AC/DC hybrid transmission infrastructure defeats important fault handling and inertia characteristics you get for free out of a fully connected AC grid. HVDC co

AC/DC hybrid transmission infrastructure defeats important fault handling and inertia characteristics you get for free out of a fully connected AC grid. HVDC converter stations cannot handle remotely the same amount of fault current that synchronized machines in an AC grid can. It's maybe 1/10th the capacity. You also don't get the same guarantees of zero crossing in a fault scenario with HVDC. If you never cross zero, it's possible some circuit breakers cannot function anymore.