Michael Weuffel

Influence of the Residual Gas Composition and the Getter-Ion Effect on the Measurement of the Internal Pressure in Vacuum Interrupters

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.

Magnetron-based on-site measurement of the internal pressure in vacuum interrupters

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 influence of current interruption operations on internal pressure in vacuum interrupters

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.

The influence of temperature on internal pressure in vacuum interrupters in a liquid nitrogen environment

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.