Feasibility Study on the Applicability of Clean Air in Gas-insulated DC Systems

Abstract:
Gas-insulated DC systems offer significant advantages in terms of space requirements. The application of compact gasinsulated DC systems (DC GIS) is of particular interest for offshore platforms as well as in converter and transition stations in the context of long DC cable lines. A gas-insulated system for 550 kV rated DC voltage, based on SF6 technology, was developed, type-tested and introduced to the market. SF6 gas allows extremely compact dimensions of the system, but it requires very careful handling throughout the life cycle, as it is a gas with a high global warming potential (GWP). In order to contribute to an enhanced sustainability of the energy supply, the demand for SF6-free solutions is increasing since several years. A consistent solution is the application of the climate-neutral and fluorine-free insulating gas Clean Air (synthetic air with 80% nitrogen and 20% oxygen). A feasibility study examined the applicability of Clean Air technology for use in DC GIS of the voltage level of 320 kV, such as that used for grid access in the North Sea. In addition to a compilation of the requirements, the suitability for dielectric strength, mechanical strength at increased pressure and current carrying capacity / temperature rise was investigated. In addition to mechanical and thermal simulations as well as electric field calculations, high voltage tests were carried out to determine limit values. In basic tests on a GIS assembly, the influence of typical imperfections on the partial discharge activity at DC voltage was investigated, where SF6 served as a reference. In addition to a configuration with a tip on the inner conductor, the behaviour of free moving (conductive) particles was examined more in detail. Based on the given requirements, the feasibility of Clean Air application in DC GIS was confirmed.