To improve the monitoring level of long-section power cables in substations, this paper proposes a leakage detection algorithm for long-section power cables in substations based on a space vector composite criterion. First, the limitations of existing cable leakage monitoring methods that cannot detect three-phase leakage faults are analyzed. Then, the concept of space vector is introduced and the residual current data is converted to a space vector circle. Residual current differential current, A-phase leakage current and space vector circle radius change rate, are proposed. A type judgment mechanism is established for cable unbalanced leakage and three-phase leakage faults. This provides accurate detection of the leakage status and type of long-section power cables. Finally, the effectiveness and superiority of the proposed method are verified by simulation and actual data of a 500 kV substation in the Jiangsu power grid. The results show that the method can accurately detect unbalanced leakage faults, fault evolution and three-phase leakage faults, enrich the information of cable leakage faults, and effectively improve the analysis and processing efficiency of leakage problems in substations.