Abstract: This paper introduces the keynote speech of the CIGRE 2020 online video conference, and the related papers,symposiums and special lectures of the study committee of DC systems and power electronics（SC B4）. The main technologies and equipment related to SC B4 are covered, including ultra-high voltage direct current（UHVDC）/high-voltage direct current（HVDC）transmission, medium-voltage DC distribution, flexible AC transmission system, etc. According to the current situation and characteristics of HVDC transmission, this paper introduces and discusses the latest development trends and challenges in the field of HVDC transmission, and prospects the development direction of this field.