What does "seamless power transfer" mean?
I would guess that it depends on context. In the example you provided, it seems to suggest that as soon as a critical load is not receiving power, a backup DC source begins providing power. Seamless in this case would be that there isn't a significant amount of time that the load is unpowered.

I think your answer could also be found in the abstract, as follows:


  The presented results show that seamless power is
  provided to the critical loads connected to the grid with
  loss/failure of generation, without any transients in voltage.