Vector Groups
Transformer nameplates carry a vector group reference such at Yy0, Yd1, Dyn11
etc. This relatively simple nomenclature provides important information about the
way in which three phase windings are connected and any phase displacement that
occurs.
Winding Connections
HV windings are designated: Y, D or Z (upper case)
LV windings are designated: y, d or z (lower case)
Where:
Y or y indicates a star connection
D or d indicates a delta connection
Z or z indicates a zigzag connection
N or n indicates that the neutral point is brought out
Phase Displacement
The digits (0, 1, 11 etc) relate to the phase displacement between the HV and LV
windings using a clock face notation. The phasor representing the HV winding is taken
as reference and set at 12 o'clock. It then follows that:
Digit 0 means that the LV phasor is in phase with the HV phasor
Digit 1 that it lags by 30 degrees
Digit 11 that it leads by 30 degrees
etc
All references are taken from phase-to-neutral and assume a counter-clockwise phase
rotation. The neutral point may be real (as in a star connection) or imaginary (as in a
delta connection)
When transformers are operated in parallel it is important that any phase shift is the
same through each. Paralleling typically occurs when transformers are located at one
site and connected to a common bus bar (banked) or located at different sites with the
secondary terminals connected via distribution or transmission circuits consisting of
cables and overhead lines
Basic Theory
An ac voltage applied to a coil will induce a voltage in a second coil where the two are
linked by a magnetic path. The phase relationship of the two voltages depends upon
which ways round the coils are connected. The voltages will either be in-phase or
displaced by 180 deg as below:
In phase 180deg displacement
When 3 coils are used in a 3 phase transformer winding a number of options exist. The
coil voltages can be in phase or displaced as above with the coils connected in star or
delta and, in the case of a star winding, have the star point (neutral) brought out to an
external terminal or not.
Example - Dyn11
We now know that this transformer has a delta connected primary winding (D) a star
connected secondary (y) with the star point brought out (n) and a phase shift of 30 deg
leading (11). Connections and vector diagrams are as follows:
HV
LV
Other Configurations
By connecting the ends of the windings in other ways a wide range of options becomes
available as set out below.
Phase shift (deg) Connections
0 Yy0 Dd0 Dz0
30 lag Yd1 Dy1 Yz1
60 lag Dd2 Dz2
120 lag Dd4 Dz4
150 lag Yd5 Dy5 Yz5
180 lag Yy6 Dd6 Dz6
150 lead Yd7 Dy7 Yz7
120 lead Dd8 Dz8
60 lead Dd10 Dz10
30 lead Yd11 Dy11 Yz11
End
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Tutorial Index
What does the vector symbol means in two winding, three phase
transformers?
faq238-1090
Posted: 22 Feb 05 (Edited 22 Apr 05)
What does Dd0, Dyn11, YNd5 etc. mean?
First symbol/symbols, capital letters: HV winding connection.
Second symbol/symbols, small letters: LV winding connection.
Third symbol, number: Phase displacement expressed as the clock hour number.
Winding connection designations
High Voltage Always capital letters
Delta - D
Star - S
Interconnected star - Z
Neutral brought out - N
Low voltage Always small letters
Delta - d
Star - s
Interconnected star - z
Neutral brought out - n
Phase displacement
Phase rotation is always anti-clockwise. (international adopted convention)
Use the hour indicator as the indicating phase displacement angle. Because there are 12 hours on a
clock, and a circle consists out of 360°, each hour represents 30°.
Thus 1 = 30°, 2 = 60°, 3 = 90°, 6 = 180° and 12 = 0° or 360°.
The minute hand is set on 12 o'clock and replaces the line to neutral voltage (sometimes imaginary) of
the HV winding. This position is always the reference point.
Because rotation is anti-clockwise, 1 = 30° lagging (LV lags HV with 30°)and 11 = 330° lagging or 30°
leading (LV leads HV with 30°)
To summarise:
Dd0
Delta connected HV winding, delta connected LV winding, no phase shift between HV and LV.
Dyn11
Delta connected HV winding, star connected LV winding with neutral brought out, LV is leading HV with
30°
YNd5
Star connected HV winding with neutral brought out, delta connected LV winding, LV lags HV with 150°
After some comments I've decided to include the following:
The phase-bushings on a three phase transformer are marked either ABC, UVW or 123 (HV-side capital,
LV-side small letters)
Two winding, three phase transformers can be devided into four main categories (Clock hour number and
phase displacement of those most frequently encountered in practice in brackets)
Group I - (0 o'clock, 0°) - delta/delta, star/star
Group II - (6 o'clock, 180°) - delta/delta, star/star
Group III - (1 o'clock, -30°) - star/delta, delta/star
Group IV - (11 o'clock, +30°) - star/delta, delta/star
(Minus indicates LV lagging HV, plus indicates LV leading HV)
Group I
Example: Dd0 (no phase displacement between HV and LV)
The conventional method is to connect the red phase on A/a, Yellow phase on B/b, and the Blue phase
on C/c. Other phase displacements are possible with unconventional connections (for instance red on b,
yellow on c and blue on a) By doing some unconventional connections externally on one side of the trsf,
an internal connected Dd0 transformer can be changed either to a Dd4(-120°) or Dd8(+120°)
connection. The same is true for internal connected Dd4 or Dd8 transformers.
Group II
Example: Dd6 (180° displacement between HV and LV)
By doing some unconventional connections externally on one side of the trsf, an internal connected Dd6
transformer can be changed either to a Dd2(-60°) or Dd10(+60°) connection.
Group III
Example: Dyn1 (-30° displacement between HV and LV)
By doing some unconventional connections externally on one side of the trsf, an internal connected Dyn1
transformer can be changed either to a Dyn5(-150°) or Dyn9(+90°) connection.
Group IV
Example: Dyn11 (+30° displacement between HV and LV)
By doing some unconventional connections externally on one side of the trsf, an internal connected
Dyn11 transformer can be changed either to a Dyn7(+150°) or Dyn3(-90°) connection.
Additional Note
By doing some unconventional connections externally on both sides of the trsf, an internal connected
groupIII or groupIV transformer can be changed to any of these two groups. Thus, an internal connected
Dyn1 transformer can be changed to either a: Dyn3, Dyn5, Dyn7, Dyn9 or Dyn11 transformer, by doing
external changes on both sides of the trsf. This is just true for star/delta or delta/star connections.
Changes for delta/delta or star/star transformers between groupI and groupII can just be done internally.
