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.

When a PtP link is expanded into a multi-terminal HVDC (MTDC) system by interconnecting an additional offshore wind farm (OWF) converter, the existing operation strategy changes. The OWF power production should be considered as a determining factor to operate the system. In this paper, a new power capability curve for the existing converters is proposed. This new power capability curve is formulated as a function of OWF power production. Furthermore, different converter control strategies are also described and compared. It has been found that a multi-slope droop control strategy is the most suitable strategy for the 3-terminal HVDC system with an OWF converter.