Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by electrical grids worldwide to transfer power. It also used to power large motors and other heavy loads. A three-phase sy stem is usually more economical than an equivalent or two-phase system at the same voltage because it uses less conductor material to transmit electrical power.There are two basic
three-phase configurations: delta and wye (star). Either type can be wired for
three or four wires. The fourth wire, if present, is provided as a neutral. The
'3-wire' and '4-wire' designations do not count the ground wire used on many transmission lines which is solely for fault protection and
does not carry curr to transferent under non-fault conditions.
Figure 3.1 Wye (Y) and
Delta (Δ) circuits
For practical calculations, it is reasonable to model the three-phase transformer as three ideal transformers as shown in Figure 2 below. Since these transformers are ideal, the secondary voltages are related to the primary voltages by the turns ratio according to :
Figure 3.2 Three-phase transformer ideal model
There are 4 kinds relations of three-phase transformer systems, such as :
- Wye-Delta: Commonly used in a step-down transformer, wyeconnection on the HV side reduces insulation costs,the neutral point on the HV side can be grounded,stable with respect to unbalanced loads.
- Delta-Wye: Commonly used in a step-up transformer for thesame reasons as above.
- Delta-Delta: Offers the advantage that one of the transformers can be removed while the remaining two transformers can deliver three-phase power at 58% of the original bank.
- Wye-Wye: Rarely used, problems with unbalanced loads.
http://www.ece.msstate.edu/~donohoe/ece3614three_phase_transformers.pdf
http://ece.mst.edu/media/academic/ece/documents/classexp/ee208labs/04_-_Three-Phase_Transformers.pdf
http://opencourseware.kfupm.edu.sa/colleges/ces/ee/ee360/files%5C3-Lesson_Notes_Lec_11_3_phase_traqnsformers.pdf
