Sarath Kumara

Surface charge decay on polymeric materials under different neutralization modes in air

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 charge decay on HTV silicone rubber: effect of material treatment by corona discharges

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.

DC flashover characteristics of a polymeric insulator in presence of surface charges

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.

Charging of Polymeric Surfaces by Positive Impulse Corona

This paper focuses on analysis of charging of polymeric surfaces by means of impulse corona discharges in air. Internal (space charge densities and electric fields) as well as external (circuit current) characteristics of corona in a point-plane electrode configuration are investigated by means of computer simulations. Two types of onset positive corona modes, namely positive glow corona and burst pulse corona are identified. The developed and verified computer model is further used to study corona charging of a 2 mm thick polymeric material sample. Both the mechanism of charge deposition and distribution of deposited charges on the surface are dependent on the mode of the corona discharge used. In the case of glow corona, charge generation is limited to the anode region and the generated charges move towards the sample surface under applied electric field. Thereafter the deposited charge cloud expands radially along a portion of the surface with fairly constant concentration. In the case of burst pulse corona, series of positive charge clouds start from the anode and move towards the sample surface in a wave-like manner. Each burst contributes to the deposited charge, which spreads over the surface less extensively than that observed during glow corona charging.

Simulation of surface charge effect on impulse flashover characteristics of outdoor polymeric insulators

Effect of pre-deposited surface charges on impulse flashover characteristics of a polymeric insulator is studied by means of computer simulations utilizing a model based on criteria for streamer inception, propagation and sustenance. A parametric study is performed to investigate the influences imposed by magnitude and polarity of the charge, its location on insulator surface and properties of insulator material on its flashover voltages. The obtained results are analyzed in terms of electric field distributions in the vicinity of the insulator surface and are compared with experimental data available in the literature.

Surface charges on cylindrical polymeric insulators

Distributions of charges are investigated on surfaces of polymeric cylindrical insulators that emerge due to dc corona discharges in surrounding air and by stressing with dc voltage. The studied model insulator consisted of a glass fiber reinforced epoxy core covered with a sheath of silicone rubber, terminated by metallic electrodes with rounded smooth edges. Surface potential distributions were measured 1 min after completing the charging process by means of a Kelvin electrostatic probe connected to an electrostatic voltmeter. The measured surface potential profiles were utilized to calculate corresponding charge density distributions. The obtained results demonstrated significant differences in the surface charge patterns for different charging conditions, which in turn may affect flashover withstand of the insulator.

Calculation of corona voltage-current characteristics in air

This paper focuses on numerical simulations of positive dc corona in air in coaxial cylindrical electrode arrangement. The study aimed at developing a computer model accounting for background physical processes in corona discharge and its verification by means of comparison of calculated corona characteristics with experimental data. Several parameters of corona, namely, inception voltages, voltage current characteristics and onset field strength values were used for evaluations of the model performance. The validated model was further utilized to investigate the effects of ion mobilities on corona current and variations of the ionization intensity on corona inception voltages.

Surface charge relaxation in silicone rubber and EPDM

The paper focuses on analyzing relaxation of charges deposited on surfaces of silicone rubber and ethylene propylene diane rubber samples by positive impulse corona in air. The developed computer model accounted for gas neutralization of the surface charges, their injection and transport in the bulk of solid materials. The simulations showed that surface charge decay takes place mostly due to neutralization by atmospheric ions while charge injection and transport in the solids led to formation of homo charge layers at interfaces. Characteristics of surface charge decay were found to be similar for both materials in the beginning of the relaxation process and slightly different at long times. The model results are in agreement with surface potential decay measurements.

Modifications of dielectric properties of polyamides induced by long-term exposure to corona

Long-time exposure of polyamides to energetic discharges and oxidizing gases can cause modifications of their dielectric properties. To quantify the effect, samples of pure and filled polyamides were aged in a corona treatment system. The corona induced changes in complex permittivity of both materials were investigated utilizing frequency domain dielectric spectroscopy. It was found that both the real and imaginary parts of the complex permittivity increased after the treatment, especially at low frequencies. This was associated with increased dc volume conductivities measured for both materials. The experimental data were analyzed by using a model incorporating Havriliak-Negami dipolar relaxation, dc conduction and low frequency dispersion for identifying the most affected by corona treatment processes.