Sample Question from PPI's practic problems (Ch.42, # 9)
Simplified Question:
A 20MVA transformer has 12,500V(wye):4160V(delta) What is the rated primary and secondary current?
Book's Answer:
Ipri = S/(sqrt(3)*Vline) = (20x10^6)/(sqrt(3)x12,500) = 923.76A
Isec = Vpri*Ipri/Vsec = (12,500x923.76)/4160 = 2775.72A
For Isec, shouldn't the Voltages be phase voltages and the equation should be Isec = (Vpri/sqrt(3))*Ipri/Vsec = (12,500/sqrt(3))*923.76/4160 = 1602.5A?
I guess the bigger question is I've assumed A = Vp/Vs means those are phase voltages. Whenever a transformer voltage ratio is given, my understanding is they're usually given as Line-Line values therefore if a 12,500V:410V wye-delta transformer is shown, then the ratio a would not be a=12,500/4160 but should be a=(12,500/sqrt(3))/4160.
Simplified Question:
A 20MVA transformer has 12,500V(wye):4160V(delta) What is the rated primary and secondary current?
Book's Answer:
Ipri = S/(sqrt(3)*Vline) = (20x10^6)/(sqrt(3)x12,500) = 923.76A
Isec = Vpri*Ipri/Vsec = (12,500x923.76)/4160 = 2775.72A
For Isec, shouldn't the Voltages be phase voltages and the equation should be Isec = (Vpri/sqrt(3))*Ipri/Vsec = (12,500/sqrt(3))*923.76/4160 = 1602.5A?
I guess the bigger question is I've assumed A = Vp/Vs means those are phase voltages. Whenever a transformer voltage ratio is given, my understanding is they're usually given as Line-Line values therefore if a 12,500V:410V wye-delta transformer is shown, then the ratio a would not be a=12,500/4160 but should be a=(12,500/sqrt(3))/4160.