Abstract: The ultra-high-voltage DC system based on voltage source converter (VSC-UHVDC) adopts the technical solution of conventional UHVDC, which is composed of high-voltage and low-voltage valve group. To facilitate balanced transmission of active and reactive power between valve groups within a single converter station without utilizing DC lines, a dual valve group power self-circulation strategy is implemented and validated through experiments. Based on the technical solution of ultra-high-voltage flexible DC double valve groups operating in a back-to-back configuration, in terms of main circuit, the buses of high-voltage and low-voltage valve groups are connected to each other. In terms of operation strategy, different from single control mode of previous transmission projects, one valve group determines the direct current voltage and the other valve group determines the active power, and one valve group adopts the DC voltage control mode while the other adopts the active power control mode, thus the active power self-circulation is achieved within a single station; meantime, the reactive power of the double valve groups can be autonomously controlled. Therefore, a power closed-loop is formed within a single station between two valve groups. The simulation results based on real time digital simulation (RTDS) indicate that this strategy can effectively verify the high-power capability of equipment and reduce the disturbance on AC grid during the power transmission period between two valve groups of VSC-UHVDC. Good prospects in application have been realized in ultra-high voltage flexible DC engineering.