Multi-Converter Reactive Power Support Strategy for Hybrid Cascaded DC Transmission Systems in Weak Grids

Introduction: To address voltage stability problems caused by integrating renewable energy into the sending-end grid, a hybrid cascaded HVDC transmission scheme and reactive power coordination support strategy are proposed. This method fully leverages the reactive power regulation ability of multiple converters to maintain the transient voltage stability of the bus. Methods: First, the system's advantages under high renewable penetration are evaluated. Then, considering MMC reactive limits and transmission features, a coupled converter model is developed to analyze how electrical quantities influence AC bus voltage. Based on this, a coordinated reactive power support strategy for the hybrid cascaded system is formulated. Results: An electromagnetic simulation model for high-renewable transmission through the hybrid system has been established. Testing confirms that the proposed strategy enhances transient voltage stability across various operating conditions. Discussion: The proposed control strategy fully leverages the reactive power regulation features of both converter types in the hybrid cascaded system, allowing them to respond simultaneously to bus voltage variations during disturbances and improve the system's overall reactive power support. Conclusion: This strategy effectively leverages the system's reactive power capacity, greatly reducing voltage fluctuations during sending-end AC faults. It provides a practical solution for maintaining stability in future long-distance transmission systems with high renewable energy integration.