Question

3rd harmonic absent from line voltages in y connecton

Answer 1

You might be knowing that the phase displacement between third harmonic phasors is 0.For Y connection the third harmonic voltages are present in each phases but between any two lines the third harmonic voltages cancel out each other. As far as third harmonic currents are concerned, for a three phase three wire system , third harmonic currents cannot flow through the lines because 3rd harmonic currents must flow through the neutral. So for Y connection, third harmonic voltages are present in each phases and third harmonic currents are present in each line if neutral is provided.

To prevent the effect of third harmonics in Y-Y transformer, either one of the winding is connected in delta or a tertiary winding is provided , connected in closed delta, which provides the path for 3rd harmonic currents and main mmf is free from 3rd harmonic.