Chuanyang Li

The control mechanism of surface traps on surface charge behavior in alumina-filled epoxy composites

To investigate the role surface traps play in the charge injection and transfer behavior of alumina-filled epoxy composites, surface traps with different trap levels are introduced by different surface modification methods which include dielectric barrier discharges plasma, direct fluorination, and Cr2O3 coating. The resulting surface physicochemical characteristics of experimental samples were observed using atomic force microscopy, scanning electron microscopy and fourier transform infrared spectroscopy. The surface potential under dc voltage was detected and the trap level distribution was measured. The results suggest that the surface morphology of the experimental samples differs dramatically after treatment with different surface modification methods. Different surface trap distributions directly determine the charge injection and transfer property along the surface. Shallow traps with trap level of 1.03–1.11 eV and 1.06–1.13 eV introduced by plasma and fluorination modifications are conducive for charge transport along the insulating surface, and the surface potential can be modified, producing a smoother potential curve. The Cr2O3 coating can introduce a large number of deep traps with energy levels ranging from 1.09 to 1.15 eV. These can prevent charge injection through the reversed electric field formed by intensive trapped charges in the Cr2O3 coatings.

Suppression of surface charge accumulation on Al2O3-filled epoxy resin insulator under dc voltage by direct fluorination

Surface charge accumulation on insulators under high dc voltage is a major factor that may lead to the reduction of insulation levels in gas insulated devices. In this paper, disc insulators made of Al2O3-filled epoxy resin were surface fluorinated using a F2/N2 mixture (12.5% F2) at 50 °C and 0.1 MPa for different durations of 15 min, 30 min and 60 min. A dc voltage was applied to the insulator for 30 min and the charge density on its surface was measured by an electrostatic probe. The results revealed significant lower surface charge densities on the fluorinated insulators in comparison with the original one. Surface conductivity measurements indicated a higher surface conductivity by over three orders of magnitude after fluorination, which would allow the charges to transfer along the surface and thus may suppress their accumulation. Further, attenuated total reflection infrared analysis and surface morphology observations of the samples revealed that the introduction of fluoride groups altered the sur...

Surface morphology and electrical characteristics of direct fluorinated epoxy-resin/alumina composite

As basic insulating material, epoxy resins are widely used as the matrix of insulators in HVDC transmission system. In most cases, alumina of micro range is blended to improve mechanical properties as well as reduce expenses, etc. However, apart from benefits, the incorporation of micro-particles could alter surface electric behaviors due to the change in material structures and the surface trap distribution under operating condition. Therefore, the surface charge behavior under dc is changed as well, which can affect the surface flashover characteristic. Direct fluorination is a method to change surface properties by introducing stable C-F structure to the surface of the dielectric material. The newly formed fluorinated layer could bring changes to electrical characteristics, thus affecting surface electrical properties under dc stress. In this paper, samples are prepared using alumina casted epoxy resin composites. The two parts are blended using the same process used to produce cone-type insulators in gasinsulated substations (GIS) in industrial application. Experiment samples were treated with F2 /N2 mixture containing 12.5% F2 in volume at 50 °C under 0.1MPa. The scanning electron microscopy (SEM) and fourier transform infrared spectroscopy (FTIR) were used to investigate surface physicochemical characteristics. Water and diiodomethane droplet contact angles were measured and surface energies were calculated. The distribution of the surface voltage potential, as well as the surface conductivity and DC flashover voltage were measured. Experiment results confirmed that the fluorination to epoxy resin and alumina composites can change the surface morphology by changing the exposed area of surface alumina particles. It also showed that fluorinated samples have higher DC flashover voltage in both air and SF6 due to the combined effect of the change in the surface morphology and the surface conductivity.

Fluorine gas treatment improves surface degradation inhibiting property of alumina-filled epoxy composite

Epoxy resins blended with micro-sized alumina show more compact surface condition and exhibit lower light emission after modified in fluorinated atmosphere. Surface morphology reflected by atomic force microscope shows that the fluorinated surface layer can overlap alumina particles inlayed in the original surface and a smoother and more compact surface condition is generated. Thermally stimulated current test exhibits that surface fluorination is able to bring more shallow traps to surface states, and the trap amount increases with the extension of fluorinating time. The photon counting technique illustrates that the fluorinated surface has lower light emission which indicates better aging inhibition properties. Therefore, we believe that the surface fluorination technique may have potential significance in modifying epoxy based insulators used in high voltage applications.

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

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.

Dynamic observation of dc surface charge dissipation for epoxy-resin/alumina composite

The low surface DC flashover voltage for the cone-type insulator has restrained the industrial application of high voltage apparatus especially for gas-insulated substation and gas-insulated transmission lines. However, the surface charge under DC, which produces uniform electric fields resulting in surface flashover under low direct voltage, are still not investigated clearly. In this paper, the dissipation characteristics of the surface charges are studied and the mechanism of the dissipation for surface voltage potential is discussed. The experiment sample is from a 550kV cone-type insulator of GIS with the surface shined by 220# sandpaper, and cut into the size of 140mm×40mm×4mm. 30kV was applied by a cylinder electrode for 10 minutes in atmosphere. To gain the dissipation characteristics of the surface voltage potential, several consecutive points along the insulator surface were chosen, and the voltage potential was detected continuously with an electrostatic probe after the voltage being removed. In addition, the overall surface potential was detected by sliding the probe along the surface of the insulator. The results showed that only negative voltage potential appeared along the surface when the voltage was applied. After the voltage was removed, the surface potential suffered great changes and positive charges were found “flooding” suddenly from both electrodes and moving towards the middle of the insulator surface with time. Only positive voltage potential was detected on the surface after more than 6 hours.

Surface charge inversion algorithm based on bilateral surface potential measurements of cone-type spacer

To study the surface charge distribution of high-voltage direct-current (HVDC) spacers for gas insulated lines (GIL), a surface charge inversion algorithm based on surface potential measurements is necessary. However, previous studies on inversion calculation of surface charge density only considered the surface potential of one side, neglecting the effect of residual surface charge on the other side of the spacer; this leads to an inaccurate inverted surface charge distribution. In this paper, the inversion algorithm methods used to determine surface charges over the past few decades were summarized, and their advantages and disadvantages were discussed. Based on the previous charge inversion algorithms, an improved surface charge inversion algorithm considering the surface potential of both sides of a cone-type model spacer was developed. The improved method was verified experimentally using a cone-type model spacer. Compared with that obtained by previous charge inversion algorithms, the charge distribution derived from the improved algorithm was closer to the situation determined from theoretical analysis. This work has practical relevance for the surface charge measurement of cone-type spacers and offers a new orientation in the surface charge analytical algorithm.

Hot electron injection regulation in Al 2 O 3 -filled epoxy resin composite using Cr 2 O 3 coatings

Al2O3-filled epoxy resin has been widely used as basic materials for insulators in ac/dc power transmission applications. Under normal working condition, charges injection from electrodes accelerates the dielectric aging rate and distorts the surface electric field, which is followed by dielectric degradation and breakdown. Therefore, to restrain the charge injection is of very important significance, especially when the dielectric was operating under temperature gradients. In this paper, to regulate the charge injection of dielectrics under temperature gradient, Cr2O3 coatings composed of dense well organized Cr2O3 molecules were created on the insulation surface. The surface potential appears low increments for modified samples, which indicates less charge injection phenomenon. Only slight surface potential increment was found using samples modified by Cr2O3 coating with thickness of 1000 nm, even when the temperature of HV electrode reaching 100 °C. Therefore, we believe that the Cr2O3 coating can effectively suppress the charge injection from the electrode, which has excellent application significance.