Climate-friendly and socially inclusive AC-DC renewable energy system with overlay multi-terminal HVDC network (OVANET): Solution with fully distributed optimization and infrastructure combination

Abstract

A large-scale integration of wind and solar energy relies on a grid that allows for the renewable electric power to securely reach the load centers. While the necessity for the extension of transmission capacity to obtain a climate-friendly and sustainable overall energy system is given by physical evidence, the realization of any extension relies on social acceptance and the support of all involved stakeholders. This is where the overall contribution of this paper comprises three dimensions. Firstly, thanks to the modular multi-level converter (MMC), it has become possible to realize multi-terminal DC overlay networks that lend themselves for connection with AC grids. The full AC-DC integration is at the core of the first dimension and aimed at offering a high flexibility of control when addressing security and efficiency. Secondly, as the AC-DC grid may stretch across multiple countries and therefore also diverse transmission system operator (TSO) control areas, it is important that a desired degree of data privacy and autonomy within those areas can be preserved. This is made possible by the proposed distributed AC-DC power flow optimization based on a modified fully parallel Alternating Direction Method of Multipliers (ADMM). The formulated local physical consensus constraints for boundary variables at the locations of tearing only rely on exchange of information with immediate neighbors. The physical consensus constraints are complemented by the newly introduced operational consensus constraints to prevent conflicting operational control actions. Thirdly, it is shown how social acceptance can be boosted by having the transmission lines share the corridors with other transport infrastructures. Of particular interest in this context are DC cable tunnels along motorways and the joint usage of bridges, leading to a combination of infrastructures. This also readily supports the development of electric vehicle charging stations along motorways. Together, these three dimensions provide the foundations of the proposed OVANET concept of socially inclusive AC-DC grids for renewable energy transmission.