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In this article, the dissipation of predeposited surface charges on the SiC/epoxy coated insulator is investigated. Similar to Part I of this study, the analysis is carried out for the system of a pair of finger-shaped metallic electrodes placed on flat material samples and different types of material precharging are considered. Specifically, two cases based on different types of electrode–sample contacts (Cases 1 and 2, similar to Part I) are examined as well as surface charging by external corona (Case 3). It is shown that surface charge dissipation in Cases 1 and 2 takes place in two stages, whereas three stages in the charge decay process are identified in Case 3. It is shown that the rate of charge dissipation is strongly affected by the increase of the content of SiC filler in the coating material. The experimental results obtained for different cases are compared and discussed by employing results of the calculations of the electric field in the experimental setups and surface conductivities and surface trap characteristics obtained in Part I. [ABSTRACT FROM AUTHOR]

Surface charge accumulation on silicon carbide (SiC)/epoxy coatings with different contents of SiC filler, 3, 5, 7, and 10 wt%, is investigated using flat samples of epoxy substrate. In the experiments, a pair of finger-shaped electrodes was attached to the sample’s surface using direct pressing and conductive adhesive providing two different types of contacts and, respectively, surface charging methods. The electrodes were energized by applying dc voltage for various time intervals and resulted surface charge distributions between the electrodes were measured. It was found that depending on the contact type, charging duration, and filler content, different patterns of homocharge or heterocharge appeared on the material surface. Physical reasons for the observed effects are analyzed based on the calculated electric field distributions in the experimental setup, results of the auxiliary tests on material charging in a limited air volume, and surface trap energy distributions measured by utilizing the isothermal surface potential decay (ISPD) method. Application of SiC/epoxy coating for controlling surface charge accumulation is discussed. [ABSTRACT FROM AUTHOR]

In this paper, an electro-thermal based surface charge accumulation model is established. Heat conduction, heat convection and heat radiation are included. The gas flow is coupled with fluid equations to describe the motion of charge carriers, and gas current density is also corrected by the gas velocity. In terms of it, the surface charge and electric field on a basin-type insulator in vertical laid DC gas insulated switchgear (GIS) and gas insulated transmission line (GIL) under different temperatures and pressures are studied. The results show that a temperature gradient is formed in the gas from top to bottom in the vertical direction, and the increase in pressure leads to a higher temperature of the lower surface. The surface charge density and tangential electric field increase with the temperature and decrease with the pressure. As a comparison, the surface charge and tangential electric field are calculated without considering temperature and without considering heat convection. The former leads to a lower tangential electric field than this model, while the latter brings about a higher electric field. This paper provides a more accurate reference for the design of DC GIS/GIL through a more comprehensive model. [ABSTRACT FROM AUTHOR]

In order to suppress surface charge accumulation, a nano-SiC/epoxy composite with nonlinear conduction characteristic was employed to coat a pure epoxy substrate in this paper. Surface potential distributions on the insulator were measured after being charged by a pair of finger-shaped electrodes (one was powered at −10 kV, and the other was grounded). For the insulators with and without coating, there were mainly positive potentials on the whole surface, and the area with higher potential located near the high voltage electrode. As the charging time increased, the high potential area began to shrink, indicating a possible charge migration. When the charging time exceeded 60 min, the potential distribution was almost kept unchanged. Compared with the pure epoxy substrate, the amount of surface charges had a slight increment after being coated with 3 wt% nano-particles. The accumulated surface charges were progressively reduced when the content of fillers became higher. Especially for the samples with 7 wt% and 10 wt% ingredients, there was a sharp decrease of accumulated surface charges in the distributed area and magnitude from 40 to 60 min. In terms of surface trapping characteristics and nonlinear conduction of the composite, the effect of coating on the surface charge accumulation was discussed. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) taking place in a solid dielectric-bounded cavity involves physical processes such as free electron supply, discharge development and surface charge decaying, which bring about memory effects and become the main reasons for stochastic behavior of PDs. This paper reviews numerical modeling of cavity PD in the past 30 years. In the first place, physical processes relevant to PD activity are summarized, and modeling methods for discharge development are classified. Then some differences of PD modeling at AC and DC voltages are distinguished. Subsequently, reproducing methods from simulations to experiments are introduced, as well as their comparison under different conditions and with the emphasis on voltage frequency and PD aging. At last, some problems about current simulation models are discussed, and our suggestions for future work are proposed. [ABSTRACT FROM AUTHOR]

In order to reveal partial discharge (PD) physical processes at DC voltage, a model with regard to PD taking place within a cavity embedded in polyethylene is established, in which the charge fluid equations and the Poisson equation are employed to simulate streamer propagation and surface charge accumulation processes after it arrives at the gas–solid interface. Meanwhile, parameters such as discharge current and discharge time during a PD sequence are obtained by considering surface charge decay and discharge time lag variation in the discharge time interval. It is found from the simulation results that the electrons generated by the discharge accumulate on the upper surface of the cavity and the time taken to accumulate is less that 1 ns when a positive voltage is applied to the upper electrode. On the contrary, positive ions accumulate on the lower surface, and the time taken is about 90 ns. Besides, the density of positive ions is slightly higher than that of the electrons. During a PD sequence, the decay of surface charges resulting from previous discharge can be considered to be the key factor contributing to the occurrence of subsequent one, and electric field within the cavity mainly depends on negative surface charges just before the subsequent PD takes place, because of the lower decay rate of the negative charges than the positive ones. The discharge time interval can be divided into two parts: discharge time lag and discharge recovery time, which depend on the surface charge decay rate. As surface charge decay rate increases or the discharge time lag decreases, the discharge time interval decreases. Moreover, the discharge current is affected by discharge time lag and the surface charge decay rate. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

Surface charges generated by previous discharges at the solid-gas interface in a void can affect the characteristics of subsequent ones. In this paper, the effect of discharge area variation on the stochastic characters of partial discharge (PD) magnitude was investigated by changing the void size. It is found that the uniformity of PD magnitude increased when void diameter was reduced. In order to clarify this, the corresponding discharge area variation was obtained by Pockels effect of Bismuth Silicon Oxide (BSO) crystal. As void diameter became smaller, due to the limitation of cavity walls, the fluctuation of discharge area became weaker, and hence the uniformity of PD magnitude increased. Besides, the result that the fluctuation of PD magnitude decreased when PD void height became larger was obtained. Considering the safety of BSO crystal, surface charge distribution was not measured by experiment for this case. Instead, a simulation model was constructed to obtain it. As a result, discharge area became larger when void height increased. According to this, it is inferred that the expanding of discharge area may be affected by cavity walls in the experiment. Therefore, the changing tendency of the uniformity of PD magnitude with the void height can be explained when the void area remained unchanged. These results suggest that the variation of discharge area can affect the stochastic character of PD magnitude and further investigation is needed to clarify the PD mechanism. [ABSTRACT FROM AUTHOR]

The distribution of surface charges at the interface between gas and insulating material relates to the partial discharge process in a void. In order to clarify this, a 2-D model is constructed in this paper. Based on this, the streamer development and the accumulating process of surface charges are obtained under both polarities of the applied voltage. It is found that the maximum surface charge density appears at the middle location of the interface, and monotonically decreases along the radial direction. As the drift velocity of the electron is higher than that of the positive ion, the accumulation of surface charges takes a much shorter time under the negative voltage than under the positive. Besides, the effect of the applied voltage amplitude and the void height on surface charge distribution is obtained. As the applied voltage or the void height becomes greater, the discharge process becomes more intense, leading to an increase of surface charge density and surface charge distribution area. [ABSTRACT FROM AUTHOR]

The accumulation of surface charges generated by a discharge at the solid-gas interface in a void is an important factor that can affect partial discharge (PD) characteristics. In this paper, with the help of the surface charge measurement system based on Pockels effect, the phenomenon of surface charge decay was observed by comparing its dynamic distribution during a PD sequence under variable voltage cycle. Moreover, the decay time of surface charge was measured. It is found that the decay rate of positive surface charge was much faster than that of negative one. The effect of surface charge decay on variation of partial discharge location was investigated when the voltage with variable cycle was applied. Two terms, the coincidence rate of hetero-discharge location (CRHEL) and the coincidence rate of homo-discharge location (CRHOL) were introduced to describe this effect. As the applied voltage cycle increased, CRHEL before and after voltage zero-crossing point decreased. Besides, CRHEL from a void between two dielectric surfaces (called VTD electrode) was larger than that from a void between a metal surface and a dielectric surface (called VMD electrode). Regarding VMD electrode, CRHEL for the case with negative discharges occurring in the former half-cycle was larger than that with positive discharges occurring in the former halfcycle. However, there was almost no difference about CRHEL for these two cases when VTD electrode was used. After the applied voltage cycle exceeded 500 ms, the coincidence phenomenon of homo-discharge location during negative half-cycle was observed in the usage of VMD electrode, and CRHOL became larger with voltage cycle increasing. As for VTD electrode, there was no coincidence of homo-discharge location. These results were attributed to the charge decay and the difference of discharge initiation position. [ABSTRACT FROM AUTHOR]

The micro-SiC/Epoxy composite coating has huge potential for application in the insulator of high voltage direct current (HVDC) gas insulated switchgear (GIS) since it can effectively suppress charge accumulation at gas–solid interface and adaptively regulate resistive electric field distribution. However, achieving stable and fruitful properties often requires high filler concentrations, which can compromise processability and long-term stability. Thus, a new method is proposed for preparing SiC/Epoxy composite coatings based on in-situ electric-field-induced assist (EFIA). The alignment process of SiC induced by the in-situ electric field, the effects of in-situ field on the physical and chemical properties of cured composites were studied. Even at concentrations far below the percolation threshold, the alignment of SiC particles along the in-situ electric field direction can induce significant non-linear conductive properties along that specific direction in the composite. Furthermore, the EFIA-coating leads to a 59.43% reduction in surface charge accumulation, more than a 50% decrease in charge dissipation time, and a 17.3% increase in surface flashover voltage. The coating discussed in this paper will be helpful in insulation design of DC components. [Display omitted] • Develop an in-situ electric field-assisted method for preparing epoxy-matrix composite coatings. • Low concentration composites with aligned additives can exhibit significant non-linear conductive properties. • Epoxy-matrix SiC composite coatings prepared based on in-situ electric field assists can effectively regulate surface charge and enhance surface flashover voltage. [ABSTRACT FROM AUTHOR]

[Display omitted] • SABMP adsorbs on the minerals surface but it can't depress the floatability of serpentine and pyrite, respectively. • SABMP can effectively eliminate the adverse effect of serpentine on pyrite flotation. • The surface charge of serpentine is changed from positive to negative in the presence of SABMP. • SABMP can not only absorb on the surface of serpentine but also accelerate the dissolution of Mg2+ on serpentine surface. The flotation of pyrite is often depressed by the slime coating of fine serpentine. To eliminate the negative effect of serpentine, N,N-bis(phosphonomethyl)-sulfamic acid (SABMP) was employed as the depressant in this study. The results of micro-flotation experiments showed that the addition of SABMP could significantly reduce the adverse effect of fine serpentine and restore the floatability of pyrite in the pyrite-serpentine system. However, the floatability of pyrite and serpentine had not been obviously reduced in single mineral flotation after the addition of SABMP. The adsorption and ICP results indicated that SABMP could be adsorbed on serpentine surface and simultaneously promote the dissolution of Mg ions on serpentine surface. Further XPS tests demonstrated that SABMP interacted with the Mg species exposed on the surface of serpentine. In addition, it was discovered that the addition of SABMP changed the zeta potential of serpentine from positive to negative at pH 8.5, which eliminated the hetero-coagulation between pyrite and serpentine. The turbidity test further suggested that SABMP could convert the aggregation to the dispersion between pyrite and serpentine. These results demonstrated that SABMP is a potential depressant in the selective flotation separation of pyrite from serpentine. [ABSTRACT FROM AUTHOR]