Analysis of waveform approximation for the AC current of a line-commutated converter

This paper analyzes mathematical approximations of the ac current of a line-commutated converter (LCC), which is the building block of conventional HVDC systems. The paper derives analytical expressions for the ac current of an LCC during the commutation and non-commutation periods and continues with calculating the percentage error of both the fundamental frequency component of the ac current and the power factor of the converter when the ac current is approximated with either a square or a trapezoidal waveform. The results quantitatively show the benefit of the trapezoidal waveform approximation in terms of its percentage error and simplicity.