We were given the following problem. We spent a night but could not even get close to solve it. It would be great if you could help us solve it.
Three identical 10 kVA, 19920/220 transformers are used to build a three-phase transformer in Y-Δ connection. An open-circuit test is performed on the low-voltage side of this threephase transformer: Vline (line-to-line)=220 V, Iline=5 A, P3ϕ(total power in three phases)=430 W. A short-circuit test is performed on the high-voltage side: Vline=500V, Iline=0.5 A, P3ϕ=86 W. a. Determine the equivalent circuit of the three-phase transformer bank. b. Find the efficiency of the transformer bank with a three-phase full-load for a power factor of 0.8 (lagging).