Isothermal Surface Potential Decay Research Articles

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.

Temperature gradient dependence on electrical tree in epoxy resin with harmonic superimposed DC voltage

Transient superimposed voltage, which is accompanied by a temperature gradient caused by Joule heat, can threaten the insulation safety of high-voltage direct current (HVDC) cable terminations. In this study, an electrical treeing experiment was conducted on epoxy resin with temperature gradient under harmonic superimposed DC voltage. The temperatures in the high voltage side and ground side were controlled by heaters within that maintained the range of 30 to 120 °C, while the DC voltage was ±12 kV. An isothermal surface potential decay (ISPD) experiment was used to obtain the trap level distribution. The effects of the temperature gradient and voltage polarity on the tree growth were depicted by the tree morphology, tree length and accumulated damage. The results show that the temperature gradient affects the tree structure and accelerates its growth. The tree growth rate has a nonlinear trend under an increased temperature in the high voltage side, and shows a steady increase under an increased temperature in the ground side. The positive voltage has a more obvious acceleration effect on the tree growth than the negative voltage does. A charge transport model is proposed to describe the tree behavior under a temperature gradient. The nature of the charge carrier accounts for the trap distribution and polarity effect. The local temperature rise and superimposed voltage pose a threat to the insulation safety, and more attention should be given to the operation of the electrical apparatus.

Trap distribution of polymeric materials and its effect on surface flashover in vacuum

The surface trap parameter can significantly affect the development of surface flashover in vacuum, but the effective mode and mechanism are not very clear yet. The trap parameters of three polymeric materials were tested and calculated by means of isothermal surface potential decay. The flashover experiment was developed under different applied voltages. The results show a positive correlation between the withstand voltage and the deep trap, i.e., the deeper trap energy level is, the higher flashover voltage is. The dynamics process of charge trapping and de-trapping was analyzed based on the charge transport model in dielectrics with a single trap level and two discrete trap levels. The time of charge trapping was compared with that of the discharge development. The results show that the charge trapping time is longer than the flashover development time. The way to influence flashover for a trap is not to decrease the secondary electrons in single discharge development, but to change the electric field distribution on the dielectric surface by charge capture.

Trap Level Characteristics and DC Breakdown Performance of Isotactic/Syndiotactic/Atactic Polypropylene Blend Insulation

Polypropylene (PP) material as a DC cable insulation material has great prospects as a recyclable alternative to cross-linked polyethylene. This paper aims at investigate the effect of PP blends crystallinity morphology and trap energy level distribution on DC breakdown strength. Five different ratios iPP/sPP/aPP blends samples are prepared by melt-blending method. Polarizing optical microscope (POM) experiments are conducted to characterize the crystallinity morphology of five iPP/sPP/aPP blends and isothermal surface potential decay (ISPD) methods are employed to measure the trap levels distribution in composite dielectric. The DC breakdown strength is tested by ball-plate electrodes equipment, and the result of DC breakdown experiment indicates that sPP30 exhibits the highest breakdown field strength, which is 29.2% higher than sPP0. The addition of sPP can act as a nucleus to help the crystallization behavior of iPP, and the distribution of spherulites in sPP30 is the most uniform and dense among the five iPP/sPP/aPP blends samples, which can improve the trap characteristics of polymers. The ISPD results show that the decay rate of the sPP30 sample is the slowest at 50 °C, 70 °C and 90 °C, indicating that sPP30 is more conducive to suppressing the dissipation process of carriers during the depolarization process. Moreover, sPP30 has higher deep trap density and lower shallow trap density and exhibits superior breakdown performance. It is deduced that the addition of sPP can effectively regulate the crystallinity morphology of iPP, and then effectively improve the trap energy level distribution of iPP/sPP/aPP blends, and it shows the improvement of DC breakdown performance macroscopically.

Effect of Moistures on Space Charge and Trap Level Characteristics of Oil-Impregnated Pressboards

Oil-paper insulation as an irreplaceable insulation system in the electrical power transformer is inevitably suffering the water intrusion on working condition, which will make dielectric performance degraded and insulation aging accelerated. In high interest of explaining the deterioration effect of absorbing moisture on the dielectric properties of oil-paper insulation system and corresponding mechanism, the space charge and trap level characteristics are investigated for the specially prepared oil-impregnated pressboards with different moisture contents. The Pulsed Electro Acoustic and Isothermal Surface Potential Decay experiments are performed to test the space charge distributions and evaluate the trap level depths, respectively. It is indicated that the polarity of injected space charges is alternated with the decaying rate being raised when the absorbing moisture is aggravated in oil-impregnated pressboards. The charge carriers are more prone to be captured by the shallow-level traps that have been introduced by water permeation near pressboard surface than the intrinsic deep-level traps inside pressboard. The captured charge carries accounts for the significantly exacerbated space charge accumulation as manifested by the surface charges of oil-impregnated pressboards.

The Evolution of Trapping Parameters on Three-Layer Oil-Paper of Partial Discharge Degradation for On-Board Traction Transformers

Trap plays an important role in the charge transportation of three-layer oil-paper and the investigation of trap parameters at different degradation degree is of great significance for further research of the partial discharge (PD) mechanism. This article considers the turn-to-turn structure of on-board traction transformer. A defect model of three-layer oil-paper insulation was designed for PD test. The PD degradation samples are selected from the inception of PD to the breakdown of PD to study the characteristics of surface potential decay on degradation samples by using isothermal surface potential decay method. Given that, the trap distribution at different degradation stages was calculated and its influence on PD characteristics was investigated. The result shows that AC fluctuation voltages caused by inter-harmonic has a great influence on PD distribution, surface charge distribution and trap distribution. With the increase of degradation, the decay rate of surface potential increases and the decay rate of AC fluctuation voltages is larger than that pure AC. And also, the number of shallow trap with AC fluctuation voltages is larger than that with pure AC. The rate of free charge increases with the number of shallow trap increasing. More free electrons involves in the process of PD, accelerating the aging rate of three-layer oil paper samples. Therefore, it can be concluded that surface charge and trap distribution are helpful to clarify PD mechanism.

Crosslinking dependence of trap distribution and breakdown performance of crosslinked polyethylene

The DC breakdown performance and trap distribution of crosslinked polyethylene (XLPE) with different crosslinking degrees were investigated in this paper. Results demonstrate that the breakdown strength initially rises from 276 to 352 kV/mm, followed by a decline to 300 kV/mm with the increase of crosslinking degree from 0 to 79.5%. The peak value occurs at the crosslinking degree of 52.1%. The deep trap density determined by isothermal surface potential decay (ISPD) displays a similar trend, sharing a same turning point with breakdown strength. In addition, an increasing crosslinking degree will lead to a lower crystallinity and an expanding amorphous region, while the content of by-products shows no obvious difference in XLPE samples due to the degassing process. It is therefore proposed that the crosslinking process can change the morphology of XLPE and influence the trap distribution. The formation of crosslinking network will increase the deep trap density, whereas the expanding amorphous region will result in the decrease of it. The increased deep trap density can cause homo-charges accumulation near the surface of XLPE and further suppress the charges injection from electrode. On the other hand, the charges in the bulk are easier to be captured and thus retard the charge transport. Dual dynamic processes contribute to the promotion of breakdown performance for XLPE.

Dielectric relaxation and trap-modulated DC breakdown of polypropylene blend insulation

This work focuses on the dielectric relaxation of isotactic polypropylene (iPP), syndiotactic polypropylene (sPP) and atactic polypropylene (aPP) blended insulation. The relationship between dielectric relaxation and trap characteristics, as well as their effects on the breakdown properties of polypropylene insulation are investigated. Frequency domain spectroscopy (FDS) is employed to characterize the dielectric relaxation of PP blends at 50, 70 and 90 °C. Isothermal surface potential decay (ISPD) measurements are carried out to obtain the trap characteristics, and the dc breakdown strength is tested; thus, the mechanisms of trap characteristics on the dielectric relaxation and breakdown performance are analysed. The dielectric relaxation processes α and δ of PP blends can be attributed to segmental motions and carrier hopping, respectively. The α relaxation strength (Δεα) decreases with the addition of sPP, since the intermolecular interaction factor (g) between PP molecules is weakened. The activation energy of δ relaxation is proportional to the shallow trap level, since the increased shallow trap level can pose higher potential barriers for carrier hopping. The higher crystallinity of PP blends can induce more deep traps in the boundaries of spherulites, and it can reduce the number of shallow traps that are mainly distributed in amorphous regions. The increased deep trap density serves a function to reduce the mean free path of charges and restrain impact ionization, resulting in an enhancement in the breakdown field.

Plasma fluorination of BaTiO3 for enhancement of interfacial adhesion and surface insulation of epoxy resin

Composite dielectrics with excellent electrical properties are integral to today’s energy industries and power infrastructure. Epoxy resin filled with inorganic nano-fillers has been extensively studied for its excellent performances. In this paper, barium titanate (BaTiO3) nanoparticles are selected as the fillers and fluorinated by plasma treatment. The morphology, element content and chemical bond of BaTiO3 and fluorinated BaTiO3 fillers are characterized by SEM, EDS, XPS and FTIR, respectively. The influence of filler concentration and fluorination on flashover voltage of epoxy resin composites was studied. We found that the fluorinated BTO could improve the compatibility of filler and matrix and surface insulation of epoxy resin composites. Finally, the relationship between filler concentration, dielectric constant, fluorination treatment and charge dissipation was analyzed, and the trap distribution on the surface of composites was calculated by isothermal surface potential decay method. Fluorinated BaTiO3 particles as fillers blended with epoxy resin have great engineering application potential.

Accelerated Charge Dissipation by Gas-Phase Fluorination on Nomex Paper

The surface charge and space charge accumulation in paper used in oil–paper insulation system may distort electric field distribution and lead to the flashover and breakdown of insulation system. In this paper, the effect of gas-phase fluorination on the surface charge and space charge characteristics of oil-impregnated Nomex paper was investigated. Nomex T410 was fluorinated at 25 °C using F2/N2 mixtures with 20% F2 in volume at 0.05 MPa for 15, 30 and 45 min. Fourier Transform Infrared Spectroscopy (FTIR) proved that the molecular chain scission and cleavage occurred during gas-phase fluorination. Furthermore, the surface charge and space charge characteristic of the original and fluorinated oil-impregnated paper were measured using an electrostatic voltmeter and Pulsed Electroacoustic (PEA) equipment respectively. Furthermore, the hole and electron trap distribution of the samples were obtained by Isothermal Surface Potential Decay (ISPD) model. The results showed that both the positive and negative charge decay rates were accelerated by gas-phase fluorination and the hole, electron trap energy and density of the fluorinated samples were reduced by fluorination. It is suggested that the space charge dissipation was also accelerated by fluorination, indicating that gas-phase fluorination is an effective approach to modify the charge dynamics of oil-impregnated Nomex paper.

Temperature and trap distribution dependence of electrical tree growth characteristics in polypropylene/elastomer blends for recyclable cable insulation

Polypropylene (PP)/elastomer blends as a promising cable insulation material have to face the challenge of electrical treeing which possibly leads to unexpected insulation failure. In this work, two types of elastomers, i.e. polyolefin elastomer and propylene-based elastomer, have been blended into isotactic PP. Effects of the elastomer content and type on electrical treeing behaviours have been investigated under different ambient temperatures. Isothermal surface potential decay measurement has been performed to explore the influence of trap distribution on the treeing behaviour, scanning electron microscope and differential scanning calorimetry measurements were performed to characterise the structure of blends. It was found that with the increase of elastomer content, the trap energy level was reduced and the shallow trap density was increased, which resulted in the enhancement of the tree growth rate and the decrease in the breakdown time. With the temperature changing from 20 to 100°C, the growth rate tended to be increased except that an anomalous reduction in the rate occurred at 80°C. It is suggested that the compatibility between PP matrix and elastomer plays an important role in determining the morphology of blends, which in turn influences the trap distribution and the temperature dependent treeing behaviours.

DC breakdown and flashover characteristics of direct fluorinated epoxy/Al2O3 nanocomposites

The effect of direct fluorination on DC breakdown and flashover characteristics of epoxy/Al 2 O 3 nanocomposites was investigated. Pure epoxy resin and epoxy/Al 2 O 3 nanocomposites with 1, 3 and 5 wt% mass fractions were synthesized, and direct fluorination of the samples was carried out at 55 °C for 30 min using a F 2 /N 2 mixture with 20 vol% F 2 at 0.05 MPa. Fourier transform infrared spectroscopy and atomic force microscopy show that molecular chain scission occur during direct fluorination, which may significantly suppress surface charge accumulation while incorporating Al 2 O 3 nanoparticles into epoxy may restrict surface potential decay of fluorinated epoxy resin. The trap characteristics obtained by isothermal surface potential decay show that 1 wt. % sample has deeper traps and higher trap density. DC breakdown and flashover of samples were measured to evaluate the influence of nano filling and direct fluorination on electrical performance of epoxy resin. The results show that epoxy resin with higher DC breakdown and flashover strength can be obtained by simultaneous application of nano filling and direct fluorination. Compared to un-fluorinated epoxy resin, the DC breakdown and flashover voltage of fluorinated epoxy/Al 2 O 3 nanocomposites with mass fraction of 1 wt% improved 17 and 23%, respectively.

Charge transport in low density polyethylene based micro/nano-composite with improved thermal conductivity

In this work, effect of nano-sized SiO2 and micro-sized hexagon boron nitride (h-BN) on charge transport behavior of low-density polyethylene (LDPE) has been investigated. LDPE based micro-nanocomposites samples were prepared through hot-press method by adding 20 nm-SiO2 and 5 µm-h-BN as the fillers. Isothermal surface potential decay measurement was carried out with sample temperature of 40 °C and relative humidity of 23%–29% to obtain trap depth, carrier mobility and field dependent conductivity, by which the effect of fillers on charge transport manners could be estimated. In addition, thermal conductivity test, scanning electron microscopy and differential scanning calorimetry had been employed to assist the analysis of the filler effect. Test results indicated that the crystal structure of LDPE was changed by the doping of the fillers. With the increase of the micro- and nano-filler content, the trap depth tended to be deeper firstly then became shallower. Both band and hopping transport of charge occurred in the composite, and the carrier mobility was dependent upon the filler content. It is suggested that the addition of the nano-sized SiO2 and the micro-sized h-BN particles has an effect to inhibit the charge transport, where a synergistic effect among the nano-sized and the micro-sized particles is expected to modify charge trapping and de-trapping behavior.

The energy distribution of trapped charges on the surface of cross-linked polyethylene thermally aged at different temperatures

In this paper, the energy distribution of trapped charges on the surface of cross-linked polyethylene (XLPE) thermally aged at 100 °C and 160 °C were studied by the isothermal surface potential decay method. It was found that the surface traps of unaged XLPE were mainly deep ones of both electron-type and hole-type. In the process of thermal aging at 100 °C, the surface traps of XLPE were still mainly deep traps. As the aging time increased, the density of shallow traps increased, and the density of deep traps didn’t change much. In the process of thermal aging at 160 °C, the density of shallow traps of both electron-type and hole-type ones increased greatly, and the density of deep traps of both electron-type and hole-type ones decreased sharply. At the same time, the energy levels of hole-type traps decreased. Under the thermal stress, the increase in the density of shallow traps may be due to an increase in aging products such as carbonyl (C=O); the decrease in the density of deep traps should be due to a decrease in chemical defects in the crystalline regions because of the destruction of spherulites. Differences of the energy distribution of trapped charges on the surface of XLPE aged at two different temperatures may be caused by the differences in the way that crystal structures deteriorate.

Aging characteristics of polymeric materials by repeated surface flashovers in vacuum under microsecond pulse

Surface flashover characteristics of insulation materials in vacuum are key factors to determine the behavior of high voltage electrical equipment. Although the surface flashover in vacuum has been investigated for decades, the surface insulation properties of polymeric materials after repeated surface discharges have shown little attention by researchers. In order to investigate the aging characteristics of polymeric materials caused by repeated flashovers in vacuum, several typical materials were used to flashover by microsecond pulses. The flashover voltages were obtained for polytetrafluoroethylene, polymethylmethacrylate and polyamide for thousands of flashovers. The surface morphology of samples was observed after different numbers of flashovers. The trap parameter distribution was obtained by the isothermal surface potential decay method. To analyze the flashover voltage change with discharge times, the Weibull distribution was used to determine the conditioning and degradation effects in repeated discharge experiments. The results show that the morphology near the cathode triple junction and deep traps together affect the subsequent flashover voltage. The conditioning and degradation phenomena often coexist in the aging process of polymers and affect the surface insulation properties. This study could provide a reference for the type selection, design, and modification of insulation materials in vacuum.

Polarization and Trap Characteristics Modification of Oil-Impregnated Paper Insulation by TiO₂ Nanoparticles.

Polarization and traps determine the electrical property of oil-paper insulation, but most attention has been paid to the modification of insulating oil with nanoparticles, so there are is little research about oil-impregnated paper, and the origin for performance variation is not understood yet. In this paper, spherical nanoscale titanium dioxide was prepared by the hydrolysis method and nanofluid-impregnated paper (NP) was fabricated through oil-impregnation. The frequency domain spectrum was measured for polarization analysis, and both thermally stimulated depolarization current (TSDC) and isothermal surface potential decay (ISPD) methods were used to reveal trap parameters. Results show that NP’s low frequency permittivity is much larger, and another peak appears in the spectrum even though the content of nanoparticles is very low. With the addition of TiO2 nanoparticles, TSDC’s amplitude and peak temperature increase, and the trap energy becomes shallower. TiO2 nanoparticles’ strong polarization and high activation energy contribute to NP’s larger interface polarization intensity and activation energy. Furthermore, because of oxygen vacancies, TiO2 nanoparticles offer a transfer site for holes and electrons to escape from deep traps; thus, the trap energy is greatly reduced.

The effect of loading ratios and electric field on charge dynamics in silica-based polyethylene nanocomposites

Nanodielectrics have been expected to improve the electrical performance and are considered as dielectrics for the future. It has been recognized that the electrical performance is closely related to charge dynamics in the dielectrics material. However, the mechanism of charge dynamics in the interphase of nanodielectrics has not been fully understood, which causes difficulty in understanding the effect of nanoparticle loading ratios and electric fields applied on the electrical properties. Recently, a model based on the tunneling process with the presence of deep traps has been suggested as one conceivable candidate for explaining charge dynamics in nanodielectrics, but the related experiment results are not discussed with the tunnelling process. In this paper, the measurements including isothermal surface potential decay and space charge are conducted for the blend polyethylene incorporated with the untreated silica nanocomposites. According to the experimental observation compared with the unfilled blend polyethylene, the electrical properties of nanocomposites with high loading ratios of 2 wt%, 5 wt% and 10 wt% are worsened, such as facilitated space charge accumulation and injection, and faster charge carriers transport. On the other hand, regarding the nanocomposites with the low loading ratios of 0.5 wt%, slow transport of charge carriers, and suppressed space charge accumulation and injection were observed. The effect of the lower and higher electric field on the electrical properties of the nanocomposites was similar for the low and high loading ratios. The tunnelling process associated with deep traps can effectively explain these observed phenomena of nanocomposites, it is therefore suggested for further explaining the electrical properties and charge dynamics in the nanodielectrics.

The control mechanism of micron and nano SiO2/epoxy composite coating on surface charge in epoxy resin

Charge accumulation on the surface of epoxy resin affects the insulation performance under DC voltage. In order to illustrate the effect of surface coating on the characteristics of surface charge accumulation in epoxy resin, micron and nano SiO 2 /epoxy composite coatings were deposited on the surface of epoxy resin by spraying, and the charge accumulation on the surface was studied by the electrostatic probe method. The surface trap parameters were analyzed by the method of isothermal surface potential decay combining with the carrier mobility and cross-section morphology of samples, the transport characteristics of surface charge and the respective control mechanism of coating layer were studied. The results suggest that the surface charge mainly accumulates near the electrodes, the main source of charges is injected from the electrodes. The surface charges near the anode are caused by charge trapping, and the surface charges near the cathode are caused by the adsorption due to the normal electric field. Surface charge accumulation can be suppressed by increasing the shallow trap density and carrier mobility on the surface of material, which is available by coating with micron and nano SiO 2 /epoxy composites. It was found that, surface charge of epoxy resin could be suppressed effectively by coating of micron SiO 2 particles with content of 1~3 wt%, or nano SiO 2 particles with content of 3 wt%.

Experimental demonstration of deep traps in silica-based polyethylene nanocomposites by combined isothermal surface potential decay and pulsed electro-acoustic measurements

The ability to suppress space charge accumulation at high electric fields makes nanocomposites attract significant research interest as potential insulation materials in high-voltage direct current cable development. At present, the deep trap introduced by nanoparticles is frequently applied to be responsible for the observed space charge suppression in nanocomposites. However, the experimental results that support deep-trap formation have not been rigorously examined. We therefore propose herein a simple and more direct approach based on isothermal surface potential decay combined with pulsed electro-acoustic measurements to verify the presence of deep traps in silica-based blend polyethylene nanocomposites. The results indicate that the deep traps are indeed introduced by filling nanosilica and the space charge suppression observed in the nanocomposite with a low loading ratio is caused by deeply trapped charges in the sub-surface region of specimens.