08_diele_scopy - 1144 A. J. Thomas and T. K. Saha: A New...

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A. J. Thomas and T. K. Saha: A New Dielectric Response Model for Water Tree Degraded XLPE Insulation – Part B 1070-9878/08/$25.00 © 2008 IEEE 1144 A New Dielectric Response Model for Water Tree Degraded XLPE Insulation – Part B: Dielectric Response Interpretation Andrew J. Thomas and Tapan K. Saha University of Queensland School of Information Technology and Electrical Engineering Brisbane, QLD 4072, Australia ABSTRACT Dielectric response measurements, in both time and frequency domains, can generate valuable information about the condition of cables affected by water tree degradation. However, the interpretation of how these dielectric response measurements relate to water tree density and length is a difficult task. This difficulty in assessing in particular the longest water tree length from dielectric response measurements is a substantial limitation, as it is well known that the remaining electrical strength of a water tree degraded cable is related to the longest tree length. This paper will detail a study whose goal was to enhance the understanding of how water tree degradation (in particular long vented trees) influences the dielectric response, and therefore seeks to reduce this interpretation limitation. The paper examines the application of a finite element model for water tree degraded insulation. This application involves using the dynamic electrical behavior as predicted by the model to generate frequency domain dielectric responses. The predicted electrical behaviour of both bow-tie and vented trees and the dielectric response that such trees will produce is examined and compared to actual Frequency Domain Spectroscopy (FDS) measurements on water tree degraded cable samples. It is shown that the developed model can accurately reproduce such measurement behavior, including the non-linear response. Because of the success of the model in reproducing actual dielectric response measurements of water tree degraded cables, it has proven itself to be a useful interpretive tool for water tree detection and assessment. Index Terms water trees, cross linked polyethylene insulation, finite element method, dielectric measurements, space charge, conductivity, dielectric loss, nonlinearities, frequency domain spectroscopy. 1 INTRODUCTION WATER trees can pose serious problems to the reliability of underground distribution networks. Therefore, it is necessary for distribution companies to diagnose their affected cables efficiently. Many studies have been dedicated to diagnosing water tree degraded cables through dielectric response measurements, in both the time and frequency domain. However, while certain good papers give strong indications of levels of water tree degradation, precise or specific knowledge such as the length and density of trees and whether the major contribution is from bow-tie trees or vented trees or both, is still elusive. The way to obtain such knowledge is undoubtedly through a clear understanding of the means in which water trees affect the electrical behaviour of cables. Perhaps the easiest way to understand such behaviour is
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08_diele_scopy - 1144 A. J. Thomas and T. K. Saha: A New...

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