Dielectric testing of equipment is performed for different duties including the lightning impulse voltage (LIV). In cases where vacuum interrupters (VI) are used, the determination of the critical LIV value poses challenges due to the statistical nature of the breakdown in vacuum [1, 2]. Consequently, it is required to evaluate the dielectric behavior of a large number of specimens in order to obtain statistically significant values for the breakdown voltage. Especially during early development testing and for quality control procedures that provide statistical significance even with smaller test batches are highly desirable. The idea behind the methods presented here is, that tests that yield a numerical value for the breakdown voltage provide greater information as compared to those that are based on a pass/fail criterion [1, 2]. In this paper, the always breakdown method was used in dielectric testing to create a set of numerical values for a number of VIs. Experimental data were reviewed and converted into a function relating breakdown probability to the applied LIV voltage.Furthermore, the statistical significance required for smaller test batches is established by assuming that a certain critical level needs to be fulfilled with a high probability. In the case of dielectric characterization, this means whether a breakdown value $\mathrm{U}_{\mathrm{d}}$ is above or below a critical value $\mathrm{U}_{\mathrm{c}}$ (limit voltage). The goal is to determine the probability of a larger number of pieces (e.g. a batch) having a breakdown voltage greater than the critical value. This method produces two probabilities as a result and defines a range or two trendlines in which a derived ratio R can be compared by means of sequential testing. Here R respectively log(R) describes the ratio of the likelihoods of a positive or negative assignment. With the help of the position of R with respect to the trendline, the desired statistical significance can be achieved.

Low current vacuum arcs have the natural tendency to interrupt currents before the natural current zero of a system, which is known as current chopping. This paper presents results of investigations comparing the chopping current performance of isotropic and anisotropic WCCu contacts for two compositions (WCCu 60 wt.% and WCCu 79 wt.%). The goal is to show the influence of the contact microstructure, and the role of the associated increase in the electrical and thermal conductivities of the anisotropic material on the chopping current. The results indicate an improved median chopping current value for the anisotropic compositions compared to the isotropic ones. It is lower of about 1A for WCCu 60 wt.% (5.1A vs 6.0 A), while is it is only slightly lower for WCCu 79 wt.% (9.0 A vs 9.4 A). The SEM investigations of the top surface structure do not show significant changes after a few arcing operations at 75A peak current between the two types of microstructures. It is assumed that the current flow localized below the contact surface is affected by the microstructure in a way that favors local heating at the last cathode spots and help to sustain the arc stability.

Due to the impact of arcing on the high purity surfaces and bound residual gases, current interruption operations may influence the internal pressure in vacuum interrupters (VIs). In this paper, we investigate the temporal development of internal pressure in VIs immediately after a current interruption operation, non-intrusively using the magnetron method. By means of an experimental test setup that consists of a vacuum test system to vary the internal pressure in punctured, non-vented VIs with active getter materials, experimental test series were performed under variation of internal pressure and arcing time. The experimental results indicate that arcing during the interruption operation predominantly causes a significant decrease in the internal pressure of up to two orders of magnitude, depending on the arcing time and the initial pressure before the current interruption operation. We also discuss the impact of the observed pressure reduction effect on the possibility of predicting the remaining service life of installed VIs.

Depending on the operating current and the ambient conditions, the operating temperature of vacuum interrupters (VIs) may vary. An increase in VI temperature can result from heat loss at electrical contact resistance points, whereas a significant decrease in temperature will especially occur if VIs are used in future applications in a liquid nitrogen (LN2) environment (e.g. in combination with high-temperature superconducting equipment). In both cases of increased and decreased VI temperature, the pressure inside the VI may vary due to the adsorption and desorption of bound residual gases and changing material properties. This paper addresses the influence of VI temperature on internal pressure and on the applicability of the magnetron method for the measurement of internal pressure. By means of experimental test series that varied the internal pressure in punctured (i.e. non-vented) VIs and the ambient temperature, we found that an increase in VI temperature leads to a limited increase in internal pressure due to desorption of bound residual gases. Correspondingly, immersing the VI into LN2 causes a significant adsorption of residual gases, resulting in a pressure decrease of up to several orders of magnitude. In both cases the magnetron method is generally applicable, although ignition behavior is considerably affected by adsorption in an LN2 environment.

The determination of the spatial arc resistance distribution is one option to gain deeper insight into the physical processes during current interruption in high voltage circuit breakers. A measurement system capable of measuring this spatial arc resistance distribution by means of capacitive field probes is applied to a circuit breaker model equipped with a Laval nozzle geometry which has previously been investigated using optical measuring methods. According to these investigations, which serve as reference case, the spatial arc resistance distribution is determined for varying current steepness values. Comparing the results a general agreement between the determined arc resistance distributions and the results from the reference case is observed. The functionality of the measurement system is confirmed for both successful and non-successful switching operations. Uncertainties and differences in the obtained arc resistance distributions are discussed.

In the coming years an increasing quantity of vacuum interrupters (VI) is expected to reach the end of its service life. Leading manufacturers can provide a service life of up to 30 years due to high quality standards and sophisticated production processes. However, their operating performance observed so far indicates the possibility to extend this service life, in case of non-intrusive diagnosis methods being available for the assessment of the remaining service life and the simultaneous estimation of the technical risks. Amongst other factors especially the internal pressure is decisive for the condition of the VI. However, currently available test devices only provide qualitative information, which is insufficient to provide a basis for the assessment of the condition of vacuum switchgear and for a decision concerning potential extension of the service life. This contribution addresses a mobile test device based on the magnetron method for the quantitative assessment of the internal pressure in VIs. It identifies the magnetic flux density and its homogeneity as critical influencing factors in the design of a mobile magnetron-based test device. Based on the results, a simplified, exemplary prototype for on-site application is designed. The application of this device on industrially manufactured VIs verifies its functionality and generally demonstrates the proof of concept.

The magnetron method is well established for the internal pressure measurement of vacuum interrupters. Its application during the manufacturing process is essential for the determination of the vacuum quality. The evaluation of the magnetron measurement requires precise knowledge of the residual gas composition and the influence of the metallic surfaces in the vacuum interrupter due to gas binding effects. Leading manufacturers having gained wide experience applying the magnetron method do consider these influences during the production process. However, it is also possible to measure the internal pressure in a vacuum interrupter by means of the magnetron method after a considerable long time in service, e.g. close to the end of the prospective service life. As the residual gas composition in these states is not precisely known, uncertainties in the pressure measurement can arise. Experiments with common residual gases and their typical compositions, which can form inside a vacuum interrupter, show that the uncertainty is less than one decade. In addition experiments performed on vacuum interrupters identify the temporary gas binding due to the Getter-Ion Effect as a factor that considerably limits the repeatability of the magnetron measurement.

Circuit breakers and gas insulated switchgear (GIS) filled with SF6 (sulphur hexafluoride) represent the state of the art in high voltage switching equipment. Nevertheless the discussion on climate change forces the search for possible SF6 substitutes resulting in first CO2 (carbon dioxide) filled circuit breakers and circuit breaker prototypes available. Therefore the optimization of the switching chamber for the thermal and thermodynamic properties of CO2 is in the focus of this contribution. For this purpose nozzle systems with two heating channels supplied from two heating volumes are investigated. Their thermal interruption capability is determined at varying filling pressures from p = 0.35 MPa to 1 MPa and current amplitudes of Ipeak > 20 kA. Here the maximum thermal interruption capability is found for an absolute filling pressure of p = 1 MPa. The experimental results are compared to those of CFD (computational fluid dynamics) simulations and first design criteria for switching chambers with two heating channels are determined.