HVDC (high voltage direct current) transmission advantages over than HVAC transmission:
1- When bulks of power are transmitted over long distances by overhead lines or under the sea by means of submarine cable, HVDC transmission will be economical and technically advantageous alternative to AC transmission.
2- In DC transmission, the stability aspects do not limit the power transmission capability of the line.
3- For a given quantity of conductor material in the line the losses in a dc line are lower than the losses in an ac line.
4- Compared to AC transmission line, DC lines can operate with narrow corridors. Large areas of farm land or forests may then be saved.
5- The conversion of alternating current into direct current allows for transmission with comparatively low losses and for the interconnection of grids of different frequencies.
HVDC Transmission components:
1-HV Overhead lines or Submarine cables
2- Two convertor station; at each end of the line or cable
Now, we only interested in the second components; Converter stations which will be explained in the following paragraphs.