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This paper evaluates all kinds of main protections for DC feeder line of subway and points out their drawbacks. On the base of analyzing characteristics of current under tow load and short-circuit in DC traction power supply of subway, it presents a main protection-rate-of-current-rise protection (di/dt- Δ I). Then, the characteristics of ROR (rate-of-current-rise) protection and the principle of setting are given. By simulation, it is proved that the design of ROR protection is rational.