# 3 phase transformer efficiency calculation

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.  • "RH = 133.5 milli-ohms, XH = 201 milli-ohms, RL = 39.5 milli-ohms, XL = 61.5 milli-ohms, RC = 240 ohms, Xm = 290 ohms." - I have no idea what parts of the transformer equivalent circuit these apply to. – Andy aka Oct 14 '20 at 11:24
• H refers to high voltage, L to low. – aviral upadhyay Oct 14 '20 at 11:25
• Although it is not mentioned, Rc and Xm are referred to low voltage, sorry for inconvenience. – aviral upadhyay Oct 14 '20 at 11:27
• If you are after efficiency all three transformers will be equally efficient so why not simplify to a single phase problem. – Andy aka Oct 14 '20 at 11:28
• Does efficiency not depend on the way we make connection? That is, star delta or delta delta? – aviral upadhyay Oct 14 '20 at 11:35