14MV would be capable of sustaining an arc 1400 feet long in normal atmosphere. I struggle to imagine how you'd build such a thing. You could maybe have a high volume sf6 pump system that would cool and quench the arc on breaker trip with a constantly replenished sf6 supply.

Even 1.1GV systems use semiconductor breakers. Basically, stacks and stacks of transistors. The actual physical breakers are only operated when the voltage is safely off.

I considered that. Considering the cheap cost of the cable, the best solution appears to simply be 'dont have a breaker'. In either over current or over voltage conditions, simply sacrifice the cable.

Obviously you engineer the convertor stations to minimize the chances of that happening - stopping the convertors automatically if anything looks abnormal. The cable has sufficient capacitance that you have multiple milliseconds to respond, so automated systems should have no difficulty.