Renewable generation is mainly connected through converters. Even if they provide more and more ancillary services to the grid, these may not be sufficient for extremely high penetrations. As the share of such generating units is growing rapidly, some synchronous areas could in the future occasionally be operated without synchronous machines. In such conditions, system behaviour will dramatically change, but stability will still have to be ensured with the same level of reliability as today. To reach this ambitious goal, the control of inverters will have to be changed radically. Inverters will need to move from following the grid to leading the grid behaviour, both in steady state and during transients. This new type of control brings additional issues on converters that are addressed in this study. A solution is proposed to allow a stable operation of the system together with a limited solicitation of inverters during transients.

Abstract-The subject of this paper is the investigation of the stability of a Voltage Source Converter (VSC) undergoing a sudden decrease of the Short Circuit Capacity of the AC system to which it is connected. A case study is reported on a simple system including an HVDC terminal and a Thévenin equivalent. First, a small-signal stability analysis is performed showing that the PLL-based vector control may become unstable at powers lower than the theoretical static stability limit. Dynamic simulations demonstrate that the stability limit may further decrease depending on the disturbance. The need for a fast instability detection method is highlighted.

This paper presents a novel control scheme for exchange of frequency support between asynchronous AC systems through a High Voltage Direct Current link or grid. The proposed controller bears the spirit of an emergency scheme. Using only locally available measurements, each converter can identify emergency situations that could potentially lead to unacceptable frequency values. Then appropriate control actions are taken to restrain the frequency decline and prevent it from reaching the thresholds of load shedding relays. Inspired of Model Predictive Control, the method uses simplified models of the AC and DC sides of the converter, and can incorporate various constraints. The effectiveness of the method is demonstrated on a test system consisting of two asynchronous AC areas interconnected through a five-terminal HVDC grid.

This paper proposes a novel control scheme for provision of frequency support among asynchronous AC areas through HVDC grids. It is based on local controllers, each acting on a voltage source converter, using local measurements only, and supporting frequency of the adjacent AC area after a significant disturbance. The new discrete control is combined with the existing DC voltage droop technique. The formulation, inspired of Receding Horizon Control, enables providing to the AC area the desired frequency support, while at the same time taking into account various constraints, such as maintaining the DC voltage between secure operating limits. Examples obtained from a test system with a five-terminal DC network connecting two asynchronous areas demonstrate the effectiveness and robustness of the proposed control scheme in various scenarios, with emphasis on component failures.

Abstract-A coordinated supervisory control scheme for future multi-terminal High-Voltage Direct-Current (HVDC) grids is proposed. The purpose is to supervise the grid and take appropriate actions to ensure power balance and prevent or remove voltage or current limit violations. First, using DC current and voltage measurements, the power references of the various Voltage Sources Converters (VSC) are updated according to participation factors. Next, the setpoints of the converters are smoothly adjusted to track those power references, while avoiding or correcting limit violations. The latter function resorts to Model Predictive Control and a sensitivity model of the system. The efficiency of the proposed scheme has been tested through dynamic simulations of a five-terminal HVDC grid interconnecting two asynchronous AC areas and a wind farm.