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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]

Expansion of the DC distribution networks in the future power system has raised concerns over their stability assessment. To assess stability of the expanded DC distribution networks, the conventional zone aggregation method requires access to not only the nodes connected to the converters but also some other nodes within the DC passive network. However, access to the nodes within the DC passive network is not always feasible as some parts of the DC network may be buried or encapsulated. To address this issue, this paper proposes a new zone aggregation method for impedance-based stability assessment of the expanded DC-distribution networks. Compared with the conventional method, the main advantages of the proposed method are as follow: i) it only utilizes the impedance/admittance information of the nodes connected to the converters. Moreover, all the impedances/admittances can be measured directly without knowing the internal structure and/or parameters; ii) it can assess stability of expanded distribution networks with minimum number of nodes. Performance and effectiveness of the proposed method are evaluated and validated by simulation studies on a 10-bus DC distribution network and a modified IEEE 37-bus system in the Matlab/Simulink software environment. [ABSTRACT FROM AUTHOR]