This document discusses high voltage direct current (HVDC) transmission systems for transmitting bulk power over long distances. It provides three key points: 1. HVDC systems use DC for transmission, which is more economical and has lower losses than equivalent AC transmission lines over long distances. HVDC also allows interconnecting power networks with different frequencies. 2. There are three main types of HVDC transmission systems - monopolar, bipolar, and homopolar - which are classified based on the arrangement of poles and use of earth return. Each type has its own advantages and disadvantages in terms of equipment costs, power capacity, and reliability. 3. HVDC transmission is preferable over AC transmission when interconnectivity between