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.

The active power gradient (APG) control of MMC-HVDC links can contribute to frequency support in AC networks affected by severe active power imbalances. This functionality is particularly convenient if the coupled AC systems (through MMC-HVDC) have similar levels of available inertia. This paper tackles the problem of determining the optimal APG parameters that entail bounding frequency excursions within acceptable limits while helping to quickly damp out electromechanical oscillations. The tuning task is tackled as a single objective computationally expensive optimization problem. Since the optimization search procedure involves repetitive time-domain (RMS) simulations, the major challenge resides in solving the problem within reduced amount of fitness evaluations. In view of this, an emerging metaheuristic algorithm, namely, the mean-variance mapping optimization (MVMO) is selected. Numerical results prove the effectiveness of MVMO in finding the optimal solution within a few fitness evaluations.

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.

This paper presents a strategy for regulation of power factor in an embedded high-voltage direct current (HVDC) system based on modular multilevel converter (MMC) technology. Field reported in existing literature are taken as a reference for qualitative assessment of the performance of the proposed strategy. The implication of adding such a type of control mode in an MMC-HVDC system is determined through software experiments performed in DIgSILENT PowerFactory. The results reveal that the power factor can be effectively controlled by an MMC-HVDC link within its power rating limits, even in the case of changes of the active power reference.