Reducing harmonics and DC-Link capacitors in cascaded multilevel converters using inter-cell magnetic couplings

Abstract

The existence of single-phase inverters in cascaded multilevel AC/DC/AC converters implies the use of large capacitors in the DC links, because the H-bridge inverters demand considerable second harmonic currents that must be filtered out by the DC stage. If the filtering out is not strictly performed, the AC input current of each module is affected with the appearance of non-characteristic harmonics, sub-harmonics or inter-harmonics; thus, the RMS currents in the multi-pulse transformer secondaries increases. This problem may become even more evident if the converter output frequency is lower than the AC mains frequency, because the capacitor presents higher impedance at low frequency, causing that part of the harmonic currents demanded by the inverter is supplied directly by the rectifier. This paper presents an alternative to eliminate the unexpected harmonics and thus reduce the RMS values of the power cells input current in cascaded multilevel converters. The proposed solution uses magnetic couplings between the power modules of the converter; such approach is presented as a low cost solution compared to the conventional oversized DC-link capacitors.