Stability of clean air under electric arcing conditions and its impact on gas handling

Insulating gases under high pressure enable compact high voltage gas-insulated systems (GIS). The common insulating gas SF6 is of outstanding technical performance but has a high global warming potential (GWP), so GIS with more ecofriendly gases were developed in recent years. The climate-neutral insulating gas mixture clean air (synthetic air) is used in the latest generation of GIS, in combination with vacuum technology for circuit-breaker switching purposes. Clean air is of high chemical stability and is non-toxic. When working on GIS with non-arced clean air, no specific personal protective equipment (PPE) is required. While GIS for up to 145 kV and gas-insulated busducts up to 420 kV are already on the market and installed in the grid, respective 420 kV GIS modules are under development. The gas may be exposed to electrical arcs during GIS service, particularly in disconnector and earthing switch duties. Furthermore, partial discharge (PD) and, in the worst case, internal arc may decompose the gas. While PD result in low gas decomposition, disconnector and earthing switch duties, and particularly internal arcs may form more decomposition products. This contribution summarizes typical conditions where gas decomposition occurs and presents respective test and gas analysis results, including long-term tests with artificial continuous PD impact and disconnector and earthing switch test duties, also in conjunction with the 420 kV GIS development, and gas measurements after internal arc tests. Decomposition products and their concentrations can be determined on site as well as based on laboratory equipment. Different detection and air quality measurement devices exist and were tested. Respective test results and recommendations are given. Furthermore, gas decomposition is of influence on gas handling. The test results are put into the respective context of air decomposition, and recommendations are given in terms of safe handling considering health, safety and environmental aspects.