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?
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
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
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Transformer Inrush
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
This might be a case where you need to measure it to know for sure.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Transformer Inrush
Rafiq Bulsara
http://www.srengineersct.com
RE: Transformer Inrush
RE: Transformer Inrush
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.
RE: Transformer Inrush
RE: Transformer Inrush
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RE: Transformer Inrush
Skosgurra, inrush current is affected very little by the rated working flux density value in core.
RE: Transformer Inrush
Skosgurra is referring to the residual flux in the core - "where you left the core when switching off"
JB
RE: Transformer Inrush
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
David Castor
www.cvoes.com
RE: Transformer Inrush
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
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
No! Definition of FLA is the "rated" full load current on the nameplate.
Rafiq Bulsara
http://www.srengineersct.com
RE: Transformer Inrush
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
RE: Transformer Inrush
Inrush will be approximately 10-12 times the FULL LOAD CURRENT for a standard transformer.
RE: Transformer Inrush
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