Impact of the Circulating Current Control on Transient Submodule Voltage Stresses for Grid-Tied Modular Multilevel Converters During Grid Faults

The maximum sub-module (SM) voltage stress is a key parameter for the modular multilevel converter (MMC) design, e.g., IGBT voltage rating selection and SM capacitor sizing. For a grid-tied MMC, the evaluation of the SM voltage stress should consider the steady-state operation and also the dynamic performances, which are significantly dependent on the MMC control system. Thus, this paper proposes a mathematical approach based on a typical MMC control strategy to investigate the interaction between the SM voltage and the circulating current suppression control. The time domain expression of the transient SM voltage during grid faults is derived. To verify the model, a downscale three-phase MMC experimental platform has been built. Comparisons of the model estimation of the SM voltage dynamics and experimental results verify the effectiveness of the proposed model.