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.

Environmental concerns related to the greenhouse effect of SF 6 have driven changes to take place in the power distribution industry. This paper discusses the main challenges for next generation medium voltage (MV) ring main units (RMU) using new technologies and materials with reduced environmental impact. Market requirements are going in the direction of equal or even higher technical ratings for new products. Replacing SF 6 with any other insulation gas in RMU requires innovative solutions to be implemented. A key challenge is to be able to maintain the outer physical dimensions of the unit, as this imposes strict conditions on the dielectric and thermal performance. Dielectric design of SF 6 free RMU targets the distribution of electrical fields within the unit, aiming to reduce the field strength of weak points to compensate for the reduced dielectric strength of alternative insulating gases. Key parameters for optimization include choice of insulating materials, geometrical shape of conducting surfaces and definition of conductor/insulator interfaces. Thermal design is further critical due to the lower thermal properties of alternative insulating gases. Computational Fluid Dynamics (CFD) analysis is used to understand and optimize the temperature distributions inside the switchgear. Simulation results are validated by temperature rise tests in full scale prototypes. The main functionality of next generation RMU relies on optimized dielectric and thermal design in order to provide a cost efficient and reliable unit. In this paper a selection of techniques are discussed with references to both simulations and full scale tests based on the challenging boundary condition of keeping the same physical dimensions as an existing SF6 product.

Puffer interrupters, which use the gas blast produced by relative movement of a piston and a cylinder, are widely used due to their compact size, simple structure and excellent interruption properties.