Resumen de Modelling and control strategies for hybrid ac/dc grids
The proposals for large-scale deployment of renewable energy sources is leading to the need for more cross-border interconnections creating a pan-European power system, at the EU level. Such interconnections may take advantage of existing AC systems, but also of the DC links or DC grids under development; hence, hybrid AC/DC grids. Although hybrid AC/DC transmission grids are far from being truly large-scale, they are beginning to evolve as key components of future transmission infrastructures. By hybrid AC/DC grids, power electronic interfaces are invariably implied. As a consequence, several challenges are beginning to emerge at the network level due to the increasing adoption of these devices.
The dynamics and interactions that may appear in such integrated systems are relatively unknown since they will interconnect at least two subnetworks of AC and DC characteristics. Understanding how several classes of interaction could occur is key to proper design of controllers to mitigate them. Further, changes to how subnetworks should be securely operated in unison is required. Therefore, a rethought is necessary in the presence of these devices. Nevertheless, the answer to the question of how to detect detrimental behaviours? Is central to any proposed solution.
Currently, traditional methods for modelling and analysis are showing inadequacies. Lack of consistent methods to model and analyse the phenomena often result to complicated solutions as existing literature suggests. Even more important are models that are tractable, flexible, and technology agnostic to allow abstraction of the underlying challenges. Hence, methods to better understand and assess the mechanism of interactions at system level, that may impact secure operation are required. In addition, methods that are intuitive and efficient to detect sources of interactions, and isolate them as rapidly as possible are preferred. This goes to the heart of flexibility and tractability. Therefore, this thesis presents methodologies and strategies for modelling and control of large-scale hybrid AC/DC transmission grids from a systematic perspective, with the consideration of the controllable devices.
This thesis employs several potent high-level methodologies that possess physical connotation, are technology agnostic, and provide tractability for control. Subsequently, recommended control strategies are easy to adopt as their physical significance can be established. The principal findings of this thesis are that, system interactions between subnetworks are dependent on the broad characteristics of the each subnetwork. Thus, manipulating any of these characteristics subject to considerations, improves the overall behaviour. Within each subnetwork, interactions depend mainly on the dynamics of existing controllers, and the interconnection between several devices. Then, questions on how to detect and mitigate interactions as efficiently as possible, while incorporating the most relevant behaviour is answered. Furthermore, the conflict of control requirements of each subnetwork of the hybrid AC/DC network is highlighted. Thus, information about these requirements are leveraged to achieve overall compromises without jeopardizing minimum performance. To conclude, following the assessment of detrimental interactions and their corresponding mechanism, control strategies that take these into consideration are proposed and demonstrated.
