This problem is taken from Fundamentals of Electric Circuits (chapter 12, problem 15).
As shown in the first blue box, I have trouble seeing why after the conversion, Z_Ye will be parallel with Z_Y. I think it's because the voltages 'emitting' from the converted network is like another 3-phase voltage source.
The Attempt at a Solution
The diagram on the right-hand side is my sketch in Microsoft Paint, where I fail to see why are them parallel after the conversion.