DC/DC Conversion and Distributed Grid based Solution of HVDC Tapping

The increasing demand for efficient and reliable energy transmission solutions has underscored the significance of High Voltage Direct Current (HVDC) technology in modern power systems. HVDC systems have proven instrumental in addressing challenges such as offshore electricity transmission, interconnection of archipelagic nations, and long-distance power transport. However, the need for innovative approaches to address specific challenges, such as connecting multiple islands' energy systems, has led to the exploration of HVDC tapping as a potential solution. HVDC tapping, a concept analogous to different voltage levels in interconnected AC grids via power transformers, presents a promising approach for connecting multiple islands' energy systems. This concept involves the utilization of a DC/DC conversion method to reduce the voltage of the main HVDC link and transmit a small portion of its nominal power. The limitations in power magnitudes and the remote locations of tapping stations introduce operational and maintenance challenges, emphasizing the need for cost-effective, reliable, and user-friendly tapping converters.

To address these concerns and fill the research gap in HVDC tapping, this paper proposes an HVDC tapping solution containing a DC/DC converter and a related DC distribution system. The proposed solution incorporates a Modular Multilevel Converter (MMC) and a diode-bridge rectifier connected by a medium frequency transformer, offering cost advantages and ease of control. Furthermore, a DC distribution grid is utilized for connecting several islands at the low voltage side, with a novel control system for the DC/DC converter to ensure the stability of the low-voltage DC grid. The significance of this study lies in addressing the challenges associated with connecting decentralized and geographically intricate power systems, with a specific focus on the potential of HVDC technology in the context of the HVDC GREEN project. The proposed HVDC tapping solution can enhance power quality, and voltage stability of islands’ power system. The study also presents novel fault-ride-through (FRT) control strategies to enhance the reliability of the DC distribution system, enabling the DC/DC converter to function as a current source during fault conditions, thereby preventing power outages.