The three-phase transformer comprises three separate single-phase transformers with shared limbs, each representing a phase consists of primary and secondary windings. The primary or secondary windings are connected in one of several ways. The most common configurations are the star, delta, and Zigzag. The phase angle between the high voltage and the low voltage windings varies between 0o and 360 o.
The vector group is the method of categorizing the high voltage HV windings and low voltage LV windings configurations of three phase transformers .
The vector group of a transformer is presented as follow:
Fig.1. Vector Group Representation
THE IMPORTANCE OF THE TRANSFORMERS VECTOR GROUP DETECTION.
The transformers vector group identification (Y-Y, Delta-Y, Delta-Delta… etc.) is an essential task for successful transformer parallel operation. Hence every electrical power transformer must undergo through the vector group test to satisfy its operation requirements .
Here, we should differentiate between two kinds of vector group test, the first is performed just to check the phase displacement between the line voltages of a known transformer type, but the second is used to find out both the transformer winding configuration and the phase displacement of an unknown transformer type. When two transformers are connected in parallel, the phase sequence in which the phases reach their maximum positive voltages, must be identical, otherwise, a short circuit will occur when the transformers are energized. For that, the good selection of the vector group of a three-phase transformer is an essential criterion for the parallel operation of the transformers, and this imposes to perform transformer vector group detection test.
Before you buy or use an electrical transformer, you must ensure the transformer vector group, whether or not it is associated with its existing system.
HOW DO WE DETECT TRANSFORMER VECTOR GROUP CONNECTION?
Performing the above-mentioned test to check the phase displacement, detects the appropriate vector group by applying an AC voltage in HV windings and making predefined test combinations. But to find out both the winding configuration and the phase displacement of the transformer, it is recommended to use the highest possible true three-phase test voltage especially with larger transformers or high ratio transformers.
HIGHTEST DEVICE USED TO DETECT TRANSFORMER VECTOR GROUP
(TURA-01+TPA-03) and TURA-03 can measure turns ratio without knowing the phase angle between the primary and the secondary windings of the transformer but to detect the vector group’s configuration for the power transformers even if they are not already known, Using HIGHTEST TURA-X device would be wiser. TURA-X is a true three phase fully automatic turns ratio test device based on a state-of-the-art technology, using the most advanced technique available today. It can perform the test for any transformer type without a need to know its winding configuration and its phase displacement.
Reference:  ISO/IEC 60076-1, international standard.  IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers,” in IEEE Std C57.12.90- 2015 (Revision of IEEE Std C57.12.90-2010) , vol., no., pp.1-120, 11 March 2016