First of all try another forum like Electric Power & Transmission & Distribution Forum.
Second, the impedance you can read on nameplate or catalogue it is the short-circuit impedance -it is not the no-load impedance of the main magnetic circuit of transformer but the leakage impedance-most of this is due magnetic field in the surrounding air.
The impedance in % is referring to a virtual impedance Z=S/Voltage^2 that means the Zsc in ohm has to be divided by this impedance and multiplied by 100.Usually it is employed in short-circuit calculation but neglecting the no-load current impedance one can use it for voltage drop calculations.
Because the impedance is a different value as expressed in ohms from the high voltage winding and the low voltage winding, we use per unit impedance which is common with both windings. Or more to a easer to express number a % Per unit impedance.
I suggest a little reading about per unit impedance for a better understanding.
The impedance is basically the leakage reactance of the transformer. Google up the equivalent circuit for a transformer. You can think of the impedance as the series reactance in the model.
The "percent" terminology is used because the impedance is expressed based on the transformer's kVA rating, not in ohms.
Plenty of info available on-line for free on this topic, I'm sure.
The impedance describes the steady state current that will flow under short circuit conditions. The actual peak offset current at the moment of the fault occurring may be almost twice as much, decaying to the value calculated from the impedance.
Distribution transformers have much lower impedances, 1.8% to about 7%.
With a high X/R ratio the X will be the predominant component of the impedance. The R may predominate for full load voltage drop or regulation.
Bill
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"Why not the best?"
Jimmy Carter
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