# What does the vector symbol means in two winding, three phase transformers

By Ralph Christie

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¦

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 divided 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.