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Correlation between inrush current and system impedance

Correlation between inrush current and system impedance

Correlation between inrush current and system impedance

(OP)
I am aware that source impedance is a determining factor when considering inrush. I was wondering if there is a correlation between the two that would provide one with a ballpark of what the inrush current will be more or less based on source impedance and transformer rated current (Ir).

I am aware that the inrush current is also dependent on other factors such as transformer design, core material, point-on-wave of energisation.

Thus allowing for all these (assuming worst case) and given that the primary voltage is x volts, Ir = y amps, source impedance = Zs (positive sequence) is there a way of deriving a ballpark figure for the inrush current?

Thanks.

RE: Correlation between inrush current and system impedance

S&C used to have a paper on primary fuse sizing that covered this. You might do a Google search or check with S&C.

RE: Correlation between inrush current and system impedance

(OP)
Excuse my ignorance but what does S&C stand for?

Thanks.

RE: Correlation between inrush current and system impedance

More than system impedance, inrush current peak of a transformer depends on following:
-Air core reactance of winding on which energized- depends on inductance ie diameter and length of winding(ie kVA rating)
- Instant of switching on the voltage sine wave (max at zero &min at max voltage)
- Magnitude and polarity of residual flux in core
- Coupling factor -depending on connection (star/delta) and core construction (single phase,3 phase 3 limb or 5 limb)

RE: Correlation between inrush current and system impedance

OP, here is the equation for peak inrush as a function of source impedance:

Iinrush = Ipeak / (1 + Ipeak * Xsource)
All values in per unit.

Source:
https://books.google.de/books?id=qW_OBQAAQBAJ&...

RE: Correlation between inrush current and system impedance

(OP)
Yes, I saw this in the Westinghouse as well, but it was not really much help as I do not know what Ipeak is. And what is Iinrush, rms or peak?

RE: Correlation between inrush current and system impedance

The best and latest tutorial on the subject is CIGRE Brochure No. 568-2014 titled "Transformer energization in Power systems: A study guide " This gives calculation and modeling for inrush current of transformers. Here also source impedance of system is not coming in the calculations.

I peak = peak of first cycle of inrush current with zero source reactance. I inrush= Peak of inrush with source impedance of X. Westighouse(now ABB) handbook is very clear on this.It is assumed that inrush will come down linearly with applied voltage. But normally source impedance too low compared to transformer impedance and such corrections are never applied.

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