Development of a multiperspective optical measuring system for investigating decaying switching arcs at the nozzle exit of circuit breakers

Abstract

High-voltage gas circuit breakers, which play an important role in the operation and protection of the power grid, function by drawing an arc between two contacts and then extinguishing it by cooling it using a transonic gas flow. Improving the design of circuit breakers requires an understanding of the physical processes in the interruption of the arc, particularly during the zero crossing of the alternating current (the point in time when the arc can be interrupted). Most diagnostic techniques currently available focus on measurement of current, voltage, and gas pressure at defined locations. However, these integral properties do not give sufficient insight into the arc physics. To understand the current interruption process, spatially resolved information about the density, temperature, and conductivity of the arc and surrounding gas flow is needed. Owing to the three-dimensional, unstable nature of the arc in a circuit breaker, especially near current zero, a spatially resolved, tomographic diagnostic technique is required that is capable of freezing the rapid, transient behavior and that is insensitive to the vibrations and electromagnetic interference inherent in the interruption of short-circuit current arcs. Here a new measurement system, based on background-oriented schlieren (BOS) imaging, is presented and assessed. BOS imaging using four beams consisting of white light sources, a background pattern, imaging optics, and a camera permits measurement of the line-of-sight integrated refractive index. Tomographic reconstruction is used to determine the three-dimensional, spatially resolved index of refraction distribution that in turn is used to calculate the density. The quantitative accuracy of a single beam of the BOS setup is verified by using a calibration lens with a known focal length. The ability of the tomographic reconstruction to detect asymmetric features of the arc and surrounding gas flow is assessed semiquantitatively using a nozzle that generates two gas jets, as described in [Exp. Fluids43, 241 (2007)EXFLDU0723-486410.1007/s00348-007-0331-1]. Experiments using a simple model of a circuit breaker, which provides optical access to an ∼1 kA arc that burns between two contacts and is blown through a nozzle system by synthetic air from a high pressure reservoir, are also described. The density in the decaying arc and surrounding gas flow is reconstructed, and the limitations of the technique, which are related to the temporal and spatial resolution, are addressed.

Optical Diagnostics of Switching Arcs Near Current-zero: Speckle Imaging and Interferometry

“…This is because a three-dimensional reconstruction based on a single projection is only possible for an object with defined symmetry, such as axi-symmetry (temporal and spatial fluctuations can also wash out features, but this effect is negligible for single, nanosecond laser pulses). One approach to overcome this problem (using almost the same circuit breaker test device) is described in [5]. In that work, multiple beams were used to probe the index of refraction distribution, permitting, in principle, a three-dimensional reconstruction from the line integrated measurements even in the absence of axi-symmetry.…”

“…Such information can be obtained from computational fluid dynamics (CFD) simulations [1], but such models often lack experimental validation (aside from comparison with the measured current, voltage, and pressure). For this reason, several spatially resolved optical diagnostic methods based on well-established techniques that measure the index of refraction (or its gradient) were recently developed or adapted to image circuit breaker arcs near current zero [2][3][4][5]. The advantages and disadvantages of interferometric techniques compared to other optical methods (such as emission spectroscopy) are discussed in detail in those studies.…”

Measuring Density Profiles of Electrons and Heavy Particles in a Stable Axially Blown Arc

“…[12], many images were acquired and those which were most cylindrically symmetric were selected. Stoffels et al [21] attempted to overcome this problem by developing a tomographic technique with four projections that permits the reconstruction of asymmetric features. However, even with four projections, the spatial resolution was too limited to reconstruct an asymmetric arc and extension to more projections would have added greatly to the cost of the experiment.…”