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close this bookCircuits, Formulas and Tables Electrical Engineering - Basic vocational knowledge (Institut für Berufliche Entwicklung, 201 p.)
close this folder4. Electrical Machines
close this folder4.3. Transformers
View the document4.3.1. Single-phase Transformers
View the document4.3.2. Three-phase Transformers

4.3.2. Three-phase Transformers

Three-phase transformers in parallel connection

Transformer station with two transformers connected in parallel

Usual groups of connection for transformers


Indicator Diagram

Circuit Diagram

Identification number

Group of connection

High Voltage

Lower Voltage

High Voltage

Lower Voltage


Three-phase power transformers

Dd 0


Yy 0

Dz 0

Dy 5


Yd 5

Yz 5

Dd 6


Yy 6

Dz 6

Dy 11


Yd 11

Yz 11

Single-phase power transformers

Explanations for the table

The table shows the commonly used circuits according to the relevant Standard of the International Electrotechnical Commission (I E C). When multiplying the identification number by 30°, the phase shift of the high voltage side with respect to the lower voltage side is obtained.

Dd 6 means:

D = high voltage side delta connection
d = lower voltage side delta connection

The lower voltage is shifted with respect to the high voltage by 6 x 30° = 180°.

Due to these different circuits, different operational behaviour is attained.

The ratio of transformation is calculated as follows: