This is an assignment. No matter how hard I try the answer is never what I expect. Here is the question:
Three single phase transformers, each rated at 12 kVA, 120/240V, 60Hz, are connected to form a three-phase, step-up, star/delta connection. The parameters of each transformer are
R(High) = 133.5 milli-ohms, X(High) = 201 milli-ohms, R(Low) = 39.5 milli-ohms, X(Low) = 61.5 milli-ohms, Rc(Low) = 240 ohms, Xm(Low) = 290 ohms. When it delivers the rated load at the rated voltage and 0.8 pf lagging, determine the line voltage, line currents and efficiency.
I will tell you what I am after. Here is what I did, but the answer is 0.92 approx. I think I am mistaken in calculating core and/or copper loss.