Investigation on Fault Current Making in Medium Voltage Switchgear in Air

Abstract

Closing a switch under a fault condition could result in extensive electrical contact wear and a possible switch failure. A load break switch (LBS), as one of the critical components in medium voltage (MV) power grids, should be able to be closed under short circuit currents of thousands of amperes. The dielectric breakdown between closing contacts creates a pre-strike arc before the contacts touch. Part of the short circuit current flows through the pre-strike arc, and the rest passes through the contacts when sliding through each other. The dissipated energy in the switching arc is partly absorbed by the electrical contact surfaces, increasing their temperature to melting and evaporation points. This increase may result in switch failure because of contacts welding when molten contact surfaces are closed. Depending on the dielectric strength of the insulation gas, the prestrike arcing time could be different. SF6 has been the most efficient insulation gas because of its high dielectric strength and thermal properties. However, it should be replaced because it is one of the most potent greenhouse gases. This PhD thesis focuses on making operations in air MV-LBS. A synthetic test circuit for making short-circuit current and a model switch are designed based on standards. Different measurement methods are employed to investigate the switch degradation and failure in short-circuit making operations. Arc emission spectroscopy is performed to characterize the pre-strike arc properties at different closing speeds. The arc dynamic motion concluded from the spatially resolved temperature profiles at different times shows that the arc gets stabilized after a few hundred microseconds in the middle of the switch between contacts. Based on the derived temperature profiles, an indirect way of estimating the arc voltage has been proposed, which was in agreement with the corrected direct measurements. Furthermore, electrical characterization of the making is performed at different stages of the operation. For the model switch, higher dissipated arc energies at the same current level result in higher contact mass loss, while an increase in the closing speed over proportionally reduces the contact degradation. The destructive energy of the pre-strike arc causing contacts degradation is measured to be at least one order of magnitude higher than the energy dissipation by ohmic loss when the current flows between sliding contacts in the closed position. Therefore, it can be concluded that the most destructive impact of making operation occurs at the first half-cycle of the short circuit current where the prestrike arc exists. The results of short circuit current making tests with one half-cycle could be extended to the test conditions according to IEC 62271-103 standard, where the minimum duration of short circuit current flow should be 0.2 s.