A Power-electronic interface for supercapacitive-storage based substation in DC-transportation networks is proposed, which allows to feed as a current-source in any voltage conditions of the line. The system has been designed as a compensation-substation to be placed at weak points like end-of-line stations, and can replace supplementary feeding substations. A dedicated power-electronic converter with associated control system is proposed, for the stabilization of the voltage level at the point of coupling. Practical results are also presented, which have been recorded from a reduced scale prototype.