Importance of Vector Groups in Transformer

The internal connections of transformer windings can be made in a number of ways. Accordingly, different types of connections have been standardized depending upon the phase displacement.
 
There are four vector groups and each group includes three methods of connection of high voltage and low voltage windings.
 
Phase displacement between EMFs of high voltage and low voltage windings is expressed as the clock hour number and is designated by symbols 0, 6, 1, and 11.
 
For example, the clock hour number 0 represents zero degrees phase displacement between primary and secondary EMFs. The clock hour number 6 is for 1800 phase displacement,  1 for  – 300, 11 for +300.
 
On the nameplate of three-phase transformer vector group is printed as Yy0, Dd1 etc. Here Yy0 will mean it’s both the windings are star-connected and phase displacement between primary and secondary EMFs is zero degree. It is belonging to Group No.1 (refer to table).
 

Group No. Winding Connection
(Primary)
Winding Connection
(Secondary)
Phase Displacement Clock – Hour Number Vector Symbol
1 Star
Delta
Delta
Star
Delta
Zig-zag
0o 0 Yy0
Dd0
Dz0
2 Star
Delta
Delta
Star
Delta
Zig-zag
180o 6 Yy6
Dd6
Dz6
3 Delta
Star
Star
Star
Delta
Zig-zag
– 30o 1 Dy1
Yd1
Yz1
4 Delta
Star
Star
Star
Delta
Zig-zag
+30o 11 Dy11
Yd11
Yz11

 
Similarly, Dy1 will mean its primary winding is connected in delta, secondary winding in the star, phase displacement is – 300 and belonging to Group No. 3.
 
Information of vector group of a transformer is very significant when it has to operate in parallel.  For a transformer working in isolation, the arrangement of its internal connections is of little importance.
 
Transformers will operate in parallel satisfactory if they have,

  • the same primary and secondary voltages
  • the same tap ratio
  • the same percentage impedance, and,
  • belonging to the same vector groups.

The two transformers may have their windings connected in star/star and yet it will not be possible to operate in parallel if one belongs to Group 1 and the other to group 2, unless the internal connections of the secondary winding of one of the transformers are changed.
 

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