Influence of grading capacitors on the breaking performance of a 24-kV vacuum breaker series design

This contribution deals with the breaking capability of a series design consisting of two 24-kV vacuum circuit breakers (VCB) with grading capacitors. The investigations were done under worst-case conditions with regard to the rate of rise of the transient recovery voltage (TRV) exceeding the values given in the relevant IEC standard. In previous publications it was shown that the breaking capability of the series arrangement is higher than doubling the voltage of a single tube. In addition to these results it is shown that in some cases grading capacitors can increase the breaking capacity even more. Tests with simultaneous opening of the contacts of both tubes are presented with the application of different grading capacitors as well as the choice of different arcing times. In the case of reignition of one of the tubes the current circulating between the two tubes is shown. As a result of the tests the breaking capability of the arrangement depending on the arcing time and the value of the grading capacitors can be classified into three zones: area of reignition of the arrangement, scatter area and area of successful arc quenching.