1.NR Electric Co., Ltd., Nanjing 211102, China;2.State Grid Economic and Technological Research Institute Co., Ltd., Beijing 102209, China

The site-division cascaded ultra-high voltage direct current (UHVDC) transmission system exhibits greater flexibility and complexity in connection configurations and return circuit transfer compared to conventional two-terminal UHVDC transmission systems. Based on an actual sending-end site-division cascaded UHVDC project, this paper designs a transfer strategy for the grounding modes of DC system, which satisfies the requirements for flexible commissioning and decommissioning of the sending-end low-voltage converter station. To address potential grounding mode transfer failures during ground return and metallic return operation modes, this paper proposes countermeasures involving small current step injection and short-term grounding of the receiving-end converter station, respectively. The effectiveness of the proposed control strategy is validated through experiments utilizing actual control and protection devices in conjunction with real-time digital simulation.