Connection group in transformers

The connection group for a three-phase electrical transformer refers to the specific arrangement of the primary and secondary windings of the transformer. It defines how windings are connected to form a particular configuration and is essential to ensure the correct voltage and phase ratios between input and output.

The choice depends on the specific application, voltage requirements, and electrical system design. Different countries and regions may have their own standards and preferences for transformer connection groups, so local practices should also be considered.

Common applications and connection groups as follows:

Yy or Y-Y:

This configuration connects the primary and secondary windings in a “Y” or “star” configuration. It is characterized by a neutral point in both the primary and secondary windings.

Distribution transformers: Yy is commonly used in distribution transformers that supply power to residential, commercial, and industrial areas. The Yy configuration allows for a grounded neutral point, which is important for safety and for handling unbalanced phase loads.
Grounding: the Yy configuration provides an accessible neutral point that can be grounded to ensure safety and protect against fault currents.
Low-voltage systems: Yy connections are often used in low-voltage systems where a neutral point is essential for practical and safety purposes.

Dd or Δ-Δ:

In this configuration, the primary and secondary windings are connected in a “delta” configuration. There is no neutral point in this arrangement.

Isolation transformers: in situations where electrical isolation is crucial, such as in sensitive electronics or in situations with strict safety requirements, the Dd configuration is used to provide electrical isolation without a grounded neutral point.
Industrial applications: Dd transformers can be found in industrial settings, particularly where it is important to maintain a constant voltage ratio between the primary and secondary sides. This is often the case in manufacturing processes and heavy industries.
Specific voltage ratios: in cases where a specific voltage ratio must be maintained without neutral connections, the Dd configuration is a suitable option.

Yd or Y-Δ:

This arrangement connects the primary winding in a “Y” configuration and the secondary winding in a “delta” configuration.

Phase splitting: the Yd configuration can be useful when you need to split a three-phase supply into two-phase supplies. This can be beneficial in certain industrial processes or loads that require such a setup.
Neutral current reduction: in cases where the load on the secondary side is unbalanced and generates significant neutral current, using a Yd setting on the primary side can help reduce this neutral current.
Transformer taps: some transformers have multiple taps in the primary winding, allowing adjustments to the turn ratio. The Yd connection can be one of the options, offering versatility in the application of the transformer.

Dy or Δ-Y:

Connects the primary winding in a “delta” configuration and the secondary winding in a “Y” configuration.

Voltage transformation: the Dy setting is often used when you need to reduce the voltage from the primary to the secondary side of the transformer while also providing a neutral point on the secondary side.
Load balancing: It can help balance unbalanced loads on the secondary side and limit the flow of zero-sequence current in the event of ground faults, which can be beneficial in certain industrial applications.
Harmonic reduction: the Δ-Y connection can help mitigate harmonics in the electrical system. Delta winding can suppress certain harmonics that may be present in the load.

The “Z” connection group:

Is a specialized transformer connection that is primarily used for applications where a balanced two-phase supply from a three-phase source is required. It’s not as commonly used as other connection groups like Yy or Dy, but it has specific applications:

Two-phase systems: the Z-connection is used when a two-phase system is needed. It is commonly employed in specialized industrial machinery, such as rolling mills and some types of motor drives, where a two-phase power supply is advantageous.
Load balancing: in applications where a balanced load is essential, such as some older types of machinery, the Z connection can provide a balanced two-phase supply, which is ideal for smoother and more stable operation.
Reduced harmonics: in some cases, the Z connection can help reduce certain harmonics in the electrical system, which can be beneficial for sensitive loads.

It is important to note that the Z connection is relatively less common and is mainly used in specialized industrial applications where a balanced two-phase supply is necessary.

Here are some examples of common connections:

The “angle” of a connection group refers to the relative phase change between the primary and secondary windings of the transformer. It is an important feature that defines how windings are connected to create specific voltage and phase relationships between the input and output sides of the transformer.

The angle of the connection group is usually expressed in degrees.

Some examples:

0° Phase Shift: in this configuration, the primary and secondary voltages are in phase, meaning they peak and trough at the same time. This is known as a “zero-phase” or “in-phase” offset connection (such as 0 in Yyn0).
30° Phase Shift: this setting shifts the secondary voltage 30 degrees relative to the primary voltage (such as 1 in Dy1).
150-degree phase shift (150° or 5×30°): this setting shifts the secondary voltage 150 degrees relative to the primary voltage (such as 5 in Dyn5).

At POLYLUX, in order to be able to offer a solution adapted to the needs of our customers and the requirements of their electrical projects, we can manufacture transformers on request with the connection groups that are necessary depending on the application.

Para más información o soporte en relación con la selección de tu transformador, puede utilizar nuestro buscador avanzado o contactarnos a través del siguiente formulario:

For more information, please download our document on transformer connection groups: