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Abstract-Previous observations on electrical tree growth in epoxy resins has shown that different types of tree growth structure, electrically conducting and non-conducting, can occur dependent on the state, glassy or flexible, of the epoxy resin. In this current study, the partial discharge characteristics were characterized experimentally at a temperature of 20C within two different epoxy resins systems having glass transition temperatures of 0C and 50C. The partial discharge activity (determined from apparent charge measurements) was characterized in terms of ~q~n patterns using statistical tools. The aim was to compare the apparent charge measurements obtained from conducting and non-conducting electrical tree structures with computer simulations of the partial discharge activity in both conducting and non-conducting electrical trees. The results show that there is a significant relationship between the local extent of the partial discharge phenomena, as determined by the conductivity of the tree channels, and the apparent charge, as shown by the experimental and simulated partial discharge patterns. The implications of this work for partial discharge detection as well as for condition monitoring in real insulating systems are discussed.

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Citation: Chalashkanov, N. M., Dodd, S. J., Dissado, L. A. & Fothergill, J. (2014). Reexamination of the dielectric spectra of epoxy resins: Bulk charge transport and interfacial polarization peaks. IEEE Transactions on Dielectrics and Electrical Insulation, 21(3), pp. 1330 -1341 . doi: 10.1109 /TDEI.2014 This is the unspecified version of the paper.This version of the publication may differ from the final published version. Permanent ABSTRACTThe dielectric properties of two amine cured bisphenol-A epoxy resin systems, Araldite CY1301 and Araldite CY1311 have been characterized using dielectric spectroscopy over the frequency range 1 mHz to 100 kHz. These two epoxy resin systems were chosen to allow the dielectric response to be studied from above and below the glass transition, as Araldite CY1311 is a modified version (with added plasticizer) of Araldite CY1301. The dielectric response was found to comprise both bulk and interfacial features. Above the glass transition temperature, two processes were identified, a low frequency process usually ascribed to interfacial polarization is shown to be a bulk process termed Quasi-DC (QDC) conduction and a dielectric dispersion usually taken to be a bulk process has been shown to be an interfacial effect.

This is the accepted version of the paper.This version of the publication may differ from the final published version. Permanent repository link ABSTRACTSamples of a dried composite oil-paper system were thermally aged in the laboratory under N 2 in a sealed container and analysed using their dielectric response. The contributing loss processes were separated using an equivalent circuit model containing constant high frequency capacitances, dc-conductances, a dispersive capacitance, and a low frequency quasi-dc (q-dc) dispersion process as circuit elements. This low frequency dispersion in oil-paper samples is a bulk property of the system that is often mistaken for a dc conduction process when only the imaginary capacitance or acconductance is measured. The Dissado-Hill response function is proposed for the frequency dependence of the dispersive capacitance in the mid-frequency range rather than the Debye function because its cluster concepts correlate with the sample morphology of a matrix of oil-filled cavities. The quasi-dc dispersion is assigned to the transport of mobile charges between charge-clusters associated with locally connected oil filled cavities, which is a process that can be represented on a global scale by the form of hierarchical circuit system that leads to constant phase angle responses, and is here modelled by the Dissado-Hill q-dc function. It was found that the two dispersions moved together to higher frequency with thermal ageing of the composite material. The characteristic frequency common to the dispersions had an activation energy that decreased with thermal ageing, whereas their amplitude increased. This behaviour is discussed in terms of possible changes to the oil-paper composite caused by the ageing process.

This is the accepted version of the paper.This version of the publication may differ from the final published version. Permanent University of Leicester Department of EngineeringAbstract-The Pulse Sequence Analysis (PSA) was carried on PD data from electrical trees grown in flexible epoxy resins. The samples used for the electrical tree experiments were conditioned in environments with different relative humidity in the range 15-100% prior to the electrical tests and the corresponding moisture content in the samples was between 0.1 and 6.9%. The electrical treeing experiments were carried out at different temperatures in the range 20-70°C. The details of PD dynamics during the electrical tree growth have been found to change significantly with temperature and absorbed moisture. In this paper, it is shown that PSA could be successfully used to discriminate between PD data from electrical trees and runaway breakdown.

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.

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