vector group

From Wikipedia:

A 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.^[1]

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.