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This is my first post and I am happy I stumbled across this forum. I am preparing for a job interview and have been given a Delta-wye transformer schematic and have been asked to discuss the phase shift between the primary and secondary windings. I know that there is a phase shift of 30 degrees, but in this particular drawing there is a neutral line on the delta side with no clear connection point marked. I haven't dealt with transformers in a while and am wondering specifically where the neutral is connected to in this schematic? I am assuming I will be asked more about this transformer configuration and would like to completely understand what is going on here with that neutral. Any help is greatly appreciated!

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Your diagram shows it connected to all it should be connected to at this transformer. It will however be connected to the generator's or preceding transformer's N, so that any phase-N connection would allow current to flow. But the transformer's primary makes no such connections. \$\endgroup\$ – Neil_UK Jul 5 '16 at 17:53
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    \$\begingroup\$ SE.EE is NOT a forum. \$\endgroup\$ – Andy aka Jul 5 '16 at 18:58
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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

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  • \$\begingroup\$ Thank you for the fast reply! Could you help me understand where the 120 degree phase shift between primary and secondary comes from? Or point me in the right direction? Thanks again! \$\endgroup\$ – nmorris Jul 6 '16 at 0:57
  • \$\begingroup\$ @nmorris. You misunderstood my wording. Because the PHASE difference is 120 degrees on the primaries, the transformer secondaries also have a 120 degree PHASE difference. The 30 degree phase lag is common to all phases but electricians normally pay no attention to it. \$\endgroup\$ – Sparky256 Jul 6 '16 at 1:02
  • \$\begingroup\$ Oh! Ok thank you for clarifying. The problem I've been given is to discuss the phase shift between the primary and secondary winding's, which must be the 30 degrees. \$\endgroup\$ – nmorris Jul 6 '16 at 1:43
  • \$\begingroup\$ @Sparky256: I think you were doing well until the last comment. The delta and wye would both have the same lag in that case. It's explained the "this article" you linked to. There's a simultaneous related question Pros and cons of 30 degree phase shift ... where I address some of this. It may help. \$\endgroup\$ – Transistor Jul 6 '16 at 19:05
  • \$\begingroup\$ @transistor. Thx, I will add that link. The OP needs both the 120 degree and the 30 degree shift clarified. \$\endgroup\$ – Sparky256 Jul 6 '16 at 19:08

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