> we are even building HVDC “corridors” embedded inside some of the sync grids
We also have that here in Brazil; the Xingu-Estreito and Xingu-Terminal Rio HVDC links run in parallel with the high-voltage AC links. On the other hand, the Madeira HVDC link (https://en.wikipedia.org/wiki/Rio_Madeira_HVDC_system) is AFAIK normally disconnected from the AC grid on the generation end (it has a direct connection, probably used to start up the power plants, but normally uses a back-to-back HVDC converter instead of it).
We also have that here in Brazil; the Xingu-Estreito and Xingu-Terminal Rio HVDC links run in parallel with the high-voltage AC links. On the other hand, the Madeira HVDC link (https://en.wikipedia.org/wiki/Rio_Madeira_HVDC_system) is AFAIK normally disconnected from the AC grid on the generation end (it has a direct connection, probably used to start up the power plants, but normally uses a back-to-back HVDC converter instead of it).