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# Electric power & transmission & distribution FAQ

## Electric Power Engineering

 What does the vector symbol means in two winding, three phase transformers? by RalphChristie 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 designationsHigh Voltage Always capital lettersDelta                     - DStar                      - SInterconnected star - ZNeutral brought out - NLow voltage Always small lettersDelta                     - dStar                      - sInterconnected star - zNeutral brought out - nPhase displacementPhase 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:Dd0Delta connected HV winding, delta connected LV winding, no phase shift between HV and LV.Dyn11Delta connected HV winding, star connected LV winding with neutral brought out, LV is leading HV with 30¦YNd5Star 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/starGroup II   - (6 o'clock, 180¦) - delta/delta, star/starGroup III  - (1 o'clock, -30¦) - star/delta, delta/starGroup IV   - (11 o'clock, +30¦) - star/delta, delta/star(Minus indicates LV lagging HV, plus indicates LV leading HV) Group IExample: 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 IIExample: 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 IIIExample: 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 IVExample: 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 NoteBy 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. Back to Electric power & transmission & distribution FAQ Index Back to Electric power & transmission & distribution Forum

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