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In this paper, a series of laboratory experiments are conducted to investigate the effect of momentary small variations in the transformer operating temperature on the dissolved gas analysis (DGA) measurement. With the increased penetration level of renewable energy sources of intermittent characteristics into electricity grids, operating power transformers are expected to experience frequent temperature variations. Sampling transformer oil during such temperature variation leads to inaccurate diagnosis. Experimental results reveal that gas evolution in transformer oil is greatly affected by the small variations in the operating temperature. Such small variation can be a result of the intermittent generation characteristics of renewable energy sources. Hence, false analysis may be reported if oil is sampled during generation or load fluctuation events. Experimental results are explained through chemical equilibrium constant theory, which indicates that dissolved gases reflect the change in aging rate of the transformer oil-paper insulation system. These results suggest a new paradigm for DGA process through correlating measurements with the transformer operating temperature through the generation and load profiles at the instant of oil sampling.

Studying the influence of short chain branch (SCB) on the crystal properties and breakdown strength of low-density polyethylene (LDPE) plays an important role in improving the electrical and mechanical properties of LDPE by adjusting the molecular structure. In this work, the SCB and long chain branch (LCB) content of LDPE are quantitatively characterized. Grain size, crystallinity and breakdown strength of LDPE (including AC and DC) are calculated through related characterization experiments. The results show that the SCB content in LDPE is dozens of times that of LCB. Reducing the SCB content can reduce the grain size of 110 face in LDPE and improve the AC and DC breakdown strength. The trap distribution characteristics measured by the thermally stimulated depolarization current (TSDC) method show that the SCB inhibits the formation of deep cavity defects in the LDPE interface area and decreases the trap energy level of crystal defects. This inspired us to improve the breakdown strength by reducing the SCB content in LDPE.

This study aims to investigate the characteristics and kinetics of cellulose degradation in oil-impregnated pressboard (OIP) under simultaneous thermal-electrical-mechanical stress accelerated aging. Thus, the multi-stress aging experimental setup is designed and established. OIP multi-stress aging experiments are performed at 130 °C, a DC electric voltage of +6 kV, vibration amplitude of ${10}{-}{50}\ \mu\mathrm{m}$ and vibration frequency of 100–300 Hz. The aging characteristics of pressboard and oil are then measured, including the degree of polymerization and aging by-products (i.e., furfural, moisture, acidity and dissolved gas). Investigations on aging characteristics reveal that thermal stress is still the dominant factor of cellulose degradation, while electrical and mechanical stresses serve as aging acceleration factors. The simultaneous thermal-electrical-mechanical aging of OIP is 10-24% faster than the single thermal aging. Moreover, the kinetics of cellulose degradation is also proposed to predict the bond scission fraction of cellulose with a goodness-of-fitting over 0.97. Finally, the correlations between the parameters in the kinetics of cellulose degradation and mechanical stress reveal that the increase of vibration amplitude generates more weak and amorphous links available for degradation, while the increase of vibration frequency accelerates the reaction rate.

To improve the efficiency and accuracy of cable ageing assessment, a new method based on residual antioxidant (AO) content is proposed. Cross-linked polyethylene (XLPE) cable insulation is thermally aged in laboratory at 110, 130 and 140°C from 240 h to 3600 h. The ageing degree of XLPE is characterized by tensile testing, AC breakdown measurement, and Fourier transform infrared spectroscopy. The residual AO content in XLPE is detected by FTIR as well as by oxidation induction time (OIT) measurement. It is observed that residual AO content measured by FTIR decreases monotonously with ageing time while OIT shows a non-monotonic transition. The results demonstrate that thermal deterioration is closely related to residual AO content and a critical value of residual AO content exists. During early stage of thermal ageing, the residual AO content is larger than the critical value and the variation in XLPE properties is relatively small. When the residual AO content decreases below the critical value, significant deterioration occurs. The close relationship between thermal degradation and residual AO content was analyzed by oxidative chain reactions.

This paper investigates the dissolved gases evolution in transformer oil under combined DC/AC electrical-thermal stress. Dissolved gases detected in transformer aged insulation oil reveal that oil under AC electric field combined with thermal stress can produce more dissolved gases than oil under DC electric field with thermal stress but less than the gases produced in oil under distorted AC or combined AC/DC voltage stress. This is attributed to the divergent migration properties of the charged components under different types of electric fields. To further understand this behavior, carrier recombination coefficient is proposed to explain the oil DGA results under various aging stresses. Results show that diagnostic parameters such as breakdown voltage, oil interfacial tension, and moisture content in pressboard should be used along with DGA results in order to accurately diagnose the insulation condition within converter transformers that impose a combined AC/DC voltage stress on the insulation system.

Here we report the combined effect of electric field with thermal stress on dissolved gas analysis (DGA). Dissolved gases detected from accelerated aging experiments show that AC electric field combined with thermal stress leads to higher gas generation than DC electric field with thermal stress or by thermal stress alone. Research reveals that the differences are caused by the divergent migration properties of the charged components under AC and DC electric fields. A mathematical explanation correlates the DGA results by proposing a carrier recombination coefficient parameter. The study suggests that dissolved gas content does not reflect the insulation performance of transformers by simply employing DGA without considering the operating condition of the transformer. It is essential that a comprehensive evaluation criterion should accompany DGA in order to accurately determine the insulation status.

Polyethylene based nanocomposites have been widely researched for being potential insulating materials in high voltage DC equipment. One of their key properties is the enhanced DC breakdown strength (E b-DC ). For a long time, the E b-DC of polyethylene nanocomposites has been attributed to the deep charge traps located at the interphase region between the nanofiller and polymer matrix. In this work, we prepared XLPE/Al(OH) 3 nanocomposites with 0.1 to 3 wt% nano-Al(OH) 3 and studied their E b-DC at 20 to 90 °C. The results show that the XLPE/Al(OH) 3 nanocomposites possess significantly increased E b-DC , although having a large amount of shallow charge traps, and more likely causing homocharge injection. The results of breakdown tests show that E b-DC first increases and then decreases at 20 to 70 °C with increasing nanofiller while it shows a totally opposite trend at 90 °C. E b-DC decreases dramatically with increasing temperature, a crossover phenomenon of E b-DC appears at about 80 °C. The elastic modulus of XLPE/Al(OH)3 at different temperatures was obtained and it shows a similar trend with E b-DC . Therefore, the mechanical properties influenced by morphology and space charge properties modulated by charge trap characteristics were analyzed to jointly determine the E b-DC of XLPE/Al(OH) 3 nanocomposites at different temperatures.

MgO/LDPE nanocomposites with various nanofiller loadings are fabricated and the morphology and trap properties are characterized to investigate their effect on DC conductivity and electrical breakdown strength. It is found that DC conductivity depends on both shallow and deep traps. As the nanofiller loading increases, the density of shallow traps increases, leading to higher shallow trap-controlled hopping conductivity, while the deep traps increase firstly due to the formation of isolated interfacial regions and then decreases caused by the overlapping of interfacial regions, resulting in the variation in charge trapping-detrapping dynamics. An equation considers both the shallow trap-controlled hopping and the charge trapping-detrapping processes is derived to calculate the DC conductivity of MgO/LDPE nanocomposites, and the calculation results are consistent with experiments. The DC electrical breakdown strength is influenced by both the morphology and deep trap properties. At low nanofiller loadings, higher crystallinity and smaller crystallite size may block the motion of molecular chains in LDPE nanocomposites, and deep traps formed in interfacial regions hinder the charge transport. The synergetic effects of nano-doping on morphology structure and charge carrier transport enhances the DC electrical breakdown strength of MgO/LDPE nanocomposites.

With the global trend to establish smart distributed-generation grids, the level of AC-voltage harmonics is significantly increasing due to power electronic converters interfacing renewable energy sources with the grid. Such harmonics will not only degrade the power quality and increase the losses in the electricity grids but will also have a severe impact on oil impregnated paper (OIP) in power transformer bushings. While much attention has been given to investigate the deterioration of OIP under thermal and electrical stresses, the impact of voltage harmonics on the OIP structure is still uncertain and calls for more investigations. Therefore, this paper is aimed at investigating the variations of the multilevel structure of an OIP aged under high levels of AC-voltage harmonics. In this context, degree of polymerization of the aged OIP is measured to reveal the changes of OIP secondary structure. Also, X-ray diffraction, deuteration treatment and Fourier transform infra-red spectroscopy are conducted to analyze the changes of the OIP aggregate structure during the aging process. Further experiments are performed to investigate the variation of the microscopic appearance, thermal behavior and dielectric properties, including scanning electronic microscopy, thermal gravity analysis, differential scanning calorimetry and frequency domain spectroscopy. This investigation on the variation of the multilevel structure of aged OIP not only promotes further understanding to the aging behavior and mechanism of OIP under AC-voltage harmonics, but also proposes crystallinity and accessibility as two new effective aging indicators for the OIP structure.

The discontinuous charge relaxation time related to the electrical conductivity and permittivity, i.e., traditional Maxwell-Wagner relaxation, is the prevailing explanation for the interfacial charge behavior in multi-layered dielectrics. However, the unipolar/bipolar charge dynamics and the corresponding mechanisms from a microscopic viewpoint remain unclear. This work focuses on the dynamics of the interfacial charges in cross-linked polyethylene (XLPE)/ethylene-propylene-diene (EPDM) dual layers to reveal the interfacial charge mechanisms using the energy band structure. The interfacial charge accumulation behavior of the XLPE/EPDM dual-layered structures is clearly observed under electrical stress. Either unipolar or bipolar charges occur at the interface, accompanied by charge injection, and trapping then occurs. The positive charges prefer to accumulate and migrate on the XLPE side. The homo-charges at the interface always remain on the EPDM side. The number of negative charges is higher than the number of positive charges at the interface. The bipolar interfacial charges are ascribed to the charge transfer process that occurs at the interface because of the energy level alignment that is revealed by the electronic energy structures of XLPE and EPDM. The trapping sites, along with the charge injection and transport, contribute to the interfacial charge behavior.

A comparative study of biaxial-orientated polypropylene (BOPP) films extracted from one failed HVDC filter capacitor (serving for 10 years) and one fresh capacitor (spare capacitor from the same batch) is reported in this paper. Experimental results show that the 10-year online operation does not bring about molecular changes; however, molecule reorganization and recrystallization was detected using differential scanning calorimetry and dynamic mechanical analysis tests. Electrical measurements including dielectric spectra, DC conductivity measurement, DC breakdown strength and were also performed. The experimental results show that the aging process causes an increase in dielectric constant, dissipation factor, DC conductivity, and the DC breakdown strength. Thermal stimulated current measurements indicate that traps were introduced by aging and are responsible for such abnormal changes. The nano-cavitivies generated by molecule reorganization could possibly act as charge traps. Therefore, homo-charges can accumulate in the vicinity of the electrodes, leading to the degradation of long-term withstand strength but an enhancement of short-term breakdown strength.

Surface tracking experiments were conducted on MgO/epoxy nanocomposites according to IEC60112. Treeing phenomenon, which was commonly observed in bulk insulators, was detected on the surface of tracked samples via optical microscopy. Tree branches were further found to be segmented tracking channels by scanning electron microscopy, which consisted of a series of ablation holes along electric field, i.e. wormholes. According to distribution of surface trees, tracking area was divided into three typical regions. Region A, close to electrodes, was barely ablated. Region B, which directly connects electrodes, exhibited paralleled tracking channels. Region C, which is the outer annular ablated region, is full of crossed tracking channels. At the beginning, the initial tracking channels were demonstrated to appear at the triple junction of Region A, B and C, where electric field was supposed to be the highest. Then tracking regions spread from the junction to Region B and Region C with tracking channels segmentally growing longer and wider, indicating a segmented energy accumulation process. Tracking trees were finally formed connecting two electrodes, leading to sample failure. With addition of nano-MgO particles, wide and deep channels turned into slight but dense ones. Correspondingly, comparative tracking index (CTI) increased from 400 to 475 V with increasing nano-MgO from 0 to 10 wt%. Improved tracking resistance was obtained in nanocomposites while reduced thermal conductivity and pyrolysis characteristics were observed. Combining with results above, a segmented worm model was thus proposed to explain surface tracking performance in MgO/epoxy nanocomposites.

In this paper, loading content was adjusted and nanoparticle surface modification by silane coupling agent was applied to obtain different XLPE/TiO2 nanocomposites. Polymer surface etching, a traditional method, was innovatively and effectively used to characterize the nanoparticle dispersion and distribution in nanocomposites. Distinct scanning electron microscope images of nanoparticle dispersion were obtained. It was proved that the surface modification improved nanoparticle dispersion, which was analyzed to relate directly to DC insulation performance and morphology. The introduction of nano-TiO2 decreased DC conductivity, suppressed homocharge injection, enhanced DC dielectric strength, increased crystallinity and changed morphology. And well nanoparticle dispersion at same loading content achieved by surface modification had positive cooperative effect to the influence above. DC breakdown strength was increased by 13.5%. Decrement of injected homocharge density was calculated, and it can reach 91.5 μC·m−2. Moreover, the non-uniform distribution of nanoparticles in semi-crystalline polymer based nanocomposites was first proposed and discussed. The nanoparticles can be divided into four regions, the boundary zone, the amorphous zone, the nucleation zone and the inter-lamella zone.

Silicone rubber is a hydrophobic polymer which is widely employed for high voltage outdoor insulation. The material offers excellent electrical performance under contaminated environments. However, pristine silicone rubber has low thermal properties and this may cause tracking and erosion failure due to severe dry band arcing and ohmic heating on the insulating surface. This work investigates the effect of material thermal characteristics on the tracking and erosion resistance of silicone rubber filled with micron sized alumina tri-hydrate (ATH), aluminum nitride (AlN) and boron nitride (BN) particles. Composites with different loading were synthesized by dispersing particles in pristine room temperature vulcanizing (RTV) silicone rubber and IEC 60587 inclined plane test (IPT) was conducted to evaluate tracking and erosion resistance. Apart from physical parameters and leakage current, an infrared thermal imager was used to measure the surface temperature distribution during the course of IPT. Experimental results showed tracking and erosion resistance is significantly enhanced with addition of BN particles followed by ATH. AlN composites exhibit poor tracking and erosion resistance, similar to pristine silicone rubber. It is concluded that addition of BN-composites improves ability to impede the tracking and erosion process, the reasons being better thermal stability and enhanced thermal conduction in the discharge region. On the other hand, infrared analysis revealed thermal accumulation is remarkably higher in AlN-composites which promotes dry band arcing and results in tracking and erosion failure.

Surface flashover properties should meet stringent requirements for the use in electrical equipment under high voltage DC environments. With the aim of enhancing surface flashover properties and understanding the mechanisms involved at ambient conditions, epoxy nanocomposites with different weight proportions of nano-TiO 2 were prepared. A silane coupling agent was selected to functionalize the TiO 2 , the effect of which on nanoparticles dispersion and distribution was perused through scanning electron microscopy and infrared spectroscopy. DC surface flashover measurements in five normally used ambient conditions (N 2 , air, CO 2 , SF 6 and vacuum) were conducted and results were analyzed relating to loading content, gas parameters, surface trap parameters as well as electrical conductivity of nanocomposites. Outcomes indicate that with low content of nanoparticles, the density of deep traps was increased, showing a positive effect on inhibiting surface flashover. When the nanoparticle content is higher than 1 wt%, the overlapped transitional region reduced the energy level of traps, resulting in a poor display of surface flashover voltages. It was found that the nanoparticle content had a greater influence on surface flashover in vacuum. Electron negativity, ionization coefficient and mean free path of gases were taken into account to explain why surface flashover in SF 6 appeared to be the highest, whereas in N 2 , it was the lowest. At last, effects of gas-solid interactions on surface flashover combining surface-trap characteristics, secondary electron-emission contributions and gas molecule impact-ionization characteristics were analyzed.

Exposure to corona discharge damages the polymeric structure of alumina trihydrate (ATH) filled silicone rubber composite insulators. In this work, an attempt has been made to enhance the resistance against sustained long term corona exposure of ATH filled silicone rubber by fabricating its co-filled composites through addition of a small amount of nano Al2O3. Fabricated specimens comprise a pristine sample of silicone rubber (SR), one micro {40wt% ATH (MSR)} composite and two co-filled {39wt% ATH+1wt% nano Al2O3 (MSR1%) and 38wt% ATH+2wt% nano Al2O3 (MSR2%)} composites with total particle loading of 40wt%. They are subjected to AC corona exposure produced from a needle electrode tip for seven days. Performance evaluation is achieved through SEM (Scanning electron microscopy), FTIR (Fourier transform infrared spectroscopy), hydrophobicity, and surface roughness. Test results show severe material ruptures occurred in SR but damage in the form of pits and voids in MSR and MSR1% in the area directly below the needle tip. Interestingly, MSR2% exhibits subdued damage and a compact lump structure. The hydrophobicity recovery rate is significantly higher below the needle tip than in the vicinity, and in the SR as compared to the other composites. Also noted is higher increment in average and peak surface roughness below needle tip, and strong resistance to roughness increment by MSR2%. FTIR results show higher concentration of hydroxylation and aluminum nitrites in MSR and the absorbance intensities of principal functional groups show fewer deviations in MSR1% and MSR2% relative to their virgin states. Based on experimental results, it is concluded that corona resistance of micro ATH filled silicone rubber can be enhanced through addition of small amount of Al2O3 by fabrication of co-filled composites.

A model based on surface potential decay (SPD) measurement was developed to study the trap energy distribution of hole-type and electron-type traps in materials by considering two distinct energy levels (i.e., shallow and deep traps) and the detrapping process. An approximated Dirac function δ((E-Em)/kT) was also introduced to obtain a consistent unit of trap density. Besides, the space charge distributions (such as uniform, linear and exponent distribution) in positively and negatively charged materials were also taken into account. In order to verify our proposed model, the different SPD curves of low density polyethylene (LDPE), LDPE/zinc oxide (ZnO) microcomposite and nanocomposite were analyzed. The results show that the proposed model is effective to obtain the trap energy distribution in materials. The SPD curves in positively and negatively charged samples show a fast decay followed by a slow decay, which is agreed with our assumption of shallow and deep traps. Two peaks are clearly observed from the trap energy distribution. The trap energy distributions of hole-type and electron-type traps in these materials are distinct from each other. This implies some essential nature of hole-type and electron-type traps (i.e., intra-chain character for hole-type traps and inter-chain character for electron-type traps). In addition, compared to LDPE and LDPE/ZnO microcompoiste, the decreasing energy level and the increasing trap density of shallow traps as well as the decreasing trap density of deep traps in LDPE/ZnO nanocomposite may be attributed to the effect of the interaction zone surrounding nanoparticles.

Epoxy resin and its microcomposite are widely used as electrical insulation in electrical equipment. Surface flashover and breakdown properties of these materials can be influenced by dielectric relaxation process and charge carrier behavior. The charge transport characteristics and dielectric relaxation process for neat EP and its microcomposites were studied by means of dielectric relaxation spectroscopy. The glass transition temperatures Tg of the two kinds of samples were investigated with differential scanning calorimetry measurement, which were approximately 105 °C and 120 °C, respectively. Both the two kinds of samples were sputtered with gold electrodes on two sides, and electrodes' diameter was 30 mm. The properties of dielectric relaxation was measured by Concept 80 Novocontrol broadband dielectric spectrometer at various temperatures from 100 °C to 180 °C. There is a relaxation peak which labeled α in the range of high frequencies on account of the molecular chain movement or motion of segmental chains when the temperature is above Tg. At the same time, the dc conductivity attributed by the charge carrier transport occurs in the range of low frequencies. Besides, there are different relaxation times for molecular chains with different length scales. Moreover, there is a broad distribution of relaxation time for both neat EP and its microcomposites and the relaxation time distribution at different temperatures was calculated. In addition, it was found that the relation between peak frequency of relaxation process and temperature follow the Vogel-Fulcher-Tammann (VFT) law, and temperature dependences of dc conductivity also obey the VFT equation. The Vogel temperatures of dielectric relaxation process and charge carrier transport were calculated by fitting the results of permittivity and conductivity. The glass transition temperatures for neat EP and its microcomposite were estimated to be 102 °C and 117 °C through Vogel temperatures, which are in consistence with DSC measurement results. It represents that free volume in the two kinds of samples increases with the increase of temperature, thus enhancing molecular chain movement and charge carrier transport process.

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.

The present work studies the space charge accumulation and decay in the degassed cross-linked polyethylene (XLPE) by detrapping mechanism and quantum chemical method. Experimentally, significant positive charge packets were observed at room temperature (300 K). Negative charges accumulated strikingly under high temperatures but with a fast decay during depolarization. The estimated bipolar carrier mobility and electron/hole trap depth that derived from the charge decay characteristics indicated that a shallow trap distribution (0.84–0.9 eV) dominates the space charge behavior. Additionally, a high temperature (>60 °C) benefits to evaluate the deep traps (0.9–1 eV). Quantum chemical analysis using density functional theory (DFT) demonstrated that both the localized states for shallow electron and hole traps exist in the electronic structure of the XLPE structures. The former is derived from the physical disorder and crosslinks. The latter is ascribed to the hopping interaction between the chain terminal and the other orientated chains. The calculation results about the charge trapping characteristics agree with the experiments. Furthermore, the role of charge injection at the electrode/dielectric interface is also discussed.

Oil impregnated paper in power transformers deteriorates in long-term operation, which leads to the variations of its dielectric characteristics. This paper focuses on investigating the generation mechanism of the dielectric processes in oil impregnated paper and their variations with thermal aging. Firstly, a set of thermally stimulated current tests with bias-voltages were conducted in distinguishing the dielectric processes and revealing their generation mechanism. Secondly, the standard thermally stimulated current spectrum and the M″ spectrum of 0–1000 h aged samples were joint analyzed in investigating the variations of dielectric processes in thermal aging process. Finally, additional experiments of moisture, acidity and microscopic observation were conducted in explaining these variations. It is concluded that three dielectric processes can be found in oil impregnated paper, i.e. the dipole polarization, the interfacial polarization and the electric conduction. These processes are all strengthened with thermal aging. The generation of molecules with strong polarity, the structural changes in insulating paper, and the generation of hydrogen ions are responsible for the strengthening of these processes sequentially. This investigation on distinguishing dielectric processes and obtaining their mechanism can promote further understanding on oil impregnated paper and provide information for insulation design in power transformers.

A satellite operates in a harsh environment including high vacuum and radiations. The vacuum-dielectric interface is the most vulnerable area to flashover discharge. Moreover, the presence of electron radiation will further decrease the flashover voltage. However, the influence of electron radiation on surface flashover remains unclear. We have performed a study on vacuum DC surface flashover during electron beam irradiation as a function of electron kinetic energy and incident direction on polyimide representative of spacecraft dielectrics. The electron radiation was accompanied by electron deposition and bombardment effects on the polyimide surface. The surface flashover voltage was experimentally found to significantly depend on the kinetic energy and incident angle of the electron beam. By calculating the trajectories of incident electron beam, we further found that the surface flashover voltages are closely connected to the irradiated area. During low energy electron beam irradiation, kinetic electrons are repelled away from the dielectric surface by the applied electric field, and deposited electrons will be dominant in the flashover process, promoting the surface flashover voltage. However, when the electron beam energy is high, kinetic electrons can overcome the applied electric field and strike the dielectric surface to generate secondary electrons, which will replace the field-emission electrons to serve as seed electrons, and initiate the flashover at a much lower applied voltage. These observations and analyses are expected to benefit the research into mitigating spacecraft discharge, and promote the knowledge of surface flashover.

This work studies the space charge mechanisms of PE and PTFE by involving the combination of electronic structures and charge transfer processes at the interfaces of electrode/dielectrics. Experimentally, significant positive charge packets were observed in LDPE when subjected to a high electric stress. However, slight charge accumulations (mainly homo-charges) were observed inside polytetrafluoroethylene (PTFE) under the same condition. The electronic structures of PE and PTFE were calculated by density functional theory (DFT). After that, we obtained the electronic parameters, such as energy level, Fermi level, band gap and electron affinity. A negative electron affinity is determined in PE, indicating an interchain electron charge transport at the bottom of the conduction band. Nevertheless, a positive electron affinity in PTFE exhibits an intrachain electron charge transport. Moreover, injection and charge transfer at electrode/dielectrics interfaces were studied based on the calculated parameters. A low hole injection barrier dominates the positive charge injection and accumulation in LDPE, while a high electron barrier hinders the electron charge injection. In contrast, the hole injection barrier of PTFE is higher than that of LDPE. We discussed these different processes in this paper.

Interface between the metal electrode and insulating material affects dielectric performance of the whole insulating system significantly. In this study, low density polyethylene (LDPE) films were treated in ozone atmosphere for different time duration to investigate the effect of interface state on breakdown performance. Fourier transform infrared spectra (FT-IR) results show that carbonyl group (C=O) is introduced into the surface of LDPE film after ozone treatment, and the amount of C=O groups increases when the ozone oxidation time extends. Surface trap distribution characterized by surface potential decay (SPD) reveals that two hole-type traps exist in the surface of LDPE and ozone treated specimens. Both trap levels become shallower and the density of deeper level traps decreases with the increasing ozone oxidation time, while that of shallower level traps increases. Space charge distribution measured by pulsed electro-acoustic (PEA) method suggests that more positive charges are injected into specimens from the anode and accumulated in the vicinity of cathode after ozone treatment. Furthermore, the dc breakdown strength declines monotonously with longer ozone treatment time, and the lowest breakdown strength of specimen occurs after 6 hours, decreasing by 26.9% compared with that of untreated LDPE. Finally, it is concluded that ozone treatment results in changing of surface chemical composition, which determines surface trap distribution (surface state), leading to the variation of charge injection and accumulation properties as well as the subsequent breakdown performance.

A dielectric breakdown model consisted of bipolar charge transport and free volume breakdown criterion is used to investigate the dc breakdown properties of polymer nanocomposites. In the model, we consider charge injection from electrodes, carrier migration with a constant mobility, charge trapping and detrapping associated with deep traps, charge recombination, and energy gained for charge carriers from the local distorted electric field. Since incorporating nanoparticles into a polymer can change its density and/or energy of deep traps, we calculate the dielectric breakdown properties of low-density polyethylene nanocomposites characterized by various densities and energies of deep traps. The simulated results show that the breakdown strength increases with an increase in the density and energy of deep traps, which is consistent with the experimental results. It is shown that the accumulation of homocharges, the distortion of electric field, and the energy gain of free carriers are regulated to improve the performance of breakdown.

In this paper, charge trapping and detrapping dynamics of PE-based materials, i.e., LDPE, HDPE, XLPE were investigated by isothermal surface potential decay method (ISPD). The whole charge transport process including ions generation process in the air, charge trapping in the top surface, the surface potential establishing process, the charge migration process in the bulk was considered in these PE-based materials. The results showed that the trapping process was closely related to the capture cross section, the injected carrier concentration and the trap level. The established surface potential was mainly determined by the captured carriers in the top surface of the PE-based materials. In addition, it was found that the mobility of LDPE was much larger than that of XLPE and HDPE. And it is shown that deep traps may play a more important role in the charge migration process than shallow traps in these PE-based materials. It is indicated that the discrepancy of charge trapping and detrapping dynamics between these PE-based materials may be attributed to the different microscopic morphology.

Gamma-ray induced changes occurring in ethylene-propylene-diene copolymer (EPDM) were analyzed through its complex permittivity spectra. First, four different processes, namely, instantaneous polarization, hopping of ions, segmental relaxation, and dc conduction are found to contribute to the complex permittivity. By fitting the spectra to theoretical equations, it has become clear that the instantaneous polarization and ionic hopping become active by the irradiation, indicating that generation of oxidative products and mobile ions and occurrence of chain scission are induced by gamma rays. Furthermore, since both the thermal expansion and the contribution of the segmental relaxation to the permittivity become less with an increase in dose, it is assumed that crosslinking is induced by the gamma irradiation.

Studies on charge transport properties of space grade insulating materials become popular recently because it is closely related to the safe operation of spacecraft. Taking charge carriers retrapping and hole injection into consideration, we simulated the surface charging and the space charge accumulation properties of PTFE under lowenergy electron-beam irradiation using bipolar charge transport model in this paper. Parameters for simulation were based on experiments and previous studies. The secondary electron yields of PTFE irradiated by electron-beam of different primary electron energies were measured, and experimental data were fitted with an empirical equation. Thermally stimulated current was measured, and the trap parameters of PTFE were analyzed from the current-temperature curve by initial-rise method. In order to investigate the space charge distribution and the surface potential of steady state, factors like primary electron energy, sample thickness, temperature and trap density were selected as variables separately. It is found that the steady surface potential is nearly linear with the primary electron energy, which also fits the experimental results. The simulation results show the sample thickness affects the charging rate of surface potential mostly, and the charging rate is slower in thinner samples. The steady surface potential fluctuates slightly in the case that temperature is not high, but it increases significantly when temperature is higher than 400 K. With the increase in trap density, the space charge accumulation tends to focus on the surface and the dielectric/electrode interface, and the steady surface potential decreases.

Polymer nanocomposites can change the density and/or energy of traps, suppress the accumulation of space charges, and enhance dielectric breakdown strength. It is of interest to reveal the influencing mechanism of trap properties on the dielectric breakdown of polymer nanocomposites. Results of thermally stimulated depolarization current and surface potential decay were reviewed, showing that incorporating a small amount of nanoparticles into a polymer can increase the density and/or energy of deep traps. Then, the relation between traps and dc breakdown field of several polymer nanocomposites were analyzed. It was found that the increase in the density and energy of deep traps contributes to the improved dielectric breakdown performance. The modifications of traps by nanoparticles and surface treatments affect the charge dynamics in the bulk of polymer nanocomposites. Then, the accumulation of space charges, the distortion of electric field, and the energy gain of free carriers are regulated to improve the performance of dielectric breakdown.

Since surface flashover of insulators in vacuum restricts the operation of the power equipment, improvement of surface flashover performance is of considerable importance in various fields. Previous studies show that surface flashover performance is closely related to the trap parameters of materials. In this paper, the crystallization behavior of low density polyethylene (LDPE) is modified by phenolphthalein. It results in the change of the trap parameters, leading to the enhancements in surface flashover performance. Results of differential scanning calorimeter (DSC) and scanning electron microscope (SEM) show that the crystallization behavior of LDPE is greatly changed by phenolphthalein. The crystallinity increases initially and then decreases with the increase of phenolphthalein concentration, and its maximum value occurs at 0.4 wt%. The spherulite size of LDPE composites decreases obviously with the increasing phenolphthalein concentration. The results of thermally stimulated current (TSC) show that both the shallow trap (γ peak) and the deep trap (α peak) become deeper with the increase of phenolphthalein concentration. Furthermore, the impulse and dc surface flashover voltages present the optimum values at 1 wt% phenolphthalein concentration, which can be enhanced by 30.8% and 48.4%, respectively. It is concluded that LDPE with the smaller spherulite size indicates the deeper trap and the better surface flashover performance. This paper reveals the relationship between surface flashover performance and crystallization behavior of LDPE, and proposes a new method to enhance the surface flashover performance of semi-crystalline polymers in vacuum.

Charge transport behaviors of dielectric under electron radiation have an important influence on surface flashover performance. In this paper, charging process and model of dielectric during electron radiation are investigated based on the synergistic effect of electron movement on dielectric surface and charge transport properties in dielectric surface layer. For one thing, electrons accumulated in dielectric surface layer will form a reverse electric field acting against kinetic electrons flowing to dielectric surface, which affects the energy and density of electrons reaching the surface. For another, the changes of electron energy and density will further affect the dynamic processes of electron deposition, transport and accumulation in the surface layer. A dielectric charging model is proposed, and charge transport characteristics in dielectric surface and surface layer are obtained, including the temporal behavior of the incident kinetic electrons flowing to dielectric surface, the temporal behavior of secondary electrons and surface conduction charge, as well as the spatial and temporal behavior of the interior potential and electric field. Based on surface potential decay curves of polyimide under different electron energy levels (3-11 keV), the surface potential decay process and model are investigated. The surface trap distribution of polyimide shows different properties under different energy levels. The shallow trap level increases slightly with the increase of electron energy, ranging from 0.81 eV-0.85 eV, while the deep trap level remains unchanged about 0.94 eV. Besides, the amount of trap charge density gradually increases with the increase of electron energy.

The vacuum surface flashover voltage of dielectric material irradiated by electron-beam is very low, which is the key scientific issue for developing high-voltage and high-power spacecraft technology. This paper proposes a novel research way of considering the combined impact of incident kinetic electrons flowing to dielectric surface and deposited electrons in dielectric surface layer on dc surface flashover performance in vacuum. The experiments on dc surface flashover of polyimide in vacuum after electron irradiation and during electron irradiation were investigated. The experimental results indicated flashover voltage showed a distinctly different variation tendency under the two cases. The dc surface flashover voltage of polyimide after electron irradiation became higher than that without electron irradiation, because of the effect of deposited electrons in dielectric surface layer. During electron irradiation, the dc surface flashover voltage exhibited a dramatic drop from 26.9 kV (energy: 5 keV) to 10.5 kV (energy: 30 keV). Furthermore, the surface flashover characteristics exerts different changes for incident electrons of different energies levels. It has been experimentally indicated that the energy and incident trajectory of incident kinetic electrons flowing to polyimide surface are altered by the combined action of the reverse electric field created by deposited electrons and the applied electric field. As a result, the surface flashover characteristics were changed. This work provides a new insight into the vacuum surface flashover of dielectric material under electron irradiation, and can be used as the research basis of surface flashover mitigation methods in space environment.

The energy distribution of trapped charges and traps in low density polyethylene (LDPE) and high density polyethylene (HDPE) were investigated based on isothermal surface potential decay (ISPD) method in this paper. Corona charging measurement was conducted by a non-contact setup, which was specially designed to obtain the decay characteristics of thin polymer films with varied charging polarity implying the dynamics of electrons or holes. Considering space charge profile of polymers under high dc electric field and detrapping mechanism after removing the field, a model based on ISPD was proposed to obtain the trap distribution of polymers. By assuming different charge spatial distribution features, two kinds of trap energy distributions of LDPE and HDPE were achieved respectively, which reveal similar distribution characteristics. It is shown that there are mainly deep traps in LDPE and HDPE both for electron-type and hole-type traps. The mean trap depth of hole-type traps is lower than that of electron-type ones in LDPE, whereas the situation in HDPE is reversed. Additionally, more trapped charges are accumulated in HDPE with deeper trap depth than that of LDPE. It is indicated that the obtained results may imply essentially nature of hole/electron traps, i.e., the electron-type traps show an inter-chain character whereas the character of hole-type traps is intra-chain. Furthermore, the energy distribution of traps is further related to the unique aggregation structure of LDPE and HDPE.

The oxidation process induced in ethylene-propylene-diene (EPDM) copolymer by gamma irradiation was simulated by solving equations on chemical reactions and gas diffusion rates. As a result, distributions of oxidative products and gases and changes in material properties were clarified. All the oxidative products such as ketones, alcohols, and carboxylic acids, crosslinks between molecular chains, and chain scissions in EPDM increase with irradiation and they show concave spatial distributions inside the sample sheets. The simulation results demonstrates that EPDM becomes hard when it was irradiated by gamma rays and the increase in hardness is more significant at the surface of the sample sheet than its inside. Moreover, it was found that a low diffusion coefficient of oxygen in EPDM leads to the appearance of a clearer diffusion-limited regime of degradation. These simulation results are in good agreement with experimental results.

It is generally known that the dc breakdown strength of low density polyethylene (LDPE) decreases with as the thickness and temperature of the sample increase. The breakdown strength is influenced by the charge transport and electric field distortion, and is also related to the molecular chain displacement and fracture. This paper investigates mutual relations among the charge transport, molecular chain displacement, and thickness dependent dc breakdown of LDPE. A model that combines the dynamics of charge transport and molecular displacement (CTMD) is used to calculate the space charge accumulation, molecular chain displacement, and dc breakdown properties of LDPE with various thicknesses at various constant voltage ramping rates. It is assumed that breakdown occurs when the molecular chain displacement reaches a critical value. The simulation results show that the breakdown field as a function of sample thickness satisfies an inverse power law with a power index of about 0.43 for various voltage ramping rates. This is consistent with experimental results. The CTMD model considers both the distortion of electric field and the displacement kinetics of molecular chains, resulting in a power index closer to the experiment than that calculated only from the electric field distortion. Adopted a Williams-Landel-Ferry type molecular chain mobility in the CTMD model, the simulation results are consistent with the results calculated by applying experimental results on polyisobutylene and polymethyl methacrylate to the free volume breakdown theory. It is also found that the CTMD model with temperature-dependent molecular chain mobility controlled by piecewise Arrhenius equations can explain well the temperature dependent breakdown experimental results of LDPE.

Due to the variation of charge transport characteristics by introduction of nanostructured filler, the conduction and dielectric breakdown properties of nanodielectrics are poorly understood. This work studies on the effect of deep trap on the dc conduction and breakdown properties of nanodielectrics. X-ray diffraction technique is conducted to study the crystallization behavior of LDPE/Al2O3 nanocomposites. Thermally stimulated current (TSC) is applied to measure the trap parameters of specimens. Breakdown strength and volume resistivity are also measured. The results indicate that small amount of nanoalumina enhances the crystallinity, volume resistivity and breakdown strength, and decreases the crystallite size. The TSC results show that the deep trap level and density both increase at low nanoparticle loading samples (

Deep dielectric charging in ring structure irradiated by energetic electrons is one of the major factors causing spacecraft anomalies. In this paper, based on the physical processes of charge deposition, transport and accumulation, we establish a 2-D deep dielectric charging physical model of a ring structure consisting of insulating materials, electrodes and aluminum shield. Deep dielectric charging properties of insulating materials irradiated by electron energy spectra are simulated and analyzed under typical space environment and extreme space environment. Furthermore, the influences of aluminum shield, insulating material properties and thickness on deep dielectric charging are investigated. When the thickness of aluminum shield is larger than 3 mm, the maximum electron-beam density reduces to 10-10 A/m2, which is so low that it can not induce high electric field within materials. Selected polyimide (PI), polytetrafluoroethylene (PTFE) and composite epoxy resin (FR4) as electrical insulating materials, space charge transport, accumulation and electric field in insulating material of the ring structure are analyzed. It is indicated that the maximum electric field obeys the sequence as follows: PTFE> FR4> PI. Finally, it is found that the electric field is obviously enhanced with the increasing thickness of insulating material. This work paves the way towards the mitigation of deep dielectric charging.