This paper gives the fundamental aspects of a power distribution scheme on railway power flow controller (RPFC) for the grid connection compatibility improvement in planning stage railway power system (RPS) via neatly adjusting the value and rotation direction of the primary three-phase current phasors for every given power factor and random load point. This method not only leads RPS to satisfy the relevant grid connection standards, but also can make RPFC have minimum designing capacity and high operating efficiency to decrease its costs in system planning stage. The analytic design method and the experimental results validate the correctness of the conception. The method of using this idea in more challenging serviced RPS will be discussed in another companion volume.