Control Strategy and Small Signal Stability Analysis of the Direct-Drive Wind Farm Transmitted Through the HC-HVdc System Under No Synchronous Power Support Conditions

The hybrid cascaded high voltage direct current (HC-HVDC) technology, which combines the advantages of the line commutated converter-based HVDC (LCC-HVDC) and the modular multilevel converter-based HVDC (MMC-HVDC) technologies, is one of the most cost-effective solutions for long-distance transmission of large-scale renewable energy. Therefore, a topology of the HC-HVDC transmission system is constructed in this paper for direct-drive wind farms in desert and gobi areas where there is no synchronous power support in the sending-end system. Then a grid-forming control strategy for the sending-end hybrid cascade converter is designed to provide AC voltage support. Besides, an active power balancing control strategy of the sending-end system based on the additional DC voltage control of the MMC is proposed to adapt to the stochastic fluctuation characteristics of wind power. After that, a small signal dynamic model of the whole system is established, and the influence of important parameters on the small signal stability of the system is analyzed. Finally, the time-domain simulations based on PSCAD/EMTDC confirm the correctness and validity of the proposed control strategies and the stability analysis conclusions.