Selection of group and connection diagram of transformer windings

The group of connecting windings of power transformers is selected so that the transformers meet the following conditions as much as possible:

  • prevented the occurrence of higher harmonics in electrical networks;
  • equalized the load between the phases of the primary winding with an asymmetric load of the secondary winding;
  • limited the resistance of the zero sequence of the short circuit circuit in the case of power supply of four-wire networks.

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.

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