Impedance in Different Bases

Hello veronica

There appears to be no "General Rule" - Refer: http://www.eng-tips.com/viewthread.cfm?qid=170856&page=1

It depends on the make and design of the individual transformer, and may also vary widely depending on the particular transformer year of manufacture.

Thus you should contact transformer makers, who may properly advise you regarding their own products.

If you have a specific transformer make in mind, then contact that maker for their comparisons.

Kind Regards....

giving an impedance in % is a system known as per unit and for a particular transformer is given on the S_base of the transformer rating and V_base of a winding. From these quantities, Z_base can be calculated as:

Z_base = (V_base)² / S_base

So for your given data, Z (in p.u) = 0.075 on S_base = 1000kVA and an abritrary V_base.

Converting Z (in p.u) to ohms is:

Z_ohm = 0.075 * (V_base)² / 1000

This ohmic value is then converted back to per unit on the new S_base and the same abritrary V_base:

Z_new(in p.u) = [0.075 * V_base² / 1000] / [V_base² / 1600]

Z_new(in p.u) = 0.120 i.e. 12%

Thankyou.it was helpful

Any text book which covers per unit calculations will contain a detailed explanation of exactly how to do this. You can even google per unit calculations and get good info. I suggest this site http://powerlearn.ece.vt.edu/modules/B4/html/index.html