J.V. Champion

Propagation of electrical tree structures in solid polymeric insulation

Two alternative theoretical approaches to electrical tree propagation exist. Stochastic models attribute tree structures to random probabilistic factors, whereas in the discharge-avalanche model mechanism-driven field fluctuations are responsible. Here we review the predictions of these approaches in the light of the available experimental evidence. It is shown that both models give the fractal structures and the form of structure distribution observed experimentally. The width of the distribution functions predicted are, however, less than those found experimentally. The quantitative formulation available to the physical model also enables it to reproduce several other features of tree propagation such as voltage dependence, growth laws, and discharge behavior patterns. This is not possible in the stochastic approach without mechanistic assumptions which are difficult to relate to the stochastic process. The connection between the discharge-avalanche model and deterministic chaos is explored. Experimental evidence is presented supporting the contention that the electrical treeing phenomenon is the result of a deterministic breakdown mechanism operating in a chaotic regime at fields lower than those required for runaway breakdown. Space-charge deposition and re-arrangement is proposed as the physical origin of the chaotic field fluctuations. Tree shapes are shown to be related to the variation in the fluctuation range available as the tree grows in accord with the predictions of the discharge-avalanche model.

The effect of voltage and material age on the electrical tree growth and breakdown characteristics of epoxy resins

Electrical tree growth (a long-term electrical breakdown process) has been investigated in Araldite CT200 and CT1200 epoxy resins as a function of voltage and material age (defined as the time between manufacture and testing of pin-plane samples). Reproducible and predictable electrical tree growth was obtained for both CT200 and CT1200 epoxy resins provided that (i) the essentially random tree initiation time is removed and (ii) the samples tested were of the same age. The tree growth and time to failure (defined as the time to breakdown from a pre-initiated 10 mu m tree) characteristics as a function of both voltage and sample age show large step changes at a critical voltage and critical age. In particular, the resin physical ageing has a large effect on the tree growth behaviour, with the time to failure varying by three orders of magnitude over a time span of 3 years. Measurements of some of the physical properties (residual internal mechanical stress, surface refractive index, glass transition temperature and dielectric loss) of CT200 epoxy resin all indicate the occurrence of physical ageing of the resin, with structural (network) relaxation as the most important ageing process. However, these measurements are unable to account for the step change (critical age effect) found in the time to failure of tree growth. The fractal nature of tree growth and its relationship with voltage and the long-term changes in the properties of the resin are briefly commented upon.

Analysis and modelling of electrical tree growth in synthetic resins over a wide range of stressing voltage

A simple accumulated damage analysis method and an empirical field-driven tree growth model are proposed to characterize and describe the spatial and temporal development of electrical trees. Examples are presented for trees grown in CT200 and CY1311 epoxy resin pin-plane samples subjected to a wide range of 50 Hz alternating current electrical stress. It is shown that a materials resistance to treeing may be described quantitatively, allowing the relative performance of different synthetic resins to be easily compared. For CY1311 epoxy resin, tree structural characteristics change progressively from branch to bush structures as the stressing voltage is increased. It is shown that the time to failure is primarily influenced by the local electric field and the resultant tree geometry and fractal dimension of tree growth and is not simply dependent on the applied voltage.

Systematic and reproducible partial discharge patterns during electrical tree growth in an epoxy resin

The partial discharge activity (the number of discharges per second) during electrical tree growth in the flexible epoxy resin CY1311 was measured using a phase-resolved synchronous counting system. The experimental conditions, voltage and pin-plane spacing were varied to produce a wide range of tree structures from branch to bush. Systematic and repeatable changes in the partial discharge activity occurred, depending on the experimental conditions and these correlated with the type of tree structure (branch density or fractal dimension) formed. The type of tree growth was principally determined by the applied electric field and the occurrence of regular changes (bursts) in the partial discharge activity which are associated with a sudden and temporary phase shift and broadening of the partial discharge phase distributions. The number of bursts an their dynamics determine the type of tree grown.

The correlation between the partial discharge behaviour and the spatial and temporal development of electrical trees grown in an epoxy resin

Combined partial discharge detection and video monitoring of the tree growth have shown a strong correlation between the partial discharge activity and the spatial and temporal development of electrical tree growth in CY1311 epoxy resin. CCD imaging of the spatial distribution of light emitted, due to partial discharges in the tree structure, has shown that the different modes of partial discharge behaviour reflect their different spatial distribution within the existing tree structure, with new growth occurring at those parts of the tree in which the partial discharges are active. The dynamics of the partial discharge behaviour, namely the frequency and duration of two modes of activity, is controlled by the experimental conditions (voltage and pin - plane spacing) and determines the type (fractal dimension) of the resultant tree. During one mode of activity, rapid low-fractal-dimension radial growth of the tree occurs. During the other mode, new growth occurs at a slower rate from the tree structure near the pin electrode, leading to an increase in the overall fractal dimension of the tree structure.

Simulation of partial discharges in conducting and non-conducting electrical tree structures

Electrical treeing is of interest to the electrical generation, transmission and distribution industries as it is one of the causes of insulation failure in electrical machines, switchgear and transformer bushings. Previous experimental investigations of electrical treeing in epoxy resins have found evidence that the tree structures formed were either electrically conducting or non-conducting, depending on whether the epoxy resin was in a flexible state (above its glass transition temperature) or in the glassy state (below its glass transition temperature). In this paper we extend an existing model, of partial discharges within an arbitrarily defined non-conducting electrical tree structure, to the case of electrical conducting trees. With the inclusion of tree channel conductivity, the partial discharge model could simulate successfully the experimentally observed partial discharge activity occurring in trees grown in both the flexible and glassy epoxy resins. This modelling highlights a fundamental difference in the mechanism of electrical tree growth in flexible and glassy epoxy resins. The much lower resistivities of the tree channels grown in the glassy epoxy resins may be due to conducting decomposition (carbonized) products condensing on the side walls of the existing channels, whereas, in the case of non-conducting tree channels, subsequent discharges within the main branches lead to side-wall erosion and a consequent widening of the tubules. The differing electrical characteristics of the tree tubules also have consequences for the development of diagnostic tools for the early detection of pre-breakdown phenomena.

Quantitative measurement of light emission during the early stages of electrical breakdown in epoxy and unsaturated polyester resins

Quantitative light emission studies of the initiation and early growth stages of electrical treeing in synthetic resins have been undertaken to gain insight into the underlying physical mechanisms responsible for these processes. Mains-synchronous photon detection techniques coupled with an ultra-sensitive photomultiplier and large-area light collection optics were used to measure the low-level light emission from pin-plane CT200 epoxy and polyester specimens subjected to 50 Hz AC step ramp electrical stress. Three types of light emission are observed consistently and correspond to (i) electroluminescence related to charge injection (Fowler-Nordheim or Schottky depending on the local electric field), (ii) microdischarge activity with formation of microchannels and (iii) conventional partial discharge activity during tree growth. No material-dependent threshold voltage/field was found for electroluminescence and hence charge injection in these materials.

An approach to the modelling of partial discharges in electrical trees

A partial discharge model has been developed to simulate experimental partial discharge activity within electrical tree structures grown in polymeric based insulation. A pin plane electrode arrangement and tree were defined on a grid representing an area 2 mm by 2.1 mm. A sinusoidal 50 Hz voltage was applied to the pin tip and 3600 time steps per cycle were used to give a phase resolution of . Partial discharges were simulated at each time step by adding one or more dipoles of charge onto the tree structure to reduce the potential difference between adjacent points in the tree from to . Each dipole of charge represents a local electron avalanche occurring over a distance equal to the grid spacing. The model parameters, and , represent the discharge properties of the decomposition gas. Partial discharge current pulses in the external circuit and the induced image charge were calculated as a function of time. The spatial distribution of the emitted light in the tree structure over one cycle of the applied voltage was also calculated. The close agreement between the calculated and experimental data suggests that the underlying assumptions used in the construction of the partial discharge model are appropriate for the case of electrical trees grown in the flexible epoxy resin.

Morphology and the growth of electrical trees in a propylene/ethylene copolymer

Growth of electrical trees under 50 Hz high electric stress has been studied in a clarified propylene/ethylene copolymer, to explore the effects of the applied field and the material microstructure. Crystallization of the copolymer at low temperatures (<128/spl deg/C) produces a continuous lamellar texture and the material consequently is optically transparent. At higher crystallization temperatures (134/spl deg/C), more sporadic nucleation occurs and, as a result of the larger scale structural features that develop, the material becomes optically scattering. Nevertheless, CCD images of evolving tree structures could be obtained in both systems. Electrical treeing was found to occur reproducibly, but in a markedly different manner in the two morphologically different but chemically identical materials. In the low temperature crystallized copolymer, electrically conducting tree structures were found to develop with a growth rate that increased monotonically with increasing applied voltage. Conversely non-conducting tree structures formed in the 134/spl deg/C crystallized copolymer that mimic the well documented decreasing tree growth rate with increasing applied voltage behavior of both low density polyethylene and a flexible epoxy resin.

An assessment of the effect of externally applied mechanical stress and water absorption on the electrical tree growth behaviour in glassy epoxy resins

The type of electrical tree growth between point and plane electrodes in glassy epoxy resins and times to failure are greatly affected by sample age, namely the time between casting and testing with a 50 Hz electrical stress. The effect has been attributed to structural relaxation in the polymer, characterized by the magnitude of the residual internal mechanical stress (RIMS). It is now shown that (i) varying the RIMS by the application of an external mechanical stress (tensional or compressional) has no significant effect on the rate of tree growth and (ii) storing samples in water affected the RIMS and the rate of tree growth. It appears that water absorption has a greater effect than does mechanical stress on electrical tree growth in these epoxy resins.