The vector group of transformer shows the phase difference between the primary and secondary sides of the transformer. There are different ways in which transformer high voltage and low voltage winding is connected. Based on HV and LV winding connections, the vector grouping of the transformer is classified.

The three-phase transformers are divided into four groups depending on the phase difference between line voltages on HV and LV sides.

The phase difference is the angle by which the LV line voltage lags behind HV line voltage and it is measured in units of 30° in the counterclockwise direction.

**Four groups are given as follows**

- Group number 1: No Phase displacement yy0, Dd0, Dz0
- Group number 2: 180° phase displacement yy6, Dd6, dz6
- Group number 3: – 30° displacement Dy1, Yd1, Yz1
- Group number 4: +30° phase displacement Dy11, yd11, yz11

Thus yd11 connection of transformer gives the following information:

Y indicates transformer HV winding connected in star Y and the “d” indicates LV winding connected in delta, 11 indicates that LV line voltage lags HV line voltage. 11×30=330° measured from the HV phase in the clockwise direction.

The clock method is also used to indicate a vector group of transformers. For this, the minute hand of the clock is considered as the line voltage on the HV side, and LV side line voltage is represented as the hour hand. In the clock method of vector group of transformer minute hand as HV line voltage always occupy to position 12 o’clock.The phase of line voltage on the LV side is considered as the hour hand and set on the dial of the clock according to its position in relation with the phaser of line voltage on the HV side.

The angle 30° is the angle between two adjacent figures on the clock dial and is taken as the unit of dial shift.

When the hour hand is at 12 O’Clock position, the phase displacement is zero. Then the hour hand is on 1’O clock then the phase displacement is – 30°

When the hour hand at 6 O’clock then the phase displacement between HV line voltage and the LV line voltage is 180 degrees.

Similarly When the hour hand on 11 O’clock the phase shift between HV line voltage and LV line voltage is +30 degrees.

Thus the numbers 0,6,1 and 11 in the vector group of the transformer indicate the primary to secondary phase shift in terms of hours of the clock.

The connection of the transformer represented by Dy11 indicates a delta star connected transformer in which the LV line voltage phasor at 11 O’clock position and has a +30°phase shift with respect to HV side line voltage.

In the case of the vector group of transformer number 1 there is no phase displacement whereas the case of group no 2 phase displacement is 180 and in group 3 line voltage of LV lags by 30 degrees.

The determination of the vector group of the transformer is very important before putting the transformer In parallel