×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Transformer Inrush

Transformer Inrush

Transformer Inrush

(OP)
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?

RE: Transformer Inrush

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

RE: Transformer Inrush

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

RE: Transformer Inrush

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.

RE: Transformer Inrush

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

RE: Transformer Inrush

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.

RE: Transformer Inrush

(OP)
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?

RE: Transformer Inrush

fwiw, I had no problem viewing the file.

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

RE: Transformer Inrush

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.

RE: Transformer Inrush

PRC,

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

JB

RE: Transformer Inrush

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

RE: Transformer Inrush

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

 

David Castor
www.cvoes.com

RE: Transformer Inrush

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

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

The peak Inrush current is a lot higher: FLApeak= 20<FLArms<60.
 
 

RE: Transformer Inrush

(OP)
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?

RE: Transformer Inrush

GadreP:

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

Rafiq Bulsara
http://www.srengineersct.com

RE: Transformer Inrush

Quote:

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
www.cvoes.com

RE: Transformer Inrush

(OP)
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..?

RE: Transformer Inrush

GadreP,

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

RE: Transformer Inrush

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
www.cvoes.com

RE: Transformer Inrush

I think for the new transformers, the material of the core is improved a lot, so the inrush current may not exceed 6-8 times FLA.
 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources