Answer / subbarayalu

The normal method
of expressing transformer impedance is as a percentage voltage drop in the
transformer at full-load current and this reflects the way in which it is seen by
system designers. For example, an impedance of 10% means that the voltage
drop at full-load current is 10% of the open-circuit voltage, or, alternatively,
neglecting any other impedance in the system, at 10 times full-load current, the
voltage drop in the transformer is equal to the total system voltage.

Answer / anonymous

It is because three phase system is most economical out of all possible polyphase system configurations...that's it...

Answer / sokib

why did electric polyphase system commonly using three phase
system? is there any electrical system more than three phase
system?

Answer / 7anoter4

Usually group of transformers presents the same short-
circuit reactance in %
For instance, for S[ rated power] up to 2500 [to 3000] kVA
the percent reactance will be vk%=4-8. Ohmically calculated
reactance will be V^2/S*vk%/100 [ V=rated voltage].
For V=10 kV and vk%=6 for S=500 kVA Xk=10^2/0.5*6/100=12
ohm
For V=6 kV and vk%=6 for S=3000 kVA Xk=6^2/3*6/100=0.72 ohm
As one could see it is more convenient to consider only
vk% instead of ohmic reactance

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