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In the last decades new technology approaches were rising until the point to make a new tendency in the electrical transmission system. Improvements in the converter’s technology have made high voltage direct current (HVDC) transmission lines attractive and viable to communicate and connect different electrical grids. This thesis deals with the design of an Active Harmonic Filter (AHF), for voltage source converters (VSC-HVDC), which is a growing filtering method. Due to the changes from the energy consumers and the energy producers in the electrical grids harmonical filtering becomes an important problem that can be solved by AHF which have better performance than the passive filters that were used commonly years ago. To reflect these mentioned behaviours the 5th and 7th harmonics are studied. In this thesis a voltage source converter (VSC) used in high voltage direct current (HVDC) systems is considered, as it provides better opportunities when compared to the conventional line-commuted converts (LCCs). The VSC is modelled with the average model assumptions. As a VSC-HVDC is used AHF can be applied. To accurately represent the VSC average model the MATLAB Simulink software is used. The considered harmonics, 5th and 7th, are mitigated with the purpose to accomplish the distortion standard regulations. The control system is implemented transforming the abc frame into the dq0 reference frame which permits independent control of the active and reactive power. Also, this structure is followed in the AHF filter technique by means of the multi reference frame method, after the harmonics are selected and controlled a current signal is introduced to the converter to obtained the desired harmonic distortion level, Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant::7.3 - Per a 2030, duplicar la taxa mundial de millora de l’eficiència energètica