A review of HVDC-based transmission congestion alleviation strategies for modern power systems

The rapid integration of large-scale renewable energy sources causes significant variability and uncertainty into modern power systems. Extreme fluctuations and randomness in wind and solar outputs, including sudden wind ramp-downs or rapid solar irradiance drops, can cause severe power imbalances, line overloads, and insufficient reserve responses, thereby threatening grid security. Conventional congestion management methods, including generation redispatch and network topology reconfiguration, often fail to adapt to fast-changing and uncertain conditions due to computational complexity and limited responsiveness. In this context, high voltage direct current (HVDC) transmission systems enable decoupled and bi-directional power flow control, rapid response, and precise regulation of interregional exchanges, outperforming other control devices in long-distance, large-capacity power transmission and congestion alleviation. This comprehensive review systematically analyses mainstream transmission congestion mitigation strategies across multiple dimensions, including key frameworks, general techniques and HVDC-based strategies for transmission congestion alleviation, highlighting the pivotal role of HVDC technologies. Furthermore, it synthesizes insights on power flow security and proposes future research directions to leverage HVDC coordination with complementary technologies. Finally, key challenges and promising research directions are identified to advance the security and reliability of future power grids. © 2026 Elsevier B.V.