@Robbie The reason why all long-distance links are HVDC is very simple: Every single generating set in a linked AC system has to be exactly* in sync, as otherwise they end up consuming power instead of generating it.*
Utter nonsense I'm afraid. An electricity grid is self-synchronizing in the same way that two people pedalling a tandem are self-synchronizing. If something synchronous (generator or motor) gets even very slightly ahead, it will start supplying energy to the grid until it is back in synch. If it gets behind, it will start draining energy until it has once again attained the same speed as everything else. If the total power drain exceeds the amount of power going in, the frequency and voltage of the whole grid will droop, until regulators at individual power plants notice this and open valves to increase the flow of steam to the turbines. The regulation is totally decentralised. There's no single control centre, no master grid-regulation station that could be attacked by terrorists.
The only time there might be trouble is if a (synchronous AC) generating plant gets disconnected from the grid and has to be reconnected. This requires careful monitoring of both the frequency and phase of the plant to make sure that they both match the grid before the connection switch is thrown. Otherwise, there would be a massive surge that would blow "fuses" and probably break many other things.
As for HVDC, as other people have observed, technology has advanced since Tesla and Westinghouse's day. Transmission losses are reduced with HVDC, and HVDC to grid-AC conversion is no longer impossible or uneconomic. Long-distance HVDC has another large bonus. It is immune to the effects of solar storms (which increase as the length of the line increases). If a huge CME induces a DC current in a DC line, it either adds to or subtracts from the power being transmitted, but it doesn't appear as any sort of abnormality to the plant at the ends. When the same happens to a conventional AC line, the DC current cannot be transformed by the transformers, and instead becomes waste heat inside the transformers. If someone doesn't break the circuit fairly rapidly, the transformers catch fire. In other words, either we suffer a short-term blackout or a long-term blackout (possibly an end-of-civilisation blackout were all the safeties to fail). Had a Carrington event occurred in, say, the 1960s, this would have been a real possibility. Today, we know to watch our sun closely, and to cut the power to save the grid if it throws another big belch at us).