The main reason for HV DC historically was that it doesn't require grids to be synchronous, or even of the same frequency. This is often desirable because a syn

The main reason for HV DC historically was that it doesn't require grids to be synchronous, or even of the same frequency. This is often desirable because a synchronous grid requires deep organizational integration. HV DC also lets you use mono-pole systems fairly easily, since they're single phase; sea or ground becomes the return path.