Protection and Fault Handling in Offshore HVDC Grids

Pre-feasibility studies have demonstrated the technical and economic benefits of a large-scale offshore grid infrastructure in the North Sea region. This offshore grid would interconnect the countries bordering the North sea and likely integrate oil rigs or offshore wind farms that already exist or are in development. Such a future large-scale power infrastructure is expected to be based on Voltage Source Converter (VSC) technology for High Voltage Direct Current (HVDC) transmission in a Multi-Terminal (M T) grid configuration. Avoiding unexpected interactions between subsystems and designing protection systems for fault handling in MT-HVDC systems are currently considered the major technical barriers for the development of large scale offshore grids. The primary objective of this project has been to establish tools and guidelines to support the design of MT offshore HVDC grids in order to maximize system availability. A first part of the project reviewed the state-of-the-art in offshore grid technologies and set the basis for the other activities. Research has been conducted by numerical simulations and experimental testing on laboratory scale systems. A library of dynamic numerical models has been created and employed for the simulation of various MT-HVDC grid configurations. In particular, the project analysed how the control of the individual terminals can affect the small signal stability of the entire grid. Moreover, the project acquired a real-time simulator that is presently used together with power converters already available to realize a three terminal grid setup. Additional research activities have been devoted to protection schemes with special focus on the requirements for fault detection and fault location in a radial grid. The project sponsored two Ph.d. candidates (one at NTNU and one at RWTH) and a post.doc at NTNU. The project also hosted a visiting Postdoc from KU Leuven for one year.

In the last years the initiatives promoting a large-scale offshore grid infrastructure in the North Sea received a growing political support by European institutions. Pre-feasibility studies already demonstrated technical and economic benefits of such an offshore grid for the connection of offshore wind farms, the electrification of oil rigs and the establishment of a pan-European electricity market. The realization of an offshore grid is already theoretically feasible since the major building blocks nec essary for its construction are commercially available. However, the operation of such a complex infrastructure represents a major knowledge need for the industry since no similar configurations are in operation today and standard practices valid for conv entional AC transmission grids cannot be directly transposed. In particular, it is still a challenge to ensure reliable operation and a selective protection. Indeed, the design of the protections is limited by the lack of commercial DC breakers with suita ble specifications. In addition, the large penetration in an electrical system of power converters can produce unexpected interactions with potential impact on the overall reliability. The project addresses these problems by establishing tools and guidel ines to support the design of an offshore grid to maximize the system availability. Focus will be on: - Develop models of offshore grid components for electromagnetic transient studies - Define guidelines to reduce the risks of unexpected interactions be tween components - Define strategies for protection and fault handling - Demonstrate the effectiveness of these tools with numerical simulations and experimental tests. The proposed project can be beneficial for the industries that are involved in the de sign, the construction and the operation of an offshore grid infrastructure.