Integrated paralleling of NPC inverters with suppressed circulating current for high-power renewable energy conversion

The development of renewable energy power generation for carbon neutrality and energy transition has been increasing worldwide,leading to an increasing demand for high-power conversion. Compared with traditional interleaved paralleling, the integrated paralleling of three-level inverters can further reduce the output harmonics. Moreover, a well-designed switching sequence ensures that the average circulating current is zero, which provides a superior and feasible solution to satisfy the demands of high-power operations. However, a large instantaneous loop current exists between shunt converters, which leads to disadvantages such as higher switching device stress and loss. In this study, by utilizing the state-distribution redundancy provided by the integrated modulation process, a new design for switching sequences is suggested for the integrated modulation of shunt three-level converters. This design aims to reduce the circulating current while better preserving the same output current harmonics than traditional parallel methods. The proposal includes an in-depth analysis and explanation of the implementation process. Finally, the proposed method is validated through simulations and prototype experiments. The results indicate that compared with traditional methods, the adoption of the improved switching sequence presented in this study leads to an average reduction of 3.2 % in the total harmonic distortion of the inverter's output and an average decrease of 32 % in the amplitude of the circulating current. Both the output harmonics and circulating currents are significantly suppressed across various modulation indices.