Short-circuit Current Making in Medium Voltage Switches with Environmentally Friendly Gas

ABB ELDS in Skien manufactures 12 - 40.5 kV compact, gas insulated switchgears for the world market. Sulphur hexafluoride (SF6) gas is used as the electrical insulation- and current interruption medium. However, since SF6 is an extremely strong "greenhouse gas", restrictions on its use are anticipated. Short-circuit current making is the ability of an electrical switch to close upon the full rated short-circuit current of the switch without being damaged. During such a closing operation, the full current (~tens of kilo amperes) will start to flow at the full rated voltage and a high energy arc will burn across the contact gap. Without the excellent thermal- and insulating properties of SF6, the energy generated by this arc is increased and the contact may fail to re-open because the contact members are welded together. Previous and ongoing research funded by ABB and the Norwegian Research Council have addressed dielectric insulation, load current interruption and thermal performance for air and air mixtures as alternatives to SF6. Now, we need to initiate R&D work on short-circuit current making in the same gases in order to facilitate a complete alternative to the existing SF6 based MV switchgear product line. The PhD student, Philipp Jabs, was working on the project more than 1 year in NTNU. Different methods to assure the commutation of the arc current to the high current transformer have been analyzed and the advantages of using different semiconductors and mechanical switches have been investigated. A synthetic test circuit using semiconductors instead of a triggered spark gap is proposed and presented in a paper at IEEE conferences. The proposed synthetic test circuit has been tested in the short circuit current laboratory at NTNU. The PhD work has been supported by researchers at SINTEF. Philipp Jabs quit the project in October 2018 and the new PhD student Naghme Dorraki, started in February 2019. The PhD student did experiments on short-circuit making operations in MV-LBS to understand pre-strike arc behavior better and improve the design parameters. Diagnostic methods including arc spectroscopy and electrical measurement are used for the investigation. A test setup has been designed to evaluate the role of arcing and main contacts at different closing speeds during making operations. More than 250 successful tests of short-circuit making operations have been performed to evaluate the design parameters under different fault conditions. The outcome of this PhD study has been published so far as two journal articles and one conference paper. Two other journal papers are also submitted for publication. In total four master students have been involved and completed their theses in this project SINTEF has performed several experiments and simulations for investigating AC breakdown strength of air and AirPlus with puffer and earthing switch contacts to better understand the pre-strike behavior and design criteria. A new current making experimental rig was built during the project, for performing fault current making tests with currents up to 80 kA, with without pre-strike. The experimental rig included sophisticated ways to measure detect current path and measure small voltage differences in a high voltage environment. Both the ABB puffer switch and the rotating earthing contacts were tested in this setup, as well as the experiments of the phd candidate. The last years, SINTEF has mainly targeted issues related to the short-circuit current making operation of the rotating copper contacts using the high-current lab in Trondheim, to support ABB's product development and passing of type tests of the short-circuit making operations for the earthing contacts. ABB EPDS have performed several development tests to explore suitable geometries for switches with high making capacity and a new product is under development. The first series of type testing were performed with acceptable results, further improvement to achieve higher rating up to 62,5kA were implemented and successfully type tested. ABB has released two new eco-friendly load break switches, one for 12 kV and one for 24 kV in December 2020. The project is a collaboration between ABB ELDS in Skien, SINTEF and NTNU.

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ABB EPMV in Skien manufactures 12 - 40.5 kV compact, gas insulated switchgears for the world market. Sulphur hexafluoride (SF6) gas is used as the electrical insulation- and current interruption medium. However, since SF6 is an extremely strong "greenhouse gas", restrictions on its use are anticipated. Short-circuit current making is the ability of an electrical switch to close upon the full rated short-circuit current of the switch without being damaged. During such a closing operation, the full current (~tens of kilo amperes) will start to flow at the full rated voltage and a high energy arc will burn across the contact gap. Without the excellent thermal- and insulating properties of SF6, the energy generated by this arc is increased and the contact may fail to re-open because the contact members are welded together. Previous and ongoing research funded by ABB and the Norwegian Research Council have addressed dielectric insulation, load current interruption and thermal performance for air and air mixtures as alternatives to SF6. Now, we need to initiate R&D work on short-circuit current making in the same gases in order to facilitate a complete alternative to the existing SF6 based MV switchgear product line. The project is a collaboration between ABB EPMV in Skien, SINTEF and NTNU.