Power transformer inrush current 8 vs 12 times FLC 4

[Tony2802]

The IEEE 141-1993 states that in general, the transformer inrush current is approximately 8 to 12 times the transformer full-load current for a maximum period of 0.1 s. However, in the IEEE 242-2001 only mention 12 times the transformer FLC. I see frequently engineer using 8 times FLC as inrush current. I could not really find a standard stating when to use 8 times or 12 times the FLC. On google, I found a document (

https://www.qualtecheng.com/docs/overcurrent-protection/QT-608.pdf)

stating that transformers > 2500 kVA should use 12 times the FLC and < 2500 kVA should use 8 times the FLC. However, I could not find the standard or reference related to the 2500 kVA threshold for inrush current. Did you ever hear about this rule of thumb?

 

[dpc]

Transformer inrush is a complicated event and varies even for the same transformer depending on voltage phase angle and residual flux. There are also significant harmonic currents during inrush. For protective relays, the goal is to set the instantaneous trip pickup just above the maximum inrush current that would ever occur to avoid nuisance trips.

Rules of thumb are just that - not precise and everyone has a different thumb. I use 10X for oil-filled transformers and that has worked well in terms of nuisance trips at least. For larger dry-type transformers I generally use 13X to 15X - based on prior experience. Many engineers use 8X for oil-filled transformers and rarely have issues.

 

[Bronzeado]

Tony2802

Please, have a look in
Cigre Technical Brochure 568 - "Transformer Energization in Power System: A Study Guide",
February 2014, Report number: TB 568 ISBN : 978-2-85873-263-0

Cheers,

Herivelto S. Bronzeado
Brasília, Brazil

https://www.linkedin.com/in/heriveltobronzeado/

https://www.researchgate.net/profile/Herivelto_Bronzeado

 

[davidbeach]

Be prepared for 12 or even 20 and Murphy says it won’t get much beyond 10. But hope for 8 and Murphy has lots of opportunity for mischief.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[unclebob]

Yeah, choosing a limiter fuse to protect your xfo's primary may give you bigger fuse than expected, if you go higher than 12 x.

 

[prc]

Inrush current as a time of rated current depends on transformer rating in kVA. The higher the kVA, lower the times inrush current. Hence for small transformers, it may be 20 times, medium-sized 10-15 times, and for large Power Transformers 5-10 times. What is quoted is wrong and quite opposite to the fact. Similarly times current will be more for inner winding ( usually LV) than for HV outer winding.

 

[RRaghunath]

prc Sir,
How do we establish the inrush current magnitude during design and further confirm during FAT!

 

[prc]

Transformer engineers can calculate the inrush current for various ratings and connections. You can refer to the book " short circuit Strength of Transformers " by ABB (now Hitachi Energy). Of course, it requires certain transformer design parameters.

 

[Parchie]

Tony2802 (Electrical)

There is a historical estimation by Schneider Electric regarding transformer inrush:
I[sub]max[/sub]=[2020 x h x A[sub]c[/sub] x (B[sub]res[/sub] + 2B[sub]max[/sub] - B[sub]sat[/sub])]/ [N x A[sub]s[/sub]]

where:

​

Imax = peak inrush current
h = exciting coil height, in inches
Bres = residual core magnetic field, in kGauss
Bmax = maximum operating flux density, in kGauss
Bsat = saturation flux density, in kGauss (~20 kGauss)
N = winding turns
As = effective area of exciting coil, in sq.inches​

Hope that helps.

 

[Tony2802]

Thanks everyone for your answers. I seems that 2500 kVA threshold comes from the McGraw-Edison Distribution System Overcurrent Protection Workshop. However, I could not find the original document.