Surface Potential Decay Characteristics Research Articles

Improvement of breakdown and flashover properties against thermal aging of oil paper insulation through addition of TiO 2 nanoparticles

TiO2 nanoparticles can improve the electrical performance of oil paper insulation. In this study, 10 nm TiO2 nanoparticles were prepared and dispersed into transformer oil, and then immerse pressboards in it to obtain nanofluid impregnated pressboards (NP). Its thermal aging was accelerated together with the reference sample at 130°C for 36 days. Analyzing the acid value in the oil, polymerization degree, surface topography, AC breakdown strength, surface flashover characteristics and surface potential decay characteristics. It was observed that TiO2 nanoparticles can reduce the acid value and improve AC breakdown strength of oil. However, the degree of polymerization and surface appearance of NP and oil impregnated pressboards (OP) during thermal aging did not differ significantly. During the aging process, the energy level of pressboard surface traps shifted first towards the shallow and then gradually towards the deep. During all the aging stages, the surface flashover voltage of NP was always higher or close to that of OP. The mechanism of TiO2 nanoparticles improving the breakdown and surface flashover characteristics of oil–paper insulation during thermal aging is discussed from the perspective of water molecules and aging by-products adsorption and introduction of shallow traps by nanoparticles.

Space Charge and Trap Distributions and Charge Dynamic Migration Characteristics in Polypropylene under Strong Electric Field

Space charge accumulation in the polypropylene will accelerate the aging of the material and lead to the degradation of its insulation performance. In the work, space charge distribution, current conduction characteristics, thermally stimulated depolarization current (TSDC) and surface potential decay (SPD) characteristics of polypropylene (PP) under strong electric field are measured and analyzed, and the bulk trap and surface trap parameters are extracted. Further, the charge transport model of PP is established to study the charge dynamic transport physical processes and characteristics under strong electric field. The experimental results show that the charge accumulation amount in PP under the action of negative polarity electric field is higher than that of positive polarity electric field, about one order of magnitude. and the corresponding trap energy levels are 0.84 eV and 0.81 eV, which both belong to deep traps. There are two obvious charge density peaks on the PP surface, which are 2.60 × 1020·eV−1·m−3 and 3.66 × 1020·eV−1·m−3, respectively, and the corresponding surface trap energy levels are 0.86 eV and 0.97 eV. The simulation results show that with the extension of the applied voltage time, the injected charges by the electrode gradually migrate to the bulk of the material and eventually the positive and negative charges are offset at the middle position. The local electric field caused by the accumulation of interfacial charges will weaken the original electric field, while the local electric field caused by the accumulation of the bulk charges will strengthen the original electric field, resulting in the distortion of the internal electric field.

Study on water droplet initiated discharges and charge trap characteristics of laser textured silicone rubber nanocomposites

AbstractSilicone rubber micro‐nanocomposites were fabricated and laser texturing was performed on their surface to investigate super hydrophobic properties. Irrespective of type of texture (square, hexagon or diamond), the contact angle is almost the same but a marginal increase in contact angle is observed with increase in wt% of nano fillers in silicone rubber. Water droplet initiated corona inception voltage (CIV) is measured by fluorescence technique and the CIV tends to increase with texturing and is high under—DC followed with +DC and AC voltages. Fast Fourier Transform (FFT) analysis of the fluorescence signal due to corona discharge indicates that the energy content of the fluorescent signal lies in the range DC to 20 MHz. Surface temperature of the sample after water droplet initiated discharge is found to be high under DC rather than under AC voltages. Textured surface has a relatively lower temperature decay rate under AC and DC voltages profiles, compared to nontextured surface. The formation of shallow charge traps is predominant with the textured specimens. In general, the diamond and hexagonal micro textured surface specimen exhibit better performance in terms of water droplet initiated CIV, hydrophobicity, and surface potential decay characteristics.

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.

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.

Trap characteristics and its temperature-dependence of silicone gel for encapsulation in IGBT power modules

Silicone gel is a prevailing material for encapsulation in insulated gate bipolar transistor (IGBT) power modules. The space charge transport behavior in silicone gel is significant to evaluate the electrical insulation characteristics. This paper focuses on the trap characteristics and electrical properties of the silicone gel, which were rarely studied before. The experiments are performed on the surface potential decay of silicone gel after the charge injection. Then, the energy distributions of electron or hole traps are determined by a double-trap energy level model, which can be fitted by the Gaussian distribution. In addition, the mobilities of positive and negative charges are determined, which are 1.38 × 10 −12 m 2 ·V −1· s −1 and 1.74 × 10 −12 m 2 ·V −1 ·s −1 , respectively. Furthermore, considering the heat as a byproduct resulting in thermal issues, the temperature-dependence of surface potential decay characteristics are also studied in this paper. When temperature rises, the decay rate of surface potential increases, especially when the temperature is higher than 80 °C. Finally, the contrastive analysis illustrates that the trap characteristics of silicone gel are between the trap characteristics in liquid-state material and solid-state material, which supports the phenomenon that silicone gel is more resistive to the sharp edges in power modules. This work can provide a useful reference for the design of encapsulation in high-voltage IGBT power modules.

Understanding the electrical, thermal, and mechanical properties of epoxy magnesium oxide nanocomposites

Epoxy MgO nanocomposites exhibit high surface discharge inception voltage (SDIV) in SF6 gas ambience. Increase in SDIV is observed with 3wt% of MgO-added epoxy nanocomposites, especially when the SF6 ambient gas pressure is high. Impact of thermal ageing of epoxy nanocomposites on SDIV is minimum. Dielectric response studies were carried out experimentally and compared with modelling studies. The surface potential decay and charge trap characteristics of epoxy nanocomposites vary with percentage of nano filler. Thermal aging of epoxy nanocomposites has less influence on surface potential decay characteristics for 1 and 3 wt% MgO-added epoxy nanocomposites, indicating anti-thermal aging characteristics. Surface discharge activity generates UHF with its dominant frequency at around 1 GHz. The UHF signal magnitude formed due to SD activity is minimum with 3 wt% MgO-added epoxy nanocomposites. Laser-induced breakdown spectroscopy (LIBS) studies indicate that plasma temperature and threshold fluence increases with increase in wt% of MgO in epoxy resin. Tensile and flexural properties are greatly improved for nanocomposite compared to epoxy. The dynamic mechanical analysis (DMA) indicates increased storage modulus and reduction of tan δ with epoxy nanocomposites. The glass transition temperature (Tg) and activation energy increases with wt % of nano filler.

Dielectric properties and surface potential decay characteristics of low density polyethylene/ZnO microvaristor composites

This paper focuses on the effects of submicron ZnO microvaristors on the microstructure and electrical properties of low density polyethylene (LDPE) based composites. The submicron ZnO microvaristors are obtained by ball milling method and are used as a filler of LDPE based composites. The submicron composites with the filler loading of 0, 10, 20, 30 and 40 wt% are prepared by melt-blending method. The morphology, crystallization behavior, dielectric properties and surface potential decay characteristics are characterized by scanning electron microscope, differential scanning calorimetry, dielectric spectrum and surface potential decay measurement, respectively. Experimental results show that the average speed of the surface potential decay characteristics of LDPE based composites increases distinctly with the increasing filler loading. The submicron ZnO microvaristors are in favorable to suppress the charge accumulation of LDPE based composites and can be used to regulate the charge distribution in electrical and electronic equipment.

Influence of Salt Fog Test on Silicone Rubber Insulating Material Under AC and DC Voltages

The present work investigates the performance of electrically stressed silicone rubber insulating material in a cyclic salt fog test. Corona Inception Voltage due to water droplet initiated discharges changes after salt fog ageing. The Ultra High Frequency signals radiated during corona discharge activity under AC voltages have a frequency span of 0.8–1.5 GHz, whereas under DC voltages span lies in the range of 0.5–1.5 GHz. Contact angle reduces with the increase in number of salt fog ageing cycles. Surface potential accumulation studies indicate that salt fog aged silicone rubber shows characteristic variation in accumulated potential magnitude and the decay characteristics. Further tdv/dt of surface potential decay characteristics enables to understand the charge penetration depth. Weight loss studies and surface roughness analysis during salt fog test are used to quantify the amount of damage.

Field-dependent electric conductivities of silicone rubbers deduced from measured currents and surface potential decay characteristics

Measurements of DC electric conductivities of four types of silicone rubber-based polymers for use in high voltage insulation systems of power components are reported. The field dependences of the conductivities obtained by two different techniques, namely by utilizing steady-state currents through materials’ samples placed between metallic electrodes and by employing surface potential decay characteristics in an open circuit configuration, are compared and discussed. It is shown that the surface potential decay technique allows for a wider range of electric field strength and reduces the time span of the measurements.

Effects of arc ablation on surface electrical performance of epoxy resin insulation

Operation of a high-voltage, direct current circuit breakers can cause irreversible damage to the surface electrical performance of insulation due to arc ablation. To analyze the failure mechanism, the epoxy resin (EP) insulation was ablated under experimental arcs of different energy values. The rated current of the experimental arc is 5 A, and the ablation energy values were set as 0 J, 31.3 J, 78.2 J, 109.6 J and 156.5 J respectively by changing the arc duration. The destruction of the EP surface molecule structure was investigated by using x-ray photoelectron spectroscopy and gas chromatography. Furthermore, the surface potential decay and distribution characteristics of surface traps were studied by using the isothermal surface potential decay (ISPD) method. The test results indicate that when arc energy reaches 156.5 J, the DC flashover voltage of EP decreases by 64.7% and the surface conductivity increases by 3.22 times, compared with that of the virgin sample. Reductive groups in EP molecules are oxidized when suffering arc ablation, thereby leading to the breakage of molecular chains and severe carbonization, forming amorphous carbides with high conductivity on EP surface. ISPD results show that the density of deep traps and the center energy level of traps on the EP surface increase when the arc energy increases. These traps can form multiple charge centers and distort the electric field distribution on the EP surface. Thus, the partial discharge is promoted and flashover finally occurs, thereby decreasing the surface electrical strength of EP.

Temperature dependent surface potential decay and flashover characteristics of epoxy/SiC composites

GIS spacers made of epoxy resin always suffer from the flashover due to surface charge accumulation. Therefore, it is important to seek a new method to improve the performance of epoxy resin. The epoxy/SiC composites have been proven to have nonlinear conductivity and the potential of being used in the spacer manufacture because of their field grading function under DC voltage. A high temperature rise on the central conductor of GIS can cause a large temperature gradient along the spacers. Exploring the effects of temperature on the electrical properties of the epoxy/SiC composites is helpful in the design the space dependent SiC filler content in GIS spacers. In this paper, three temperatures 20, 50 and 70 °C were studed on the temperature dependent surface potential decay (SPD) and flashover characteristics of the epoxy/SiC composites. At 20 °C, the epoxy/SiC composites, especially the 14 vol % samples, have much better SPD performances and higher flashover voltages than the pure epoxy. As the temperature increases, the SPD performances of all samples are greatly improved. The SPD rate of the 0 vol % samples approaches that of the 6 vol % samples at 50 °C and the 14 vol % samples at 70 °C. When the temperature is over 50 °C, flashover of the epoxy/SiC composites occur at lower voltages compared with the pure epoxy resin. With a further increase of temperature from 50 to 70°C, the flashover voltages of all samples have a significant drop, indicating that the temperature effects on flashover fault must be included in the spacer design and manufacture.

Dielectric and Carrier Transport Properties of Silicone Rubber Degraded by Gamma Irradiation.

Silicone rubber (SiR) is used as an insulating material for cables installed in a nuclear power plant. Gamma rays irradiated SiR sheets for various periods at temperatures of 145 and 185 °C, and the resultant changes were analyzed by examining complex permittivity spectra and surface potential decay characteristics. Three different processes, namely, instantaneous polarization, electrode polarization due to the accumulation of ions to form double charge layers at dielectric/electrode interfaces, and DC conduction caused by directional hopping of ions, contribute to the complex permittivity. By fitting the spectra to theoretical equations, we can obtain the dielectric constant at high frequencies, concentration and diffusion coefficient of ions and DC conductivity for the pristine and degraded samples. The instantaneous polarization becomes active with an increase of dose and ageing temperature. The thermal expansion coefficient estimated from the temperature dependence of dielectric constant at high frequencies becomes smaller with an increase in dose, which is in good agreement with the experimental results of the swelling ratio. Additionally, trap distributions are calculated from surface potential decay measurements and analyzed to explain the variation in conductivity. Trap energy increases firstly, and then decreases with an increase in dose, leading to a similar change in DC conductivity. It is concluded that generations of both oxidative products and mobile ions, as well as the occurrence of chain scission and crosslinking are simultaneously induced by gamma rays.

Nonlinear conduction and surface potential decay of epoxy/SiC nanocomposites

The nonlinear conduction of polymeric insulation may solve the surface charge accumulation, avoiding the distortion of electric field and the flashover phenomenon. As the silicon carbide (SiC) powder is usually used to improve the nonlinear conduction of polymer composites, the epoxy/SiC nanocomposites are prepared and the dc conductivity, permittivity and isothermal surface potential decay characteristics are investigated. The experimental results show that the conduction of high filler loading is field dependent and is attributed to a new conduction mechanism. The permittivity is closely related to the interaction zone around nanoparticles. The isothermal surface potential decay characteristics are greatly affected by the charging polarity, charging level and filler loading. The investigation indicates that the nonlinear conduction plays an important role in the surface potential decay.

Dielectric charging research status and prospect

Dielectric charging phenomenon, caused by the interaction effect between space environment and spacecraft material, has a severe threat to the normal operation of spacecraft. Especially, the problem becomes more serious with the improvement of spacecraft operation voltage. An overview of dielectric charging research status, exsiting problems and prospect are reviewed. Firstly, dielectric charging phenomenon, hazards, research history and new challenges are introduced. There have five development stages: the initial attention, the focus of surface dielectric charging, the focus of deep dielectric charging, the rapid development of dielectric charging and new challenges and demands. The development demands of spacecraft dielectric in China are analysed. There have several insulation positions, which are easy to occur fault in practice. They include surface flashover on insulation material bewteen solar cells of solar array, surface flashover on insulation material between electrodes of solay array drive assembly, the insulation material in cables and pulse discharge of circuit board. Secondly, the specific research status and exsiting problems of surface dielectric charging and deep dielectric charging are summarized from the following aspects: dielectric charging mechanism, dielectric charging mitigation and numerical simulation. Next, the top issues about dielectric charging experiments and material characteristics are introduced. They include in-situ surface potential decay measurement and characteristics of dielectric radiated by electrons, in-situ space charge measurement and material characteristics under electron radiation, surface flashover characteristics of dielectric radiated by electrons in vacuum. Two kinds of space charge in-situ measurement, short-circuit PEA (Pulsed Electro-Acoustic) and open PEA are introduced. At present, there do not have the mature in-situ space charge measurement setup under electron radiation in China, the difficult points in design and debugging are analysed. It is interesting of the surface flashover of dielectric under electron radiation. The latest experimental results indicated that the flashover voltage shows a distinctly different variation tendency under the two cases (after electron radiation and during electron radiation). The first case of after electron radiation, flashover voltage becomes higher than that without electron radiation. Because of the effect of deposited electrons in dielectric surface layer and the changes of trap distribution inside material. The trap distributions of polyimide after electron radiation are calculated based on surface potential decay measurement and Simmons theory. The results show that the shallow trap level increases slightly with the increase of electron energy (3–11 keV), ranging from 0.81–0.85 eV, while the deep trap level remains unchanged about 0.94 eV; the second case of during electron radiation, flashover voltage exhibited a dramatic drop from 26.9 kV (energy: 5 keV) to 10.5 kV (energy: 30 keV), due to the combined effect of incident kinetic electrons flowing to dielectric surface and deposited electrons in dielectric surface layer. Finally, the key scientific issues in the filed of space dielectric charging are put forward and prospected based on the present research status.

Surface potential decay on material samples taken from in-service aged HVDC silicone rubber composite insulators

Surface potential decay characteristics of specimens extracted from in-service aged and reference HVDC composite insulator sheds are reported and analyzed in this paper. In the experiments, surfaces of the insulator samples were charged by dc corona and the decay was recorded utilizing a non-contact technique. In addition, surface and bulk conductivities of the specimens were determined and used for analyzing by means of computer simulations their impact on the potential distribution profiles and its decay. Based on the performed experiments, trap density distributions, mobility of charge carriers and field dependent bulk conductivities are deduced for the investigated samples with the aim to evaluate the ageing severity.

Charge Transport in LDPE Nanocomposites Part I-Experimental Approach.

This work presents results of bulk conductivity and surface potential decay measurements on low-density polyethylene and its nanocomposites filled with uncoated MgO and Al2O3, with the aim to highlight the effect of the nanofillers on charge transport processes. Material samples at various filler contents, up to 9 wt %, were prepared in the form of thin films. The performed measurements show a significant impact of the nanofillers on reduction of material’s direct current (dc) conductivity. The investigations thus focused on the nanocomposites having the lowest dc conductivity. Various mechanisms of charge generation and transport in solids, including space charge limited current, Poole-Frenkel effect and Schottky injection, were utilized for examining the experimental results. The mobilities of charge carriers were deduced from the measured surface potential decay characteristics and were found to be at least two times lower for the nanocomposites. The temperature dependencies of the mobilities were compared for different materials.

Research on surface potential decay characteristics of epoxy resin charged by direct current corona

Surface charge accumulation and decay behaviors of dielectric materials are the key factors restricting the development of high voltage direct current power equipment. For flat samples, the density of surface charges deposited by corona can be regarded as a linear change with the surface potential. For this reason, the behavior of surface charge decay can be directly related to that of surface potential. According to the corona charging process, the surface charge deposition and detrapping process, as well as the charge transport process in the bulk, we may establish a physical model dynamic response to the surface potential. Influences of grid voltage, relative permittivity, and bulk conductivity on the surface potential decay process can be obtained through calculating the surface potential decay behaviors of epoxy resin. The higher the grid voltage, the faster the surface potential decays. At the typical parameter value of epoxy resin (relative permittivity 3.93, bulk conductivity 10-14 S m-1), the normalized decay rate can be fitted by two straight lines in a log-log plot; moreover, the calculated results show a linear variation of power factors with the grid voltage, while the power function shows a relationship between the characteristic time and the grid voltage. The bigger the relative permittivity, the slower the surface potential decays. In the typical parameter area of epoxy resin (relative permittivity 3-4), the surface potential decay time constant increases from 1720 s to 2540 s, showing a linear variation. Also the bigger the bulk conductivity, the faster the surface potential decays. In the typical parameter area of epoxy resin (bulk conductivity 10-15-10-13 S m-1), the surface potential decay time constant decreases from 24760 s to 260 s, showing a power function relationship.

Potential decay on silicone rubber surfaces affected by bulk and surface conductivities

A decay of electric surface potential on a pre-charged gas-solid interface involves several charge transport processes and each of them may become dominant under certain environmental conditions (temperature, pressure, humidity). In the present paper, surface potential decay on flat samples of different kinds of silicone rubber used in HVDC applications is analyzed at reduced pressures of ambient air that allows for minimizing the involvement of the gas phase, i.e. surface charge neutralization by gas ions. Effects imposed solely by bulk and surface conduction in the solid material are studied experimentally and by means of computer simulations. The results allow for evaluating threshold values of volume and surface electric conductivities at which these mechanisms become most essential. Field dependent bulk conductivities are deduced from the surface potential decay characteristics obtained for the studied materials.