In electrical engineering, a vector group, officially called a connection symbol, 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.
Transformers connected in parallel must have the same vector group; mismatching phase angles will result in circulating current and other system disturbances.
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 numeric digits.
The only factor affecting the choice between one or the other is system phasing, i.e. whether parts of the network fed from the transformer need to operate in parallel with another source.