I haven't studied power systems yet but i keep wondering about that. What would happen if there was a short circuit fault BEFORE the main circuit breaker(on the grid itself)? what protction is supposed to stop that kind of faults?
You can't stop the fault but you can limit the damage.
The distribution transformer will have fuses on the primary (high voltage side) and secondary (low voltage side) to prevent the wires running to your house from vaporizing and to prevent damage to the transformer and disruption to the grid.
Everywhere through the distribution system there will be similar protective devices, they just have to handle more kVA the further you get up in the food chain.
Depends in what context you call it "main circuit breaker". If you are talking about the main breaker on your household, then the distributor allways installs a protection of their own between your house input and the grid (usually a fuse).
So, the fuse would blow up, and you'd have to call the distributor to come and replace it.
Distribution transformers have large cylindrical fuses in them.
Sub-stations have fast smart protection gear with breakers and reclosures that must swing wide open to quench the holdoff arc current.
A Power transmission line model appears as follows;
From Telegrapher's Equations a surge impedance can be determined. This is often rated by % per unit.
The short circuit power can be defined by ;
The source end of transmission can look like this with propagation delays;