Coordinated Supervisory Control of Multi-Terminal HVDC Grids: a Model Predictive Control Approach

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

Tuning of Cascaded Controllers for Robust Grid-Forming Voltage Source Converter

From the origin of the grid, energy has been delivered to electrical loads mainly by synchronous machines. All the main rules to manage the grid have been based on the electromechanical behavior of these machines which have been extensively studied for many years. Due to the increase of HVDC link and renewable energy sources as wind turbine and PV, power converters are massively introduced in the grid with a fundamentally different dynamic behavior. Some years ago, they were connected as simple power injector. Then, they were asked to provide some ancillary services to the grid, in the future, grid forming capability will be required. Even if gridforming converters had been extensively studied for microgrids and offshore grids, it has to be adapted to transmission grid where the topology may be largely modified. This paper presents an algorithm for calculating the controller parameters of a gridforming converter which guarantee a stable behavior for many different configurations of the grid.MIGRATE, Horizon 202

Intégration massive d'électronique de puissance : synchronisation et stabilité des grands systèmes électriques

Renewable generation is mainly connected through converters. It can provide more and more services to the grid such as voltage support or frequency control. However, these services 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 be occasionally operated without synchronous machines. In such conditions, system stability will have to be ensured with the same level of reliability as today. Today, operation of power systems is based on the presence of synchronous machines. Frequency is linked to the balance between consumption and generation of electricity via the rotating masses equation. This will not be inherently valid for grids without synchronous machines. The issue of operating a network with 100 % power electronics is quite well solved for small isolated systems. The same doesn’t apply for large transmission systems where grid topology and power injections are highly variable and are not known at any time by all system components or even by a centralized entity. This paper describes the research that needs to be achieved to remove barriers to high penetrations of converters

A Receding Horizon Approach to Incorporate Frequency Support into the AC/DC Converters of a Multi-Terminal DC 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

A dynamic simulation approach to identify additional reactive reserves against long-term voltage instability

peer reviewedA simple method is proposed to identify additional reactive reserves enabling to counteract long-term voltage instability after a large disturbance. In contrast to the many references based on power flow calculations, the method resorts to dynamic simulation. The post-disturbance system evolution is simulated in the presence of time-varying shunt susceptances with specified rate of change. This allows to deal with dynamic issues such as the required speed of the additional reactive power sources, or the onset of unstable electromechanical oscillations. Furthermore, a compromise is sought between the speed of action and the volume of additional compensation. The method is demonstrated on a detailed model of the Nordic test system

Local Control of AC/DC Converters for Frequency Support Between Asynchronous AC Areas

peer reviewedThis paper proposes a novel control scheme for frequency support among asynchronous AC areas through HVDC grids. It is based on local controllers, each acting on a voltage source converter, using locally available measurements only, and supporting frequency of the adjacent AC area after a significant disturbance. The controller is combined with the existing DC voltage droop technique and is inspired of Model Predictive Control, taking into account various constraints. The coordination of the proposed control scheme with the existing secondary frequency control of an AC area is also discussed. Examples obtained from a test system with a five-terminal DC network connecting two asynchronous areas demonstrate the effectiveness of the proposed control scheme

Simplified representation of a large transmission network for use in long-term expansion planning

This paper concerns the development of a new approach to the simplification of representation of the spatial dimension of a large transmission network in order that the influences on bulk power transfers can be assessed in a practical way and the main routes that should be reinforced readily identified. The main challenge is to achieve a satisfactory clustering to deliver a number of zones that is small enough to make subsequent analysis of the expansion panning problem manageable but not so small as to neglect key regions of the original system. Two particular methods that have previously been proposed are described: a K-means algorithm and Dodu’s mixed integer linear programming based approach. Each of them has some disadvantages, in particular that a direct interface between two zones might be derived that has no equivalent on the real network; or that it is difficult to control the number of zones. Hence, this paper describes a new hybrid method that ensures that resulting zonal delineations make engineering sense from the point of view of physical connections and allow some control over the number of zones. Results are presented in respect of the transmission network in Great Britain. Applications of the simplified network are discussed, not only in long-term planning but also in respect of the potential for use in transmission charging

Stability of a Voltage Source Converter subject to Decrease of Short-Circuit Capacity: a Case Study

peer reviewedThe 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