An Improved Three-Phase Voltage Source Converter With High-Performance Operation Under Unbalanced Conditions

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.