Performance and testing of multi-unit circuit breakers switching low inductive currents

Earlier studies of the performance of a single gas-blast interrupter when switching low inductive currents have been extended to multi-unit circuit breakers. Results for two and four interrupters in series have been compared under equivalent unit capacitance conditions. In the practical case, when most parallel capacitance is connected across a full phase of the circuit breaker, the interrupters can be considered as operating independently in determining the overall stability of the string. Both average instability and average chopped current levels are consequently lower than for individual units. If the grading components are connected across individual units, a complex interaction between the interrupters and the network is observed. However, the results for the last unit to clear for each operation are also well described by a model assuming that units operate independently. Other published data on air-blast circuit breakers comprising four and eight interrupters and oil circuit breakers, with up to four units, can also be explained on the basis of an independent stability model. Data from oil circuit breakers confirm that the statistical spread of the chopping level is reduced by series connection as predicted by the simple model.