SERIOUSLY, you should get the basics figured out before coming here. If you don't know how to perform a search, maybe that should be your first question.
When your power system is inter-connected with other power systems, any fault in the power system external to your plant should not be fed by your generation plant. You must isolate your generation plant from the faulty grid as soon as possible. Else, your generation plant also would trip causing a total black-out. For no fault in your plant, your plant loads will also be deprived of supply. This action of isolating your generation plant from a faulty grid is called islanding. There are high speed islanding schemes available today.