Phase shift between pole voltage and line-to-line voltage (3 phase)
This site has a nice picture that explains why there is a 30 degrees phase shift between phase and line voltages: -



For instance, Vca is the addition of Voc and Voa and this produces a phase shift that is 30 degrees lagging to Voa. 

If you are not familiar with phasor diagrams then I recommend that you become familiar. It will be time well-spent and it's just half an hour of your life and a few simple lessons in geometry.