Transformer Inrush

[GadreP]

Guys, I have a step-up transformer 100 kVA, 415/11000V. The impdance is 5%. What is a value of inrush current? I read some articles but everybody has different views which is really confusing me. Some people say 10-12 times rated primary current & some people say 10-12 time secondary rated current.
To my opinion,inrush current we assume is 10 times the primary FLA. Can you please suggest me a correct answer. And what would be the value of inrush current for above said transformer? Would it be 1.4 kA or 1.68 kA?

 

[Skogsgurra]

1.4 or 1.68 - does that really matter?

You cannot say much about inrush without knowing what flux density you are running the transformer at. Impedance alone will not tell.

And, inrush current depends a lot on where on the sinewave you close the breaker and also in what state you left the core when switching off.

It is only natural that you get differing answers.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rbulsara]

Transformer inrush is only seen by the primary side device.
It is true that typically the fuse or the breaker shall be able to withstand 10 to 12 times the FLA (of the primary, of course) for 0.1 second. This does NOT equate to the fuse or breaker trip rating. You need to check this on a TCC. "Average" inrush for this size transformer would be in the range of 5-7 times FLA. Typically an inverse time device rated for 1.25-2 X FLA on for 415V primary would suffice, but you need to check both TCC (with transformer inrush point and its thermal damage curve) and local codes. And of course coordination with other devices may influence the final selection.

Rafiq Bulsara

http://www.srengineersct.com

 

[TurbineGen]

I strongly second that post! In our testing lab here we tested closing our breakers at exactly the zero voltage crossing and the peak voltage on the wave. The zero crossing close pegged out ammeters. Peak of the wave close had significantly lower current and less harmonics.

This might be a case where you need to measure it to know for sure.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[rbulsara]

Search S&C's website for the technical bulletin for fuse selection for transformer protection. It has good description.

Rafiq Bulsara

http://www.srengineersct.com

 

[davidbeach]

And in this case, "Primary", is the side the transformer is being energized from and has nothing to do with high side or low side.

 

[cuky2000]

Enclosed is a range of inrush current for various type of transformers and connections.

At 0.1 second, the factor of 11 or 12 times the FLC is somehow within the expected range for a wide variety of units. However, if more accuracy is desired, the enclosed table could help.

 

[GadreP]

Cuky2000, I am unable to download this file; windows is giving an error. Can u please upload in some other format or provide a patha of the website?

 

[electricpete]

fwiw, I had no problem viewing the file.

=====================================
(2B)+(2B)' ?

 

[prc]

When we say 10-12 times inrush,what does it mean? Are we taking the peak of inrush with peak of FLA?or RMS of FLA.
Skosgurra, inrush current is affected very little by the rated working flux density value in core.

 

[JBinCA]

PRC,

Skosgurra is referring to the residual flux in the core - "where you left the core when switching off"

JB

 

[rbulsara]

prc:
I believe that would be RMS. Based on the way they are compared on the TCC, where the rms current is plotted.

Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

Power engineers are almost always talking about rms current, not peak, unless specifically noted otherwise.



David Castor

http://www.cvoes.com

 

[cuky2000]

Q] When we say 10-12 times inrush, what does it mean?

A] It mean RMS value: 12xFLA_rms@0.1 sec.
_{NOTE: (FLApeak= SQRT(2)xFLArms}

The peak Inrush current is a lot higher: FLA_peak= 20<FLA_rms<60.

 

[GadreP]

Dear All, thanks for your replies. But please confirm one thing:

1) If we are charging a transformer on No-Load then 10-12 times what we say should be 10-12 times of No-Load current which is 5-6% of Full load current (FLA).
2) If we charge a transformer on loaded condition then that 10-12 times will be 10-12 times of respective load current.

Am I correct? Because I could not understand when we charge a transformer on No-Load, how come we calculate a 10-12 times of FLA?

 

[rbulsara]

GadreP:

No! Definition of FLA is the "rated" full load current on the nameplate.

Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

how come we calculate a 10-12 times of FLA

Because that is how high the inrush current can be. This is just a rough approximation that is used to set primary overcurrent relays and size primary fuses to avoid tripping on inrush.



David Castor

http://www.cvoes.com

 

[GadreP]

dpc: For relay setting & feeder sizing 12 times of primary full load current is ok, but please tell me whether I am saying is correct or not? When I am charging a transformer on no-load, inrush should be 10-12 times of its No-Load Amps..?

 

[jimbofitz]

GadreP,

Inrush will be approximately 10-12 times the FULL LOAD CURRENT for a standard transformer.

 

[dpc]

No, you are not correct. If the transformer is energized unloaded, the inrush will be (very approximately) 8 to 12 times the FULL LOAD amp rating (self-cooled) of the transformer.

The loading could have some impact on the duration of the inrush, but not much on the initial magnitude.



David Castor

http://www.cvoes.com