Transformer’s connections

The various schemes of transformer connection are identified systematically with an alpha-numeric string. Two letters represent the method of connection followed by a number which represents the phase shift of the secondary voltage relative to the primary voltage, expressed in hours, where one hour is equal to an angle of 30 degrees. The first letter is a capital letter, which relates to the higher winding, where Y=star, D=delta and N=neutral lead. The second letter is a small letter, which relates to the lower winding, where y=star, d=delta, z=zigzag and n=neutral lead. So Dy5 specifies a transformer with a delta higher and a star lower, where the lower voltage lags the higher voltage by 150 degrees.

Winding selection for 3-phase transformers
In practice the most commonly used transformer connections are Yy, Dy, Yd, Yz and Dz. In addition the star and zigzag have a neutral point which is either lead-out or latent. Correct connection of a 3-phase transformer depends on several factors:

  • Supply system: 
    • 3 or 4 wire mains network
    • from converter
  • Transformer Power
  • Voltage level
  • Load asymmetry
    • symmetrical voltage
    • asymmetric voltage
  • Economic reasons (cost of construction and connection method) 

1) Yy system is applied mainly for low power applications for symmetrical 3-phase loads, however its is sometimes used in high power applications where the neutral point must be earthed. This system has the disadvantage that the third (and other zero-sequence) harmonics are not isolated and can cause problems of overvoltage and imbalance especially at low loads. These problems can be reduced by the addition of a compensation winding connected in delta.

2) Dy system is usually applied to step-down transformers with high rated power, connected to a low voltage 3-phase distribution network. If the neutral wire is provided the user has the benefit of both line to line and phase voltages. It has the advantage of reduction of zero-sequence fluxes and associated currents.

3) Yd system is used mainly for step-up applications where the primary side needs to be earthed either directly or via an inductor. This system has the advantage of eliminating zero-sequence currents.

4) Yz and Dz systems are used for step-down applications of low rated power. Phase voltages can be used where the neutral lead is available. This solution is rarely used due to economic reasons. Comparing zigzag to star windings the number of turns required is 2/v3 higher, thus costing about 15% more. So this is limited to supplying asymmetric single phase loads whilst maintaining symmetry on the supply side.

Only the basic properties of the various connection systems have been presented above. For proper operation of the transformer it is important to select correctly. Sometimes it is necessary to make a detailed analysis of the entire system, considering converter type, load asymmetry and supply type, before correct selection can be made.

The most commonly applied connection systems of 3-phase, 2-winding transformers