The role of bulk charge transport processes in electrical tree formation and breakdown mechanisms in epoxy resins

N M Chalashkanov,S J Dodd,L A Dissado,J.C Fothergill

doi:10.1109/tdei.2016.006141

Abstract

Electrical treeing experiments have been conducted at different temperatures and levels of absorbed moisture in Araldite CY1311 epoxy resin samples above their glass transition temperature, i.e. when the resin was in a flexible state. The fractal dimension of the electrical trees obtained and the rate of tree growth were found to depend on the environmental factors: temperature and humidity. It has also been found that at certain levels of temperature and moisture absorbed in the samples, a transition occurs from electrical treeing degradation to breakdown by thermal runaway. Complementary investigations of the dielectric properties of the same epoxy resin system have revealed that a bulk quasi-dc (QDC) charge transport mechanism takes place above the glass transition temperature, and we show that the characteristic features of the dielectric response are related to the shape of the electrical treeing degradation and the transition to thermal breakdown. This is explained qualitatively through the effect of the bulk QDC charge transport process in modifying the local space charge electric field distribution.

Highlights

ELECTRICAL treeing is a pre-breakdown degradation mechanism that is initiated by local high stress points such as metallic asperities, discharging voids [1], and water trees [1, 2]
The charge deposited at the tips of the electrical tree as a result of the partial discharges (PDs) activity can be effectively transferred deeper into the material by this mechanism, and as a result the field within the tree structure is increased on the same half-cycle as the initial charge depositing PD
The shape and growth rate of electrical trees in epoxy resins has been correlated with their bulk electrical properties, and in particular with the way in which these are affected by temperature and absorbed moisture; increases in both of which have been shown to affect the electrical tree growth in a similar way

Summary

Introduction

ELECTRICAL treeing is a pre-breakdown degradation mechanism that is initiated by local high stress points such as metallic asperities, discharging voids [1], and water trees [1, 2]. Electrical tree propagation is governed by partial discharges (PDs) occurring in the tree channels, or in the case where the channels gain a thin graphitic conducting layer at the top of the channels [3, 4]. The shape of the electrical trees can be characterized by their fractal dimension, df [1, 3]. In some cases a bush tree develops a small number of long branched structures extending from its periphery and is called a bush-branch tree, whose overall fractal dimension can be taken as df≈2. In epoxy resins electrical tree growth has been found to depend on various factors, including applied electric stress [5, 6], frequency of the applied voltage [7], temperature [8], absorbed moisture [9], material state (rubber-like or glassy) [4], and others

Results

Discussion

Conclusion