I've heard that a delta-star transformer produces a phase shift of 30 degrees between the primary and secondary coils. My question is very basic but I can not find its answer on the internet. Why there is a delay between the secondary coils and the primary coils?
Thank you very much.

 

Here's an explanation.

 

It's all in the vectors

 

@crutschow Thank you very much for the fantastic explanation, it is really helpful. In conclusion, What I understand is:

A line voltage of Wye is the sum of the two phase voltages of Wye.

The two phase voltages of Wye: One of them is in phase with the line voltage of delta, and the other in Not in phase with the line voltage of delta.

So, The line voltage voltage of Wye (which is the sum of them) is Not in phase with the line voltage of delta. The line voltages of delta are only in phase with the phase voltages of Wye.

Is that correct ?

Sorry about my english