I. Introduction
In earlier power system harmonic simulation the harmonic sources, such as power electronics conditioners, were normally represented by harmonic current injections, calculated from specified ac and dc system conditions and assuming steady firing angles. In practice, the presence of some waveform distortion can not be specified in advance and the firing angle (and commutation angle in the case of line commutated conversion) will not be steady. Therefore, harmonic (and non-harmonic) voltages and currents will be transferred through the conversion process, which will be often amplified through the variation of the valves switching instants.