Stability Analysis of Grid-Forming Mmc in All-Dc System Considering Harmonic Coupling

All-DC offshore wind is regarded as an effective solution for large-scale and long-distance transmission of offshore wind power. However, its stability analysis is rarely mentioned, and the harmonic characteristic analysis of the DC transformer is also rarely reported. Meanwhile, the rise of grid-forming (GFM) controls makes power electronic devices change the control structure, there is insufficient research on the interaction stability between GFM control converter and DC transformer. This paper first analyzes the harmonic coupling characteristics of the DC transformer. Then, based on the harmonic state space, impedance matrices for the DC transformer and the onshore modular multilevel converter are established, respectively. Subsequently, the matrices are transformed into single-input single-output impedances to facilitate stability analysis. Finally, a simulation system is constructed to validate the impedance models, uncovering the potential low-frequency stability issues.