The testing of the vector group of the Transformer is a very important check.  It is crucial to know the vector group of the transformer when trying to parallel the transformer.

Even when purchasing a transformer, it is necessary to confirm whether the vector group of the transformer is the same as that mentioned in the nameplate.

The vector group of a transformer may be found by a simple test using a 3 phase power supply of about 440 volts and a multimeter.

The 'R' terminal of the HV winding and the 'r' terminal of the LV winding. are shorted together.  The 3 phase supply is connected to the HV winding (It should never be connected to the LV winding as that word cause dangerously high Voltages to appear on the HV side).

Let us assume for a moment that we have a Ynd11 transformer. when one of the terminal on both the HV and the LV winding are shorted and a LV Voltage applied, we will get the following vector notation.

 The connection is verified by taking voltage measurements between each terminal and every other terminal. 

The following table would be helpful

A vector diagram can be drawn graphically based on the data in the table.  This helps to verify the vector group.