Most high voltage power lines have ground fault detection, generally provided by having a Peterson transformer connected to provide a Y connection to ground, and a great big inductor in series with the tap, so that any fault to ground will cause a voltage drop across this coil. AC or DC plus AC, not really any difference, as the current flowing in the coil as a result of imbalance will pull in a solenoid, and close a contact to the SCADA system of the line, telling of a ground fault. Might take 5 seconds to respond, so as to ignore transients, as the voltage will kind of self clear the general fault, by flash cooking the big bird or errant tree branch, or the metal kite string somebody flew into the power line. But a prolonged fault will trip out the line, and, so long as the sending end and receiving end still have power available, they can be commanded remotely, using the fibre networks most utilities have now, run along the middle cable of the long transmission lines as they have no need then to do any EIA report on installation, and also it is easy to install, as they have the equipment and pulleys to draw along the line, and droop down by pylons to follow the existing cable, and then tie off to the existing as well. Long lines will have issues, unless they are HVDC, which already have DC fault blocking and detection, and long AC lines will merely need a few minutes notice to cut power, island the power plants to local generation supply only, and thus not need them to go offline fully, but keep a low spinning load, and run with a largish circulating current instead, excess supplying local loads. Otherwise you need to black start them, and most power plants cannot do this, they need power to run.
By me not an issue, 15 years of rotational load shedding means there is a pretty robust system of alternative power already, thanks to this. USA and Europe has never really had this except short term over relatively small areas, but by me being without power a quarter of the time is somewhat accepted to be normal.