The group of connecting windings of power transformers is selected so that the transformers meet the following conditions as much as possible:
To fulfill the first and second conditions, one winding of power transformers is connected into a star (Y), and the other into a triangle (A).
At GPP enterprises, a high voltage winding (35-220 kV) is usually connected to a star, as this may be required by a neutral grounding system in networks of this voltage; the low voltage winding is connected in a triangle. The star connection of the primary winding also makes it easier to regulate the voltages by switching the taps. For these reasons, transformers with a star-delta connection group (Y / A) or a star with a neutral point removed - delta (Y0 / A) are mainly used at industrial enterprises. The same transformers are used in workshop substations supplying three-wire low voltage networks (for example, 220 or 660 V networks without a neutral conductor).
To power four-wire networks with a voltage of 220/380 or 380/660 V, transformers are used, in which the secondary winding is connected in a star with a neutral point removed (Y0) or in a zigzag with a neutral point removed (Z0). To fulfill the above three conditions, the primary winding should be connected in a delta, and the optimal group for connecting the transformer would be A / Y0; the same requirements, especially in terms of balancing, are also met by the Y / Z0 group, which is used at a rated power of transformers from 25 to 100 kVA.
The Y / Y0 group does not possess these positive properties and, in particular, is distinguished by an increased zero-sequence resistance, which makes it difficult to protect networks from single-phase short-circuits arising from short circuits to the case, etc. Therefore, transformers with a group of connection of windings Y / Y or Y / Y0 in most cases are not recommended for supplying shop low voltage networks.