How to base change percent impedance values (and why the formula looks backward compared to the base change formula)

[Zach Stone P.E.]

Hi Everyone, I just published two new videos that I'd like to share to help address one of the questions that I get asked the most lately about base changing the percent impedance values of machines (transformers, generators, motors, etc).

Have you ever noticed that the percent impedance base change formula below: 

seems to have the old and new power bases on the wrong side of the fraction compared to the standard per unit base change formula: 

 
Or, have you ever seen this version of the per unit impedance base change formula and were not sure where it came from:

 ​

Click play to see why all three of formulas are actually the same as long as a very specific condition is met that we will discuss in the video:





If you'd like to visit the full article that these videos were made for, please visit: https://www.electricalpereview.com/base-changing-percent-impedance-and-per-unit-impedance/. 

I hope you enjoyed the new videos and learned something new that you can take with you to the up coming October 2019 electrical PE exam.

 

[DLD PE]

Zach,

Thank you for posting this video. I was able to follow and understand it.  I tried turning this into a fault current analysis problem to see if I could solve it. What if there was a 3-phase fault on the right side of the bus? If I use the MVA method, I get 18404 amps.  If I use the Per Unit method, I get 18265 amps.  Is that a big enough difference to be concerned?  

 

[Zach Stone P.E.]

Zach,

Thank you for posting this video. I was able to follow and understand it.  I tried turning this into a fault current analysis problem to see if I could solve it. What if there was a 3-phase fault on the right side of the bus? If I use the MVA method, I get 18404 amps.  If I use the Per Unit method, I get 18265 amps.  Is that a big enough difference to be concerned?  

I would check your rounding. Both methods should yield the exact same number. You can try storing values in your calculator as variables to get a more precise answer.