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

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

Surface charges have the effect of changing the electric field over spacers in compressed gases, which may affect the stable operation of gas insulated equipment and could possibly trigger a catastrophic surface flashover without warning. In this paper, we review the literature on surface charge accumulation and related surface flashover phenomenon on spacers in compressed gases. A summary of experimental research of surface charge accumulation is presented and, the surface charge accumulation mechanism is also summarized, with some newly identified factors in mind that may affect surface charge transport and measurements thereof. Subsequently, the correlation between surface charge accumulation and the surface flashover phenomenon is thoroughly discussed, and the effect of surface charges on surface breakdown under applied voltage pulses of various waveforms is analyzed. Finally, several important research topics in this area are proposed. In addition, some ideas on potential surface charge control methods are presented to aid the design of a better spacer free from surface charges in future studies.

Understanding surface charge accumulation and surface flashover on spacers in compressed gas insulation

This report intends to reveal the role of electron migration and its effects in triggering direct current (DC) surface flashover under temperature gradient conditions when using epoxy-based insulating composites. The surface potential and the surface flashover voltage are both measured using insulators that are bridged between two thermo-regulated electrodes. The space charge injection and migration properties under different temperature are detected. The results show that the surface potential rises significantly because of electron migration near the high voltage (HV) electrode under high temperature conditions, thus creating an "analogous ineffective region". The expansion of this "analogous ineffective region" results in most of the voltage drop occurring near the ground electrode, which serves as an important factor triggering positive streamers across the insulation surface. This work is helpful in understanding of DC surface flashover mechanism from a new perspective and also has important significance in design of a suitable DC insulator to avoid surface flashover problem.

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The potentially neglected culprit of DC surface flashover: electron migration under temperature gradients.

Charge cluster triggers unpredictable insulation surface flashover in pressurized SF 6

Surface charge decay kinetics on corona-charged polymeric insulators were studied. In the experiment, the sample surface exposed to corona was kept open to ambient air whereas the opposite surface was in contact with a grounded plate. The resulting surface potential distribution was scanned in a raster mode using a Kelvin electrostatic probe, from which the relief map of charge density was obtained by an inverse calculation method. The developed procedure allowed for implementation of two study cases: (i) gas neutralization took place under natural conditions, with enhanced concentration of free ions, and with limited gas volume; (ii) surface charge decay occurred only due to bulk neutralization. Three mechanisms were considered responsible for surface charge decay, i.e., bulk neutralization, gas neutralization, and surface conduction. They were discussed separately with the help of numerical models, which quantified the relative importance of each mechanism on the total process of charge decay. The resul...

Interpretation of the surface charge decay kinetics on insulators with different neutralization mechanisms

https://iopscience.iop.org/article/10.1088/2053-1591/ab3f0d/pdf

Aging of silicone rubber-based composite insulators under multi-stressed conditions: an overview

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.

Surface charges on cylindrical polymeric insulators

A decay of electric surface potential on a pre-charged gas-solid interface involves several charge transport processes and each of them may become dominant under certain environmental conditions (temperature, pressure, humidity) In the present paper, surface potential decay on flat samples of different kinds of silicone rubber used in HVDC applications is analyzed at reduced pressures of ambient air that allows for minimizing the involvement of the gas phase, ie surface charge neutralization by gas ions Effects imposed solely by bulk and surface conduction in the solid material are studied experimentally and by means of computer simulations The results allow for evaluating threshold values of volume and surface electric conductivities at which these mechanisms become most essential Field dependent bulk conductivities are deduced from the surface potential decay characteristics obtained for the studied materials

Potential decay on silicone rubber surfaces affected by bulk and surface conductivities

Tungsten disulfide (WS2) due to its layered structure and high capacitance is an attractive electrode material for supercapattery application. In this study, different thickness of WS2 is deposited by magnetron sputtering technique. The thickness of the sputtered layer is also optimized. The sputtered WS2 is characterized by various techniques such as X-ray diffraction and atomic force microscopy to examine the structural morphology and to study the surface morphology. The electrochemical performance of sputtered WS2 is investigated through three-electrode assembly via cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy. Sample S2 (WS2 with 250 nm thickness) shows the best performance in comparison with other samples. The S2 exhibits the maximum specific capacity of 346 C/g at 0.5 A/g. The hybrid device is designed by keeping S2 as positive and activated carbon as the negative electrode. The device exhibits maximal specific capacity of 190.2 C/g and after 3000 galvanostatic charge-discharge, cycles it retains 98.6% of its initial capacity. The maximum specific energy of the device is 45.2 Wh/kg which is high enough and an exceptional maximum specific power of 10,200 W/kg. Furthermore, by applying Dunn's model the diffusive and capacitive contributions of the hybrid device are studied. Moreover, b values are calculated by employing power law, the trend of b values confirms the asymmetric nature of supercapattery device. The excellent results of magnetron sputtered WS2 makes it a favorable electrode for its application in supercapattery devices. 

Shahid Alam

Papers

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

Aging of silicone rubber-based composite insulators under multi-stressed conditions: an overview

Surface charges on cylindrical polymeric insulators

Potential decay on silicone rubber surfaces affected by bulk and surface conductivities

Magnetron sputtered tungsten di-sulfide: An efficient battery grade electrode for supercapattery devices