A linear and direct method of determining the major components of noncharacteristic waveform distortion at the terminals of an HVDC convertor is presented. The method is a development of the frequency-domain transfer function technique, and relates the AC current and DC voltage to distortion in the firing angle, AC voltage and DC current. The consequential effects of the commutation period and its dynamic variation are included. It is demonstrated that the dynamic variation of the commutation period is an important factor. The model is validated for low-order frequencies by comparison with results obtained by dynamic simulation for a simple test case.