This study presents an investigation of the impact of the quasi-stationary voltage support provided by a voltage source converter (VSC) connected to a single point of a power system. Based on the directional derivative concept, an analytical method is developed to quantify the sensitivities of the AC bus voltage with respect to the VSC reactive power control modes. Based on a real case study, it is shown that the method applies to VSC units that are part of VSC-HVDC systems, which can operate in a point-to-point or multi-terminal configuration. Time-domain simulations are performed to verify the findings from the application of the analytical method on a reduced size power system.

This paper studies the inclusion of averaged VSC-based grid interfaces and HVDC networks into stability type simulations, and compares the accuracy and speed of three multi-terminal DC dynamic models: 1) a state-space based model, 2) a multi-rate improved model, and 3) a reduced-order model. The accuracy comparison is conducted by qualitative time-domain analysis taking the state-space model as a reference whereas the computational aspects are investigated by the respective execution times. The models are demonstrated on a three VSC terminal, two synchronous area system. It is shown that the reduced-order model is too inaccurate to investigate detailed large-disturbance stability-related dynamics. The multi-rate model shows the best trade-off between simulation accuracy and speed.