Three-wire bipolar high-voltage direct current line using an existing single-circuit high-voltage alternating current line for integrating renewable energy sources in multiterminal DC networks

When compared with conventional point-To-point high-voltage direct current (HVDC) systems, multiterminal HVDC (MTDC) systems are more economical and flexible. MTDC systems can tie distributed high power renewable energy sources (such as offshore wind energy and high-power photovoltaic systems) into AC grid(s). To save in the required infrastructure for DC line installations, existing high-voltage alternating current (HVAC) lines can be converted into DC links. The bipole HVDC concept can be used easily for converting the HVAC lines into DC links, but with a limited transmission capacity. Alternatively, three-wire bipole structure (TWBS) can be used, yet it requires a power electronics-based converter connected in series with one of the wires (modulating wire), which affects the system reliability and adds more complexity. In this study, a new configuration is proposed for converting the HVAC lines into DC links with enhanced power capability compared with the two-wire bipole HVDC system. The proposed approach provides a lower power enhancement compared with that provided by TWBS. However, a higher reliability can be guaranteed as it avoids the series-connected converters. Analytical and simulation studies using Matlab/Simulink platform have been presented in this study to validate the proposed concept.