Vector Group

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 HV winding is designated with a capital letter, followed by medium or low voltage windings designated with a lowercase letter. The digits following the letter codes indicate the difference in phase angle between the windings, with HV winding is taken as a reference. The number is in units of 30 degrees. For example, a transformer with a vector group of Dy1 has a delta-connected HV winding and a wye-connected LV winding. The phase angle of the LV winding lags the HV by 30 degrees.

The point of confusion is in how to use this notation in a step-up transformer. As the IEC60076-1 standard has stated, the notation is HV-LV in sequence. For example, a step-up transformer with a delta-connected primary, and star-connected secondary, is not written as 'dY11', but 'Yd11'. The 11 indicates the LV winding leads the HV by 30 degrees.

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.

Hope that helps.

Hello RDGRNR,

Regarding your 'point of confusion':

I don't think there is any confusion. A transformer itself is not designated as 'step-up' and 'step-down' transformer. Only its terminals are defined as HV and LV. The terms 'step-up' and step-down' are used only for the use of the transformer. A 33/11KV (D winding in 33 KV and Y winding in 11 KV) transformer can be designated as 'step down' when used in the distribution side and 'step-up' when used in the generation side. But, 33 KV side is always called HV and 11 KV is called LV.

As the vector group symbol always starts with HV winding type with capital letter followed by the LV winding type with small letter, the above transformer's vector group is always Dy…, there is no way it can be yD…

Hope it clears you and helps to get rid of the 'point of confusion'.

- MS

How to find vector group.

Example. A transformer with delta(primary) and wye,(secondary) has 13.8.kV-0.48/0.277kV,3ph, vector group is unknown.

1. Supply 220volts to the primary delta side.

2. Secondary voltage will become (220V*277V)/13800V = 4.42V

3. Assume it is Dyn1 then you need to show its vector group diagram.See Fig. 1

4. Connect "a" of wye to "A" of delta transformer.See fig. 2 then measure the voltage accross C-c, C-b, and B-c of transformer.

5. Calculate the voltage

VCc = VAC<60+Van<-120+Vcn<180 = 220<60+4.42<-120+4.42<180 = 213.4<61.03

VCb = VCA<60+Van<-120+Vbn<-60 = 220<60+4.42<-120+4.42<-60 = 213.4<58.9

Note: VCc = VCb check the diagram .

VBc = VAB<120+Van<-120+Vcn<180 = 220<120+4.42<-120+4.42<180 = 220.13<122

6. If the measure voltage matches to the calculted voltage above then our guess is correct which is Dyn1. If it does'nt match then you have to guess another vector group and repeat the procedure.