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Unformatted text preview: About the dimensioning of a needle plane electrode arrangement for comparative investigations of partial dischar!ges in air. W. Pfeiffer, R. Plessow Institut fur Hochspannungs- und Meljtechnik Darmstadt University of Technology Landgraf-Georg-Str. 4 64283 Darmstadt Gemany Abstract: Needle plane electrode arrangements are often used to perform basic investigations of the discharge physics. The main reasons and advantages are, that any kind of parameters influencing the measurement are well known and also limited in number, if compared to solid insulation. However, taking into account the existing parameters of this arrangement will lead to the assessment, that commonly an important part is neglected. Usually only the needle-tip- radius, the gap distance and the magnitude of the voltage-stress are used for specification of the arrangement. This is due to the fact that based on those data the electrical field is calculated by a simplified equation and the values are used to evaluate the measurement results. But in practice the needle has to be mounted and connected within a conductive part. This may lead to a strong influence on the field distribution. By the use of finite element method (FEM) the performance of the needle-plane arrangement, as used in practice, is investigated. The result is, that the calculation of the electrical field is easily possible by this equation if some requirements are fulfilled. Introduction Development and quality improvement of insulating materials closely depends on testing procedures via suitable methods. To N. Kolev,, P. Darjanov, D. Darjanova Faculty of Automation Technical IJniversity Sofia 1756 Sofia Bulgaria ensure the insu1atio:n lifetime when in service, a widely used method is to measure the partial discharge (PD) inception voltage Ui. There are certain reasons to perform such tests under non homogeneous electrical field stress. The main reason is to consider the physical phenomena being responsible for the PD initiation and its fmher development. A needle-plane electrode arrangement seems to be usefid to reproduce the technical requirements for the necessary non homogeneous fields. From practical viewpoint a needle plane electrode arrangement is quite realistic to model the discharge phenomena between two conductors, starting from the glow discharge initiation and its further transition into spark or complete breakdown of the gap. Compared with real insulating arrangements, one conductor can be looked upon as a plane while a surface discontinuity (protrusion) can play the role of the needle. Additionally to evaluate some measurement results one should be able to calculate the field of the stressed arrangement easily, by using a simple equation. Such an equation (l), derived theoretically under the assumption that the needle has a fonn of hyperboloid, is recommended in [Florkowska]: 30 1 Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY KANPUR. Downloaded on August 15,2010 at 13:10:20 UTC from IEEE Xplore. Restrictions apply. Restrictions apply....
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- Spring '11