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Neutral-point (NP) balancing issues in the neutral-point-clamped three-level converters have been widely discussed in the last decades. Numbers of prior NP control algorithms have been proven working well in terms of NP potential balancing. However, it seems that the impact of these algorithms on the ac side did not draw much attention in the past, e.g., some algorithms inject additional common mode voltages with considerable amplitude that may saturate the duty cycles or inject low-order harmonic currents into the ac load. As a consequence, compared with the original system without NP controls, the total harmonic distortion (THD) performance at the ac side becomes worse, i.e., the THD standards are violated and a redesign of the system (e.g., ac filter) may be essential. Therefore, particularly for industrial applications, low distortion and high margins for duty cycles are typically important criteria for designing an NP controller. In this paper, a novel NP controller with full power-factor (PF) range and low distortion is proposed, which has the minimum common voltage injection at no unbalance or slight unbalance conditions. On the other hand, at heavy unbalance conditions, the controller has the maximum balancing ability at any PFs. Its superior performance is summarized via a comparison between different NP controls in this paper.
Date of Publication: Jan.-Feb. 2013