In practice it depends a lot on how a bunch of specific power equipment is set up. Remember that the A/C power grid needs to be synchronized across the entire

In practice it depends a lot on how a bunch of specific power equipment is set up. Remember that the A/C power grid needs to be synchronized across the entire continent, so each A/C generator that makes A/C power and runs at 50Hz needs to have the position of a physical object (the rotors) match the position in the current power cycle. If it's badly wrong, you have power-plant sized energy feeds fighting each other — imagine a train wreck, two massive oversized freight trains smashing right into each other. It's like that, but with electricity instead of a physical collision. This can obviously cause major damage, so once you start deviating from the reference frequency by enough, it's safer to just cut off the interconnects and go offline.