A comparison of the short circuit interruption performance using transverse magnetic field contacts and axial magnetic field contacts in low frequency circuits with long arcing times i

Abstract

During the development of a vacuum interrupter for a 63k.A, 15kV generator breaker it was observed that the transverse magnetic field (TMF) contact structure operated well under the difficult short circuit conditions expected from such a circuit breaker. The TMF contact successfully endured the long arcing times associated with no current zero periods of very high asymmetric currents and also interrupted them successfully. This development resulted in the question: how would the interruption performance of axial magnetic field (AMF) contacts compare with the TMF designs for short circuit currents and for long arcing times? A controlled experiment using both contact structures was thus developed. Contacts using TMF and AMF structures with Cu-Cr contacts and the same dimensions were manufactured into experimental vacuum interrupters that had the same diameter. A 12kV, 16Hz ac circuit was arranged using our tuned capacitor circuit in our High Power Test Lab. The two contact structures were then evaluated for symmetric short circuit currents in the range 16kA (rms.) to 2SkA (rms.). It was observed that the TMF contact successfully interrupted the circuit for currents all the way up to the 25kA level 100% of the time. The AMF contact interrupted the circuit 100% of the time for currents up to 22.5kA, but had a distinct drop in its ability at 25kA where it was successful only 80% of the time. These data will be discussed in terms of the quite different vacuum arc characteristics expected from the TMF and AMF contact structures.