SF6 high voltage circuit breaker contact systems under lightning impulse and very fast transient voltage stress

In the past few years gas insulated high voltage circuit breakers have improved considerably, in particular with respect to required drive energy for operation, compactness and reliability. A further reduction in size implies increasing operational field stress inside the breaker and, thus, a reduction in built-in safety margins. If the design comes closer to its physical limits, in particular the dielectric coordination of the contact systems becomes more and more challenging. The aim of the present work is to identify factors that may influence the dielectric coordination of contact systems with reduced coordination margin. The breakdown probability distribution of SF₆ insulated multi-contact systems was investigated with the focus on two aspects. First of all, the breakdown probabilities under lightning impulse (LI) stress were measured. These indicated that it is not feasible to extrapolate the breakdown probability curves of single contacts to a combined full contact system. Secondly, the findings on the same contact system stressed with very fast transient voltages (VFT) showed a considerable reduction in breakdown voltage and suggested a significant increase in the main contact breakdown probability, compared to the LI tests. The results and interpretation within this work identified the polarity and voltage shape dependent breakdown initiation of different contacts as causes for circuit breaker contact system failure.