Analysis of Same-Polarity Lightning Strike Faults in Three-Terminal Hybrid HVDC Transmission Systems and Research on Fault Line Selection Strategies

A three-terminal hybrid HVDC transmission system is a DC transmission system composed of three converter stations connected via ultra-high-voltage DC transmission lines. In engineering operation and maintenance, lightning strikes are a leading cause of unplanned outages in HVDC transmission systems. Due to the principle of opposite charges attracting, reverse-polarity lightning strikes are generally more common in HVDC transmission systems. The fault line selection logic in the protection and control systems of three-terminal hybrid HVDC systems is typically designed under the default assumption that lightning strikes are of reverse polarity. However, since same-polarity lightning strikes have different impacts on the system, conventional fault line selection logic for three-terminal DC transmission systems cannot be applied, necessitating the development of new countermeasures. This paper primarily analyzes the effects of same-polarity lightning strikes on three-terminal DC transmission systems and proposes a universal fault line selection strategy tailored to same-polarity lightning strikes for three-terminal hybrid HVDC transmission systems.