Electronic – Star delta phase difference

1) This is a typical delta to wye configuration. I might add that normally these 3 transformers would be built as one chunk of steel with 3 sections, one for each phase. The floating Vn wire is just to show that the primary is delta only, neutral is not used.

2) Because this is 3-phase the difference in phases is 120 degrees. The 30 degree phase shift is the same in each transformer so X1/X2/X3 are also 120 degrees out of phase.

3) A typical application for this would be 480vac main feed from a service entrance panel to risers (buss-bars) going to each floor of a 5 story building. Each floor has its own 3-phase step-down transformer to convert 480vac delta to 120vac wye (208vac delta) to power the outlets and lights on each floor from a 3-phase 42 breaker panel. There is plenty of heavy machinery that uses 208 single or 3-phase for power such as machine shops, ultrasonic welders, etc. This is typical of what I did as an electrician so it is easy to write about.

4) The common neutral (shown)for X1/X2/X3 is grounded at its sub-panel, and though it is not shown a ground runs back to the main service entrance panel. In large buildings it is mostly a distribution panel for all the sub-panels on each floor.

5) If Va/Vb/Vc is 480 vac delta then the phase to neutral voltage is 277vac, typically used for lighting and HVAC blower motors and heaters. It is not unusual to have a double-neutral feed to such a building, to account for imbalances in the loading of the phases.

EDIT: For more details about the 30 degree phase shift between phase windings, please see this article:Why does a delta/wye transformer make 30 degrees phase shift ? and this article:What are the Pros/Cons on the 30 Degree voltage phase shift between the primary and secondary of a Delta-Wye Transformer

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.