Physics of microarcs in low-voltage switching devices

• Applicants: Dr. Margarita Baeva; Dr. Dirk Uhrlandt
• Subject Area: Electrical Energy Systems, Power Management, Power Electronics, Electrical Machines and Drives ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 524731006

The project is aimed at the description and the physical understanding of low-current arcs (between 0.5 and 20 A) in low-voltage direct current (DC) switchgears. Low-voltage DC switching finds an increasing importance e.g. for electro mobility as well as in local grids that use photo-voltaic systems and batteries. Arc switching devices are attractive as low-cost solutions or in combination with semiconducting devices in hybrid switchgears because they permit a galvanic separation and a high insulation level. Although switchgears have been used for many decades, the basic understanding of the physical processes in switching arcs is still incomplete or even very poor in particular at low currents. An unexpected behaviour of the arc voltage and the arc radiation has been found for short gap lengths in investigations preliminary to this project, which cannot be described by conventional electric arc models. Moreover, low-voltage switchgear typically works with non-refractory electrode materials like copper alloys. The transfer of electric current from a non-refractory cathode to a low-current arc at atmospheric pressure is an open theoretical issue mainly because of the low thermionic electron emission even for temperatures close to the boiling temperature of non-refractory metals. These problems should be tackled in the project by combined experimental and theoretical studies. The focus is on arcs between flat cylindrical electrodes in ambient air at atmospheric pressure as a representative of most common low-voltage and low-cost switching devices. Copper composites as well as pure copper should be used as electrode materials. In particular, the voltage-current characteristics of the current breaking process for small and increasing gap distances, the spatial structure of the arc including electrode sheath regions and the arc attachment at the electrodes should be evaluated. A dominant role of metal vapour, distinct deviations from local thermodynamic equilibrium and the establishment of spot modes at both electrodes are expected and should be confirmed even for low currents. Therefore, a model switch will be set up, and the three-dimensional arc structure will be studied by high-speed imaging, tomography and spectroscopy. A multi-fluid non-equilibrium numerical simulation of the arc including the electrode sheath areas and the self-consistent coupling with models of the electrodes will be developed considering in particular the plasma chemistry in copper vapour. It is aimed in a self-consistent description of the current transfer and arc attachment at non-refractory cathodes for low currents, which is not available up to now. The model should explain the current-voltage characteristic and should be validated by the measurements at the model switch. Conclusions on the switching performance, the erosion of electrodes and the lifetime and long-term behaviour of the low-voltage switching devices should be drown.

• DFG Programme: Research Grants
• Major Instrumentation: High-speed camera
• Instrumentation Group: 5430 Hochgeschwindigkeits-Kameras (ab 100 Bilder/Sek)