  The converters in an AC/DC grid form actuated boundaries between the AC and
DC subgrids. We show how in both simple linear and balanced dq-frame models,
the states on either side of these boundaries are coupled only by control
inputs. This topological property imparts all AC/DC grids with poset-causal
information structures. A practical benefit is that certain decentralized
control problems that are hard in general are tractable for poset-causal
systems. We also show that special cases like multi-terminal DC grids can have
coordinated and leader-follower information structures.
