AC/DC convertors used in HVdc transmission systems inject harmonics into their associated AC and DC systems. A conventional approach to filter these harmonics is to use passive filters on both AC and DC sides of the convertor. Apart from their technical problems, harmonic filters constitute a considerable part of the volume and cost of present DC terminal stations. Another possible alternative of harmonic suppression is to increase the number of pulses of AC/DC convertors by transformer phase-shifting techniques, but the resulting complicated circuitry together with its problems of insulation produce significant technical and economical disadvantages. In this thesis a novel method of harmonic suppression for HVdc transmission systems is presented. It is a technique producing the same effect as transformer phase-shifting but without complicated circuitry. Moreover, the possibility of increasing the number of pulses of AC/DC convertors to any desired extent is proposed. The concept can be applied to parallel and series connections of convertors. Further applications, other than HVdc transmission, are also suggested.