A-C delta-wye phase shift

Question

^{simulate this circuit – Schematic created using CircuitLab}

My question is, because the delta primary in this case is connected A-C, rather than A-B, does this effect the direction of the 30 degree phase shift? I have read that if the primary is connected A-B then the primary phase is leading, and if it is connected A-C the primary phase is then lagging the secondary by 30 degrees. Can somebody please shed some light on this for me? I would like to know exactly what is shifting, leading or lagging, and why. Thank you !

Answer 1

You'll be in trouble at the interview. Read on ...

^{simulate this circuit – 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.