Adaptive Voltage Control Strategy of Hybrid Cascaded DC Transmission System Considering Voltage Regulation Dead Zone

Introduction: The adoption of hybrid cascaded direct current (DC) interconnection for transmission from renewable energy bases is regarded as a key future direction. Most new energy bases are located in remote areas, with extremely weak electrical connections to the main alternating current grid. This makes the voltage security and stability issues of the sending-end system increasingly critical challenges. Method: A model is established to quantitatively characterize the voltage coupling relationship between the modular multilevel converter (MMC) bus and the line-commutated converter (LCC) bus. On the basis of analyzing the reactive power support capability of the MMC, an MMC reactive power adaptive control strategy based on a voltage regulation dead zone and its main parameters is proposed. Result: For the hybrid cascaded DC transmission system, this paper proposes an MMC adaptive reactive power control strategy, which enhances the system voltage security and stability while enhancing the power transmission capacity of the hybrid cascaded high-voltage direct current (HVDC) system. Discussion: The validity of the bus voltage coupling relationship between the MMC and LCC stations is verified through simulations. In addition, a comparative simulation analysis is conducted between the proposed method and traditional methods. The results show that the proposed method not only suppresses transient voltage fluctuations but also suppresses active power fluctuations. Conclusion: This method establishes a quantitative voltage coupling relationship and designs a voltage regulation dead band, thereby effectively solving the problems of insufficient selectivity and adaptability in the MMC-based voltage support strategy. The proposed control strategy can significantly improve the voltage stability of the sending-end system with minimal impact on power transmission, providing a theoretical basis for the safe and stable operation of hybrid cascaded HVDC systems.