# NCEES Power Question # 132



## majorever (Jun 8, 2016)

I am trying to understand this question about transmission line impedances change questions. 

Z1 = 16.75&lt;71

Z2 = 13.4&lt;71 and system impedance = 31.25&lt;81. what is the fault current at station C? 

Ok to solve this I immediately search for base change formula. which is 




, but book did not use this formula they use this formula  Z p.u new = Zpu_old (S_new / S_old). I did not find this formula any where. Can someone explain it to me why they use this formula ?

thank you


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## TWJ PE (Jun 8, 2016)

majorever said:


> I am trying to understand this question about transmission line impedances change questions.
> 
> Z1 = 16.75&lt;71
> 
> ...


They "omitted" voltage because it didn't change... so it's 1.


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## nukem2k5 (Aug 20, 2016)

I came here to look for this exact NCEES practice problem.  To address your question, they aren't converting any impedances to different bases.  It's simply a voltage / impedance calculation.

That being said, the thing I'm curious about is why they're using sqrt(3) in the equation.  The voltage is a line voltage, these are (I assume) per-phase impedances (on the transmission lines), but it asks for 3-phase fault current.  Why is it not just 60kV/(Z1+Z2+Z3) ?


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## trochreo (Sep 23, 2016)

nukem2k5 said:


> I came here to look for this exact NCEES practice problem.  To address your question, they aren't converting any impedances to different bases.  It's simply a voltage / impedance calculation.
> 
> That being said, the thing I'm curious about is why they're using sqrt(3) in the equation.  The voltage is a line voltage, these are (I assume) per-phase impedances (on the transmission lines), but it asks for 3-phase fault current.  Why is it not just 60kV/(Z1+Z2+Z3) ?


Hoe I am not late for this.

remember, that three phase  Fault current calculation is base on phase values. Thus, 60KV is line value, that nees conversion to phase value.


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