What are the Pros/Cons on the 30 Degree voltage phase shift between the primary and secondary of a Delta-Wye Transformer
What is so important about this phase shift? Are there Pros/Cons?


Probably the main point is to be aware that there is a phase shift. For example, series connecting delta and wye secondaries will give strange results with the resultant midway (15°) between the other two. (That's from memory and needs confirmation.)

One useful application is to generate a DC with less ripple. A standard 3-phase rectifier will give 6-pulses of DC per 360°.



Figure 1. 3-phase rectification gives six pulses per 360°.

By rectifying both delta and wye secondaries we can get 12-pulses per 360° and further reduce ripple.


  Is it possible to obtain more pulses than twice the number of phases in a rectifier circuit? The answer to this question is yes: especially in polyphase circuits. Through the creative use of transformers, sets of full-wave rectifiers may be paralleled in such a way that more than six pulses of DC are produced for three phases of AC. A 30° phase shift is introduced from primary to secondary of a three-phase transformer when the winding configurations are not of the same type. In other words, a transformer connected either Y-Δ or Δ-Y will exhibit this 30° phase shift, while a transformer connected Y-Y or Δ-Δ will not. This phenomenon may be exploited by having one transformer connected Y-Y feed a bridge rectifier, and have another transformer connected Y-Δ feed a second bridge rectifier, then parallel the DC outputs of both rectifiers. (Figure below) Since the ripple voltage waveforms of the two rectifiers' outputs are phase-shifted 30° from one another, their superposition results in less ripple than either rectifier output considered separately: 12 pulses per 360° instead of just six: 




Figure 1. 12-pulse rectifier generated by delta and wye secondaries.

As I hinted earlier, I think further improvement can be obtained by series connecting delta and wye secondaries to create other phases for even more improvement.

Diagrams and quotation from Lessons In Electric Circuits -- Volume III where there is a lot more information.