Modulation Schemes for a 30-MVA IGCT Converter Using NPC H-Bridges

This paper demonstrates the impact of modulation schemes on the power capability of a high-power converter with low pulse ratios. This integrated gate-commutated thyristor (IGCT) converter uses a five-level neutral-point-clamped H-bridge topology. It is concluded that phase-shifted carrier modulators are not attractive for such converters at low pulse ratios due to the poor total harmonic distortion (THD) performance and poor power capability. The THD performance is a key indicator for optimizing the performance of IGCT converters. Therefore, low-THD patterns such as the level-shifted carrier and optimized pulse patterns are proposed. It is explored that a smart and instantaneous pattern distribution is a key factor to ensure a robust modulator design. Compared with the standard interleaving concept, the proposed modulation schemes allow a power capability increase up to 40%. Meanwhile, the current THD is reduced from 11% to 4%-7%. The concept is implemented and experimentally validated up to 30 MVA on an industrial IGCT converter.