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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]

The increasing power/load demand is becoming a new concern for the grid operators as it can potentially overload transmission lines. Among advantages of a multi-terminal dc (MTDC) grid, its high controlability makes it a promising asset for ac line overload alleviation. This paper proposes a model predictive control based strategy, which regularly updates the power setpoints of the MTDC converter stations and ac generators to alleviate ac line overloads. To compute and update the power setpoints, necessary measurements are received by the controller within regular time intervals. The proposed controller ensures that subsequent to any ac line overload, changing the power setpoints of the MTDC converter station does not cause any operation violation in the MTDC grid. Furthermore, it ensures that the ac voltage limits and voltage stability criteria are met to avoid voltage instability. Performance and effectiveness of the proposed controller are evaluated by simulation studies on two systems, i.e., the 39-bus New England system integrated with a 5-bus MTDC system and the IEEE 118-bus system connected to a 6-bus MTDC system. Simulation results confirm that the MTDC grid can play a major role in controlling active powers of most of the ac transmission lines. [ABSTRACT FROM AUTHOR]