Surface charge characteristics of high voltage insulators and their flashover performance
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Date
2020-08-25
Authors
Amer, Mohammed
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Abstract
Surface charging is a common occurrence during service for all high voltage (HV) insulators as well as live-line tools during live-line maintenance work. This research focuses on studying the DC charging characteristics of different insulating materials as well as the flashover performance of such pre-charged insulating materials. The main charging sources are corona discharges from energized objects and cloth wiping by tribo-charging effect. The used samples are cylindrical in shape with different diameters and 130 mm length. The surface potential of the charged samples is measured with the aid of electrostatic voltmeter and is converted into surface charge density by applying the probe response matrix method. The tribo-charging effect of cloth wiping results in much more deposited charges on the sample’s surface than corona discharges from energized objects. However, those charges consume several hours to decay by 80%. Furthermore, the withstand characteristics of pre-charged insulating samples increase when negative charges are deposited on their surfaces while decrease with positive charges.
The research is furthered extended to investigate the combined effect of surface charging and air humidity on the DC flashover of insulating materials. The relative humidity inside the climate chamber is controlled and varied between 20 and 80%. Afterwards, the pre-charged samples are stressed with negative or positive HVDC values until flashover takes place. Increasing humidity results in a reduction of the DC flashover voltage as well as a shorter time to flashover regardless of the voltage polarity. The effect of very light pollution on the flashover performance of insulating samples is studied according to the up and down method test procedure. The withstanding characteristics of the polluted samples decrease dramatically as compared to the clean samples. Moreover, their DC pollution flashover performance decreases as the pollution severity increases. The importance of the research on insulation surface charging comes with covering the lack of knowledge related to the effect of atmospheric conditions on the charging characteristics of insulating materials and their flashover performance. The outcomes of this research forms hypotheses and recommendations which may improve the live-line maintenance procedures in ways that protect live-line workers from accidents.
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Keywords
corona charging, surface charging, surface potential, DC flashover voltage, live-line work, polymer insulators, FRP tools, Pollution flashover, Relative Humidity