There is a certain formula that the game runs through to determine if each enemy wave will destroy your sector if you aren't there. I think it was something along the lines of your building HP and damage+healing+unit health and damage versus enemy HP and damage. If you have a higher total, you win the wave, and if you have enough healing, the base won't even take damage. Only buildings on the main attack path are counted though, but all enemies are calculated as one number, so no naval/air attack paths. Build a solid defence and it should be able to shrug off most attacks until you get to the higher difficulties.

This is actually a way many take over low diff sectors, drop in a schematic with high dmg and healing with some power, the leave it to finish on its own.