Usually transmission lines (above 11kV) are delta connected because of 2 factors:
- To save copper without needing an extra conductor for neutral.
- To reduce current per phase (as Current per Phase = 0.866 times Line Current value).
However, if one of the phases goes out, then the connection reduces to an open delta connection, thereby power reduces to 57.7% of the rated power capacity. Below link explain the open delta phenomenon in power transformers in a better way.