Effect of energy absorption systems on internal arc testing of medium voltage switchgear

The international market of air-and gas-insulated switchgear for electrical transmission and distribution demands high requirements with respect to reliability of operation and safety. The probability of the occurrence of an internal arc in metal-enclosed medium voltage switchgear during its entire service lifetime is rather low but it cannot be completely disregarded. The internal arc heats the surrounding insulation gas, which results in an overpressure in the switchgear compartment because part of the electric energy will be transferred into thermal energy After exceeding a predetermined pressure limit the hot gases under high pressure will expand via pressure relief devices into the surroundings. To increase safety by reducing temperature and pressure, energy absorbing systems may be integrated between switchgear compartment and building. The systems are often composed of cooling grids from expanded metal which are placed in the flow of hot gases. The cooling grids absorb via heat conduction and convection a part of thermal and kinetic energy of gas flow. Exploratory investigations are performed to analyse the efficiency of such absorption systems and its influence on different design types. For this several internal arc tests were performed in a model arrangement and in gas-insulated ring main units. Additionally the influence of the insulation medium air and sulphur hexafluoride (SF₆) were analysed. A mathematical model has been evolved, which describes the effect of energy absorbers and is integrated in a Computational Fluid-Dynamic (CFD) calculation. The simulation results are compared with measurement results.