A-C delta-wye phase shift
You'll be in trouble at the interview. Read on ...







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A delta-wye (delta-star) transformer connection and phasor diagrams.


Figure 1 shows a standard delta-wye 3-phase transformer connection. The useful aspect of showing it wired this way is that it is very easy to see that each of XFMR1 primary and secondary pairs must be in phase as they are on the same core. 
Figure 1(i) shows the primary and secondary phasors. Note that we join the phasors wherever they are electrically connected. The secondaries are smaller (representing a step-down in voltage) for clarity and haven't been connected yet. Note that 'a' is in-phase with 'A', etc.
When we wye-connect the secondaries we join the secondary phasors at the neutral point. This is shown in Figure 1(ii). Note that the phase-neutral voltages are in still phase with the primaries.
Figure 1(iii) shows the phase to phase phasors in black. Note that these are 30° out of phase with the phasors in Figure 1(i). This is the phase-shift you will be interrogated on.


Draw the diagram a few times, then do it from memory. Show by trigonometry that the shift is actually 30°. Then explain it to a friend. If you can do that you should be ready for the interview.