This problem tells you that a wye-wye transmission line is stepped down from 230KV to 200KV using three auto xfmrs connected line to neutral. For each phase, the auto Xfmr is rated 500 MVA, and the turns ratio is 5:2 (common:series). Then it asks to find Apparent Power in common coil. The solution uses the formula S (at) = S (tw) * (1 + N common/ N series). When they ask to find the KVA in common coil, I assumed that it's asking you to.find the KVA of auto Xfmr...But as per solution, they solved for the S (tw) instead, which is the two winding transformer. Can someone tell me why this is correct?