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The Optimal Two Degrees-of-Freedom Based Neutral Point Potential Control for Three-Level Neutral Point Clamped Converters
Bo GuanĘ”,* and Shinji Doki**
Abstract Although the dual modulation wave method can solve the low-frequency neutral point potential (NPP) fluctuation problem for a three-level neutral point clamped converter, it would increase the switching frequency and limit the zero-sequence voltage. That makes it harmful dealing with the NPP drift problem when the converter suffers from a long dead time or asymmetrical loads. By introducing two degrees of freedom (2-DOF), an NPP control based on the search optimization method can display its ability for coping with the above-mentioned two types of NPP problems. However, the calculation amount for obtaining the optimal 2-DOF is so large that the method cannot be applied to certain industry applications with an inexpensive digital signal processor. In this paper, a novel optimal 2-DOF-based NPP control is proposed. The relationships between the NPP and 2-DOF are analyzed directly and a method for directly determining the optimal 2-DOF is also discussed. Using the direct calculation method, the calculation amount is reduced significantly. Simultaneously, the proposed method is also able to maintain the strongest control ability for the two types of NPP problems. Finally, some experimental results are given to confirm the validity and feasibility of the proposed method.
Keyword Control ability, Direct calculation, Neutral point potential, Three level converter, Two degrees of freedom
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