This book focuses on the latest development of ultra-high-voltage direct current (UHV DC) technology, which is one of the most advanced power transmission technologies in the world. Both principles, key technologies, and engineering practice have been addressed, with more weight placed on engineering practice of the Zhundong-Wannan ±1100 kV UHV DC power transmission project. This mega project set a series of world records such as the highest voltage, the largest capacity, the longest distance and so on. ±1100 kV UHV DC power transmission technology can realize the large-scale and long-distance optimal allocation of clean energy, which is of great significance for guaranteeing energy security, and promoting low-carbon transformation. This book is the first monograph in the related field, which comprehensively exhibits the principles and key technologies, the equipment of the converter station, the general layout of converter station, key technologies of construction, and engineering commissioning. It can benefit researchers and engineers engaged in the construction, design and operation of high voltage power transmission projects. The basis of English translation of this book, originally in Chinese, was facilitated by artificial intelligence. The content was later revised by the author for accuracy.