Determination of Axial Electric Field Distribution in Blown Arcs With Differential Method

DC switchgear using gas circuit breakers (CBs) in parallel to a LC-resonant path is used for decades already. This “passive oscillation” topology relies on negative damping of the oscillating current made possible by the fact that the voltage drop over the CB decreases with increasing current. This concept is cheap and reliable since it does not require any active components besides the mechanical breaker. However, the achievable breaking times and maximum interruptable current amplitude highly depend on the u(i) characteristic of the axially blown arc inside the breaker. Improvements to the breaking chamber could move the maximal breakable current to higher values and reduce the time to current zero. Analyzing which parameters influence the arc voltage is crucial for this optimization. Knowing which axial segments contribute how much to the total arc voltage enables a deeper understanding and possibilities to improve performance. Direct measurements of voltages inside the nozzle of a CB are not feasible; therefore, an indirect method was developed. The contact positions in a model CB were varied systematically, and by evaluating the u(i) curves of 13 different configurations, the voltage drop of eight segments of 2 cm length was calculated. It is shown that most of the voltage drops over the converging nozzle part, where gas density and acceleration is high. These segments also exhibit beneficial behavior with du/di being negative. The method was validated by comparing the extracted results with suitable direct measurements.