Why it is recommended to use Dyn11 in most transformers?

The Dyn winding vector groups do not allow zero sequence currents (commonly known as earth fault current) to pass across the transformer.





Thus, the yn side of the transformer is a 'separately derived system' in which all earth fault current must flow through the neutral of this transformer. With this configuration, earth fault protection is possible and practical. Likewise, earth fault protection on the primary side is not affected by earth faults on the secondary side.





The choice of Dyn11 vs Dyn1 or some other angle (5,7, etc) is simply due to user standardization.