Low switching frequency modulation strategies with low impact on the mains, suitable for a for a large power voltage sourced converter that employs reduced VA-rating coupling magnetic elements

This paper presents a review of modulation strategies that can be applied on a novel large power voltage source converter arrangement. This arrangement employs four three-level, three-phase neutral-point-clamped inverters, combined by means of low volt-ampere rating intermediate magnetic elements. The 12 resulting inverter poles are paralleled up in pairs by using six interphase transformers (IPTs), and their outputs are connected in series by using a simple openwye/ delta transformer, rated to 50% of the power throughput. Due to the inherent harmonic cancelation provided, the converter is suited for the use of various low-switching frequency modulation strategies. In this paper full, wave modulation and three different Selective Harmonic Elimination strategies are described, all obtaining a clean output waveform, in some cases comparable to that of a 48-pulse converter. The converter components and its operation are explained, and experimental results are provided.