605 / 2022-03-30 17:46:27

Polarity Reversal of Surface Charges at SiC/Epoxy Coating due to Increase of Voltage Amplitude

surface charge,HVDC,Coating,voltage level,Sic/Epoxy

Polarity Reversal of Surface Charges at SiC/Epoxy Coating due to Increase of Voltage Amplitude

 


 The accumulation of charges at the interface between gas and insulator has been considered as the major factor that limits the development of HVDC gas insulated switchgear (GIS) because it distorts the tangential electric field along the insulator surface and even may result in flashover. At present, several methods have been proposed to inhibit the surface charge accumulation at DC voltage, among which surface coating is considered to be a promising way. In order to estimate its effectiveness, the surface charge distributions of SiC/Epoxy coating were measured at different levels of DC voltage in this paper.


A two-step pouring method was used to fabricate cylindrical samples. A basal insulator made of epoxy resin was firstly prepared, and epoxy resin with ingredient of SiC was then coated to the base with the help of a mould. After cured, the samples were pressed by a pair of finger-shaped electrodes so that there was a strong tangential electric field on the coating surface between electrodes. After a period of DC voltage application, the power supply was removed, and surface charge distribution was measured by an electrostatic probe.


The surface charge distributions of coating with 0, 5wt% and 10wt% SiC ingredient were obtained under different levels of voltage. When the voltage was -10 kV, most surface charges were positive, while few were negative.  The positive charges distributed near the high-voltage electrode, and the negative distributed near the grounded electrode. After the voltage increased to -20 kV, negative charges accumulated near the high-voltage electrode, and positive charges accumulated near the grounded electrode. As for the charge density and distributed area, the negative was much larger than the positive. It was inferred that the gas conduction contributed to the surface charge accumulation at -10 kV, while it was dominated by electrode injection at -20 kV. In addition, the surface charge amount was reduced when the content of SiC became higher. This change trend was attributed to the increase of conductivity and shallow traps.   


Voltage increase leads to the polarity reversal of surface charges. With the application of -10 kV, many positive charges located near the high-voltage electrode through the gas conduction. After the voltage increased to -20 kV, the electrode injection was enhanced, so negative charges dominated near the high-voltage electrode.  For both -10 kV and -20 kV, the coating has an inhibition effect on surface charge accumulation.