01_multistres_epoxy - IEEE Transactions on Dielectrics and...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 8 No. 2, April 2001 COMMUNICATION On Multistress Aging of Epoxy Resins PD and Temperature M. Di Lorenzo del Casale Universiti di Palermo Dipartimento di Ingegneria Elcttrica Palermo, Italy ABSTRACT The results of partial discharge (PD) aging at different temperatures performed by means of PD electrical detection on specimens with a flat cavity are reported. First, polymethylmethacrylate (PMMA) resin was used to set up a suitable test methodology under multistress conditions, e.g. in a divergent electrical field and at a temperature higher than the ambient. Next, an epoxy resin of electrical and industrial interest was tested. To analyze the aging phenomena during the life tests and to identify the PD statistical features, physical and statistical off-line analysis of the acquired data were carried out, Temperature strongly changes PD activity in pulse num- ber and amplitude, affecting the material lifetime as well. At the examined test conditions, the skewness parameter applied to the PD phase distribution was found to be useful in the dielectric aging progression analysis, as was found earlier by other authors. The reported re- sults are considered useful for the PD performance study of electrical systems under working conditions. 1 INTRODUCTION HE study of the partial discharge (PD) mechanism within gas filled T voids in solid insulation has received considerable attention in the past [l, 21. However, PD research in most cases is confined to studies performed at ambient temperature and very little data are available of PD studies performed at higher temperatures, e.g. at real working con- ditions of many materials employed in electrical machine manufacture. Therefore the underlying study was focused on aging mechanisms due to pulse-shape discharges in different sizes of flat cavities at different thermal conditions. The final goal was the formulation of numerical life models for electrical power components. During the first stage of this work, the standard Cigr6 Method 11, with an air gap of 0.125 mm, was used to test a polymethylmethacry- late (PMMA) resin in order to setup a suitable methodology [3]. Next, the cell air gap size was doubled to determine its influence on discharge activity both at ambient and higher temperatures [4]. Furthermore, an epoxy resin of electrical and industrial interest was tested using a cell air gap s=0.250 mm. For complete PD phenomena characterization, physical and statistical parameters were studied for a reliable interpre- tation of data. 299 distribution H,(cp), mean charge distribution as a function of volt- a e phase Hqm(p) and total charge distribution as function of voltage surements were performed by a statistical discharge phase analyzer built in our laboratory [6]. The test cell assembly employed was the CIGRE specimen holder (Method 11) depicted in Figure 1.
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01_multistres_epoxy - IEEE Transactions on Dielectrics and...

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