## Percentage Impedance of Transformers

Dr. Transformer: Back from the holidays transformed!  Got on the scale this morning and.. oh, my! Certainly transformed!  Enough!  The questions about impedance continue to come in unimpeded! So let’s get to it!

It is very common to see transformers specified with ‘Percent Impedance’. Because stating impedance in percentage terms is as a result of the ‘per unit’ system favored by power engineers, it isn’t surprising that it causes confusion in the greater engineering community.  The concept of ‘per unit’ system is breath-takingly (if there is such a word) simple: Reduce voltage/current/resistance to unity; i.e. V Line-1, I Line =1,R Load =1.  This saves the hardworking engineer, i.e. you, the chore of having to reflect ohmic impedance (which is completely equivalent) by the turns ratio squared (Npri/Nsec)².

Example

Equivalent per unit Circuit

Special Note: In 3 phase transformer systems, divide VLL by √3 (square root of 3)

Also, in the ‘per unit’ system, the short circuit current multiple is the inverse of % impedance.

So there you have it my dear students of transformers.  Please let me know if you have any further questions!