Abstract: The co-phase interconnected power supply system of traction substation group is a feasible technical solution to electrified railway power supply problem of long and steep slope, external power supply with poor short circuit capacity and lack of interface. Aiming at the problem of negative sequence current, a treatment scheme and its control strategy based on Dd transformer and SVG were proposed. The structure of co-phase interconnected power supply system of traction substation group was introduced, and the relationship of electrical quantity of its each port was analyzed. Then, two modes of negative sequence treatment were proposed, which were separately compatible to different traction load power factor. Meanwhile, a mathematical model was derived. The limit of three-phase unbalance was taken into consideration, and the reactive power capacity resolved by each SVG on secondary ports of the Dd transformer was analyzed when they were working under different conditions. Based on research above, a switching strategy between different modes was put forward. According to the scheme, the negative sequence current detecting methodology was determined and an SVG control strategy of double closed loop was proposed. Finally, the simulation model was built. Simulation results verify the effectiveness of the scheme and the accuracy of the control strategy.