|The attention in this paper is directed to the de-sign of a passive damped LCL filter intended to connect the voltage source inverter of an active DC-traction substation to the AC-power supply. Based on the tasks to be fulfilled by the passive connecting filter related to the current filtering and its dynamics, the mathematical foundation of the LCL filter design consists in taking into consideration the trans-fer function of the currents, the switching frequency, the resonance frequency and the compensated harmonics by active filtering. A design algorithms is formulated, by con-sidering the imposition of the attenuation corresponding to the switching frequency. Besides this, the position of the resonance frequency relative to the frequency of the highest order harmonic to be compensated is imposed. The design algorithm is fully formulated, and specific indicators are defined and used to evaluate the performance of the whole active filtering system. The case study taken into considera-tion is of the active DC-traction substation, where the un-controlled traction rectifier is of 12-pulse parallel type. Based on Matlab-Simulink modeling, the simulation of the whole active DC-traction substation operating in both active filtering and regeneration regimes illustrates the proper behavior of the system and the influence of the LCL filter parameters on the power quality performance.