Three-Phase Transformer Winding Arrangements that Commonly Used for Primary and Secondary Sides:

Transformer is the key equipment for electrical energy transfer from one step to other step in power transmission and distribution system. Transformer may construct as single or three phases. Our discussion limited on three-phase transformer winding arrangements

There are three methods of arranging the windings of three-phase transformers are commonly used; such as star winding, delta winding and zig-zag winding which are presented in the diagram. Each method can be applied to either or both of the primary and secondary windings.

Star winding contains four terminals are used when a neutral connection is required for earthing. Normally in low voltage distribution transformer primary and secondary winding and primary distribution transformer secondary windings are used star arrangement.  

Delta winding contains tree terminals are most frequently used on the high voltage winding, which is usually the winding connected to the supply. The delta connection also allows third harmonic currents to circulate which improves the waveforms of the line currents and voltages on both side of the transformer. Delta windings may be slightly more expensive because the insulation has to withstand the full line-to-line voltage.

The zig-zag winding requires each three-phase winding to be split in half. Each half is interconnected with a half-winding on another core limb. Zig-zag windings are used to suppress third harmonics or to provide a neutral connection as an earthing transformer as power rectifiers when high order harmonic can be trouble and have to be minimized.
Three-Phase Transformer Primary & Secondary Winding Arrangements
Fig- Transformer Winding Arrengement

As per standard IEC60076 some specified numbers and letters are used, such as for high voltage (HV) terminals have upper-case letters
e.g: A-B-C, R-Y-B, U-V-W, L1-L2-L3 and for low voltage (LV) terminals have lower-case letters e.g: a-b-c, r-y-b, u-v-w, l1-l2-l3. Each winding has a start numbered 1 and a finish number 2. Tapings are numbered 3, 4, 5 etc. from the start terminal.

 The choice of letters and numbers depends of different nations or countries; like as-
Nations                           Letters or Numbers Used
USA                                   L1            L2           L3
           1                2               3
Europe                                 U               V             W
            R                S              T
UK                                       R               Y             B
             A                B             C

The type of winding e.g. star, delta is given a letter, again upper case for HV and lower case for LV windings. The letters are,
D for Delta HV                                              d for delta LV
Y for Star HV                                                 y for star LV
Z for Zig-zag HV                                          z for zig-zag LV
Since a phase angle displacement can occur across the transformer due to its method of connection it is necessary to identify this displacement. The numbering system for this is based on the hands of a clock. Each five-minute position on a clock gives 30◦ phase displacement, such as

12 o’clock gives zero displacement
1 o’clock gives -30◦ displacement
6 o’clock gives 180◦ displacement
11 o’clock gives +30◦ displacement

These are the commonly encountered displacements. (Note that the phasor rotation is anti- clockwise).
For example a transformer has a delta HV winding a star LV winding and a +30◦ displacement, It is described by letters and numbers as a Dyll transformer.

Transformer in the field of Electrical Engineering is very big field to discuss and wide range in used. It is quite impossible to make sense in a single article or even in a book about Transformer. But hope, this portrayed article on three-phase transformer winding arrangements will help you in your works and make you hungry to learn more about transformer in your working field. 

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