Each periodic non sinusoidal signal consist many sinusoidal harmonic with different frequency. For example as you know the difference phase angle of main harmonic of line and phase voltage in three phase system is 30 degree because the difference angle of two phases is 120 degree. But the difference phase angle of two phase of third harmonic is zero; therefore all signal contain triplen harmonics, they are eliminated from the line voltages because they are co-phasal, Thus the difference phase angle of line and phase of third harmonic voltage in three phase system cannot be defined.

The Fourier series for v

_{A0 }and v

_{B0} are given by:

Note that the triplen harmonics are removed in vAB although they are present in v

_{A0} and v

_{B0}
Generally we can speak about difference phase angle of two same sinusoidal harmonic in line and phase signals.