DC Breakdown Strength Research Articles

Optimization of the Temperature-Dependent Electrical Resistivity in Epoxy/Positive Temperature Coefficient Ceramic Nanocomposites

Nano-size positive temperature coefficient (PTC) ceramic particles and epoxy composites doped with 0, 0.5, 1, 5, 10 and 20 phr of PTC nanoparticles are prepared to suppress the decrease of electrical resistivity with temperature. Electrical resistivity under 30, 50, 70, 90, and 110 °C, DC breakdown strength, and trap characteristic are measured. DC electric field distribution under radical temperature gradient is simulated using a simplified bushing model insulated with prepared materials. The mechanism for the regulating of electrical resistivity-temperature characteristic of epoxy composites is discussed. The electrical resistivity at 110 °C, DC breakdown strength at 110 °C, and electric field distortion of the epoxy composite with 1 phr PTC nanoparticles is 454, 131 and 74%, respectively of the neat epoxy resin, which exhibits potential to be used as insulating materials in high voltage DC electrical systems.

Grafting of Antioxidant onto Polyethylene to Improve DC Dielectric and Thermal Aging Properties

To suppress space charge accumulation and minimize the antioxidant migration, polyethylene (PE) grafted with reactive hindered amine antioxidant MC prepared by melt radical grafting is investigated in this work. Fourier transform infrared spectroscopy is used to investigate the grafting reaction, which shows that the antioxidant MC is successfully introduced onto PE. Space charge distribution, DC conductivity, DC breakdown strength and thermally stimulated current are measured and the electronic structure is calculated from first-principles. The results show that PE grafted with antioxidant MC can effectively suppress space charge accumulation, reduce DC electrical conductivity and enhance DC breakdown strength. The polar functional groups of antioxidant MC anchored on PE macromolecular chain form immobile deep traps at the grafted MC sites, which can capture space charges and scatter charge carriers to reduce the amount, mobility and energy of charge carriers. Seen from the oxidative induction time of the specimens before and after acetone extraction, the level of retention of the grafted antioxidant MC are significantly higher than those of conventional antioxidant 300 and antioxidant 4020, which indicates that PE grafted with antioxidant MC has excellent long-term thermal oxidation stability.

Effect of temperature on electric‐thermal properties of semi‐conductive shielding layer and insulation layer for high‐voltage cable

A semi-conductive shielding layer plays an important role in the uniform electric field for a high-voltage cable. The electric-thermal properties of the semi-conductive layer and insulation layer directly affect the overall insulation performance of the cable. The physicochemical performances of semi-conductive composites are firstly analysed herein. Furthermore, electric-thermal properties of the semi-conductive layer and insulation layer are discussed. The experimental results show that the thermal conductivity of the commercial semi-conductive layer is about twice that of the insulation layer, owing to the effect of carbon black. The thermal expansion coefficient of the insulation layer rises from 1.86 × 10−4/K at 25°C to 3.20 × 10−4/K at 90°C. By contrast, the semi-conductive layer begins to slowly decline at a certain temperature, and decreases significantly to 2.25 × 10−4/K at 80°C, owing to the effect of ethylene-vinyl acetate copolymer (EVA). The electrical experiments show that the resistivity of semi-conductive composite gradually rises with an increase in temperature, and gradually declines with an increase in the carbon black content. The dc breakdown strength of the composite structure of the semi-conductive layer/insulation layer decreases significantly with an increase in temperature, decreasing from 307 kV/mm at 25°C to 203 kV/mm at 90°C. At four typical temperatures, the breakdown strength reaches the maximum value when the carbon black content is 25 phr. It is about 15% and 19% higher than carbon black contents of 20 and 30 phr. These findings have reference significance for high-voltage cable breakdown fault analysis and material selection in cable design.

Improvement of DC Breakdown Strength of the Epoxy/POSS Nanocomposite by Tailoring Interfacial Electron Trap Characteristics.

To date, breakdown voltage is an underlying risk to the epoxy-based electrical high voltage (HV) equipment. To improve the breakdown strength of epoxy resin and to explore the formation of charge traps, in this study, two types of polyhedral oligomeric silsesquioxane (POSS) fillers are doped into epoxy resin. The breakdown voltage test is performed to investigate the breakdown strength of neat epoxy and epoxy/POSS composites. Electron traps that play an important role in breakdown strength are characterized by thermally stimulated depolarized current (TSDC) measurement. A quantum chemical calculation tool identifies the source of traps. It is found that adding octa-glycidyl POSS (OG-POSS) to epoxy enhances the breakdown strength than that of neat epoxy and epoxycyclohexyl POSS (ECH-POSS) incorporated epoxy. Moreover, side groups of OG-POSS possess higher electron affinity (EA) and large electronegativity that introduces deep-level traps into epoxy resin and restrain the electron transport. In this work, the origin of traps has been investigated by the simulation method. It is revealed that the functional properties of POSS side group can tailor an extensive network of deep traps in the interfacial region with epoxy and enhance the breakdown strength of the epoxy/POSS nanocomposite.

Improved Breakdown Strength of Polypropylene Film by Polycyclic Compounds Addition for Power Capacitors.

In this paper, an improved method for the electric performance of polypropylene (PP) film was proposed to promote the safety and stability of power capacitors. Modified PP films containing three different polycyclic compounds were prepared, which showed good thermal properties and decreased DC conductivity. The DC breakdown strength of the modified PP films under both positive and negative voltage is increased compared with that of the original film. The deep traps introduced by polycyclic compounds and the decreased carrier mobility give an explanation of the decreased DC conductivity. A quantum chemistry calculation was further performed to clarify the mechanism for improving electrical performance, presenting that polycyclic compounds with a high electron affinity and low ionization energy can capture high-energy electrons, protecting the PP molecular chain from attack, and then increase the breakdown strength. It is concluded that the modified PP films by polycyclic compounds have great potential in improving the insulating properties of power capacitors.

Effects of Hindered Phenolic Antioxidants on Space Charge and Breakdown Properties of Polypropylene

This paper focuses on the effects of hindered phenolic antioxidants AO300 and AO736 with similar molecular structure on the electrical properties of isotactic polypropylene (iPP). Experiments of conductivity, space charge distribution, and DC breakdown strength after the hetero-polarity DC prestress are performed. The experimental results show that these two antioxidants can increase the conductivity-temperature coefficient, reduce the accumulation of space charge and improve the DC breakdown strength after the hetero-polarity DC prestress of iPP. By calculating the molecular orbitals of these two antioxidants, it is found that the antioxidants introduce local states in the forbidden band of iPP matrix to capture high-energy charges. Besides, the bond dissociation energy (BDE) of the hydrogen-oxygen bond in AO736 is lower than that in AO300, representing a stronger ability of proton transfer, which scavenges the radicals, inhibits the growth of microvoids and prevents the process of breakdown. It is concluded that the improvement of space charge and breakdown properties is attributed to the coupling effects of deep trap sites and radicals scavenging introduced by antioxidant. This research shows that AO736 has potential applications in PP insulation.

Effect of Crystallization Regulation on the Breakdown Strength of Metallized Polypropylene Film Capacitors

In this work, polypropylene (PP) film samples doped with an organic phosphorus nucleating agent under three cooling processes are examined for the effects of regulating the crystallization. The conductivity and DC breakdown strength of the film samples were tested at 25, 55 and 85 °C. The average breakdown strength with 0.01 wt% nucleating agent increased by approximately 25% compared to un-nucleating samples and the DC conductivity decreased slightly. For the three cooling methods in these tests, the slow process increased the crystallinity of the film samples and stabilized the electrical properties of the PP samples. It is concluded that improving the insulation performance through crystallization control is feasible, and this method shows great potential for the modification of PP films.

Dielectric behaviours of bio‐derived epoxy resins from cashew nutshell liquid

In this study, four commercially available bio-derived epoxy systems (extracted from cashew nutshell liquid) were prepared and characterised. The glass transition temperature (T g), dielectric spectroscopy, DC conductivity and breakdown properties of these epoxy resins were studied. Differential scanning calorimetry (DSC) demonstrated that the T g of the investigated systems ranged from 67 to 122°C. The DC conductivity was very low (<10−16 S cm−1) and comparable to the conventional dielectrics at room temperature (RT). However, all systems showed a strong temperature dependence of the electrical conductivity and exhibited sharp increase around their respective T g. Arrhenius analysis led to activation energy, E a, values around 1 eV; higher E a values were observed in systems with a lower T g. Dielectric spectroscopy revealed a flat and low response at temperature below T g. However, both the real and imaginary permittivity increased with decreasing frequency at mid to low frequencies as the temperatures approached T g. The variations of AC breakdown strength of all samples were not statistically significant, but the DC breakdown strength of sample 2503A + 2002B was higher than the others, which might be due to reduced charge transport in this system. The results indicate that novel bio-derived epoxy systems from renewable sources are potential alternatives for traditional petroleum-based epoxy systems in certain insulation applications.

DC Breakdown and Surface Potential Behavior of Epoxy/Al2O3 Nanocomposites at Cryogenic Temperature

For high temperature superconducting (HTS) dc cables, one key issue is the necessity of using polymer dielectrics with higher dielectric strength at cryogenic temperature in liquid nitrogen. Nanocomposites have played an important role in improving the dielectric properties of epoxy resin (EP). This article investigates on the dc breakdown strength and the surface potential behavior of the epoxy/Al2O3 nanocomposites (0, 1, 3, 5 wt%) at the room and cryogenic temperatures. Moreover, the trap distributions are analyzed from the surface potential decay (SPD) measurement. The possible effect of the trap distributions on the dc breakdown strength is discussed with the valid results. The measurements show that the dc breakdown strength and the initial surface potential continually increased with the nanoparticle addition up to 3 wt% at both room and cryogenic temperatures, and decreased thereafter. The enhancement of the breakdown strength and the initial potential is also found at cryogenic temperature. Besides, the addition of the nanoparticles decreased the rate of SPD compared with the pure epoxy. It is proposed that the higher trap density and trap level enhances the dc breakdown strength. Interestingly, under the cryogenic environment, the nanoparticles addition significantly affects the trap distributions, which further influences the dielectric strength. The enhanced dc breakdown strength is attributed to the reduced space charge formation, the charge carrier mobility and the energy of the charge carriers within the sample bulk.

Temperature Stability of DC Conductivity and Breakdown of Crosslinked LLDPE

In this paper, a way to improve the temperature stability of DC conductivity and breakdown characteristics of polyethylene (PE) is proposed using a crosslinked linear low density polyethylene (LLDPE). The ratio of DC conductivity, as well as breakdown strength values under 90 to 30°C is used to determine the temperature stability of DC properties. The closer it is to 1, the better the performance of temperature stability. The results show that when using crosslinked-LLDPE material, the ratio of DC conductivity under 90 to 30°C is 10.5, while it is 461.6 for crosslinked-LDPE samples with the same DCP content. And the ratio of DC breakdown strength is 0.97 for crosslinked-LLDPE, while it is 0.32 for crosslinked-LDPE. The gel permeation chromatography (GPC) method, hot-set test, differential scanning calorimetry (DSC) analysis and scanning electron microscopy (SEM) images show that the fine arrangement and smaller molecular weight between crosslinks of molecular and morphology structure that LLDPE owns contribute to the improvement in comparison with LDPE. The findings may contribute a solution to the development of HVDC power cable insulation materials.

Dielectric properties dependent on crystalline morphology of PP film for HVDC capacitors application

In this paper, the effects and mechanism of crystalline morphology on dielectric loss and breakdown strength of PP film were studied. The crystalline morphology was modified by adding the β-nucleating agent (β-NA). The crystalline types and crystalline morphologies of samples were analysed by the X-Ray diffraction (XRD) and the polarized optical microscopy (POM), respectively. The current integration, representing the dielectric loss, was investigated using the Q(t) method. The DC conductivity was measured at temperature from 25 to 85 °C. The effects of crystalline morphology on the trap energy level distribution were analysed by the isothermal discharge current (IDC). The DC breakdown strength was tested using a pair of ball-plate electrodes at 85 °C. The results of XRD and POM indicate that the addition of β-NA introduced β-spherulites in PP film and the spherulite size of samples with β-NA are smaller the spherulites size of PP without β-NA. The results of IDC indicate that the special structure of β-spherulites introduced deep trap energy levels. The results of Q(t) and DC conductivity indicate that β-spherulites can effectively suppress the current though samples. Besides, the addition of β-nucleating agent increases the DC breakdown strength at 85 °C.

Filler concentration effect on breakdown strength and trap level of epoxy resin–Al2O3 nanocomposites

In this work, epoxy resin samples incorporated with nano-Al2O3 of various concentrations 0, 1, 3 and 5 wt% were prepared. Surface treatment of nano-Al2O3 was performed by the saline coupling agent of γ-aminopropyltriethoxysilane (KH550) to restrict the aggregation. The thermal gravimetric analysis (TGA) of the epoxy resin/Al2O3 nanocomposites was observed at different filler concentrations. TGA analysis indicated that incorporation of nano-Al2O3 increases the decomposition temperature as compared to pure epoxy resin. Moreover, 3 wt% filler concentration showed increased decomposition temperature because nanoparticles form close connections between molecules due to which thermal stability improved at higher temperatures. Atomic force microscopy was employed for the surface morphology of epoxy resin/Al2O3 nanocomposites which has indicated that surface roughness of epoxy resin/Al2O3 nanocomposites increased slightly with the filler concentration. The distribution of trap energy and trap density obtained by isothermal surface potential decay measurements revealed that 1 wt% filler concentration results in decrease in surface potential decay rate. In addition, significant increase in deep trap energy level and trap density was obvious at 1 wt% filler concentration. AC and DC breakdown strength of the samples was measured to evaluate the effect of filler concentrations on the electrical characteristics of epoxy resin. The results showed that breakdown strength of the epoxy resin/Al2O3 nanocomposites increases with the nano-Al2O3 concentration. Moreover, increase in trap energy results in an increase in the breakdown strength of the nanoparticles.

Interfacial space charge characteristic of PPLP insulation for HVDC cables

High-voltage DC (HVDC) cables show more efficiency than high-voltage AC cables over the long distance power transmission and have attracted significant attention recently due to rapid developments in off-shore wind farm. Polypropylene laminated paper (PPLP) insulation based HVDC cables have provided a reliable operation for many years and are the highest voltage level in operation at the moment. In this study, the authors intend to understand the electrical performance of PPLP insulation system by examining space charge in detail, especially at the interfaces. It has been found that the interface between polypropylene and Kraft paper acts as trap for charge carriers, resulting in an optimal electric field distribution in PPLP insulation system. The optimal electric field distribution enhances overall electrical performance of PPLP insulation, including higher dc breakdown strength and lower electrical conductivity.

Enhanced electrical insulating properties of polyethylene by incorporating polyethylene‐g‐polystyrene graft copolymers

AbstractAiming at utilizing the electron energy absorption characteristic of the benzene ring in polystyrene (PS) to enhance the electrical properties of insulation, here linear low density polyethylene grafted PS graft copolymers (LLDPE‐g‐PS) have been synthesized by pre‐irradiation and suspension grafting technology. Then, LLDPE‐g‐PS was added in LLDPE to prepare LLDPE/LLDPE‐g‐PS blends, of which the microstructure and thermal, rheological, electrical and mechanical properties were systematically investigated. Scanning electron microscopy showed that no interfacial debonding occurred in the blends. Rheological measurements showed that the viscoelastic properties of the blends were roughly the same as those of LLDPE. In terms of electrical properties, as LLDPE‐g‐PS was added, the injection of space charges was markedly suppressed. Meanwhile, all blends presented higher DC breakdown strength and the maximum was 1.45 times higher than that of LLDPE. Besides, blends exhibited little change in mechanical properties compared with LLDPE when PS contents were lower than 10 wt%. This investigation proposes an efficient method to improve the electrical properties of insulating materials. © 2020 Society of Industrial Chemistry

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

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

Dielectric and Thermal Conductivity Characteristics of Epoxy Resin-Impregnated H-BN/CNF-Modified Insulating Paper.

High-voltage direct-current (HVDC) dry bushing capacitor-core insulation is composed of epoxy resin-impregnated insulating paper (RIP). To improve the thermal conductivity, breakdown strength, and space charge characteristics of RIP, 0.1 wt % nano-cellulose fiber (CNF)-modified RIP (CNF/RIP), 2.5–30 wt % hexagonal boron nitride (h-BN)-modified RIP (h-BN/RIP), and 2.5–30 wt % h-BN + 0.1 wt % CNF-modified RIP (h-BN + 0.1 wt % CNF/RIP) were prepared. Scanning electron microscopy (SEM) was implemented; the thermal conductivity, DC conductivity, DC breakdown strength, and space charge characteristics were tested. The maximum thermal conductivity of h-BN + 0.1 wt % CNF/RIP was 0.376 W/m.K with a h-BN content of 30 wt %. The thermal conductivity was 85.2% higher than that of unmodified RIP. The breakdown strength and charge suppression were the best in the case of 10 wt % h-BN + 0.1 wt % CNF/RIP. The maximum breakdown strength was 11.2% higher than that of unmodified RIP. These results can play a significant role in the research and development of insulation materials for HVDC dry bushing.

Influence of Ungrafted Monomers in Graft Copolymers on Electrical Insulating Properties of Polyethylene

Herein, the effects of ungrafted acrylic acid (AA) monomers in graft copolymers on electrical properties were investigated. Linear low-density polyethylene-grafted AA (LLDPE-g-AA) was synthesized by preirradiation and melt-extrusion grafting. Then, LLDPE/LLDPE-g-AA and LLDPE/u-LLDPE-g-AA blends were prepared based on purified LLDPE-g-AA and untreated LLDPE-g-AA, respectively. The rheological, thermal, and electrical properties of blends were studied and compared. The results showed that the suppression of space charges mainly resulted from grafted polyacrylic acid (PAA) branches. Ungrafted AA monomers in blends had no apparent effects on space charge distribution but featured positive influence on the DC breakdown strength within the concentration range of this study. The dielectric strength for different accelerated migration periods showed that as ungrafted AA monomers migrated from the blends, the influence of ungrafted AA monomers on the dielectric strength weakened, and PAA branches played a dominant role in the enhancement of dielectric strength.

Simulation of thickness controlled DC breakdown of XLPE regulated by space charge &amp; molecular chain movement

In this study, XLPE electrical breakdown depending on insulation thickness is analyzed by simulations and experiments. The bipolar space charge and molecular chain displacement models are investigated to explain the experimental results. The results from experiments indicate that the electrical DC breakdown field decreases with increasing insulation thickness. For simulations, carrier mobility and trapping parameters are obtained from experiments. The simulation results of both models present an inverse power relationship between the DC breakdown strength and insulation thickness. The simulation results of molecular chain mobility model show consistency with experimental results as compared to a bipolar space charge model. The movement of polymer chain containing deep traps expands the free volume and this enlargement in free volume can be a major reason for the variation in the DC electrical breakdown strength of XLPE with insulation thickness.

The Role of Silicon-Based Nanofillers and Polymer Crystallization on the Breakdown Behaviors of Polyethylene Blend Nanocomposites

Good breakdown strength is an important feature for the selection of dielectric materials, especially in high-voltage engineering. Although nanocomposites have been shown to possess many promising dielectric properties, the breakdown strength of nanocomposites is often found to be negatively affected. Recently, imposing nonisothermal crystallization processes on polyethylene blends has been demonstrated to be favorable for breakdown strength improvements of dielectric materials. In an attempt to increase nanocomposites’ voltage rating, this work reports on the effects of nonisothermal crystallization (fast, moderate and slow crystallizations) on the structure and dielectric properties of a polyethylene blend (PE) composed of 80% low density polyethylene and 20% high density polyethylene, added with silicon dioxide (SiO2) and silicon nitride (Si3N4) nanofillers. Through breakdown testing, the breakdown performance of Si3N4-based nanocomposites was better than SiO2-based nanocomposites. Since nanofiller dispersion within both nanocomposite systems was comparable, the enhanced breakdown performance of Si3N4-based nanocomposites is attributed to the surface chemistry of Si3N4 containing less hydroxyl groups than SiO2. Furthermore, the breakdown strength of SiO2-based nanocomposites and Si3N4-based nanocomposites improved, with the DC breakdown strength increasing by at least 12% when both the nanocomposites were subjected to moderate crystallization rather than fast and slow crystallizations. This is attributed to changes in the underlying molecular conformation of PE in addition to water-related effects. These results suggest that apart from changes in the nanofiller surface chemistry, changes in the underlying molecular conformation of polymers are also important to improve the breakdown performance of nanocomposites.

Evaluating the dielectric strength of promising SF6 alternatives by DFT calculations and DC breakdown tests

In this paper, a calculation method is proposed to predict the dielectric strength (DS) of gases and the DS of some promising SF 6 alternative gases and their mixtures is further evaluated by DC breakdown tests. The calculation method is based on the density functional theory (DFT), so that the DS of the gases can be obtained by only calculating their molecular parameters, including the average static electronic polarizability and adiabatic ionization energy. The paper also presents the DC breakdown strength of the C 4 F 7 N, C 5 F 10 O, and HFO-type gases, and it is found that the experimental DS values are in good agreement with the calculated values. In addition, DC breakdown tests for gas mixtures are also conducted and their synergistic effect is discussed in detail. The results show that the DS of C 4 F 7 N mixed gases is equivalent to that of pure SF 6 gas even under high pressures, indicating their potential applications in HV electrical equipment. The DS of C 5 F 10 O mixed gas is slightly lower than that of C 4 F 7 N mixed gas. Compared with CO 2 , mixing with air brings stronger synergistic effect in C 5 F 10 O and C 4 F 7 N in our experiment. The DS of HFO mixture is substantially less than that of SF 6 because of the weak synergistic effect when mixed with CO 2 or N 2 . This paper presents an effective method to evaluate DS of SF 6 alternative gases and the results are useful for their future engineering applications.