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

Threephase 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 
Threephase 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 




_{} 
Singlephase 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:
_{}
_{}