What are the advantages and disadvantages of high voltage DC transmission lines?

High voltage DC transmission lines avoid problems related to the wavelength and reactive power of AC transmission. At best an AC transmission line will need a unity transformer every 5000km to avoid voltage changes over distance. In addition, capacitive and inductive losses in AC transmission lower efficiency. DC power avoids these issues, yet have a significant drawback that it is much harder to change voltage, power electronics must be used instead of a normal transformer.