Experimental performance of passive and hybrid filters applied to AC/DC converters fed by a weak AC system

This paper deals with an experimental comparative analysis of the filtering and power factor correcting capabilities of a passive filter versus a hybrid filter, when each one of them is separately connected to a weak AC system feeding an AC/DC three-phase power converter load. Two physical models, that represent respectively a passive and a hybrid filter, were constructed and tested for a variety of operating firing angle control modes of the AC/DC converter. The passive filter was designed to cope with the inherent characteristic harmonic currents of the rectangular waveshaped AC current. The hybrid filter (a combination of an active filter and a passive coupling branch) was designed so that the active filter injects the right amount of counterharmonic current into the AC mains via the passive coupling branch. The AC system was deliberately weakened by inserting an inductance between the AC source and the AC/DC power converter, as the goal of the research was to test the filtering performance of each model under conditions of highly distorted AC voltage and current waveforms. By recording experimental voltage and current data, in both the time and in the frequency domains, a comparative performance of the filters was made. The paper includes a selected number of experimental results to compare the performance of the filtering systems. These results demonstrate that, with some differences, the constructed filtering models reduce conveniently the total harmonic content and improve the power factor of the AC voltage and current waveforms.