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Growing concerns with the harmonic oscillation phenomenon of multiterminal DC (MTDC) systems calls for appropriate harmonic stability analysis methods. However, the conventional ac or dc admittance-based stability assessment methods cannot effectively protect trade secret and user privacy, distinguish different system instability root causes, and put forward corresponding instability mitigation methods. To address the aforementioned issues, this article first establishes a hybrid ac/dc admittance-based stability assessment model for a MTDC system. Then, by introducing the determinant-based General Nyquist Criterion (GNC), the system stability is assessed and three types of instability root causes, including the ac side, the dc side, and the coupling interaction between the ac and dc sides are identified. Regarding the instability caused by the ac side, the dominant interlinking converter is directly identified, in which the instability can be mitigated by reshaping its q-axis admittance. In contrast, for the instability caused by the latter two root causes, the most influential interlinking converter is recognized by an additional sensitivity analysis. The instability is then mitigated by reshaping the dc-side equivalent admittance of the most influential interlinking converter. Compared with the conventional methods, the proposed method offers advantages as follows: 1) trade secret and user privacy is effectively protected, 2) different system instability root causes can be effectively distinguished, and 3) instabilities caused by diverse root causes can be mitigated by corresponding methods. Finally, the proposed method is verified by simulations in the MATLAB/Simulink environment. [ABSTRACT FROM AUTHOR]

Medium-voltage dc (MVdc) grids are attractive for electric aircraft and ship power systems, battery energy storage system (BESS), fast charging electric vehicle (EV), etc. Such EV or BESS applications need isolated bidirectional MVdc to low-voltage dc (LVdc) or LVac converters. However, the existing Si-based solutions cannot fulfill the requirements of a high-efficiency and robust converter for MVdc grids. This article presents a 5-kV SiC-based universal modular solid-state transformer (SST). This universal current-source SST can interface either an LVac or LVdc grid with an MVdc grid in single-stage power conversion, while the conventional dual-active bridge (DAB) converter needs an additional inverter. The proposed SST module using 3.3-kV SiC MOSFETs and diodes is bidirectional and can serve as a building block in series or parallel for higher voltage higher power systems. The topology of each module is based on the soft-switching solid-state transformer (S4T) with reduced conduction loss, which features reduced electromagnetic interference electromagnetic interference (EMI) through controlled dv/dt, and high efficiency with full-range zero-voltage switching for main devices and zero-current switching for auxiliary devices. Operation principle of the modular S4T (M-S4T), capacitor voltage balancing control between the cascaded modules, design of components including a medium-voltage (MV) medium-frequency transformer (MFT) to realize a 50-kVA, 5-kV dc to 600 V dc or 480 V ac M-S4T are presented. Importantly, the MV MFT prototype achieves very low leakage inductance (0.13%) and 15-kV insulation with coaxial cables and nanocrystalline cores. The proposed universal modular SST is compared against the DAB solution and verified with dc–dc and dc–ac simulation and 4-kV experimental results. Significantly, the MV experimental results of a modular dc transformer with each module at MVdc are rarely covered in the literature and reported for the first time. [ABSTRACT FROM AUTHOR]