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Large convertors such as DC drives or HVDC schemes will generate noncharacteristic harmonics if fed from unbalanced AC busbars. In particular, high levels of third harmonic may be generated. The paper describes the mechanism for the generation of this third harmonic both for conventional and equally pulse spaced (phase locked) control systems. The mechanisms are shown to be different. It is also shown that noninfinite reactances on the DC side of the convertors can increase the harmonics, in particular if there is a second-harmonic series resonance between the DC reactors and the cable or line capacitance.