Internal arc fault testing of gas insulated metal enclosed MV switchgear

Abstract

An investigation on internal arc fault tests in gas-insulated metal enclosed MV switchgears is described and discussed. The influence of different gases (SF 6 and air) and electrode materials (Cu, Al, and Fe) has been put into evidence. This is highly relevant based on the fact that IEC Standards and utilities technical specifications allows that internal arc fault tests are performed with air replacing SF 6 with some precautions. The experimental tests were carried out at NEFI High Power Laboratory in Skien, Norway. Full-scale test objects representative of typical gas insulated metal enclosed MV switchgear were prepared, filled with air or SF 6 and tested. The short circuit current was 16 kA rms and the duration was 1 second. Electric input energy, internal gas pressure in different locations, opening time of bursting discs and temperature were acquired. The arc was ignited at the end of the simplified, but still representative, 400 mm long bus bars that were located in the middle of the metal enclosure. The full-scale experiments were additionally analyzed by means of Finite Element Method. The comparison between experiments and simulations show that there was possible to set up an arc model and tune it in to the results. The main difference between testing of units filled with air versus SF 6 is the significant faster pressure increase in air. As a result of this and the fact that the bursting discs need a certain time to open, the pressure inside the test object will be higher at the time the bursting discs open when testing with air. The maximum pressure reached during a test with air however may be equal or lower than in SF 6 depending on dimensional parameters of bursting disc and encapsulation. Furthermore it is evident that there is a clear difference in the exhaust characteristics of SF 6 and air from an internal arc test.

High Current Arc Erosion on Copper Electrodes in Air

“…At a first approximation it may be concluded that the extra energy input caused by Cu vapors reacting exothermally with the surrounding air is most probably negligible. This is however in contrast to the strong exothermal reaction between vaporized aluminum and dissociated SF 6 reported by Bjørtuft et al [5]. For the moving arc with no gross melting, relevant information is relatively scarce in the literature.…”

“…To get a better understanding of the influence of chemical reactions, it is assumed that the reactions contribute significantly only if the released energy is more than 10 % of the total arc energy. Table V shows the assumed amount of copper, ∆m Wchem , which will have to react chemically, according to equation (5), for a 10 % extra contribution to the arc energy input. It may be assumed that an exothermic reaction will to a large extent depend on Cu being present in vapor form.…”

“…The metal vapor may react chemically with the insulating medium, in our case air, resulting in an energy release W chem (exothermic reaction). The possible chemical reactions of copper vapor and air (oxygen) are Cu+1/2O 2 →CuO + 155 kJ/mol (5) 2Cu+1/2O 2 →Cu 2 O + 167 kJ/mol.…”

Pressure Rise due to SF6 Arcing and Energy Balance in a Closed Container

“…The ratio of energy contributing to the internal pressure rise and the arc energy depends on a number of parameters, such as the physical properties of the arc and gas, the material and shape of electrodes, the size and shape of closed container, and so on . Here, we report the results obtained in these experiments regarding in air and SF 6 gas.…”

Analysis of Pressure Rise in a Closed Container Due to Internal Arcing

“…For air at ambient temperature, the adiabatic coefficient κ is about 1.4 [22]. c v is the specific heat of the gas at constant volume.…”