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A modeling approach is presented in this paper in order to investigate instabilities originating from dc-network resonances in voltage-source converter HVDC (VSC-HVDC) systems. Through frequency-domain modeling, it is shown that the VSC-HVDC system can be represented as a feedback interconnection of two subsystems. These subsystems are defined in this paper as a dc network impedance connected to a VSC dc-side power-dependent admittance. The conditions in which this admittance has a negative conductance are investigated since they can amplify dc network resonances. It is found that when the VSC that controls the direct voltage injects power into the dc network, the mentioned conductance becomes negative above certain frequencies. In addition, the impact of weak ac grids, control system delays, and other direct voltage controllers on the studied conductance is investigated. The theoretical findings are verified through time-domain simulations.
Pinares et al. (Fri,) studied this question.