Abstract
Existing polymeric cable sealing end (CSE) typically contain silicone oil as an insulating liquid between the cable core and the outside of the CSE. In recent years, a number of performance issues have been identified with silicone oil filled CSEs. To understand the reasons for the observed behavior of silicone oil, a range of characterization methods are reviewed, to allow investigation of the influence of moisture, potential chemical contaminants and the compatibility with CSE materials. The water content of all the oils studied was above the limit given in IEC 60836 (2015) for new silicone oil. There is no significant difference between the moisture content, whether the oil is used in CSE or new. Neither did the Fourier infrared (FTIR) spectroscopic analysis show difference between the oils. Preliminary results from gas chromatography connected to mass spectrometer (GC/MS) clearly show structural differences between unused expired oil and new oil samples particularly that species suspected to be cyclic siloxane appear to be present in the expired sample.
Highlights
Silicone oil is used in cable sealing end (CSE) as an insulating liquid, due to its very low dielectric losses and high resistivity
It is hard to replicate the conditions that prevail in the CSE; and in general, the results of electrical testing are very dependent on the test geometry and conditions
The water content is above the limit of 50 mg/kg given in IEC 60836 (2015)
Summary
Silicone oil is used in CSE as an insulating liquid, due to its very low dielectric losses and high resistivity. When considering dielectric properties of liquids, it seems that often the first value presented is breakdown strength as a function of field. It is not appropriate for evaluating the behavior of insulating material in actual applications [1]. The electrode configuration can vary from spherical, through VDE type “mushroom” electrodes to disc electrodes This measurement geometry is not representative of CSE conditions. Electrical testing in a way that oil is subjected to electrical stresses and analysed by other means, is more feasible than judging it on often non reproducible electrical tests in conditions that are not even prevailing in the CSE
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