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The use of three-level converters with low switching frequency (SF) modulation is urgently needed for medium voltage high power applications. However, since it is not possible to frequently adjust the injected zero-sequence voltage and duty time of redundant vectors, the neutral point (NP) voltage problem of three-level converters becomes a tricky issue under low SFs. In this paper, an optimized low SF modulation with NP voltage control is proposed, which is based on half-wave symmetry selective harmonic elimination pulse width modulation. The proposed modulation specifically addresses the low-frequency NP voltage fluctuation of three-level converters, using the amplitude and phase of the 3n-order harmonics as degrees of freedoms (DOFs). In addition, it actively optimizes the control problem into constraint equations. By revealing the relationship between these new DOFs and low-frequency NP voltage fluctuation, and obtaining the optimum settings for the new DOFs, the newly established constraint equations can naturally suppress the low-frequency NP voltage fluctuation, without the need for real-time switching angle adjustments. It fits very well with the feature that switching angles can only be calculated offline and used via a table online. Experimental results show the validity and feasibility of the proposed modulation strategy. [ABSTRACT FROM AUTHOR]

This paper proposes a compensation method to improve the distorted space vectors when a 3-level Neutral Point Clamped (NPC) inverter has an unbalanced neutral point voltage. Since both the neutral point voltage of the DC link and the space vector of a 3-level NPC inverter are closely related depending on the output load connecting state, a distorted space vector can occur when the neutral point voltage of a 3-level NPC inverter is unbalanced. The proposed method can improve the distorted space vectors by adjusting the injection time of the small and medium vectors and by modulating the amplitude of the carrier waveforms. In this paper, the proposed method is verified by both simulation and experimental results based on a 3-level NPC inverter. [ABSTRACT FROM AUTHOR]