Surface charge accumulation on insulators becomes an important bottleneck to restrain the development of a DC energy transmission system. This study investigates the effects of SiC/ epoxy coating on surface charge behaviors and flashover performance of alumina/epoxy spacers. A nonlinear conductive coating consisting of different contents of SiC fillers and epoxy matrix was fabricated and sprayed on only half surface of the spacers. The dependence of surface charges on the contents of SiC fillers is characterized by three stages. Surface charge accumulation is aggravated at deterioration stage, then almost unaltered at balance stage and inhibited at suppression stage. These three stages are determined by the competition between the increased surface trap density from interface region and the improved surface conductivity from SiC fillers. The variation trend of flashover in air and SF 6 /N 2 mixtures is distinguished by three stages as well. Flashover voltage decreases at descent stage where deficient contents of SiC fillers are appended and then goes up at improved stage with moderate supply of SiC fillers. Finally, it ends up with drastically collapse at degenerated stage when affluent SiC fillers are brought in. The theoretical analysis is proposed based on surface trap distribution and conductivity measurement results to reveal the regulation mechanism of SiC/epoxy coating on surface charge behavior and flashover performance.

Charge accumulation phenomenon on gas–solid interface greatly restricts the development of HVDC energy transmission system. In this study, the surface charge transport behavior and flashover performance on alumina/epoxy spacer coated by SiC/epoxy composites are experimentally investigated under DC stress. SiC/epoxy composites with varied SiC particle size are fabricated and deposited on spacer surface. Nearly a charge free surface is achieved especially at smaller SiC particle size, even when metallic wires are adhered on spacer surface and connected to high voltage electrode. The DC flashover voltage increases with the decrease of SiC particle size. On the one hand, smaller SiC particle size exhibits more outstanding nonlinear conductivity characteristics, contributing to accelerating charge dissipation, whereas on the other hand, it would introduce plenty of shallow traps due to the increased interfacial regions between SiC particle and epoxy matrix, resulting in aggravating charge accumulation. A theoretical model is proposed to reveal the control mechanism of SiC/epoxy coating with different SiC particle size on flashover performance. The validity of this model can be confirmed based on the surface trap distribution, carrier mobility and our previous investigations on gas–solid interface flashover development process.

Surface flashover properties of alumina/epoxy spacers, involving a surface charge accumulation process, are critical for the safe and reliable operation of a high-voltage direct-current (HVDC) gas-insulated transmission line (GIL). This study reports surface charging behavior and flashover performance of alumina/epoxy spacers with different surface conductivity graded coating (SCGC) schemes in SF 6 /N 2 mixtures under DC stress. Four kinds of SCGC schemes, i.e. localized coating near high voltage (HV-coating), near grounded electrode (GND-coating), at the middle of spacer surface (SPM-coating) and near both high voltage and grounded electrode (HV-GND-coating), are designed by partially spraying SiC/epoxy composites on the spacer surface. Surface charge distribution patterns exhibit varied features with different SCGC schemes. The HV-coating and GND-coating schemes lead to aggravated homo-charge and hetero-charge accumulation respectively, whereas in the SPM-coating scheme surface charge shows a multi-tier distribution pattern with alternating polarity. A transition of the dominant surface charge mechanism from bulk conductivity to surface conductivity with increasing conductivity on the coated area is found. Flashover performance differs a lot with different SCGC schemes: the HV-coating and HV-GND-coating schemes increase the flashover voltage while the SPM-coating and GND-coating schemes degrade it. The optimal surface insulation strength is achieved in the HV-coating scheme with a coating width of about 10 mm. The impact of different SCGC schemes on flashover performance is revealed based on the electric field analysis by considering the effect of surface charges.

Experimental research has shown that much less charge injection occurs in polytetrafluoroethylene (PTFE) compared to polyethylene (PE). To clarify the mechanisms of charge injection from metals into polymer insulators, we comparatively studied charge injection in PE and PTFE oligomers using first-principles calculations. Two different models were studied: chemisorption (bonding) and physisorption (nonbonding). The results show that the electron injection barrier of the metal/PTFE interface is larger than that of the metal/PE interface only in the case of chemisorption. The larger electron injection barrier of the metal/PTFE oligomer interface is mainly affected by the positive vacuum level shift of the metal/PTFE interface induced by electron transfer from the metal to PTFE along the chemical bonds. In the case of physisorption, the hole injection barrier of the metal/PTFE interface is larger than that of the metal/PE interface. This is attributed to the larger ionization potential of PTFE compared to PE. The calculated results reasonably explain the experimental phenomena. The agreement between the experimental and calculated results verifies the rationality of our calculation models. The models used herein can likely be applied in other metal/polymer interfacial systems with acceptable accuracy.

The backward discharge induced by residual charge has attracted considerable attention, but the complex mechanism requires further investigation. This article analyzes the surface discharge and the residual surface charge accumulation characteristics in atmospheric air under a positive impulse. The photographic figure method, the dust figure method, and the electrostatic probe method are employed, allowing the residual charge distribution to be quantitatively determined. One main forward discharge is observed at the impulse front and several subsequent backward discharges are observed at the impulse tail. The propagation length of the forward streamer is closely related to the voltage at the moment of discharge. A model illustrating the evolution process of surface forward and backward discharges as well as the corresponding residual charge accumulation characteristics is established. The residual surface charge plays a significant role in the development of the streamer discharge. The backward discharge is incurred by the positive charge that is deposited by the previous forward discharge.