Comparison of approximate methods for estimate harmonic currents injected by AC/DC converters

This paper concerns a comparative study on available estimators of harmonic current amplitudes injected in the AC network by six-pulse AC/DC converters. Current estimates are reached by approximate methods that resort to simplified current waveforms, as well as by an exact procedure that carries out the harmonic decomposition of the actual waveform. It has been shown that approximate procedures assuming ripple-free DC current can provide amplitude values significantly different from actual ones. In particular, it has been ascertained that the rectangular waveshape does not always correspond to the worst case as far as harmonic current amplitudes are concerned. The approximate methods that take ripple into account are generally more accurate, but the algorithms required for current amplitudes estimates are not as easy to apply as those valid for ripple-free waveforms. These findings should be kept in mind when systems for harmonic compensation, particularly filters, are to be designed. Moreover, national and international standards, when based on ripple-free DC current assumptions, should be carefully considered.