Inrush current calculation

[sobeys81]

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

 

[dpc]

The values you are using for fuse selection are intended to reflect a worst case to avoid fuse clearing during inrush. The actual inrush is not so simple to determine.

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

 

[wareagle]

"If the contacts close at the peak voltage, there is no inrush. "
DPC
Is that what you meant to say?

 

[dpc]

I think so. In an inductive circuit, the current lags the voltage by 90 degrees. Zero current (and flux) at max voltage. Ignoring the residual flux, if a transformer is energized at the peak of the voltage wave, the transformer is starting from its normal operating condition and there is no transient inrush.

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.

 

[wareagle]

I agree with your comment but I was thinking that if you energize all 3 phases at once you would have inrush current.

 

[sobeys81]

Thanks all for your valuable input.

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.

 

[davidbeach]

Yep, unless you can do point on wave of each phase independently you will get significant inrush on at least one phase. All three phases interrupted at a voltage peak (max positive or max negative) because each interrupted at its own current zero. But then, with a 3 pole breaker, they all close simultaneously.

 

[slavag]

Hi Sobeys81.
Before few days we had intresting thread on the topic.
Please see attached
thread238-222884
Regards.
Slava

 

[lansford]

The inrush also depends on the system source impedance.
JIM

 

[Zapped77]

I'm also looking at a similar situation to Sobeys81.

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.)

 

[dpc]

I'd make sure the manufacturer's data is in asymmetrical amps, or at least that you know how they are giving you the data.

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.