Vector group
In electrical engineering, a vector group is the International Electrotechnical Commission (IEC) method of categorizing the high voltage (HV) windings and low voltage (LV) winding configurations of three-phase transformers. The vector group designation indicates the windings configurations and the difference in phase angle between them. For example, a wye HV winding and delta LV winding with a 30-degree lead is denoted as Yd11.