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Dodd, Stephen John (1992) Electrical breakdown and structural characterization in synthetic resins using optical techniques. Doctoral thesis, City of London Polytechnic.
As part of a National Power/National Grid funded collaborative project on electrical breakdown in synthetic resins, various non-evasive optical techniques were developed to quantitatively characterise resin microstructure and residual internal mechanical (RIMS) and to study the physical processes occurring during electrical tree initiation and tree growth in the pin-plane geometry.
Light scattering measurements have been used to determine the origins and magnitudes of the microstructure content (MC) of the Epoxy (CT200) and unsaturated polyester rains, and in conjunction with FTIR spectroscopy, the polyester resin cure reaction kinetics have been established.
Polarisation microscopy with the aid of an interpretive fictive force model have enabled the RIMS to be quantified for the first time. It is shown that the RIMS relaxes with a time constant of some four years for CT200 and polyester rains and is associated with structural relaxation (physical ageing). The large variations of MC and RIMS depend on the supplied material, preparation procedures, sample age etc., and have important ramifications for any study of electrical tree initiation and growth.
Light emission measurements, using phase synchronous photon counting and CCD imaging during tree initiation and growth shows that three distinct types of emission occur. Type A; associated with charge injection processes which may be explained in terms of a deep trap charge recombination model. Type B; with microdischarges as a prelude to tree propagation and Type C; associated with the conventional partial discharge activity in growing tree channels. Electrical tree initiation follows slow material degradation and debonding of the metal/polymer interface with the formation of the first microchannels.
A study of tree initiation and growth in the CY1311 epoxy resin details some of the many factors that influence electrical breakdown in resins. The early tree growth is shown to be driven by the high electrical field near the pin tip, which is itself dependent on the pin-plane electrode separation as well as the applied voltage. Significantly, at least two different tree growth mechanisms occur and compressive mechanical stress acts to retard tree initiation. Tree growth may also be impeded by high compressive or tensile mechanical stress and a region of zero RIMS may completely passivate a growing tree.
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