NCEES Power Question # 132

[majorever]

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

Z1 = 16.75<71

Z2 = 13.4<71 and system impedance = 31.25<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 

 

[TWJ PE]

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

Z1 = 16.75<71

Z2 = 13.4<71 and system impedance = 31.25<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 

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

 

[nukem2k5]

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) ?

 

[trochreo]

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.