A Short-Circuit Current Calculation Method for Hybrid MMC Considering DC-Side Control Strategies

The hybrid modular multilevel converter (MMC) employing dc-side control exhibits significant differences in dc output characteristics compared to the half-bridge (HB) MMC, leading to different dc short-circuit current characteristics. However, there is currently a lack of research on the dc short-circuit current analysis and calculation method of the hybrid MMC employing dc-side control. This paper proposes a short-circuit calculation method (SCCM) for hybrid MMC-based dc grids under various control strategies, with comprehensive short-circuit current characteristic analysis. First, calculation of single-terminal hybrid MMC short-circuit characteristics is performed based on analytical formulations. The quantitative analysis evaluates the effects of controller parameters on dc fault characteristics. Subsequently, an RLRC equivalent circuit model is proposed to analyze the influence of outer-loop controllers on fault characteristics. Based on the RLRC model, a SCCM for multi-terminal dc (MTDC) grids containing hybrid MMC(s) is proposed. The proposed SCCM is used for analyzing the impact of the hybrid MMC under different strategies on the short-circuit characteristics of MTDC grid, significantly reducing computational complexity. The accuracy of the SCCM is verified through electromagnetic transient (EMT) model and experiments. Based on the SCCM, case studies demonstrate that the dc-side control of hybrid MMCs can effectively suppress short-circuit currents of dc grids.