In recent years, major power engineering equipment manufacturers such as Siemens and ABB have proposed the use of active filters in place of traditional passive filters in their newer HVdc schemes. The compact design and many other advantages offered by active filters over their passive counterpart have increased the appeal for this new technology. The aims of this thesis are to develop detailed PSCAD/EMTDC$\sp{\rm TM}$ models of active filters for both the ac- and dc-side of an HVdc scheme. The models are then integrated into the CIGRE HVdc Benchmark Model through suitably designed de-coupling elements, to evaluate their effectiveness in reducing harmonic currents in the system. Transient simulations have been carried out to examine the active filter controller responses to transient conditions typical to such an HVdc scheme. This thesis also includes an investigation into the feasibility and performance of an ac-side active filter installation within the Capacitor Commutated Converter (CCC) HVdc scheme.