Analysis of Switching Effects of Vacuum Circuit Breaker on Dry-Type Foil-Winding Transformers Validated by Experiments

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.