Surface Potential Decay Research Articles

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.

Understanding the surface condition of gamma irradiated epoxy alumina nanocomposites adopting wavelets and fractal technique

The influence of alumina nanofiller and gamma irradiation on the surface potential variation of epoxy-alumina nanocomposites was investigated. The surface potential decay rate of nanocomposites has increased and the trap depth decreased with alumina nanoparticles addition to the matrix as well as upon exposure to gamma irradiation, Surface roughness was estimated using the wavelets and fractal technique. Daubechies wavelet of order 4 (db4) wavelet was chosen as the most suitable mother wavelet for surface roughness measurement. Multi resolution signal decomposition (MRSD) analysis of surface profile has revealed that with increasing wt% of alumina nanofiller in the nanocomposites, reduction in surface roughness of nanocomposites was observed. Upon gamma irradiation, the surface roughness factor at each level of MRSD has increased marginally. Fractal dimension and lacunarity were calculated for unaged and gamma ray irradiated samples and it exhibits inverse correlation.

Enhanced thermal and dielectric performance of epoxy resin/aluminum nitride nanocomposites at high temperatures

AbstractThe effects of nano‐aluminum nitride (nano‐AlN) on the thermal conductivity and dielectric performances of epoxy resin (EP) are investigated. The addition of nanoparticles in the materials is characterized. The trap characteristics of the samples are estimated using the surface potential decay (SPD) measurements at high temperatures. The results show that the nano‐AlN fillers are well dispersed in the nano‐AlN composites. The thermal conductivity consistently increases with an increase of the nano‐AlN content. The DC breakdown strength of the EP is enhanced with the addition of the nanoparticles as well, especially for the 0.5 and 1 wt% EP/AlN composites. The DC conductivity of the EP/AlN composites increases more obviously at the high temperature. The initial surface charge density of the samples with the same content of nano‐filler decreases with the increase of the temperature. Both the addition of nanoparticles and high temperature can improve the apparent carrier mobility of the EP sample. The trap density and trap level of the deep traps decreases with the increase of the temperature. It is postulated that shallow traps influence the voltage decay in the EP samples and increase the activation energy and the apparent carrier mobility of the specimens at the high temperature.

Comparative analysis of isothermal decay of the surface potential of fluoroethylenepropylene electrets and of the sensitivity of electret microphones at elevated temperature

It is well known that fluorocarbon electrets, although thermally very stable, suffer a surface potential decay if exposed to high temperatures. This potential decay is of considerable interest in applications, for example, in the so-called prepolarized microphones. As a result, these devices suffer a loss in sensitivity approximately proportional to the decay of the electret surface potential. Since the potential and the related sensitivity losses are very slow at room temperature, a common approach in the literature is to perform accelerated isothermal depolarization experiments at elevated temperatures, and extrapolate the results to lower temperatures by assuming an Arrhenius-type behavior. In this paper, we investigate experimentally the potential decay of differently pre-annealed fluoroethylenepropylene electrets of different thicknesses, as well as the drop of sensitivity of commercially available measurement microphones from several manufacturers by the exposure to an ambient temperature of 95 °C for up to three years. Until now, no other reports compare electret and microphone decays over such a long period. The experimental data presented here could not be fitted with only one exponential decay function over the whole time-span investigated. However, assuming two or more discharge processes results in a good agreement between measurement and model functions. The time constants of these decay processes are specified in the text.

Effect of Aging under AC Superimposed Harmonic Voltage on Trap and Carrier Transport Properties of Silicone Rubber

The rapid growth of power grid capacity and the widespread use of a large number of power electronics and non-linear loads have led to harmonics in the power system. Harmonics in the power system will cause safety hazards to the normal operation of power equipment, exacerbate the aging of insulation materials, and reducing the overall operation reliability of the system. In the present work, we used power frequency ac voltage superimposed harmonics to carry out ageing experiments on power cable terminals. Then, we tested the infrared spectra, dielectric spectra, electrical conductivity, and surface potential decay characteristics of silicone rubber insulation materials on the cable terminals aged for different times. The experimental results show that the dielectric constant and dielectric loss of silicone rubber gradually increase with the aging time. In particular, the dielectric loss of silicone rubber changed greatly at low frequencies. The effect of dc conductance of aged silicone rubber on dielectric loss is significantly enhanced at low frequencies, which causes the dielectric loss to increase as the frequency decreases following an inverse power law. The surface potential decay rates of silicone rubber insulation after positive and negative corona charging accelerate with increasing the aging time, which is consistent with the experimental results of electrical conductivity. By analyzing the distribution characteristics of electron and hole traps in silicone rubbers, it is found that the trap energy levels of electron and hole traps become shallower as the operating time increases. The calculation of the carrier hopping conduction model shows that the shallow trap formed with increasing the aging time will lead to increases in both carrier mobility and conductivity. When the conductivity rises to a certain value, the silicone rubber will lose its insulation performance, resulting in insulation failure.

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.

Surface potential decay and DC surface flashover characteristics of DBD plasma-treated silicone rubber

This paper presents an investigation on DC flashover voltage of silicone rubber (SiR) improved by dielectric barrier discharge (DBD) plasma treatments under ambient atmospheric pressure air. DC surface conductivity, surface potential decay (SPD), DC surface flashover voltage, partial discharge magnitude, Fourier transform infrared (FT-IR) spectrograms, and surface water contact angles are measured to analyze the influence of plasma treatment on the SiR. It is found that the speed of SPD increase consistently with the plasma modification time. The tendency of flashover voltage is increasing at first and then decreasing with the increased time of the plasma treatment. The magnitude and number of partial discharge pulses increase apparently with the increased plasma treatment time. Physicochemical measurements indicate that more amount of polar groups appear on surface after the DBD plasma modification, whereas the surface water contact angles decline continuously with the increased plasma modification time. However, the hydrophobicity is recovered after 30 d exposure in the air. It is demonstrated that the SPD is accelerated significantly due to the increased surface conductivities and density of shallow traps. However, the reduction of flashover voltage after longer time of the plasma treatment is attributed to the increased mobility of charge carriers on the sample surface.

Temperature and Field Induced Variations of Electric Conductivities of HTV Silicone Rubbers Derived from Measured Currents and Surface Potential Decay Characteristics

The temperature and field dependencies of electric conductivities of two types of silicone rubber-based polymers intended for use in high voltage direct current applications are presented and discussed. The conductivities obtained with the standard method by measuring a current through the material sample placed between metallic electrodes in response to the applied voltage are compared with those deduced from the measured potential decay on pre-charged material surface in an open circuit configuration. The measurements were conducted in the range of the applied electric field strength (0.5–5) kV/mm and temperatures ranging from 22 °C to 70 °C. It is shown that the values of the conductivities obtained by the two methods are in agreement and their temperature dependences obey Arrhenius law yielding similar activation energies.

Understanding the impact of gamma irradiation of epoxy titania nanocomposites on surface and bulk charge characteristics

Epoxy titania nanocomposites were prepared under optimum process conditions through shear mixing of titania nanoparticles in to epoxy resin, for its potential application as insulant in nuclear power plants and space applications. The complex intrinsic nature of properties, its characteristic variation due to ageing of nanocomposite insulating material upon its continuous exposure to gamma irradiation, and their charge trap and space charge characteristics are explored. Surface potential variation studies were carried out under DC voltage. In the present study, the charge trap performance was assessed under switching impulse voltage. It is observed that surface potential decay and shallow trap formation are high with gamma irradiated specimen. In addition, the potential decay is high under switching impulse voltage compared to DC voltage. Also, the trap depth formed is less under switching impulse voltage compared to DC voltage and it is high under negative DC voltage. The space charge analysis through Pulsed electro acoustic (PEA) studies has shown increase in accumulation of space charge and enhancement of electric field with increase in dosage of gamma-irradiation. Polarity reversal tests have revealed that the electric field enhancement is high before reversal of polarity, irrespective of level of gamma irradiation dosage. The direct correlation between characteristic variation in trap depth values with the gamma irradiated specimen and its contact angle was observed.

Investigation of surface strain by digital image correlation and charge trap characteristics of epoxy alumina nanocomposites

Epoxy alumina nanocomposites were fabricated through shear mixing of alumina nanoparticles in to epoxy resin under optimum process conditions for its potential application as insulator in transformer. The fundamental insight in to the influence of ageing of nanocomposite insulator due to its continuous exposure to UV irradiation and water in terms of their charge trap characteristics was explored. The surface potential decay and strain variation of nanocomposite samples were measured by using Electrostatic voltmeter and Digital Image Correlation technique (DIC), respectively. Diffusion of water into epoxy alumina nanocomposite is less compared with pure epoxy resin. It has been observed that surface potential decay rate and trap depth have shown increasing and decreasing trend, respectively, with the addition of alumina nanoparticles in to epoxy resin. The UV and water aged nanocomposites exhibited a decreasing trend of surface potential decay rate and the governing reasons could be due to surface damage causing increase in deep trap formation. The tensile strength and stiffness of the samples have increased with the addition of alumina nanoparticles in to the epoxy resin. It could be due to the better bonding of alumina particles with epoxy resin. The surface strain induced in to the sample during tensile loading and the surface potential decay rate of nanocomposite has shown a direct correlation, as observed in the present work.

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.

Electrical tree in silicone rubber: Roles of silicone grease and switching impulses

This study investigates the characteristics of electrical trees in silicone rubber (SiR) samples after soaking in silicone grease (SG) for various times and exposed to switching impulses. The results show that the tree initiation probability increases from 60 to 100 % after SG soaking for 340 h when the impulse amplitude is 38 kV and the tree fractal dimension increases from 1.16 to 1.45. A similar trend is observed under negative impulse. It is suggested that SG soaking leads to a higher tree initiation probability and a larger tree fractal dimension. In addition, isothermal surface potential decay measurements are employed to obtain the trap parameters of SiR samples. Equilibrium swelling procedures are used to test the crosslinking density. The energy level and density of deep trap both decrease with SG soaking time, which is beneficial for the transport of charge carriers injected from the needle tip and further results in more severe scission of SiR molecular chains. Moreover, the expansion of free volume originating from the broken physical crosslinking structure allows more violent impact ionization of charges to occur, which can also aggravate the scission of SiR molecular chains. Therefore, the decreased tree resistance can be found in SiR after SG soaking. Furthermore, the polarity effects can be observed during the tree initiation and propagation process, which are mainly rooted in the different distribution of hole­type and electron-type traps.

Causation of ultra-high surface insulation of Bi0.95Y0.05FeO3/epoxy composites: Simultaneous sine-variations of dielectric and trap properties with filler content

Composite dielectrics with high vacuum insulation properties are integral to today's advanced energy transmission. In this study, 5% mol Yttrium doped BiFeO3 sub-particles were synthesized by a facile sol-gel route and then were blended with epoxy resin as filler to prepare the epoxy composites (BY5FO/EP) with a wide range of filler content. The multi-electrical investigations were employed, and the results show the BY5FO/EP composites present ultra-high surface insulation performance. As the concentration of filler increases, the dielectric constant of the BY5FO/EP composites exhibited sine-shaped variations. Considering the multiple-layers of filler particles, we determined the dielectric model of composites for a wide range of filler contents by simulation. Meanwhile, the trap depths of BY5FO/EP composites were calculated according to Isothermal Surface Potential Decay, and we found the trapping stability is of sinusoidal relevance with the filler content. The simultaneous sine-variations of dielectric polarization and trap depth lead to the vacuum surface breakdown strength of the composite increased by 111% with the optimal filler content. The results in this paper provide a prospective strategy to realize ultra-high surface insulation in a vacuum and evidence to the applicability of the Electron Triggered Polarization Relaxation theory.

Electrical modeling of dielectric barrier discharge considering surface charge on the plasma modified material**Project supported by the National Key R&D Program of China (Grant No. 2018YFB0904400), the National Natural Science Foundation of China (Grant No. 51977187), the “Science and Technology Innovation 2025” Key Project of Ningbo City, China (Grant No. 2018B10019), the Natural Science Foundation of Zhejiang Province, China (Grant

We present the variations of electrical parameters of dielectric barrier discharge (DBD) when the DBD generator is used for the material modification, whereas the relevant physical mechanism is also elaborated. An equivalent circuit model is applied for a DBD generator working in a filament discharging mode, considering the addition of epoxy resin (EP) as the plasma modified material. The electrical parameters are calculated through the circuit model. The surface conductivity, surface potential decay, trap distributions and surface charge distributions on the EP surface before and after plasma treatments were measured and calculated. It is found that the coverage area of micro-discharge channels on the EP surface is increased with the discharging time under the same applied AC voltage. The results indicate that the plasma modified material could influence the ignition of new filaments in return during the modification process. Moreover, the surface conductivity and density of shallow traps with low trap energy of the EP samples increase after the plasma treatment. The surface charge distributions indicate that the improved surface properties accelerate the movement and redistribution of charge carriers on the EP surface. The variable electrical parameters of discharge are attributed to the redistribution of deposited surface charge on the plasma modified EP sample surface.

On the Discharge Transport Mechanisms Through the Dielectric Film in MEMS Capacitive Switches

A method that allows the investigation of discharge transport mechanism in dielectric films used in MEMS capacitive switches or MIM capacitors or even bare dielectric films is presented. The method is based on monitoring the dielectric film surface or MIM top electrode potential decay rate and the dependence of conductivity on surface potential. The method has been applied in MEMS and MIMs with simultaneously deposited dielectric film to confirm the closeness of transport data in MIM and MEMS devices. The impact of non-uniform charging on local discharge rate and different dielectric film stoichiometry has been also assessed. Finally, the effect of surface leakage due to environment humidity has been also investigated. [2019-0159].

Surface charge dissipation and DC flashover characteristic of DBD plasma treated epoxy resin/AlN nanocomposites

This paper presents the mechanism of the surface charge dissipation and flashover voltage of epoxy resin (EP) with nano aluminum nitride (AlN) composites enhanced by dielectric barrier discharge (DBD) plasma treatment in open air. Surface conductivity, surface potential decay (SPD), flashover voltage, Fourier transform infrared (FT-IR) and surface roughness were measured to evaluate the electrical and physicochemical properties of EP/AlN nanocomposite samples before and after the plasma treatments. It is found that surface conductivity is evidently increased with the increase of the plasma treatment time. Moreover, the SPD of the plasma treated samples after positive and negative corona charging are enhanced. However, the positive charge dissipation is influenced more significantly by the treatment. Both the positive and negative flashover voltage are improved after the plasma treatment, and the amplitude of negative flashover voltage is higher than the positive flashover voltage in each case. Physicochemical measurements show that the content of polar groups is improved after the plasma treatment, and the surface roughness is continuously increased with the increment of the plasma treatment time. The calculated trap distribution demonstrates that the amount of both shallow holes trap and shallow electron trap are increased after the plasma treatment. This investigation attributes the enhanced SPD and improved flashover voltage to the increased surface conductivity, the more amount of shallow traps and the improvement of the sample surface roughness.

Temperature-dependent DC conductivity and space charge distribution of XLPE/GO nanocomposites for HVDC cable insulation

Nanocomposites have been proven to be an important dielectric material, which provides advanced dielectric properties of insulation to power equipment. For the use of polyethylene (PE)-based nanocomposites in HVDC cable insulation, their temperature dependence needs to be considered, because it could deeply affect the conductivity and space charge behavior. To investigate the temperature dependence of conductivity and space charge properties of the cross-linked polyethylene (XLPE)/graphene oxide (GO) nanocomposites, XLPE samples with GO mass fractions of 0, 0.001, 0.01 and 0.1 wt% were prepared. The conductivity, space charge behavior and breakdown strength of the nanocomposites were tested. Then, the surface potential decay (SPD) was measured to elucidate the mechanism of the GO nanoparticles in the XLPE matrix according to the trap theory. The obtained experimental results showed that the XLPE/GO nanocomposites with the mass fraction of 0.01 wt% exhibited a lower conductivity, lower space charge accumulation, and higher DC breakdown strength than those of the neat XLPE. This observation is related to the nanoparticle-polymer interaction regions and the deep traps introduced by the GO nanoparticles. In addition, the temperature dependence of the electrical properties of nanocomposites decreased when appropriate GO amounts were added. This result indicates that the XLPE/GO nanocomposites are more suitable for use in HVDC cable insulation.