Inrush Current Exceeding SCA 2

[farmape]

Is this a correct statement?
"In general, if the inrush current exceeds the available SCA, the inrush event would take more time and voltage drop would be larger"
Thanks.

 

[dpc]

Well, the actual inrush current cannot exceed the available SC amps. If the estimated inrush current AT RATED VOLTAGE exceeds the available SC current, the actual inrush current will be reduced, the starting voltage will be greatly reduced and accelerating time will be longer.

 

[jraef]

Well, the actual inrush current cannot exceed the available SC amps. If the estimated inrush current AT RATED VOLTAGE exceeds the available SC current, the actual inrush current will be reduced, the starting voltage will be greatly reduced and accelerating time will be longer.

Been there, done that, got the T-shirt and the hat... Your supply transformer becomes, in effect, a motor starting auto-transformer.

But this is assuming here that you are defining "inrush" as motor starting current, not it's true definition as MAGNETIZING current, which applies to any induction device like an AC motor or a transformer. In the true definition of "inrush current" the statement would be true, albeit impossible because, as dpc noted first, the inrush CANNOT be higher than the SC current. The SC current should represent the MAXIMUM current available in the circuit. So a "true-er" way of saying that is that if the rated inrush current of an inductive device is higher than the available SC current, the SC current BECOMES the inrush current value, which may cause a voltage drop on the circuit.

"Will work for (the memory of) salami"

 

[waross]

Available Short Circuit Current, by some definitions is the symmetrical component of the short circuit current. The peak asymmetrical or offset current may be over twice the symmetrical RMS current.
The inrush may exceed the defined symmetrical short circuit current but it may not exceed the asymmetrical peak current.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[farmape]

Thanks for the replies, got me thinking about transformer inrush (i.e. energizing inrush or Magnetizing inrush current). In the prescribed scenario, It will be limited by the SCA, and, although smaller in magnitude, will cause a greater voltage drop. Understanding this inrush's decay time is mostly dependent on the resistance (e.g. winding resistance), the event duration would not lengthen?

 

[7anoter4]

Usually the transformer main magnetic flow impedance is about 30 times the leakage magnetic flow-which determines the transformer short-circuit reactance-so the no-load current working on unsaturated reactance of the transformer core will be about 3% of the rated current. If the voltage is applied on the remaining magnetic field the magnetic flux density will be
very elevated and the core will be saturated. The inductance will decrease dramatically and the inrush current will be elevated.
Now, even if we will neglect this inductance, the primary winding impedance will exist since the resistance and the leakage magnetic flow reactance will not be changed.
In this case, the maximum short-circuit current peak value[Ip=1.7-2*sqrt(2)*Ik] upstream the transformer location will be more than transformer inrush current, any way.
Only if we could supply the transformer with the rated voltage-unchanged-then the inrush current would be more.
There are some studies how an inserted resistance could mitigate the inrush current.See [for instance]:
Analytical Study of Transformer Inrush Current Transients and Its Applications

http://www.researchgate.net/publica...nrush_Current_Transients_and_Its_Applications