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

Abstract

DC switchgear using gas circuit breakers (CB) 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 (CZ). 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 circuit breaker 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.

Controlling the differential resistance of axial blown arcs

“…Since the model breaks down in the downstream section, this was done only up to position 100 mm, which is just upstream the throat and shown in figure 10b. These values are around 400 V to 600 V, which agrees well with the experimental arc voltage up to the throat position as shown in [5], which were in the same range for the flat part of the u(i) curve.…”

“…There is little variation in the converging segment, only around the throat there are significant changes. Figure 6 shows the electric field as function of axial position, which is in line with the results of [5]. The highest field is around the throat, with a gradual increase towards that position in the upstream part.…”

“…This energy balance equation reduces the free parameters to one. For each plasma temperature, there is only one possible arc area that fulfills equations (1) to (5), and vice versa. To get a unique solution for T and A, the combination is chosen which minimises the voltage drop over that segment.…”

“…This clearly violates the second law of thermodynamics, and therefore the downstream segment was not used in the evaluation. This flaw of the model was tolerated, since previous work showed that the converging segments are the ones which contribute most to the overall arc voltage [5], and the downstream segments are not of high interest.…”

“…Previous work by the author includes investigating the influence of nozzle wall ablation [4] and determining which axial segment of the arc contributes how much to the total arc voltage [5]. Both papers indicated that ablation onset might be unfavorable for the u(i) characteristics, and that the voltage drop of upstream segments is higher and additionally has more A c c e p t e d M a n u s c r i p t 2 negative du/di.…”