| High voltage composite insulation and its sensitivity to moisture absorption was studied for various materials and conditions. The influence of ions in the water penetration of composite insulation specimens, partially immersed in water, is reported. The uptake of water by such specimens under the application of an electric field has been shown to depend on the nature and concentration of ions in the water. In general, increasing the salt concentration increased the water absorption. Water is believed to ingress the dielectrics by diffusion in the absence of an electric field and also by diffusion and dielectrophoresis with ionised water in the presence of a field. The effect of the frequency of the applied field on water ingress is presented for a range of frequencies from 5kHz to 15kHz with a constant applied voltage of 5kV[p-p]. The uptake was found to increase with frequency up to about 10kHz and then decrease at higher frequencies. This is attributed to the frequency response of the composite material to the mechanical stresses occurring during water ingress. Since the durability of composites is dictated by the durability of the components and the interface, the effect of various reinforcements, having different adhesive strengths with resin, on moisture absorption is illustrated. The field distribution within the composite specimen and its relationship to water absorption is investigated. Regarding the role of temperature in water uptake, glass fibre composite and pure polyester specimens were completely immersed in water under ambient and high temperatures. The results showed an increase of water ingress with the increase in temperature. The role of moisture in the initiation of water trees in various composites is presented via a number of photographic images demonstrating the shapes of these trees. A study of the effect of absorbed water on the growth and configuration of electrical trees in composite insulation is also described using the point-plane specimen arrangement. The trees were grown to demonstrate the response of composite materials under wet and dry conditions. The speed of tree growth between the electrodes and at the barrier was measured under a variety of experimental conditions. The relationship between the barrier type and its resistance to tree growth when the composite had been subjected to water was investigated. The change in the internal strain pattern is used to monitor the effect of water on the material resistance to electrical treeing phenomenon. The mechanical strength of adhesion between the reinforcing material and the surrounding resin was measured for dry and wet specimens using standard mechanical tensile tests. The effect of moisture absorption, both on tree growth and mechanical properties was conducive with the water acting as a plasticising agent. To confirm this hypothesis, composite samples containing 50% of a plasticising polyester were also studied and comparative observations made. |
