Direct Current Breakdown Strength Research Articles

Electrical Characterization of Epoxy Nanocomposite under High DC Voltage.

This work studies the direct current breakdown characteristics of unfilled epoxy and epoxy nonconductive nanocomposites (SiO2,MgO and Al2O3). It also examines the variation of electrical properties in epoxy nanocomposites. The novel aspect of this study is that the samples of Epoxy nanocomposite were exposed to high voltages of up to six kilo volts for three hours using field electron microscopy under high vacuum conditions (10-5 mbar). The current emitted from these samples was measured at three different intervals of time. In addition, the influence of high voltage on the permittivity, loss factor (tan(δ)), and conductivity of the epoxy nanocomposite was studied. This evaluation was conducted before and after applying the voltage at room temperature, The frequency range extends from 10-2-10-7 Hz using the Novo Control Alpha-A analyzer. Current-voltage characterization was performed through field electron microscopy. The samples were characterized by scanning electron microscopy-energy dispersive X-ray spectroscopy and Fourier Transform Infrared Spectroscopy. The unfilled epoxy exhibited structural degradation, resulting in the formation of holes when exposed to high voltages of up to six kilo volts, leading to a reduction in electrical properties. Nevertheless, the addition of nanoparticles shows a significant increase in the operational lifetime of the epoxy nanocomposite. The degree of increase in the lifetime of epoxy composite varied depending on several factors such as the type of NPs introduced and their respective sizes. The epoxy/Al2O3 nanocomposite comparing with epoxy/MgO and epoxy/SiO2 nanocomposite showed elevated resistance to direct current breakdown strength and maintaining its dielectric.

Study on thermal cycling ageing resistance of performances of polypropylene/elastomer/boron nitride nanocomposites for high voltage direct current cable insulation

AbstractThe influences of thermal cycling ageing on structures and insulation performances of polypropylene (PP)/elastomer/boron nitride (BN) nanocomposites are investigated. The Melt blending method was used to prepare the nanocomposites, in which propylene‐based elastomer (PBE) or ethylene‐octene copolymer elastomer (EOC) was contained for comparison. Then, the samples were treated using a thermal cycling process with a temperature range from −30 to 150°C, and the number of thermal cycles was set from 0 to 15. Scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry and X‐ray diffraction measurements were taken to facilitate the comprehension of structural changes. Additionally, measurements were taken to assess the trap distribution and direct current (DC) breakdown strength of the samples. The obtained results revealed that following thermal cycling ageing, the morphology and crystallinity of PP/PBE/BN remained almost unchanged. In contrast, PP/EOC/BN exhibited substantial microstructural damage accompanied by a significant reduction in crystallinity. As the number of thermal cycles increased, the trap level and DC breakdown strength of PP/PBE/BN were maintained at high levels, while those of PP/EOC/BN initially remained stable but then experienced a sharp decline. It is suggested that the addition of BN nanoparticles enhances the thermal cycling ageing resistance of the PP/PBE blend, whereas it weakens this resistance of the PP/EOC blend.

Blending Modification Technology of Insulation Materials for Deep Sea Optoelectronic Composite Cables

The insulation layer of deep-sea optoelectronic composite cables in direct contact with high-pressure and highly corrosive seawater is required for excellent water resistance, environmental stress cracking resistance (ESCR), and the ability to withstand high DC voltage. Although high-density polyethylene (HDPE) displays remarkable water resistance, it lacks sufficient resistance to environmental stress cracking (ESCR). This article is based on a blend modification approach to mixing HDPE with different vinyl copolymer materials (cPE-A and cPE-B). The processing performance and mechanical properties of the materials are evaluated through rheological and mechanical testing. The materials’ durability in working environments is assessed through ESCR tests and water resistance experiments. Ultimately, the direct current electrical performance of the materials is evaluated through tests measuring space charge distribution, direct current resistivity, and direct current breakdown strength. The results indicate that, in the polyethylene blend system, the rheological properties and ESCR characteristics of HDPE/cPE-A composite materials did not show significant improvement. Further incorporation of high melt index linear low-density polyethylene (LLDPE) material not only meets the requirements of extrusion processing but also exhibits a notable enhancement in ESCR performance. Meanwhile, copolymerized polyethylene cPE-B, with a more complex structure, proves effective in toughening HDPE materials. The material’s hardness significantly decreases, and when incorporating cPE-B at a level exceeding 20 phr, the composite materials achieve excellent ESCR performance. In a simulated seawater environment at 50 MPa, the water permeability of all co-modified composite materials remained below 0.16% after 120 h. The spatial charge distribution and direct current resistivity characteristics of the HDPE, cPE-A, and LLDPE composite systems surpassed those of the HDPE/cPE-B materials. However, the HDPE/cPE-B composite system exhibited superior dielectric strength. The application of composite materials in deep-sea electro–optical composite cables is highly promising.

Dielectric Breakdown Strength of Polypropylene Blended with Ethylene Octene Elastomers

Polymer blends have become a major focus of research in exploring the potential for new dielectric material applications, particularly in the insulation of high voltage systems. In the current research area, polypropylene (PP) is being investigated as an insulation material for high voltage alternating current and direct current power cables. However, PP must be treated with rubbery materials such as elastomers to lessen its overall stiffness for power cable extrusion. While various promising results have been published on the potential use of PP/elastomer blends as dielectric materials, the dielectric effects of adding ethylene octene elastomer to PP have been less explored. This paper discusses the alternating current (AC) and direct current (DC) breakdown strength of PP that contains different amounts of ethylene octene elastomer (10, 30, and 50 wt%). The addition of ethylene octene elastomer to PP affects the breakdown performance of the blend systems under both AC and DC fields. Significantly, an increase in elastomer contents results in a notable reduction in the AC and DC breakdown strength. A discussion is presented on the mechanisms underlying the change in the breakdown strength caused by the different amounts of ethylene octene elastomer blended with PP.

Insulation properties enhancement of crosslinked polyethylene by grafting electron‐buffering voltage stabilizers

AbstractGrafting a voltage stabilizer on molecular chains of cross‐linked polyethylene (XLPE) can effectively improve the materials' insulation properties. Four different samples (XLPE, XLPE grafted with the voltage stabilizer, low‐density polyethylene (LDPE) blended with the voltage stabilizer, and LDPE grafted with the voltage stabilizer) are prepared in this paper. Several physicochemical and dielectric tests are performed to investigate the properties of the samples. The results demonstrate that the m‐aminobenzoic acid is successfully grafted onto the XLPE molecular chain. Moreover, the crosslinking and grafting processes are equally important for enhancing the materials' performance of the XLPE. In addition, the grafted voltage stabilizer negligibly affects the gel content and thermal properties of XLPE. However, it increases the crystallinity but decreases the lamella thickness. Moreover, the XLPE grafted with a voltage stabilizer has lower direct current (DC) electrical conductivity, less aggregation of space charge, and higher DC breakdown strength. The trap distribution results indicate that the voltage stabilizer introduces shallow traps in the sample and the quantum chemical calculations describe that the material bandgap is decreased. Thus, the high‐energy charge carriers are buffered by the newly‐introduced traps and the functional groups, leading to the enhanced material properties of the XLPE.

The Improved DC Breakdown Strength Induced by Enhanced Interaction between SiO2 Nanoparticles and LLDPE Matrix.

Direct current (DC) power transmission systems have received great attention because it can easily integrate many types of renewable energies and have low energy loss in long-distance and large-capacity power transmission for electricity global sharing. Nanoparticles (NPs) have a positive effect on the insulation properties of polymers, but weak interaction between NPs and polymer matrix greatly decreases the effort of NPs on the enhancement of insulation properties, and thereby limits its engineering application. In this work, grafting strategy was used to link the modified NPs and polymer matrix to improve their interactions. Silica NPs (SiO2-NPs) were modified by 3-(methacrylyloxy) propyl-trimethoxysilane (MPS) to introduce highly active groups on the SiO2-NPs surface, followed by the pre-irradiated linear low-density polyethylene (LLDPE) being easily grafted onto the MPS modified SiO2-NPs (MPS-SiO2-NPs) in the melt blending process to obtain LLDPE-g-MPS-SiO2-NPs nanocomposites. Fourier-transform infrared (FT-IR) spectrum and X-ray photoelectron spectroscopy (XPS) confirm the successful incorporation of MPS into SiO2-NPs. Transmission electron microscopy (TEM) verifies that the modified SiO2-NPs exhibits more uniform distribution. The rheology result shows that the interaction between MPS-SiO2-NPs and LLDPE significantly improves. More importantly, the LLDPE-g-MPS-SiO2-NPs nanocomposites displays superior DC breakdown strength to that fabricated by conventional modification methods. When the addition of MPS-SiO2-NPs is 0.1 wt%, the highest DC breakdown strength values of 525 kV/mm and 372 kV/mm are obtained at 30 °C and 70 °C, respectively, and high DC breakdown strength can be well maintained in a wide loading range of NPs.

Effect of Surface-Modified Nanosilicas on the Electrical Breakdown Strength in Epoxy Nanocomposites

Hydrophilic surface of fumed nanosilica was treated with a trimethylsilane coupling agent to achieve its hydrophobic surface, and the surface modification was confirmed by thermogravimetric weight loss and increasing contact angle of water droplet. The effects of the surface-modified nanosilica content on the electrical and mechanical properties of epoxy nanocomposites were studied to apply to the insulators for heavy electrical equipment. Epoxy nanocomposites were prepared by mixing epoxy base resin/curing agent/accelerator/trimethylsilane-modified nanosilica in the weight ratio of 100:80:1:0–7 g. High-voltage direct current breakdown strength increased with increasing modified nanosilica content. High-voltage alternating current breakdown strength was maximum when the nanosilica content was 1 phr (part per one hundred of epoxy resin), and thereafter, the value gradually decreased as the nanosilica content increased. Tensile strength of epoxy nanocomposite with trimethylsilane-modified nanosilica (7 phr) is 78.76 MPa, which is 20% higher than that of neat epoxy resin. The dielectric permittivity increases with increasing amount of the trimethylsilane-modified nanosilica.

Influence of Bismaleimide HVA-2 Grafting on the Direct Current Dielectric Properties of XLPE

In this paper, N, N’-m-phenylene dimaleimide (HVA-2) grafted crosslinked polyethylene (XLPE) insulation materials with different HVA-2 contents were prepared. The grafting, crosslinking, and crystalline structure were characterized by Fourier-transform infrared spectroscopy (FTIR), Soxhlet extraction, and differential scanning calorimetry (DSC), respectively. The space charge distribution, direct current (DC) breakdown strength, and DC conduction current density were tested and the electronic structure was calculated from first-principles. HVA-2 grafting modification can significantly reduce the accumulation of space charges and the conduction current density of XLPE, but have a negative effect on DC breakdown strength. The polar groups of the grafted HVA-2 anchored on XLPE by the grafting reaction can introduce deep traps densely and evenly in XLPE, which would capture and scatter charge carriers, thus reducing the carrier concentration and mobility and further improving the space charge distribution and reducing conduction current density. However, the grafting of HVA-2 can increase the crosslinking extent of XLPE to make the crystallinity decrease and the crystallization inhomogeous, leading to a certain decrease in the breakdown strength of the grafted XLPE.

Influence of phosphorus-based flame retardants on polypropylene insulation for high-voltage power cable applications

For high voltage (HV) power cable applications, various studies have been performed to improve the mechanical and electrical properties of polypropylene (PP)-based insulation materials to replace crosslinked polyethylene. However, studies on the effect of additives to yield additional PP properties are still lacking. Herein, we prepared PP blends by melt-mixing widely used commercial flame retardants for PP with isotactic PP (iPP) and investigated their electrical breakdown, flame retardancy behaviors, and UV stability. Among the five kinds of flame retardants employed, aluminum hypophosphite (AHP), aluminum diethyl phosphinate, melamine pyrophosphate, ammonium polyphosphate (APP), and APP treated with silane, AHP was very effective in minimizing the decrease of the direct current breakdown strength of iPP at both 25 °C and 110 °C in the range of 5–20 phr. Particularly, only AHP afforded V-2 grade flame retardancy to iPP, and the flame retardancy was maintained even when the content was reduced to 3 phr. Furthermore, upon exposure to ultraviolet (UV) rays for 5 d, the tensile strength of pristine iPP decreased by approximately 44%, while that of a blend with 3 phr AHP decreased by only 10%. The study results will contribute to the optimization of power cable products through the use of appropriate flame retardants in the design of high-performance PP-based HV insulation materials.

Spraying EPDM@NPs as an efficient strategy for polyethylene-based high-voltage insulation

The synthesizing strategy of grafting a structure similar to the polymer matrix on the nanoparticles (NPs) surface is highly efficient in improving the insulating properties of polymers. The comprehensive study consisting of quantum chemical simulations and electrical properties at elevated temperatures shows that under high temperatures and strong direct current (DC) electric fields, spraying an ultra-low concentration of 0.0001 wt% EPDM@ZnO at the cross-linked polyethylene (XLPE) interface can reduce its DC electrical conductivity, suppress space charge accumulation and increase the DC breakdown strength. The EPDM@ZnO inherits the hydrophobic properties of graft material EPDM. Adding very low concentrations such as 0.0001 wt% minimizes the impact on material cleanliness, i.e., impurities, and facilitates the uniform dispersion of nanoparticles in the host polyethylene matrix, thereby overcoming nanoparticle agglomeration. The reported synthesis strategy of nanoparticles and preparation process is believed to provide new insights for developing future ultra-high voltage direct current cables and can be widely applied to nano-polymer modification applications of coatings, structural materials, and interface materials.

Improving the DC Dielectric Properties of XLPE with Appropriate Content of Dicumyl Peroxide for HVDC Cables Insulation

In this research, crosslinked polyethylene (XLPE) is developed with selective content of dicumyl peroxide (DCP), and the influence of microstructural properties and chemical composition on the mechanical and direct current (DC) dielectric properties are investigated. The measurements for the microstructural analysis are taken by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), gel content test and Fourier transform infrared (FTIR). The mechanical properties of XLPE are evaluated by hot-set test. The results of microstructural and chemical composition show that the increase in DCP content increases the crosslinking degree from 74.3% to 81.6%, reduces the crystallinity/lamella thickness from 36.8% to 35.5%/7.6–7.1 nm, reduces the average molecular weight between two crosslinks by 0.01 kg/mol and reduces the oxidation level/carbonyl index. The increase in DCP in XLPE samples decreases the permanent elongation from 2.2% to 0% and elongation rate from 300% to 80% of the cable insulation. The rise in DCP content increases the crosslinking degree due to which the DC resistivity and activation energy is increased. The DC breakdown strength at 30–90 °C is increased due to the increase in crosslinking degree and reduction in carbonyl index/oxidation level. The space charge accumulation is measured at 30 °C under 20–60 kV/mm, resulting in less homo-charges and hetero-charges with the increase in DCP. It is proven that the role of appropriate DCP content is vital in increasing the DC dielectric performance, internal material characteristics and mechanical performance of XLPE.

Excellent thermal stability and energy storage properties of lead‐free Bi0.5Na0.5TiO3‐based ceramic

AbstractThe ceramic capacitors with excellent energy storage properties and wide operating temperature are the main challenges in power system applications. Here, the lead‐free (1‐x)Bi0.5Na0.5TiO3‐xCaTiO3 (abbreviated as BNT‐xCT) ceramics were synthesized through solid‐state reaction method. The introduction of CT reduced the temperature of permittivity peak of BNT ceramic, guaranteeing excellent thermal stability over a wide temperature range of −100 ∼ 136°C. Meanwhile, the long‐range order structure of BNT was destructed by structural distortion, and the relaxor behavior was enhanced after doping CT. Moreover, the direct current breakdown strength was improved from 203 to 455 kV/cm, and the high recoverable energy density (Wrec ∼ 2.74 J/cm3) with high efficiency (η ∼ 91%) was achieved for BNT‐0.25CT ceramic, along with a fast discharge speed (t0.9 ∼ 110 ns) superior cycle stability and thermal stability. Those properties enabled a promising practical prospect of BNT‐CT ceramics.

Enhanced direct‐current breakdown strength of Al2O3/epoxy resin composites through plasma fluoridation of fillers interface

AbstractHerein, a method for improving the direct‐current (DC) breakdown strength of the epoxy resin (ER) was proposed. Nano‐Al2O3 was treated with CF4/Ar plasma and doped into the epoxy resin matrix to obtain epoxy composites with high DC breakdown strength in both extremely uneven and slightly uneven electric fields. The reaction essence of plasma fluoridation was characterized, and the dielectric spectrum, volume conductivity, space charge, and carrier trap characteristics of the modified composites were studied. The plasma fluoridation at the fillers‐matrices interface is an effective pretreatment method to enhance insulation, exhibiting good applicability to ceramic/polymer composites.

Regulation of PANI nanofiber conductivity and its influence on the DC dielectric properties of LDPE

To study the effect of electrical conductivity of nanomaterials on the space charge accumulation in low density polyethylene (LDPE), doped and dedoped polyaniline (PANI) nanofibers with the same diameter and different conductivity were prepared and added into LDPE. The PANI nanofibers and nanocomposites were characterized by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscope, thermogravimetric analysis (TGA), X-ray diffraction analysis (XRD) and differential scanning calorimeter (DSC). The space charge distribution, direct current (DC) conductivity and DC breakdown strength of the nanocomposites were measured. The addition of either PANI nanofibers enhanced the crystallization, improved the space charge distribution and reduced the conductivity of LDPE. Doped PANI nanofibers had higher conductivity and could more effectively suppress space charge accumulation than dedoped PANI nanofibers. However, doped PANI nanofibers reduced the DC breakdown strength of PANI/LDPE nanocomposites. Overall, dedoped PANI nanofibers were superior for DC dielectric properties of LDPE.

Influence of polypyrrole nanospheres on the direct current dielectric properties of LDPE

AbstractTo study the effect of nano conducting materials on the direct current (DC) dielectric properties of low density polyethylene (LDPE), polypyrrole nanospheres are prepared by the soft template method and then melt‐blended with LDPE to obtain LDPE/polypyrrole nanocomposites. The micro‐structure, space charge distribution, thermally stimulated depolarization current, DC conductivity and DC breakdown strength of the nanocomposites are investigated. The results show there is interaction between polypyrrole nanospheres and LDPE, which will not affect the crystal structure of LDPE, but slightly increase the crystallinity of LDPE/polypyrrole nanocomposites. Polypyrrole nanospheres can introduce deep traps in LDPE to capture space charge carriers, which will suppress the accumulation of space charges in LDPE and reduce the DC conductivity. The DC breakdown strength of LDPE/polypyrrole nanocomposites is improved slightly by adding a small amount of polypyrrole nanospheres.

Preparation and properties of LLDPE/LLDPE-g-PS/MgO@PS Nanocomposites

ABSTRACT Polyethylene (PE) is widely used in the electrical insulation systems because of its excellent mechanical and electrical properties. However, space charge will accumulate in insulating materials and cause electrical field distortion under high voltage. Linear low density polyethylene/linear low density polyethylene grafted polystyrene/polystyrene coated magnesium oxide (LLDPE/LLDPE-g-PS/MgO@PS) nanocomposites were prepared by melt blending. The morphology, thermal, mechanical, and electrical properties of LLDPE/LLDPE-g-PS/MgO@PS nanocomposites were investigated. Scanning electron microscopy indicated that the MgO@PS nanoparticles were well dispersed in the matrix, and the direct current breakdown strength was increased about 51.7% when 2.0 wt % MgO@PS added.

Effect of Surface State on DC Breakdown Characteristics of Thermally Aged Double-Layered Polyester Films

The surface state of polymer films has a significant influence on direct current (DC) breakdown characteristics, but its mechanism needs further investigation. In this paper, DC breakdown strength of thermally aged two-layer polyester films is studied. The charge trap and their space distribution characteristics are analyzed by an isothermal relaxation current and the pulsed electro-acoustic method. Furthermore, a bipolar charge transport model suitable for two-layer dielectrics is established, and the correlation between surface state and DC breakdown performance is dynamically simulated. The experimental results show that as the aging time increases, DC breakdown strength first increases and then decreases. The trap depth and its density first increase and then decrease, and the change mainly comes from dielectric surface area. The simulation results show that through modulating the injection and accumulation behaviors of space charges, the change of trap depth, density and extended depth of surface state have an important effect on the space charge and electric field distribution at the pre-breakdown time.

Crosslinking Dependence of Direct Current Breakdown Performance for XLPE-PS Composites at Different Temperatures

In this paper, crosslinked polyethylene-polystyrene (XLPE-PS) composites with different degrees of crosslinking were fabricated by using different crosslinking agent contents and their direct current (DC) breakdown performance at 30~90 °C was investigated. Results show that with the increase of the degree of crosslinking, the crystallinity of XLPE-PS composites decreases gradually, but their DC breakdown strength demonstrates an increasing trend at 30~90 °C and the enhancement also increases with the rise of temperature. And as the degree of crosslinking increases, the elastic modulus of XLPE-PS composites is reduced and the loss tangent peak temperature decreases but the peak shifts to a lower value, which reveals the suppression of the relaxation process for crystallites. It is believed that high DC breakdown strength with good temperature stability for XLPE-PS composites with a larger degree of crosslinking is attributable to the presence of PS and suppression in the formation of crystallites due to crosslinking.

Enhanced Thermal Conductivity and Dielectric Properties of h-BN/LDPE Composites.

Low-density polyethylene (LDPE), as an excellent dielectric insulating material, is widely used in electrical equipment insulation, whereas its low thermal conductivity limits its further development and application. Hexagonal boron nitride (h-BN) filler was introduced into LDPE to tailor the properties of LDPE to make it more suitable for high-voltage direct current (HVDC) cable insulation application. We employed melt blending to prepare h-BN/LDPE thermally conductive composite insulation materials with different contents. We focused on investigating the micromorphology and structure, thermal properties, and electrical properties of h-BN/LDPE composites, and explained the space charge characteristics. The scanning electron microscope (SEM) results indicate that the h-BN filler has good dispersibility in the LDPE at a low loading (less than 3 phr (3 g of micron h-BN particles filled in 100g of LDPE)), as well as no heterogeneous phase formation. The results of thermal conductivity analysis show that the introduction of h-BN filler can significantly improve the thermal conductivity of composites. The thermal conductivity of the composite samples with 10 phr h-BN particles is as high as 0.51 W/(m·K), which is 57% higher than that of pure LDPE. The electrical performance illustrates that h-BN filler doping can significantly inhibit space charge injection and reduce space charge accumulation in LDPE. The interface effect between h-BN and the substrate reduces the carrier mobility, thereby suppressing the injection of charges of the same polarity and increasing the direct-current (DC) breakdown strength. h-BN/LDPE composite doped with 3 phr h-BN particles has excellent space charge suppression effect and high DC breakdown strength, which is 14.3% higher than that of pure LDPE.

Effects of alkyl/vinyl‐modified nanosilicas on negative or positive high voltage direct current breakdown strength and tensile properties in silicone rubber nanocomposites

AbstractTo develop silicone/nanosilica insulation materials for high voltage direct current (HVDC), hydrophilic surface of fumed nanosilica was changed to hydrophobic by modifying with various ratios of alkylsilane and alkylsilane/vinylsilane coupling agents and the effects of the modified nanosilicas on the HVDC breakdown strength under negative or positive polarities were studied. Dielectric and tensile properties were also studied. The surface modification was confirmed by Fourier‐transform infrared spectroscopy (FT‐IR) analysis and the weights of the alkyl and alkyl/vinyl groups on the modified nanosilicas were measured by thermogravimetric analysis (TGA). Silicone rubber nanocomposites were prepared by mixing a liquid silicone rubber (LSR) and the modified nanosilicas, in which the mixing ratio of the LSR to the nanosilicas was fixed to be 20 wt%. Transmission electron microscopy (TEM) was used to observe the even dispersion of the nanosilica particles in the LSR matrix, and it was found that the surface‐modified nanosilicas were well dispersed in the form of nano‐clusters with 20–60 nm in size. Electrical properties (±HVDC breakdown strength and dielectric properties) and mechanical properties (tensile strength and elongation‐at‐break) were estimated, and it was found that ±HVDC breakdown strength and tensile strength were maximal when the surface modification ratio of alkyl: alkyl/vinyl groups was 50: 50 wt%.