In this paper, the prestrike and the restrike effects during switching a vacuum circuit breaker (VCB) connected to a dry-type foil-winding transformer are theoretically and experimentally analyzed. The analysis is conducted by performing a wide range of experimental three-phase switching ON/OFF tests for a 3.75-MVA prototype test transformer connected to a VCB by a cable with different lengths and for different loading conditions. A refined transformer model is successfully applied for the computation of the voltage distribution along the transformer windings. In particular, the winding properties are separately investigated, for which the core influence for frequencies above some tens of kilohertz is studied, and the terminal impedance variation for a broad frequency range is accurately calculated. The simulated results are excellent. Based on these results, an analysis was performed to evaluate the severity of the switching operations by applying spectral analysis of the recorded signals. The statistical analysis of the overvoltages with respect to the cable length and the load value shows that long cables do not guarantee lower overvoltages because of the load and its interaction with the system that makes each case unique.

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While the short-circuit current of vacuum interrupters (VIs) is a key performance parameter, very high current operations rarely occur in actual applications. In contrast, switching load and/or capacitive currents comprise virtually all the practical operations. Load current switching requires a low contact erosion and low metal vapor generation of the arc to achieve a long electrical life and is achieved by contacts in vacuum. However, the excellent performance of contacts in vacuum makes the testing and determination of the ultimate electrical life at load currents challenging. Tests on Cu-Cr and Cu-W contacts in VIs were performed at 2500-2700 A rms with 20 000-30 000 switching operations. The VIs with both types of contacts retained their dielectric strength, contact shape and composition, and low contact resistance. Even after arcing operations with total transferred charges >200 kA • s per interrupter, the contacts could continue providing good dielectric and switching performance with low resistance. Contact erosion was minimal and of limited practical significance. These results demonstrate the very long electrical life of VIs when the vacuum arc is naturally diffuse.

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