Converters and nonlinear loads absorb reactive power and produce harmonics on both sides of the d.c. transmission systems. The demand of reactive power and harmonics cancelation are usually met by employing passive and active power filters. In this paper, a conventional passive filter and a new active power filter topology are suggested in order to improve the power quality of the d.c. transmission systems. The nonlinear application chosen here is the 12-pluse Line Commutated Converter High Voltage D.C. (LCC-HVDC) link. The passive filter is tuned at fixed harmonic and constant transmitted d.c. power while the active power filter is dynamically controlled for different values of d.c. power flow through the transmission line. To effectively control the active power filter, a modified harmonic pulse width modulation algorithm is suggested in order to minimize the source harmonics and force the a.c. source current to be in-phase with the a.c. mains. Comparison of simulation results using MATLAB/SIMULINK show that the suggested active filter is effective for transient and steady-state operating conditions.