COMPARISON OF APPROXIMATE METHODS FOR ESTIMATE HARMONIC CURRENTS INJECTED BY AC DC CONVERTERS

This paper concerns a comparative study on available estimators of har monic 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 proced ure that carries out the harmonic decomposition of the actual waveform . It has been shown that approximate procedures assuming ripple-free d c current can provide amplitude values significantly different from ac tual ones. In particular, it has been ascertained that the rectangular waveshape does not always correspond to the worst case as far as harm onic current amplitudes are concerned. The approximate methods that ta ke 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, wh en based on ripple-free dc current assumptions, should be carefully co nsidered.