# PU & X/R ratio relationship



## Sthabik PE (Feb 16, 2019)

Cut sheets are taken form old thread (link at the end)

Question)


Ans: (only initial description, no calculation)


Any idea how PU impedance of generator is 0.1 pu? I couldn't understand how identical X/R ratios leads to ratio of powers that provides PU impedance of GEN.

Please go through the link for the solution provided by Fpar.

Source; http://engineerboards.com/topic/29389-pu-and-xr-ratio/?tab=comments#comment-7451245


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## Messi (Feb 19, 2019)

I am guessing since the Vbase and actual voltage is the same, they are simply dividing the Sactual/Sbase to get Zpu for generator. If we calculate it in ohms and convert it to pu we would do the same division.


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## Sthabik PE (Feb 20, 2019)

Messi said:


> I am guessing since the Vbase and actual voltage is the same, they are simply dividing the Sactual/Sbase to get Zpu for generator. If we calculate it in ohms and convert it to pu we would do the same division.


Lets say Sbase is gen MVA, but what is the logic of getting Zpu of generator by dividing transformer MVA to Sbase. I don't see any logic.

Can you please elaborate more?


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## Messi (Feb 20, 2019)

Sdhabik said:


> Lets say Sbase is gen MVA, but what is the logic of getting Zpu of generator by dividing transformer MVA to Sbase. I don't see any logic.
> 
> Can you please elaborate more?


the only logic I see is that the voltage is cancelled when we divide Zactual/Zbase and we end up dividing 2/20 which gives 0.1 pu for Gen. I am not stressing at that statement they wrote, not sure if that is true because even though X/R ratio is same doesn't means that the Z is same for both. May be others can comment who understood that part ?


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## Sthabik PE (Feb 20, 2019)

@Messi I appreciate your point of view.


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