A coordinated overvoltage suppression strategy for sending-end hybrid cascaded HVDC systems

The sending-end hybrid cascaded (SE-HC) HVDC system formed by a DC-side series connection of a line-commutated-converter (LCC) and a modular-multilevel-converter (MMC) has become a crucial solution for large-scale renewable energy long-distance transmission due to its advantages of large transmission capacity, high operation flexibility, and strong grid support capability compared with conventional LCC-HVDC systems. However, SE-HC MMC may face DC overvoltage under sending-end (SE) AC system faults, which is caused by the HVDC system's active power surplus. To address this issue, this paper first analyzes the mechanisms and influencing factors of DC overvoltage based on the power balance model. Then, an overvoltage suppression strategy is proposed by coordinating the transmitted active power from SE and receiving-end (RE) converters, renewable energy bases (REBs), and energy dissipation devices. This method does not rely solely on the energy dissipation device to absorb surplus active power. Instead, by fully leveraging the control flexibility of the SE MMC, it enables the RE MMCs and REBs to detect SE AC system faults, thereby increasing active power transmission and reducing surplus active power during the fault. As a result, more effective suppression of DC overvoltage is achieved, and the required capacity of the energy dissipation device in the system is reduced. A monopolar model of the SE-HC HVDC system has been established in PSCAD/EMTDC, and the feasibility and superiority of the proposed control strategy have been verified through this model.