Electrical trees in silicone gel: A combination of liquid and solid behaviour patterns

Abstract

Electrical trees in liquids (streamers) occur on the nano-second timescale and produce structures that dissipate on voltage removal and sometimes even during voltage application. In AC fields the structures are a combination of fine filaments from the positive half-cycle and spheroidal cavities from the negative half-cycle. In contrast electrical trees in solids are permanent filamentary structures with a fractal geometry that grow on timescales of hours or longer at typical field values. Here we present the results of tree formation in a silicone gel under an AC applied electric field. These grow on timescales of minutes and possess a fine branched filamentary structure as well as spheroidal cavities. As in liquids, the cavities can collapse during tree growth. In contrast the filamentary structure is permanent as in solids. However the whole tree contracts following the removal of the applied voltage. The observed stability of the partial self-healing tree features are discussed in terms of the mixed liquid and solid features of the gel structure.

Self-healing during electrical treeing: A feature of the two-phase liquid-solid nature of silicone gels

“…[5,[7][8][9][10][28][29][30]). However the behaviour of gels has received little attention [2][3][4]. The data obtained here indicates that the Low Molecular Weight (LMW) liquid content of the samples is instrumental in determining the structure and behaviour of the electrical trees in the gel samples whereas in the silicone rubber elastomer the flexibility of the 3D matrix becomes important.…”

“…Samples with ratios between 1:0 (pure liquid of part A) and 0:1 (pure liquid of part B) have been studied using FTIR and dynamical mechanical analysis to characterize the physicochemical nature of the samples. FTIR shows that the maximum amount of cross-linking was not located at the 1:1 ratio used for the gels previously studied [4]. Unexpectedly therefore increasing part A close to 2:1 ratio yielded a highly cross-linked silicone elastomer, reaching the highest curing efficiency between active agents inside each liquid precursor component.…”

“…In liquids, PDs cause jets or "streamer" projections from an initiating point electrode constituted by ionized gaseous cavities that disappear quickly, usually after some microseconds (μs) [5] at atmospheric pressure. The semi-liquid, semi-solid, structure of a silicone gel means that both of these features can be expected when it is exposed to fields high enough to cause electrical tree formation [4]. For this reason silicone gels are interesting to study in terms of the formation of electrical trees and sample failure, and also electrical ageing.…”

“…The mixture is cured to ensure the final desired sticky and soft polymer, with a curing time that depends on the curing temperature. In a preliminary study [4], we presented results for these gels showing the self-healing behavior that is observed under electrical treeing testing. In this work we purposely change this ratio, obtaining a continuous set of samples with different physicochemical properties.…”

“…The self-healing capability of silicone gels is of some importance in high voltage (HV) applications where partial selfhealing has been observed when the gel was subjected to partial discharges (PDs) [2][3][4]. In a solid polymer, a cavity resulting from PDs will be a permanent defect and usually will propagate and grow as a fractal or tree structure leading eventually to failure.…”