The current proposed UHV DC systems are not sophisticated switching regulators.  They still use 50&#x2F;60 Hz AC transformers to change the voltage but use electronic commutators to convert this to DC and back to AC (and is reversible).  The commutators actually use thyristors (they can only be turned on, and depend on AC momentum to turn off).  It means the receiving end of the power must have large AC machines (power generators).<p>Take a look at: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;HVDC_converter" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;HVDC_converter</a>
It mentions IGBT based commutators which do not require AC machines, but which are less efficient.<p>I think all the thoughts about the switching harmonics being horrendous are correct.