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

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Zach Stone P.E.

Learn how to Pass the Power PE Exam at electricalp
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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.

 
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DuranDuran

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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.

Learn how to Pass the Power PE Exam at electricalp
Joined
Oct 25, 2016
Messages
338
Reaction score
144
Location
USA
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. 

 

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