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The unbalance of the three-phase grid-voltages/loads can cause double-frequency power ripples in the voltage source converter (VSC) system. Such power ripples can further lead to current and/or voltage ripples on the dc link, which may create instability of the system, lower its efficiency and shorten the lifetime of dc sources (e.g., batteries). Existing control solutions cannot well solve this issue and even bring collateral damage. Several hardware solutions have recently been proposed to tackle these issues, but these methods suffer from large size, high cost, and reduced system reliability. Instead, this paper proposed an improved topology to handle the problems faced by the VSC under unbalanced conditions. Although, hardware modification is also needed, it will not increase the size and cost of the system, since only one connecting line is added, which can be easily made on the power-stage printed circuit board. Moreover, the improved topology can also be used to reduce the leakage current in transformerless photovoltaic system and to eliminate the high frequency circulating current in multiple parallel converters. Theoretical analysis, simulation, and experimental results are provided to explain the principle and to validate the effectiveness of the improved topology.INDEX TERMS Grid-connected, islanded, power ripple, three-phase unbalance, voltage source converter.FEN TANG received the B.S. degree in electrical engineering and the Ph.D. degree in power electronics and electric drives from

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The centred space vector modulation scheme is considered to be the best since it produces the least switching harmonics among the modulation techniques developed for neutral-point-clamped (NPC) inverters. However, this common belief was concluded in the condition when inverters interfaced to the grid with a simple inductor filter. Nevertheless, to achieve an excellent harmonic attenuation performance, higher-order filter is much preferred for NPC inverter. Consequently, the aforementioned common belief may lose its correctness due to the distinctive resonance characteristics accompanied by highorder filters. The objective is to review and re-evaluate the performances of the commonly used modulation techniques for NPC inverters when different high-order filters are applied. Furthermore, this study provides a guide to choose an appropriate modulation scheme when high-order filters are used. The sinusoidal pulse-width modulation with alternative phase opposition disposition shows the best performance since it always ensures high-quality current waveforms when high-order filters are used in NPC inverters. Apparently, this conclusion is against the common belief. Moreover, different conclusions in terms of the performances have also been found for other modulation techniques considered in this review. Finally, to validate the correctness of the analysis and the findings, simulated and experimental results are provided and discussed.

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