Inrush Calculations 2

[sky2020]

What does it mean 10x @ 0.1s?

It means the current maximum (top wave point) will go to 10 x FLA and it lasts 0.1s, right?

So the maximum point is 10x FLA, no need to divide it by sqrt(2).

Can someone confirm? Is there a reference of transformer size and inrush value? Can it really go to 25X?

 

[waross]

The peak limit of a fully offset sine wave is 2 x √2 x RMS, so an inrush may approach a peak value of 283% of the RMS inrush value.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

This is what IEC 60787 says about transformer inrush:
Quote "For practical purposes this may be taken from 10 to 12 times the transformer rated current for
a duration of 0,1 s" Unquote
So, here the reference is transformer rated current (RMS).
Hope this clarifies.

 

[waross]

To expand RRaghunath's information.
12 times RMF offset peak may approach 12 x 2.8 = 33.6 times full load current.
In practice this limit is reduced by the X/R ratio to 25 times or less.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

That's the reason IEC also stipulates that the protection shall remain stable for 10ms at 25x current, I suppose, Bill. Thanks for mentioning.

 

[prc]

1) I have seen old documents stating inrush current of 25 times full load current at 0.01 second, 12 times FLC at 0.1 seconds, 3 times FLA at 10 seconds, etc. Frankly, today's transformers have different inrush features.

2) Answers to your specific queries:
- 10 times means the peak of inrush current to RMS value of rated current (full load current). More exact, this should be to the peak of RMS rated current. But people are using it,like that for so long.
- 0.1 sec means inrush current decays to 10 times FLC value by 0.1 sec

3) In today's transformers, inrush peak is 20-25 times for control transformers, 10-15 times for distribution transformers and 4-8 times for power transformers. Inrush will decay to normal excitation current with in 5 sec (control trf) - 40 sec ( for large transformers). Inrush will decay to half of the peak value with in 1/10 of the total inrush current duration. Time constant for inrush decay depends on 2L/R where L= air core inductance of energised winding R= Winding resistance.As transformers become more and more energy efficient,decaying time constant increases.