Hierarchical Circulating Current Control Method for Modular Multilevel Converter

Modular multilevel converter (MMC) has become a promising topology to be applied in medium and high voltage system due to its superior performance, also accompanied with some serious challenges, such as external harmonic circulating currents. In this paper, a hierarchical circulating current control method (HCCCM) is proposed which is composed of four levels. And the circulating current controller (CCC) based the HCCCM is evaluated from point of view of the harmonic impedance. The CCC employs 2nd band stop filter (BSF) in both inner and outer controlling paths. The dynamic performance and experiment comparisons of the controllers which only use inner 2nd BSF or outer 2nd BSF and the controller which simultaneously uses inner and outer filters are carried out. The parameters of CCC are designed properly and the experiments are tested.