A review of the current state of understanding of dielectric mixture properties, and approaches to use numerical calculations for their modeling are presented. It is shown that interfacial polarization can yield different non-Debye dielectric responses depending on the properties of the constituents, their concentrations and geometrical arrangements. Future challenges on the subject are also discussed.

Effect of surface charges on dc flashover characteristics of a composite polymeric insulator is studied by means of experiments and theoretical calculations. The considered insulator consisted of a glass fiber reinforced epoxy core covered with a layer of silicone rubber and terminated by metallic electrodes with rounded smooth edges. In the experiments, the insulator surface was charged by external corona while keeping the electrodes grounded and different charging levels were realized by varying its intensity. A series of disruptive discharge tests were carried out on the charged insulator under negative dc voltages. It was revealed that negative deposited surface charges led to an enhancement of the flashover performance whereas positive ones reduced the flashover voltage level. A theoretical model has been developed and utilized for analyzing the experimental results. In the model, surface charge density profiles deduced from measured surface potential distributions were used as boundary conditions for calculations of electric fields. The measured and calculated flashover voltages were found to be in agreement indicating that the observed variations in the flashover characteristics could be attributed to the modifications of the electric field produced by the surface charges.

Surface charge decay on thick flat samples of HTV silicone rubbers charged by impulse corona is studied. In the experiments, surfaces of the materials exposed to corona were kept open to ambient air whereas the opposite surfaces were in contact with a grounded copper plate and surface potential distributions on the samples were measured using Kelvin type electrostatic probe. The developed procedure allowed for implementation of three study cases when (i) neutralization of pre-deposited charges by free ions present in air was prevented and surface potential decay occurred mainly due to bulk neutralization; (ii) gas neutralization took place under natural conditions and (iii) gas neutralization was enhanced due to increased amount of free ions in ambient air provided by nearby corona. Potential decay observed only due to bulk neutralization was used to evaluate voltage dependent conductivity of the materials and allowed for comparing them with those measured by the standard method. Comparison of decay characteristics observed for different test conditions were used to evaluate the relative importance of each mechanism on the total process of charge decay.

Surface potential dynamics on corona charged surfaces of thick samples of EPDM and silicone rubbers have been studied and analyzed by examining the so called tdV/dt characteristics. This methodology allows for directly obtaining information about charge trap distributions in materials from measurements of potential decay without a-priori knowledge about trap parameters. More importantly, the combination of the above mentioned methodology in conjunction with measurements of other material parameters may provide information desired when analyzing the surface charge behavior on composite insulators for HVDC applications.

This paper presents how the frequency response analysis (FRA) measurements on a transformer can be influenced by magnetization condition of the core. Measurements were performed on two transformers at different magnetization levels to show effects of remanent magnetization in the core due to removal of 3-supply, of relaxation demagnetization and of dc flux on the frequency response of winding impedance. The most important observations were that any sudden change of excitation field (applying or removing magnetization current) yields a slowly varying magnetic relaxation, which causes the impedance to change with time due to magnetic viscosity. Results of diagnostic measurements of transformer winding impedance can therefore depend on the level of remanent magnetic flux appearing in the core limbs after switching-off a transformer. These observations lead to a conclusion that, for avoiding the effects of core magnetization, diagnostic FRA measurements on power transformers should not be performed directly after disconnection from network.

Surface charge decay on thick flat samples of high temperature vulcanized silicone rubber is studied prior and after ac and dc corona pre-treatments. It is found that the charge decay rate on the material exposed to ac corona becomes much higher and sensitive to moisture content in the surrounding air. These features are associated with an increased surface conductivity and formation of a silica-like layer on the polymeric surface, both resulting from ac corona treatment. In contrast, characteristics of the charge decay on the material exposed to dc corona are found to be similar to that measured on untreated samples.