Switching group

The switching group is a key feature of three-phase transformers and describes the electrical connection and the position of the phases between the primary and secondary windings. It enables the user to recognize the type of connection and the phase shift at a glance. The labeling is based on a system that is used worldwide and enables a uniform and comprehensible description of the transformers.

The switching group is made up of letters and numbers. The first capital letter describes the connection and the phase position between the primary and secondary windings, while the second lower-case letter indicates the connection of the secondary or output winding. Common designations are, for example, “D” for delta connections, “Y” for star connection and “Z” for zigzag connection. In addition, an “n” can be specified to indicate that the star point is brought out.

The following figure describes the phase shift of the individual phases between the primary and secondary sides. This is indicated in so-called “clock positions”. Each hour on the clock corresponds to a phase shift of 30 degrees. A switching group such as “Dyn5” therefore means that the primary winding is delta-connected (D) and the secondary winding is star-connected with a neutral conductor (yn), with a phase shift of 150 degrees between the two sides.

If no other order specifications with a more precise assignment of upper and lower voltage are formulated, this standardized type of marking is used. In many applications, three-phase transformers are preferably manufactured in the Dyn5 switching group, unless there are specific requirements. This design offers a good combination of mains compatibility, stability and practical application in an industrial environment.

Importance of the gearshift assembly in practice

Choosing the right wiring is crucial for the safe and trouble-free operation of transformers and chokes. Among other things, it influences the voltage quality, the behavior under unbalanced load and the possibility of operating transformers in parallel. For parallel operation, it is essential that transformers have the same switching group, as different phase shifts can lead to equalizing currents and thus to damage.

The switching group also has an influence on earthing and the handling of the neutral conductor. This plays an important role for the supply of single-phase loads, for example in 24V control circuits, and for protective measures in the network, particularly in star connections with an outgoing neutral point.

Load capacity of the neutral conductor (neutral point)

An important aspect in connection with the switching group is the load capacity of the neutral conductor, also known as the neutral point. In the star-wye circuit (Yy), this point may only be loaded with the full rated current if the neutral conductor of the supplying network is connected to the primary-side transformer neutral point. A symmetrical load is only guaranteed under this condition.

If this is not the case, the neutral point can only be loaded with around 10 % of the phase conductor current. A higher load can lead to undesirable voltage shifts and overloads. This is particularly important for unbalanced loads, which often occur in practice.

Special features of autotransformers

Similar conditions apply to a three-phase autotransformer designed as a star-connected autotransformer (YNa0) with regard to the load capacity of the neutral conductor. Here too, the connection to the supplying network is decisive for the permissible load of the neutral point.

If a fully loadable neutral point is available but this is not available on the mains side, an alternative connection must be selected. In such cases, the double zigzag connection (ZZan0) is often used. This enables an even load distribution and a significantly higher load capacity of the neutral conductor, even with asymmetrical loads.

Overview of typical shift groups

In practice, there are numerous switching groups that are selected depending on the application. The most frequently used include

• Dyn5 or Dyn11: widely used in industry and energy supply
• Yy0: simple star-star circuit without phase shift
• YNa0: Star connection with neutral conductor brought out for autotransformers
• ZZan0: Zigzag circuit with high neutral conductor load capacity

The selection depends on various factors, including grid structure, load behavior, earthing concept and voltage quality requirements.

Summary

The switching group is a central feature of a three-phase transformer and provides important information about its wiring and phase position. It has a significant influence on the function, safety and possible applications of a transformer in the electrical network.

Correct selection and design of the switching group is crucial for trouble-free operation, especially when several transformers are used in parallel and with unbalanced loads. The load capacity of the neutral conductor and the choice of suitable wiring also play an important role.

The standardized marking system allows users worldwide to clearly identify transformers and use them according to their requirements. The switching group is therefore an indispensable part of the technical description and planning of electrical systems and an important technical basis.