That's correct what Ianh sais.
Depends upon POCO requirements.
Here in Auckland a historical error was made between the 11 kV networks from the former Auckland Electric Power Board and the Waitemata Electric Power Board.
One uses Dy11 the other Dy1 for 11 kV hence we can't tie 11 kV feeders across the borders which are 60° out of phase instead of 120° which only requires swapping phases to get it corrected.
With the 60° phase difference special interphasing transformers are needed to get it right and this method is not used, because of unnecessary costs.
It is therefore very important to check that phasing is correct when backfeeding utility networks.
