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 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 THD (Total Harmonic Distortion) performance and poor power capability. The THD performance is a key indicator for optimizing the performance of IGCT converters. Therefore, low-THD patterns like 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 to 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.