Evolution of Gas Composition under Switching of Fault Currents in a 420 kV SF6-free Circuit Breaker

SF6-free circuit breaker when switching fault currents in the high kiloampere range. The investigation is based on an extensive development and type testing series performed on a commercial circuit breaker design, combined with offline gas analysis in a laboratory. We draw conclusions on the influence of gas composition on the electrical lifetime of the circuit breaker.

For this study, the change in concentration of C4-FN and the formation of decomposition products such as carbon monoxide CO is analyzed and correlated with the test duty. The total arc energy is determined from the voltage and current traces of the test and correlated with the changes in gas composition. While the changes are clearly measurable, e.g., formation of CO in the percentage range after 100% short circuit current terminal faults, no significant reduction in performance was observed based on the dielectric and arc quenching performance of the resulting mixture.

The gas composition measured after the power tests is compared with the limit composition defined in safety data sheets for the CO2/O2/C4-FN mixture. Directly after power testing with very high arc energy, the gas classifies as heavily arced. For all in-service conditions found in the field, the normally arced condition applies.