Circuits, Formulas and Tables Electrical Engineering - Basic vocational knowledge (Institut für Berufliche Entwicklung, 201 p.)
 4. Electrical Machines
 4.3. Transformers
 4.3.1. Single-phase Transformers 4.3.2. 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

 Designation Indicator Diagram Circuit Diagram Identification number Group of connection High Voltage Lower Voltage High Voltage Lower Voltage Transformation Three-phase power transformers Dd 0 0 Yy 0 Dz 0 Dy 5 5 Yd 5 Yz 5 Dd 6 6 Yy 6 Dz 6 Dy 11 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: