Two-stage power conversion system (PCS) for energy storage systems has been considered in islanded operation mode. A three-level T-type three-leg three-phase four-wire topology (3LT ² 3L3P4W) is employed as AC/DC part and a three-level buck/boost converter is used as DC/DC interface. This study is mainly focused on balancing the neutral-point potential (NPP) of the PCS with asymmetrical load connection. An NPP low-frequency ripple model of 3LT ² 3L3P4 inverter is proposed considering various load conditions firstly, which includes detailed expression of low-frequency ripple components at fundamental frequency and fundamental frequency multipliers. Then, a novel NPP balancing method based on front-end three-level buck/boost converter under proportional resonant control is presented, in which the resonance points are set at fundamental frequency and triple fundamental frequency to suppress the dominant components of NPP ripple. Furthermore, a small-signal model of three-level buck/boost NPP control system is established and stability analysis is carried out. Simulation and experimental results have been provided to verify the accuracy of the NPP low-frequency ripple model and the effectiveness of the NPP balancing method.

中文翻译：

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两级三相四线电力转换系统中三电平功率转换器的中点电位平衡方法

在孤岛式运行模式中已考虑了用于储能系统的两级功率转换系统（PCS）。 AC / DC部分采用三电平T型三脚三相四线拓扑（3LT ² 3L3P4W），DC / DC接口用作三电平降压/升压转换器。这项研究主要集中在通过不对称负载连接平衡PCS的中性点电位（NPP）。3LT ²的NPP低频波纹模型 首先提出了3L3P4逆变器，它考虑了各种负载条件，其中包括基频下的低频纹波分量和基频倍增器的详细表示。然后，提出了一种在比例谐振控制下基于前端三电平降压/升压转换器的新型NPP平衡方法，该方法将谐振点设置在基频和三基频处，以抑制NPP纹波的主要成分。建立了三级降压/升压NPP控制系统的小信号模型，并进行了稳定性分析。通过仿真和实验结果验证了NPP低频纹波模型的准确性和NPP平衡方法的有效性。