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A modeling approach is presented in this paper in order to investigate instabilities originated 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 are investigated. The theoretical findings are verified through time-domain simulations.