Isothermal Surface Potential Decay Research Articles

Effect of Atmospheric Pressure Plasma Treatment on the Charging Behavior of ITO and Fto Surfaces

Over the past two decades, many different cold atmospheric pressure plasmas (CAPP) sources have been developed in research labs and brought to industrial use. Due to their versatility, CAPPs are now employed in a wide range of applications, including water or exhaust gas purification [1], sterilization of medical equipment [2], or the surface treatment of sensitive materials [3]. Plasma-surface interaction can induce various types of modifications, such as etching, deposition, cross-linking, or functionalization. All these different treatments have proven to be suitable to modify and tune the surface properties of materials used in organic photovoltaic (OPV). The possibility of operating those plasma devices at atmospheric pressure makes them particularly attractive for further industrial scale-up, allowing, for example, treatments of significant amounts of materials in roll-to-roll systems compared to the batch approach used at low pressure.Indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) are the most common transparent electrodes used in OPV cells. The substrate on which the electrodes are usually deposited is glass or polyethylene terephthalate (PET). To optimize the interface between the electrode and the electron transport layer, the charge transfer needs to be maximized. Therefore, the work functions of these two materials need to be matched, and charge traps at the interface must be passivated [4]. Preliminary results suggest that these objectives can be reached by using plasma treatment at atmospheric pressure [5]. Still, the exact mechanisms leading to the increase of the work function induced by the plasma remain unclear. Possible explanations can be related to the removal of organic contaminants, change in the electronic state of the surface and the band bending or the modification of surface charge traps affecting the work function.In this work, the effect of different plasma treatments, such as volume, surface or coplanar dielectric barrier discharge, on ITO and FTO is analyzed. More specifically, the influence of the substrate, as well as its thickness is correlated to the surface charges and the related modification of the work function. The different modified samples are analyzed by isothermal surface potential decay (ISPD), and the work function is determined by conductive atomic force spectroscopy. Such a comprehensive study is of interest to tune the materials used in OPV and obtain the desired properties to optimize the final cells. [1] Zhang et al. Environmental Technology & Innovation, Volume 29, 103007, https://doi.org/10.1016/j.eti.2023.103007[2] Halfmann et al. ,J. Phys. D: Appl. Phys., 40, 4145, https://doi.org/10.1088/0022-3727/40/14/008[3] Brandenburg et al., Contribution to Plasma Physics, Volume47, 72, https://doi.org/10.1002/ctpp.200710011[4] Polydorou et al., Journal of Materials Chemistry A, Issue 30, 4, https://doi.org/10.1039/C6TA03594A[5] Shekargoftar et al. Materials Science in Semiconductor Processing, Volume 75, 95, https://doi.org/10.1016/j.mssp.2017.11.022

Increased Deep Trap Density in Interfacial Engineered Nanocomposite Revealed by Sequential Kelvin Probe Force Microscopy for High Dielectric Energy Storage.

Nanocomposites combining inorganic nanoparticles with high dielectric constant and polymers with high breakdown strength are promising for the high energy density storage of electricity, and carrier traps can significantly affect the dielectric breakdown process. Nevertheless, there still lacks direct experimental evidence on how nanoparticles affect the trap characteristics of nanocomposites, especially in a spatially resolved manner. Here, a technique is developed to image the trap distribution based on sequential Kelvin probe force microscopy (KPFM) in combination with the isothermal surface potential decay (ISPD) technique, wherein both shallow and deep trap densities and the corresponding energy levels can be mapped with nanoscale resolution. The technique is first validated using the widely-used commercial biaxially oriented polypropylene, yielding consistent results with macroscopic ISPD. The technique is then applied to investigate polyvinylidene fluoride-based nanocomposites filled with barium titanate nanoparticles, revealing higher deep trap density around surface-modified nanoparticles, which correlates well with its increased breakdown strength. This technique thus provides a powerful spatially resolved tool for understanding the microscopic mechanism of dielectric breakdown of nanocomposites.

Electrical conduction properties of PP-based nanocomposite with high thermal conductivity for HVDC cable insulation

This paper reports on thermal and electrical conduction properties of polypropylene (PP)-based nanocomposite used for high voltage direct current cable insulation. To improve the thermal conductivity of the PP/propylene-based elastomer (PBE) blend (noted as PB), boron nitride (BN) nanosheets with high thermal conductivity are added. The pretreatment process through template method is to obtain a three-dimensional (3D) network structure composed of BN nanosheets (noted as 3DBN). Then the 3DBN is added into the PP/ PBE blend, in which the 3D networks are locally arranged and the other BN nanosheets are randomly dispersed. Thermal conductivity is estimated, while electrical conductivity is measured under electric fields of 5–40 kV mm−1 at various temperatures. Carrier trap distribution is derived from isothermal surface potential decay method. In addition, the microstructure and crystallization characteristics of the nanocomposites are measured. The results show that the thermal conductivity of the PP/PBE blend is improved obviously by the addition of the locally arranged 3DBN, and that of PB-3DBN-10 is increased by 16% compared with that of PB. Deep traps are introduced by the dispersed BN nanosheets and the electrical conduction is reduced, which becomes increasingly evident with the filler content and temperature. It is suggested that the introduction of thermal conductive fillers with a 3D network structure and well-dispersed state can synergistically enhance the thermal conductivity and insulation performances of the PP-based material.

Enhanced surface flashover performance of oriented hexagonal boron nitride composites via anisotropic charge transportation

AbstractSurface flashover is a crucial issue for the miniaturisation of electronic facilities in military, industrial, and aerospace engineering. The oriented hexagonal boron nitride (hBN) composites, due to excellent thermal and electrical insulating properties, show a potential application in high‐voltage power equipment, while the surface flashover performance of hBN composites dependent on oriented hBN texture is rarely reported. The effects of hBN orientation and contents on the surface flashover performances of oriented hBN composites are investigated. The isothermal surface potential decay of the oriented hBN composites was also studied. It is found that the charge transportation could be adjusted by the hBN orientation, thus regulating surface flashover strength. The DC flashover voltage of the in‐plane oriented hBN composites with a thickness of 15 μm reached the maximum of 27.6 kV at the hBN loading of 20 wt%, 14.5% higher than that of the pure resin. The carrier mobility of out‐of‐plane oriented hBN composites is about three times greater than that of the in‐plane oriented composites, indicating that the charges are easily transported along the hBN basal plane. The larger carrier mobility causes charge dissipation in composites near the electrode at the hBN basal plane parallel to the axis of electrodes and inhibits the distortion of the surface electric field on the composites, thus enhancing the surface flashover. Consequently, developing oriented insulators for high‐voltage applications and enabling an optimum insulation design would be beneficial because of the compactness and high reliability of power apparatus for use in power grids.

Methyl and carbon–carbon double bond in anhydride molecules modulated surface charge trap depth of epoxy materials

Epoxy insulators in gas insulated switchgear and gas insulated transmission lines tend to accumulate surface charges, leading to insulation flashover. Improving the surface trap characteristics of epoxy materials, which can accelerate the surface charge dissipation of epoxy insulators, is a promising method to improve the surface insulation performance. The surface trap characteristics of epoxy materials are strongly influenced by the chemical groups in the acid anhydride molecules. In this work, by quantum chemical calculations and isothermal surface potential decay tests, taking six organic anhydrides that differ only in the methyl and carbon–carbon double bonds, we find the modulation laws of methyl and carbon–carbon double bonds on the charge trap depth within and between molecular chains. The regulation mechanism is revealed from the microscopic perspectives of electron energy structure and electron cloud offset. The changes of surface charge trap depth of epoxy materials are primarily attributed to the changes in the spatial distribution of the electron cloud density between and on the valence bonds caused by the interaction between the electron-donating methyl group and the electron-absorbing carbon–carbon double bond.

Improvement of partial discharge resistance of polypropylene under AC voltage by blending elastomer

This paper reports on the partial discharge (PD) erosion characteristics of polypropylene (PP)/elastomer blends where ethylene-octene copolymer elastomer (EOC) and propylene-based elastomer are involved. The PD erosion is performed through a pair of needle-to-plane electrodes by applying 5 kV AC voltage for 2 h. A high-frequency current transformer, optical recorder, and ozone sensor are used to capture the information released during the PD erosion process. In addition, a 3D optical profilometer is employed to record the erosion profile, from which the erosion depth and the surface roughness can be extracted. Carrier trap distribution is derived from the isothermal surface potential decay measurement. The influences of elastomer type and content on the PD resistance are analyzed. The results show that the resistance is improved with the addition of elastomer, and the blend containing EOC of 20 wt% has the best PD resistance among all samples, which is 61.9% higher than that of the pure PP. It is proposed that the carrier trap characteristics, the property of elastomer, and the free volume in the material play important roles in the improvement of the PD resistance. This work provides a potential method to enhance the PD resistance of PP by blending elastomer with proper type and content.

Effects of nanofiller and voltage stabilizer on partial discharge resistance of polypropylene/elastomer blends: A comparison study

AbstractThis paper reports on the individual and synergistic influence mechanisms of nano‐silica (SiO2) and voltage stabilizers on partial discharge (PD) resistance of polypropylene blended with ethylene‐octene copolymer elastomer. The SiO2 nanoparticles and three types of voltage stabilizers, that is, benzoin, 4,4′‐dimethoxybenzil, and 3‐aminobenzoic acid are selected as fillers to prepare samples. PD is initiated in a rod‐to‐plane electrode system with an alternating current voltage of 5 kVrms. PD characteristics are captured with multi‐sensors, and the erosion profile of the test sample is measured using a three‐dimensional (3D) optical profilometer. Carrier trap distribution is extracted with isothermal surface potential decay measurement to assist the analysis of PD eroding material. Fourier transform infrared spectrum, scanning electron microscopy, X‐ray diffraction spectrum, and differential scanning calorimetry analysis are performed to reveal the change in material structures caused by the fillers. Test results indicate that the PD resistance of the blend can be improved by the addition of nano‐SiO2 and voltage stabilizers individually and synergistically. The PD resistance of PO is increased by 71.2% at most in the sample with 3 wt% nano‐SiO2 addition. The sample filled with SiO2 exhibits better PD resistance than that with the voltage stabilizer or co‐added, which should be ascribed to the introduction of deep traps, the formation of the packaged layer, the suppression of oxygen diffusion of the nanofillers, and the good compatibility between additives and the polymer matrix.

Influence of SiC/Epoxy Coating on Surface Charging Phenomenon at DC Voltage—Part I: Charge Accumulation

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.

Electrical Aging Effect on Breakdown and Charge Trap Characteristics of Polyimide Films With Different Thickness in LN2

The Polyimide (PI) film, as an excellent insulating material, is broadly utilized in superconductivity power equipment, especially under low temperature. There is an enormous influence on the breakdown performance of polymers because of the presence of surface and space charge. In order to investigate the electrical aging impact on breakdown and trap characteristics of PI films with 0.1 mm and 0.125 mm thickness, the samples suffer from diverse degrees of electrical aging in liquid nitrogen (LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). The insulation strength test has been carried out on the PI film samples under different electrical aging stages in LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . The isothermal surface potential decay (ISPD) method is performed on the PI films to explore the behaviors of trapped charges, and the trap characteristics of the PI films are obtained. Those results indicate that the breakdown strength of the thinner PI film (0.1 mm) in LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is higher than that of the thicker one (0.125 mm), and the breakdown field strength reduces with the electrical aging time increasing. The ISPD measurement results indicate that the average decay rate of surface potential in the 0.125 mm PI is faster than the other thickness during the whole decay process.

Skin-core structure of thermally aged epoxy resin: Roles of oxidation and re-crosslinking

The skin-core structure of thermally aged epoxy resin consisting of the aged surface-layer and the unaged inner-layer, was studied in this paper. A new tanδ peak appeared in the results of dynamic mechanical analysis (DMA) after the specimens were thermally aged. It steadily diminished when the surface of the specimens was gradually removed, which indicates that the newly detected peak arose from formation of surface-layer during ageing. Thickness of the surface-layer could reach up to 0.09 mm after thermal ageing for 480 h. According to Fourier transform infrared spectroscopy (FTIR) results, formation of the surface-layer was found to closely correlated with thermal oxidation and re-crosslinking processes. The former induced oxidized molecular chains while the later might be related to the production of carboxylic acid dimers and etherification reactions to result in strengthened networks. This was further ascertained by the results of dielectric spectroscopy and isothermal surface potential decay (ISPD) tests. The relaxation process of aged epoxy resin displayed higher relaxation time, strength, and activation energy than unaged specimens, and evident growths of both deep and shallow level traps densities were also observed after thermal ageing. This casts new light on the thermal ageing mechanism of epoxy resin and could provide a feasible method for its insulation degradation assessment.

A Comparative Study of Gas-phase Fluorination and Nano-Al2O3 Doping on Space Charge Behavior and Trap Level in Epoxy Resin

In this work, the effect of gas-phase fluorination concurrent with nano-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> doping on space charge distribution, as well as trap level and density of epoxy resin, is investigated. Epoxy resins with different concentrations of Al2O3 nano-fillers (0, 1, 3, and 5 wt%) are prepared and fluorinated by a gas mixture of F2/N2 (20/80 v/v) for 30 min at 40 °C and 0.05 MPa. Fourier-transform infrared (FTIR) spectroscopy confirms the breakage of molecular-chain during the gas-phase fluorination process. The space charge behavior is observed by the pulsed electroacoustic (PEA) method before and after fluorination. PEA results indicate that 30-minute fluorination is useful against space charge injection into the bulk of epoxy resin/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanocomposites. The trap density and trap energy level distribution are determined by isothermal surface potential decay (ISPD) measurement. The results indicate that fluorination introduces shallow traps on the surface, which can increase the conductivity of surface, and, thus, the trap depth is considerably reduced after fluorination. Moreover, it is found that incorporating 1 wt% nano-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> particles into epoxy resin can level up the energy level of deep traps. However, further increasing the nano-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> concentration, e.g., 3 and 5 wt%, promises the overlap of interaction zones between polymer matrix and nanoparticles, reducing the trap energy level.

Influence of thermally induced self‐assembly shish‐kebab crystal on charge transport behaviour in polypropylene/elastomer blends

This paper reports on the influence of thermally induced self-assembly shish-kebab crystal on charge transport behaviour in polypropylene (PP) and PP/propylene-based elastomer (PBE) blends. The film samples with the shish-kebab crystal were prepared by adding a β nucleating agent TMB-5 under thermally induced self-assembly. Polarised optical microscope （POM）, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) measurements were performed to understand the crystallisation characteristics of samples. Carrier trap distribution was analysed by isothermal surface potential decay (ISPD) method, and DC breakdown strength measured as well. The decrease of shish-kebab crystal size, the reduction of trap level, the increase of shallow trap density and the decrease of deep trap density were obtained with the TMB-5 content. The hopping distance of charges decreased with the shallow trap density increasing. The DC breakdown strength for PP/PBE/TMB-5 was higher than that for PP/TMB-5. It is suggested that deep traps are formed on the clear shish-kebab crystal boundaries, while shallow traps are induced by the elastomer and the un-crystallisation nucleating agents. Both the carrier trap formation on the crystal boundary and the physical channel orientation of the shish-kebab crystals affect the charge transport behaviour.

Charge migration of multilayer oil paper on the process of partial discharge under AC voltage

As one of the main reasons for the deterioration of Nomex paper and charge migration, partial discharge (PD) plays a critical role during PD degradation. In this study, the PD characteristics of multilayer oil paper are focused on. In order to explore the characteristics of charge migration, the experiments of the relative permittivity, conductivity, isothermal surface potential decay, scanning electron microscopy and Fourier infrared spectroscopy have been carried out. The results show that the discharge branches of the second layer are two and three times longer than those in the first layer. In addition, the trap level of the first layer for PD degradation samples increases with the degree of PD degradation. The charges captured by deep traps provide seed charges for PD on the surface of the first layer. The charge transportation will accelerate the formation of shallow traps. The charge distribution and shallow traps contribute to the expansion of discharge branches.

Effect of polarity‐reversal voltage on charge accumulation and carrier mobility in silicone rubber/silicon carbide composites

Polarity-reversal voltage in high-voltage direct current transmission system would cause serious electric field distortion, accelerating charge injection. The effect of polarity reversal on charge transport characteristics in silicone rubber (SiR)/silicon carbide (SiC) composites with nonlinear conductivity at different temperatures is studied. The charge transport characteristics of silicone rubber/silicon carbide composites under unipolarity voltage and polarity-reversal voltage at 30°C, 70°C and 90°C are obtained by the isothermal surface potential decay method, and the carrier mobility and trap characteristics of the composites are further calculated. Studies have shown that a large amount of hetero-charge remains in silicone rubber materials under polarity-reversal voltage, which in turn leads to intensive charge neutralisation and severe electric field distortion. The silicon carbide doped composites exhibit excellent performance in suppressing the accumulation of residual charges and weakening the charge neutralisation under polarity-reversal voltage. In addition, high temperature enhances the conductivity of silicone rubber/silicon carbide composites and reduces the accumulation of residual charges, which restrains the distortion of the local electric field under polarity-reversal voltage. This research shows that the application of non-linear conductivity material is an effective measure to solve the problem of cable insulation under polarity-reversal voltage.

Effect of mineral oil on flashover voltage of HTV silicone rubber under AC corona ageing: from the view of trap level distribution

The flashover voltage of high temperature vulcanised (HTV) silicone rubber will be decreased after corona ageing, and it further decreases under the effect of mineral oil. The mechanisms for the phenomena can be explained by considering the role of trap level distribution. In this study, the corona ageing of silicone rubber was conducted and the flashover voltage was tested. The isothermal surface potential decay was tested to explore the internal relationship between trap distribution and flashover voltage. Besides, the surface microtopography and chemical structure of silicone rubber were gained by scanning electron microscopy and Fourier infrared spectroscopy to explain the change of trap distribution. The test results show that the flashover voltage of HTV silicone rubber decreases with AC corona ageing time, the reduction of flashover voltage for an oil-impregnated sample is more serious. The density of shallow electron traps increases at first and then decreases, while the density of shallow hole traps increases with ageing time. The density of deep traps decreases after corona ageing. These trap level distribution characteristics are beneficial to promote the development of flashover, leading to lower flashover voltage. The effect of mineral oil can be regarded as an accelerated ageing effect.

Filler concentration effect on breakdown strength and trap level of epoxy resin–Al2O3 nanocomposites

In this work, epoxy resin samples incorporated with nano-Al2O3 of various concentrations 0, 1, 3 and 5 wt% were prepared. Surface treatment of nano-Al2O3 was performed by the saline coupling agent of γ-aminopropyltriethoxysilane (KH550) to restrict the aggregation. The thermal gravimetric analysis (TGA) of the epoxy resin/Al2O3 nanocomposites was observed at different filler concentrations. TGA analysis indicated that incorporation of nano-Al2O3 increases the decomposition temperature as compared to pure epoxy resin. Moreover, 3 wt% filler concentration showed increased decomposition temperature because nanoparticles form close connections between molecules due to which thermal stability improved at higher temperatures. Atomic force microscopy was employed for the surface morphology of epoxy resin/Al2O3 nanocomposites which has indicated that surface roughness of epoxy resin/Al2O3 nanocomposites increased slightly with the filler concentration. The distribution of trap energy and trap density obtained by isothermal surface potential decay measurements revealed that 1 wt% filler concentration results in decrease in surface potential decay rate. In addition, significant increase in deep trap energy level and trap density was obvious at 1 wt% filler concentration. AC and DC breakdown strength of the samples was measured to evaluate the effect of filler concentrations on the electrical characteristics of epoxy resin. The results showed that breakdown strength of the epoxy resin/Al2O3 nanocomposites increases with the nano-Al2O3 concentration. Moreover, increase in trap energy results in an increase in the breakdown strength of the nanoparticles.

Trap level distribution dependence of lifetime for polyimide films under repetitive impulse voltage

In this paper, polyimide (PI) films were modified by using non-thermal plasma, which was generated by dielectric barrier discharge (DBD) in atmospheric air. Under repetitive impulse voltage, the lifetime of plasma treated PI films increases obviously, which reaches the maximum value of 16.8% higher than that of untreated PI films, obtained by 20 s’ treatment. For further understanding lifetime improvement mechanism, energy level distribution of both electron-type and hole-type traps was calculated based on isothermal surface potential decay (ISPD) measurement. It is found that energy level of both shallow and deep traps decrease after treatment, reaching the lowest value for the films treated for 20 s. Lower energy for shallow trap is beneficial to charge dissipation, so that local electrical field would be mitigated. Furthermore, lower energy for deep trap results in less amount of trapped surface charges. Thus, surface PD intensity would be suppressed, ending up with longer lifetime. Analysis of chemical bonding structure reveals that active groups rich in oxygen and nitrogen were introduced into the near-surface region of plasma treated sample, which are responsible for corresponding change of trap energy level distribution. However, in order to prolong lifetime effectively, plasma treating time must to be controlled in an appropriate range.

Dielectric properties and thermal conductivity of micro-BN-modified LSR used for high-voltage direct current cable accessories

Thermal breakdown is the main form which leads to direct current (DC) cable accessories insulation invalid. In order to enhance the insulation of DC cable accessories, liquid silicone rubber (LSR) is improved by filling micron boron nitride (BN) to promote the dielectric properties and thermal conductivity in this paper. The electrical conductance, DC dielectric breakdown strength (DBS), dielectric spectrum and thermal conductivity of micro-BN/LSR composites with various filling concentrations are analyzed in combination with the charge-trap energy-level distributions by means of isothermal surface potential decay (ISPD) method. By filling micron BN into LSR, the insulation performance and thermal conductivity can be improved simultaneously, thus achieving a higher insulation reliability. When the concentration of BN micron-filler is increased to 40 wt%, the electrical conductivity can be reduced by three orders of magnitude compared with pure LSR, and the DC DBS and thermal conductivity rises by 52.82% and 141.38%, respectively, while the relative dielectric permittivity and dielectric loss are slightly increased. The ISPD analyses indicate that trap-level depth in BN/LSR composites can be further raised by increasing the concentration of BN micron fillers, thereby getting a higher DBS.

Nano ZnO/epoxy coating to promote surface charge dissipation on insulators in DC gas-insulated systems

This paper describes the addition of ZnO nanoparticles to epoxy resin at concentrations of 20, 25, and 30wt%, and the application of this ZnO/epoxy coating to the surface of an epoxy insulator. This coating significantly suppresses the charge accumulation on the surface of the insulator, reduces the number of charge speckles, and increases the flashover voltage. These effects are enhanced as the doping concentration of ZnO nanoparticles increases. Through isothermal surface potential decay tests, we find that the ZnO/epoxy coating introduces numerous shallow traps, which help to dissipate the surface charge. Analysis shows that the ZnO/epoxy coating promotes surface charge dissipation through its nonlinear conductivity. The nonlinear characteristics of the 30wt%-doped samples are 55.3% greater than those of the 20wt%-doped samples. The coating adapts to the external electric field and can achieve the effect of a uniform electric field. This article analyzes the feasibility of using ZnO/epoxy coatings to suppress the accumulation of charge on insulator surfaces, and provides a feasible solution for practical engineering applications.

Numerical analysis of thermo-electric field for AC XLPE cables with different service times in DC operation based on conduction current measurement

This paper presents an estimate for the influence of service time on the conduction current characteristics of a 10 kV AC XLPE. The experimental conduction current results show that the space charge accumulation threshold increases with service time. Fourier transform infrared (FT-IR) and differential scanning calorimetry (DSC) experiments show increased thermo-oxidative aging and reduced crystallinity as the service time increases. This results in deeper traps in the XLPE material which can be verified from isothermal surface potential decay experiments. Based on the experimental results, the numerical analysis of thermo-electric fields is carried out for 10 kV AC XLPE cables with different service times under bipolar DC operation mode. It shows that the DC power for 10 kV AC XLPE cable in bipolar DC operation is higher than the AC power, whereas the DC operating voltage and DC power increases with the service time. In order to ensure a safe and long service life of the cables in DC operation, a conductor temperature of 60 °C and DC operating voltage of 11 kV are recommended for 10 kV AC XLPE cables in bipolar DC operation.