Vector groupA Vector group is the International Electrotechnical Commission (IEC) method of categorizing the primary and secondary winding configurations of three-phase transformers. Within a polyphase system power transformer it indicates the windings configurations and the difference in phase angle between them.

The phase windings of a polyphase transformer can be connected together internally in different configurations, depending on what characteristics are needed from the transformer. For example, in a three-phase power system, it may be necessary to connect a three-wire system to a four-wire system, or vice versa. Because of this, transformers are manufactured with a variety of winding configurations to meet these requirements.

Different combinations of winding connections will result in different phase angles between the voltages on the windings. This limits the types of transformers that can be connected between two systems, because mismatching phase angles can result in circulating current and other system disturbances.

Symbol designation

The vector group provides a simple way of indicating how the internal connections of a particular transformer are arranged. In the system adopted by the IEC, the vector group is indicated by a code consisting of two or three letters, followed by one or two digits. The letters indicate the winding configuration as follows:

D: Delta winding, also called a mesh winding. Each phase terminal connects to two windings, so the windings form a triangular configuration with the terminals on the points of the triangle.

Y: Wye winding, also called a star winding. Each phase terminal connects to one end of a winding, and the other end of each winding connects to the other two at a central point, so that the configuration resembles a capital letter Y. The central point may or may not be connected outside of the transformer.

Z: Zigzag winding, or interconnected star winding. Basically similar to a star winding, but the windings are arranged so that the three legs are "bent" when the phase diagram is drawn. Zigzag-wound transformers have special characteristics and are not commonly used where these characteristics are not needed.

III: Independent windings. The three windings are not interconnected inside the transformer at all, and must be connected externally.

In the IEC vector group code, each letter stands for one set of windings. The primary (input) winding is designated with a capital letter, while the other winding or windings are designated with a lowercase letter. The digits following the letter codes indicate the difference in phase angle between the windings, in units of 30 degrees. For example, a transformer with a vector group of Dy1 has a delta-connected primary winding and a wye-connected secondary winding. The phase angle of the secondary lags the primary by 30 degrees.

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Transformers built to ANSI standards usually do not have the vector group shown on their nameplate and instead a vector diagram is given to show the relationship between the primary and other windings.

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.High Voltage Always capital letters Delta - D Star - Y Interconnected star - Z Neutral brought out - N

Low voltage Always small letters Delta - d Star - y Interconnected star - z Neutral brought out - n

for auto transformer - a

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° YNa0d11 Star connected HV winding with neutral brought out, auto transformer with 0° displacement. LV winding delta conected leading HV by 30°

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)

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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 transformer, 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 transformer, 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 transformer, 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 transformer, 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 transformer, 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 transformer. 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.

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