Coupling of Three-phase Transformers

The coils of both, primary and secondary sides of the three-phase transformers can be coupled on three ways:
- star (Y);
- triangle (D);
- broken star (Z - "Zickzak");

Picture 1: Coupling diagrams of three-phase transformers: Star (Y), Delta (D), Broken star (Z-"Zickzak")


At star coupling configuration, the inputs taken to the isolators are connected to the phase voltages, while outputs are connected at the neutral point, see Picture 1 (Y). At the second coupling configuration, (D) delta configuration, cyclically are connected the start of the coil of one phase with the end of the coil of the second phase, and so on, see Picture 1 (D). Finally, at the third coupling configuration, each of the coils is divided into two half-coils which are coming on two different cores (phases). There are 6 half-coils in total, which are usually coupled in a way that the end of the first half-coil from the first phase is connected to the end of the second half-coil from the second phase, the end of the first half-coil from the second phase is connected to the end of the second half-coil from the third phase, and so on. The inputs of the second half-coils of all phases are all connected together in the neutral point, while the inputs of the first half-coils are connected on the isolators or on the network, see Picture 1 (Z).
At the star or broken star coupling configurations, sometimes there is a lead from the neutral point with a neutral conductor to the isolator which allows grounding usually at higher voltages, while at lower voltages it allows two voltages available, for example 380/220 V.

On Picture 1, besides the coupling schematics, there are also and vector diagrams of the voltages of the three-phase transformer's coupling. Of course, the convention of the vectors directions according to the phases can be different than the one used on this diagram.

So, depending on the coupling used, the coupling of the transformers can be different. The three usually used couplings in practice are star-star (Yy), when both, primary and secondary coils of the transformer are coupled in star configuration, delta(triangle)-star (Dy) or the opposite one, star-delta (Yd), and star-broken star (Yz).