inrush current.
Please refer me to any post that explains this concept or I would appreciate a word.
Thanks
I am sure that there might have been discussions about inrush current but I did not find any information relevant to my question.
I am trying to justify an information received from a manufacturer about inrush current calculation on a 1 MVA 13.8 kv transformer Dry type transformer. It talks a value of 6 to 8 times of full load current. The transformer is Delta/ Star Neutral grounded
As you know traditionally the concept of fuse selection for primary of transformer is to use 12X I(FL) for .1 sec and 25X I(FL) for .01 sec.
So how do I agree to the above value of 6 to 8 times inrush.
After talking to the manufacturer, I found out that while calculating the value of Inrush current theoritcally they
assume certain value of residual flux, which might lead the transformer in saturation for a short period of time and increase the inrush current.
Where in these calculations can we consider the impact of "Instand of Closing". Will that increase the value of inrush current and how does residual flux increase the inrush current.
Please refer me to any post that explains this concept or I would appreciate a word.
Thanks






RE: Inrush current calculation
If the contacts close at the peak voltage, there is no inrush.
I see no serious conflict between the fuse selection criteria you are using and the manufacturer's prediction of inrush current.
Maybe this will help:
http://www.sandc.com/webzine/2002/111102_1.asp
RE: Inrush current calculation
DPC
Is that what you meant to say?
RE: Inrush current calculation
Of course in a three-phase core, your luck runs out after one phase (unless you have some really fancy circuit breakers), since the other two phase are separated by 120 degrees.
RE: Inrush current calculation
RE: Inrush current calculation
So could you explain; How residual flux impacts the inrush current. How does it makes it worse.
So, if I energize a brand new transformer, Will this be a worse case
or a transformer reenergized that was running for an year.
I would appreciate a word.
RE: Inrush current calculation
RE: Inrush current calculation
Before few days we had intresting thread on the topic.
Please see attached
thread238-222884: Transformer residual flux remains or decays with time?
Regards.
Slava
RE: Inrush current calculation
JIM
RE: Inrush current calculation
When the manufacturer provides peak inrush versus time plots that indicate 6x FLA at 0.1s and about 15x at 0.01s can I use a fuse with a minimum melt around 8x FLA at 0.1s and 18x around 0.01s? (The fuse nicely solves some arc flash issues downstream.)
RE: Inrush current calculation
As lansford says, the actual inrush current is influenced by the source impedance, so the actual inrush may be less than the transformer supplier is predicting.