Space Charge Behavior Research Articles

Investigation of space charge behavior in self-healing epoxy resin composites

Investigation of space charge behavior in self-healing epoxy resin composites

Effects of SiC nanoparticles on space charge behaviors of LSR/nano-SiO2/nano-SiC composites for insulating material of HVDC cable joints

In order to develop insulating materials for high-voltage direct current (HVDC) cable joints, silicone nanocomposites are prepared by mixing liquid silicone resin (LSR) with the surface-modified nano-SiO2 and the surface-modified SiC nanoparticles, and the effects of SiC nanoparticles on space charge behaviors of the silicone nanocomposites are studied. The hydrophilic surface of the fumed nano-SiO2 is modified into a hydrophobic surface by treating with an alkyl silane coupling agent in order to be uniformly dispersed in LSR, and the surface of the SiC nanoparticles is modified with a vinyl silane coupling agent in order to improve the interfacial characteristics through covalent bonds between SiC nanoparticles and LSR. LSR is mixed with the modified nano-SiO2 (20 wt%) and the modified SiC nanoparticle (0 wt%-9 wt%). To observe the even dispersion of the modified nano-particles in LSR matrixes, a transmission electron microscope is employed and it is found that nano-SiO2 and nano-SiC particles are somewhat uniformly dispersed regardless of their content. When an appropriate amount of semiconductive nano-SiC particles is uniformly dispersed in the LSR/SiO2 nanocomposite, space charges are evenly dispersed. Therefore, the HVDC breakdown strength of the LSR nanocomposite would increase. However, if nano-SiC is mixed in excess, the conduction paths are formed, therefore the HVDC breakdown strength would decrease. HVDC breakdown strength under positive polarity condition shows the maximum value, 99.8 kV/2 mm in the nanocomposite with 3 wt% nano-SiC, which means that the space charge distribution is optimal in the 3 wt% nano-SiC system. However, excessive nano-SiC adversely affects the HVDC breakdown strength.

Effect of Thermal Ageing on Physico-Chemical and Electrical Properties of EHVDC XLPE Cable Insulation

This paper presents the influence of thermal ageing on the physico-chemical and electrical properties of 500 kV extra high voltage direct current (EHVDC) XLPE cable insulation. EHVDC XLPE cable sections are placed in a thermal oven and thermally aged at 106 °C (melting peak temperature) for 250, 500 and 1000 h. After ageing, the inner, middle and outer layers of the cable insulation are characterized by Fourier transform infrared (FTIR) spectroscopy, oxidation induction time, X-ray diffraction (XRD), space charge, DC conductivity and DC breakdown measurements. The physicochemical analysis results show that the cross-linking by-products in the cable insulation are difficult to be completely removed during the ageing process, whereas the thermal ageing cannot completely consume the antioxidants in the materials. The space charge measurement results show that the hetero charges are accumulated inside the unaged middle layer samples. However, the space charge accumulation and electrical conductivity of the samples are significantly reduced after 1000 h ageing. Moreover, the DC breakdown strength of the inner and middle layer samples is continually increased with the increase of the ageing time. It is elucidated that the thermal ageing leads to the re-crystallization and re-degassing of samples, improving the space charge behavior and the charge carrier trapping phenomenon. It is proposed that the temperature of 106 °C could be used as a suitable degassing temperature for the EHVDC XLPE cable production.

Space Charge Behavior of Quantum Dot-Doped Polystyrene Polymers

This paper deals with the role played by the interface and bulk volume of the nanofiller about affecting the electrical properties of a nanocomposite material. For this purpose, a simple and completely amorphous matrix, polystyrene (PS), is used as base material, and core-shell quantum dots are exploited for simulating the structure of nanocomposites: CdSe core and CdSe-ZnS core-shell semiconductor quantum dots (QDs) are added into a PS matrix. The latter is to highlight the effect of the ZnS interface and as contrast to the core material. Dispersion and distribution of QDs are first microscopically observed and optimized by including isopropyl alcohol in the manufacturing phase as an additional solvent. Among electrical properties the focus is on space charge accumulation, tested by means of the pulsed electroacoustic technique at 10 kV/mm and 50 kV/mm on CdSe and CdSe-ZnS doped PS composites. Results are then compared with a reference PS without QDs. Trap depth and density are also obtained by space charge measurement results. When CdSe QDs are added to PS, the trap density increases with respect to the baseline values measured on the unfilled polymer. In contrast, the ZnS shell around the CdSe core creates an additional trap level with lower trap depth, which increases charge mobility, thus turning homocharge into heterocharge accumulation. Therefore, the surface shell-structure of QD nanocrystals appears to significantly influence the space charge behavior of the nanocomposite, independently of the polymer.

Modelling the impact of electrode roughness on net charge density in polyethylene

Predicting the electric field distribution under dc stress within polymeric insulations remains a challenge, as space charge behaviour is still difficult to understand in these materials. Charge generation is often thought to arise from injection at the electrodes. Hence, surface roughness should be taken into account, as it strengthens the electric field locally and therefore promotes charge generation at some points. A charge transport model has been developed in 2D to account for surface roughness. The model is first validated with the help of previous results, obtained with a one-dimensional charge transport model. Then, with simple shapes accounting for roughness, the simulated results show that surface roughness has a significant impact on the net space charge behaviour. The impact of shape, and size of protrusions are presented, as well as a more realistic case where a large surface of the electrode is considered to be rough.

Effects of Trapping Characteristics on Space Charge and Electric Field Distributions in HVDC Cable under Electrothermal Stress

Space charge behavior has a strong impact on the long-term operation reliability of high voltage–direct current (HVDC) cables. This study intended to reveal the effect of trap density and depth on the space charge and electric field evolution behavior in HVDC cable insulation under different load currents and voltages by combined numerical bipolar charge transport (BCT) and thermal field simulation. The results show that when the load current is 1800 A (normal value), the temperature difference between the inside and the outside of the insulation is 20 °C, space charge accumulation and electric field distortion become more serious with the increase in the trap depth (Et) from 0.80 to 1.20 eV for the trap densities (Nt) of 10 × 1019 and 80 × 1019 m−3, and become more serious with the increase in Nt from 10 × 1019 to 1000 × 1019 m−3 for Et = 0.94 eV. Simultaneously decreasing trap depth and trap density (such as Et = 0.80 eV, Nt = 10 × 1019 m−3) or increasing trap depth and trap density (such as Et = 1.20 eV, Nt = 1000 × 1019 m−3), space charge accumulation can be effectively suppressed along with capacitive electric field distribution for different load currents (1800 A, 2100 A and 2600 A) and voltages (320 kV and 592 kV). Furthermore, we can draw the conclusion that increasing bulk conduction current by simultaneously decreasing the trap depth and density or decreasing injection current from conductor by regulating the interface electric field via simultaneously increasing the trap depth and density can both effectively suppress space charge accumulations in HVDC cables. Thus, space charge and electric field can be readily regulated by the trap characteristics.

Investigation on the impact of nano MgO addition on dielectric properties and space charge behavior of epoxy resin AlN nanocomposites

Epoxy nano composites were fabricated with the incorporation of nano AlN and MgO fillers to investigate their dielectric and space charge properties. Epoxy AlN nanocomposites with the addition of nano MgO have shown lower permittivity with reduced tanδ, good space charge behaviour, during poling period. Higher wt% of AlN in epoxy resin exhibited higher permittivity. The nanocomposites with AlN nanofiller have revealed higher space charge accumulation than on addition of MgO nano filler. Also, the addition of nanofillers to the base polymer has led to enhanced space charge decay performance compared to the base matrix.

High Temperature Insulation Materials for DC Cable Insulation — Part III: Degradation and Surface Breakdown

The first two parts in this series gave full analysis and discussion of five high temperature insulation materials, i.e. PTFE, FEP, ETFE, PEEK, and PI over aspects of space charge behavior, conduction property as well as partial discharge, respectively. In this part, the degradation and surface breakdown properties are investigated and the related mechanism is discussed. The results showed that due to stable formation of C-F structures, PTFE and FEP melted locally during arc erosion test, with no carbon element being precipitated. At 25 °C, PI showed the best surface anti-arc erosion property among ETFE, PEEK and PI. However, compared with other samples, the arc withstand property of PI can be much more influenced at higher temperature and in case of surface contamination. Compared with the results measured at 25 °C, the surface flashover voltage decreased for all samples when measured at 150 °C, which can be explained by the expansion of “analogous ineffective region”. FEP has the highest surface flashover voltage at both 25 °C and 150 °C, which is due to higher surface roughness. The content of this paper provides a reference for aging characterization and evaluation of high temperature materials for DC cable and circuit board insulation.

Dielectric Performance of Silica-Filled Nanocomposites Based on Miscible (PP/PP-HI) and Immiscible (PP/EOC) Polymer Blends

This study compares different polymer-nanofiller blends concerning their suitability for application as insulating thermoplastic composites for High Voltage Direct Current (HVDC) cable application. Two polymer blends, PP/EOC (polypropylene/ethylene-octene copolymer) and PP/PP-HI (polypropylene/ propylene - ethylene copolymer) and their nanocomposites filled with 2 wt.% of fumed silica modified with 3-aminopropyltriethoxysilane were studied. Morphology, thermal stability, crystallization behavior dynamic relaxation, conductivity, charge trap distribution and space charge behavior were studied respectively. The results showed that the comprehensive performance of the PP/PP-HI composite is better than the one of the PP/EOC composite due to better polymer miscibility and flexibility, as well as lower charging current density and space charge accumulation. Nanosilica addition improves the thermal stability and dielectric properties of both polymer blends. The filler acts as nucleating agent increasing the crystallization temperature, but decreasing the degree of crystallinity. Dynamic mechanical analysis results revealed three polymer relaxation transitions: PP glass transition ( $\beta$ ), weak crystal reorientation ( $\alpha 1$ ) and melting ( $\alpha 2$ ). The nanosilica introduced deep traps in the polymer blends and suppressed space charge accumulation, but slightly increased the conductivity. A hypothesis for the correlation of charge trap distribution and polymer chain transition peaks is developed: In unfilled PP/EOC and PP/PP-HI matrices, charges are mostly located at the crystalline-amorphous interface, whereas in the filled PP/EOC/silica and PP/PP-HI /silica composites, charges are mostly located at the nanosilica-polymer interface. Overall, the PP/PP-HI (55/45) nanocomposite with 2 wt.% modified silica and 0.3 wt.% of antioxidants making it $a$ promising material for PP based HVDC cable insulation application with $a$ reduced space charge accumulation and good mechanical properties.

Influence of charge emission behaviors of semi-conductive shielding layer on charge accumulation properties of insulation layer for HVDC cable

Space charge accumulation is one of the key factors restricting the manufacture and application of high-voltage direct current (HVDC) cables. Semi-conductive shielding layer plays an important role in uniform electric field in HVDC cable, and its own emission properties directly affect the space charge behaviours in the insulation material. In the paper, charge injection property from semi-conductive layer to insulation layer have been focused emphatically based on experiment and simulation. The charge injection characteristics of semi-conductive layer under two typical electrode configurations of ‘Metal-Insulation layer-Metal’ (M-I-M) and ‘Semi-conductive layer-Insulation layer-Metal’ (S-I-M) have been studied. The conductive current and depolarization current of the insulation layer are compared. Finally, effect of particle size and concentration of carbon black (CB) in the semi-conductive layer on the interfacial electric field has been studied by electric field simulation. The experimental results show that the charge injection from the semi-conductive layer is more obvious under the action of low electric field of 5 kV mm−1 due to effect of CB particles. It can be seen from the depolarizing current curve that the charge injection quantity with S-I interface is greater than that of M-I interface. Further calculation results show that the maximum distortion electric field increases slightly with the increase of CB size. When CB content is 25%, the electric field distortion caused by interface CB particles is about twice of the average electric field (5 kV mm−1), increasing from 9.61 kV mm−1 at 10 nm to 10.15 kV mm−1 at 50 nm. The reduction of CB size in the semi-conductive layer within a certain range is conducive to the reduction of the interface electric field. The maximum distortion field gradually decreases with the increase of CB content at a certain range.

Moisture‐dependent interface and bulk behaviour of space charge in DC‐stressed oil‐impregnated pressboard insulation

Moisture‐dependent interface and bulk behaviour of space charge in DC‐stressed oil‐impregnated pressboard insulation

Space charge behaviors in cable insulation under a direct current‐superimposed pulsed electric field

Space charge behaviors in cable insulation under a direct current‐superimposed pulsed electric field

Space Charge Accumulation Characteristics in HVDC Cable under Temperature Gradient

One of the main issues that affect the development of high-voltage direct-current (HVDC) cable insulation is the accumulation of space charge. The load operation of an HVDC cable leads to the formation of a radially distributed temperature gradient (TG) across the insulation. In this study, the space charge accumulation in a cross-linked polyethylene (XLPE) cable is measured under a DC electric field and TG using the pulsed electro-acoustic (PEA) method, and the effect of the TG on the space charge behavior is investigated. In addition, the bipolar charge transport (BCT) model and the conductivity model based on an improved cylindrical geometry are used to simulate the charge behavior in the HVDC XLPE cable under TG, and the experimental and simulated results are compared. The results show that the higher temperature of the cable conductor promotes the accumulation of homocharge near the side of high temperature. Additionally, with the increase of the TG, not only does more heterocharge accumulates adjacent to the side of low temperature, but more space charge also extends into the bulk of the cable insulation. More attention should be paid to the conductor shield layer and the insulation shield layer in HVDC cables. Moreover, the BCT model can more accurately describe the experimental results than the conductivity model.

Impact of accelerated aging of epoxy‐Ni nanocomposites on space charge variation adopting pulsed electro acoustic technique

AbstractEpoxy‐Ni nanocomposites of different weight percentages are subjected to accelerated aging by water aging, thermal aging and UV irradiation to understand the influence of aging conditions on their space charge behavior. Homo‐charge accumulation is observed for unaged epoxy‐Ni nanocomposites. The magnitude of accumulated space charge is found to be lesser in 0.5 wt% epoxy‐Ni nanocomposite specimen compared to other unaged specimens. Water diffusion coefficient of epoxy‐Ni nanocomposites decreased with increasing weight percentage of filler addition. Space charge accumulation tends to increase with water aging. Increment in the space charge accumulation of 0.25 wt% epoxy‐Ni nanocomposite specimen is lesser compared to other water aged specimens. Exposure to thermal stress resulted in deterioration of the molecular structure of insulating material leading to an increase in trap‐controlled mobility, further leading to the reduction in space charge density with aging period. UV irradiation marginally assisted the charge transportation phenomenon, which resulted in the slight decrease of space charge density of epoxy‐Ni nanocomposites after UV irradiation. Hetero‐charge formation is found for all the three aging procedures. Overall, the specimen S1 reflected better space charge characteristics after being subjected to different aging conditions.

Effect of moisture content on space charge behavior in oil-paper insulation under DC electric field

This study investigates the effect of moisture content on space charge characteristics based on the dynamic space charge response and conductivity characteristics of oilpaper insulation under a DC electric field. Moisture contents ranging from 0.34–2.89% are considered under an electric field of up to 20 kV/mm. The results show that homocharge accumulates inside the oil-paper insulator near both electrodes, with the amount of charge gradually increasing as the moisture content rises. It may be related to the decreased charge injection barrier at the electrode-sample interface as the moisture level increases. The conductivity increases with the rise in moisture level. However, as the conductivity increases, the corresponding space charge accumulation during polarization does not decrease, but increases significantly. This indicates that space charge accumulation may be related to both charge injection and conductivity. The electric field distortion inside the oil-paper insulator increases as the moisture content rises, with the maximum electric field distortion rate going from 16.9 to 92.9% at 5 kV/mm and from 29.7 to 62.5% at 20 kV/mm.

Effects of Inorganic ZnO Particle Doping on Crystalline Polymer Morphology and Space Charge Behavior

This study further investigated the synergistic effect of micro- and nanofiller doping on matrix material space charges and breakdown characteristics. Accordingly, low-density polyethylene (LDPE) was used as the matrix material, and spherical ZnO particles with sizes of 30 nm and 1 µm were used as additives. Micro-ZnO/LDPE, nano-ZnO/LDPE, and micro-nano-ZnO/LDPE composites were prepared through melt blending. The crystalline morphologies of the composites were observed via polarized light microscopy. The composite crystallinity and melting peak temperature were measured via differential scanning calorimetry, and the micro- and nanoparticle dispersions in the matrix were observed via scanning electron microscopy. The test results showed that the particles were uniformly dispersed in the polyethylene matrix. The filler acted as a heterogeneous nucleation agent in the matrix. The crystal size decreased, thereby increasing the crystal quantity. The doping of inorganic ZnO particles improved the composite crystallinity. The ZnO/LDPE composites were subjected to DC breakdown, space charge, and dielectric spectrum tests. When the crystal arrangement of the sample was loose and its size was large, the breakdown process developed along a shorter path, and the field strength of the composite breakdown decreased. The order of AC and DC breakdown field strengths of the samples was as follows: micro-ZnO/LDPE < pure LDPE < micro-nano-ZnO/LDPE < nano-ZnO/LDPE. The DC and AC breakdown field strengths of the micro- and nano-ZnO/LDPE were 4.7% and 3.2% higher than those of the pure LDPE, respectively. Moreover, the DC and AC breakdown field strengths of the nano-ZnO/LDPE were 11.02% and 15.8% higher than those of the pure LDPE, respectively. The doping of inorganic ZnO particles restrained the space charge accumulation, and the residual charges decreased after short-circuit treatment. The dielectric constant of all nanocomposites was lower than that of LDPE, and the dielectric loss of all composites was higher than that of LDPE.

Investigation of charge transport and breakdown properties in XLPE/GO nanocomposites part 2: Effect of polarity reversal

In this paper, the effect of polarity reversal on the space charge behaviors and breakdown properties of cross-linked polyethylene (XLPE)/graphene oxide (GO) nanocomposites are studied. Five kinds of XLPE based nanocomposites doped with different combinations of GO quantum wells (QWs) and the polycyclic aromatic compounds (PAC) were prepared by mechanical blending. The dynamic space charge distributions and breakdown strength of the XLPE samples under polarity reversal were measured. The experimental results indicate that the injection and accumulation of space charges in XLPE show polarity dependence under polarity reversal. The breakdown strength of the XLPE samples decreases with the increase of the pre-applied hetero-polarity electric field. Both the GO QWs and PAC can inhibit charge transport and improve the breakdown properties of XLPE, and the synergistic effect between the two dopants is observed. As a result, XLPE samples modified by the PAC functionalized GO QWs show a minimum space charge accumulation and the highest breakdown strength under polarity reversal. The quantum chemistry calculation method was employed to clarify the charge transport characteristics in XLPE dielectrics, and the relationship between space charge distributions and breakdown properties of the nanocomposites under polarity reversal was discussed.

Investigation of charge transport and breakdown properties in XLPE/GO nanocomposites part 1: The role of functionalized GO quantum wells

The charge transport characteristics and breakdown properties of cross-linked polyethylene (XLPE)/graphene oxide (GO) nanocomposites are investigated in this study. The functionalized graphene oxide (fGO) quantum wells (QWs) with quantum confinement effects were produced by chemically grafting the polycyclic aromatic compounds (PAC) onto the surface of GO QWs. The XLPE/GO nanocomposites doped with different kinds of dopants were prepared by mechanical blending, of which the DC conductivity, space charge behaviors, and breakdown strength were measured. The experimental results reveal that the GO QWs show a synergistic effect with PAC on the modification of insulating properties in XLPEs. The grafted PAC on the surface of GO QWs still retains its original function of attenuating the energy of charge carriers, while the interlayer distances of fGO QWs increase after functionalization. As a result, the XLPE/fGO nanocomposites show a much lower conductivity, fewer space charge accumulation, and higher breakdown strength than those of the other XLPEs. Based on the quantum chemistry calculation method, the role of fGO QWs in the regulation of charge transport and breakdown properties in polyethylene was revealed.

Space charge observation under periodic stress — Part 2: Signal improvement and corresponding procedure

Several signal processing methods are introduced to improve the signal detected under periodic stress in terms of the signal-to-noise (S/N) ratio and behaviour in an arbitrary phase. In the filtering process for the averaging and fitting methods, it is assumed that the variation of space charge can be neglected for a given detecting phase. The averaging process is associated with the phase identification under periodic stress. The fitting process is shown to further enhance the S/N ratio based on space charge behaviour at different positions. A fitting model is chosen based on the signal characteristics, such as the dynamic response of the charge signal and the voltage waveform in the excitation source. The process is based on the premise that the space charge distribution can be detected without significant mutation near the interpolation phase. The procedure proposed optimises signal processing that is associated with space charge.

An equivalent investigation of space charge for plane samples and coaxial cables under a temperature gradient

In this research, the equivalence of space charge for coaxial cables and plane samples are analyzed through simulation. Bipolar charge transport models of coaxial cables and plane samples are established with the same material-dependent parameters, the bulk charge and total space charge distributions are simulated and comparatively researched under isothermal and temperature gradient conditions. The presented results indicate that the space charge behavior shows a basic equivalence in terms of charge variation laws and distribution. In addition, few discrepancies in the heterocharge amount and charge waveform amplitude are observed and mainly ascribed to the sample geometry, mobility gradient distribution and the spatial resolution of the measuring system, respectively. Investigations of space charge characteristics with a plane pulsed electro-acoustic measurement system may be accepted as a practice for providing the space charge characteristics of coaxial cable insulation, unless it is necessary to investigate the cable space charge distribution under actual operating conditions.