Space Charge Suppression Research Articles

Flexible High‐Temperature Polymer Dielectrics Induced by Ultraviolet Radiation for High Efficient Energy Storage

AbstractPolymer‐based dielectrics with fast electrostatic energy storage and release, are crucial for advanced electronics and power systems. However, the deterioration of insulation performance and charge–discharge efficiency of polymer dielectrics at elevated temperatures and high electric fields hinder the applications of capacitors in harsh environments. Herein, a facile and scalable approach is reported to fabricating flexible high‐temperature polymer dielectrics for high‐efficiency energy storage by ultraviolet irradiation. The resultant dielectric films exhibit an augment of 493% in energy density and exceeding 800% in discharge efficiency at 200 °C (3.2 J cm−3 and over 90% discharge efficiency at 480 M V−1m for irradiated polyetherimide (PEI), 0.54 J cm−3, and below 10% discharge efficiency at 400 MV m−1 for pristine PEI) and excellent cycle performance. The injected space charge is found to be the dominant contributor to energy loss during the charge–discharge process. Free radicals introduced by ultraviolet irradiation can act as deep traps to capture injected charge and suppress space charge migration. This work clarifies the contribution of space charge to energy loss and demonstrates the effectiveness of ultraviolet irradiation in improving the capacitive performance of high‐temperature polymer dielectrics. These findings provide a novel paradigm for the rational design of high‐temperature polymer dielectrics for high‐efficiency energy storage.

Hindered phenolic antioxidant grafting on tailoring the DC electrical characteristics of polypropylene cable insulation

AbstractThe authors focus on the impact of melt‐free radical grafting with hindered phenolic antioxidants (AO3052) on the electrical properties of polypropylene (PP) for DC cable insulation. The DC conductivity, space charge distribution and breakdown characteristic tests of grafting‐modified PP are performed by comparing unmodified PP. The results demonstrate that the grafting of antioxidants can effectively suppress space charge injection, owing to the deeper trap sites at the grafting molecule. The breakdown strength of the grafted PP is significantly enhanced from 30°C to 90°C and especially achieves a 5.3%–6.7% increase after the same DC‐prestressed time at 90°C. The surface electrostatic potential and molecular orbitals of the grafted PP are calculated. Simulation shows that the antioxidant introduces multi‐level local state traps that can effectively trap the injected space charge, thus decreasing the destruction of molecular chains by electrons and increasing the breakdown strength level. In conclusion, antioxidant grafting modification can improve the breakdown characteristics with or without DC prestress, and thus it appears to be promising in the application of PP‐insulated cables.

Realizing Outstanding Energy Storage Performance in KBT-Based Lead-Free Ceramics via Suppressing Space Charge Accumulation.

The great potential of K1/2Bi1/2TiO3 (KBT) for dielectric energy storage ceramics is impeded by its low dielectric breakdown strength, thereby limiting its utilization of high polarization. This study develops a novel composition, 0.83KBT-0.095Na1/2Bi1/2ZrO3-0.075 Bi0.85Nd0.15FeO3 (KNBNTF) ceramics, demonstrating outstanding energy storage performance under high electric fields up to 425kVcm-1: a remarkable recoverable energy density of 7.03Jcm-3, and a high efficiency of 86.0%. The analysis reveals that the superior dielectric breakdown resistance arises from effective mitigation of space charge accumulation at the interface, influenced by differential dielectric and conductance behaviors between grains and grain boundaries. Electric impedance spectra confirm the significant suppression of space charge accumulation in KNBNTF, attributable to the co-introduction of Na1/2Bi1/2ZrO3 and Bi0.85Nd0.15FeO3. Phase-field simulations reveal the emergence of a trans-granular breakdown mode in KNBNTF resulting from the mitigated interfacial polarization, impeding breakdown propagation and increasing dielectric breakdown resistance. Furthermore, KNBNTF exhibits a complex local polarization and enhances the relaxor features, facilitating high field-induced polarization and establishing favorable conditions for exceptional energy storage performance. Therefore, the proposed strategy is a promising design pathway for tailoring dielectric ceramics in energy storage applications.

Molecular Dynamics Simulations on Epoxy Resin Composite via Grafting Acryloyl-chloride to Inhibit Electron Transport and Improve Thermal-mechanical Properties

Electrical, thermal, and mechanical properties of cross-linked epoxy resin (EP) modified by the chemical grafting of acryloyl chloride (AC) were studied to explore the trapping mechanism of charge transport inhibition. The bound state traps deriving from grafted molecules were analyzed by first-principles calculations combined with electron transmission spectra to study the underlying mechanism of the electrical properties. In contrast to pure EP, the EP-graft-AC (EP-g-AC) represents significantly depressed conductivity due to the electron scattering from polar-groups of the grafted AC molecule. The substantial deep traps are generated in EP-g-AC molecules by the polar group of grafted AC and accordingly decrease charge mobility and raise the charge injection barrier, consequently suppressing space charge accumulation and charge carrier transport. EP-g-AC polymer acquires a significant amelioration in thermal and mechanical properties, as indicated by the higher cohesive energy density, glass transition temperature, and decomposition temperature in consistence with the lower thermal vibrations compared with pure EP polymer, except that the resulting higher fractional free volume is not preferable, which is attributed to the mixing incompatibility of the grafted AC molecules with EP molecular-chains.

Improved Electrical Properties of Organic Modified Thermoplastic Insulation Material for Direct Current Cable Application.

To achieve exceptional recyclable DC cable insulation material using thermoplastic polypropylene (PP), we have introduced the organic polar molecule styrene-maleic anhydride copolymer (SMA) into PP-based insulation materials following the principles of deep trap modification. PP, PP/SMA, PP/ethylene-octene copolymer (POE), and PP/POE/SMA insulating samples were prepared, and their meso-morphology, crystalline morphology, and molecular structure were comprehensively characterized. The results indicate that SMA can be uniformly dispersed in PP with minimal impact on the crystalline morphology of PP. The DC electrical properties of the materials were tested at temperatures of 30, 50, and 70 °C. The findings demonstrate that the introduction of SMA can improve the DC properties of the material in both PP and PP/POE. The thermal stimulated depolarization current results reveal that SMA can introduce deep traps into the material, thereby improving its DC properties, which is in agreement with the quantum chemical calculation results. Subsequently, a bipolar carrier transport model was employed for coaxial cables to simulate the space charge distribution in the insulation layer of the four sets of insulation samples as well as the actual cable in service. The results highlight that SMA can significantly suppress space charge in PP and PP/POE systems, and it exhibits excellent electric field distortion resistance. In summary, the results illustrate that SMA is expected to be used as an organic deep trap modifier in PP-based cable insulation materials.

Constructing interfacial barrier from tribo-positive shell microcapsules to suppress space charge in thermochromic phase change composites for smart electronics

Thermochromic phase change composites (TPC) exhibit versatile functions under electrothermal stimuli, which are extensively applied as packaging, insulation and circuit components in considerable advanced smart electronics. However, space charge accumulation under DC voltages is still a critical and universal issue for the serious deterioration of functionalities and life-span of TPC and electronics. This study presents a novel strategy of suppressing space charge in thermochromic phase change epoxy composites (TPCE) by constructing interfacial barrier from tribo-positive shell thermochromic phase change microcapsules. The strong tribo-positive melamine-formaldehyde (MF) shell of microcapsules can establish negative charge barriers in the interfaces between microcapsules and epoxy matrix through triboelectrification, which is directly verified by Kelvin probe force microscopy. Pulsed electroacoustic measurement results exhibit the space charge accumulation in TPCE under an extreme electric field strength of 50 kV/mm and an elevated temperature of 70 °C can be significantly diminished by 39.3% compared to that of neat epoxy resin after incorporating merely 1 phr MF shell microcapsules, affirming the effect of interfacial barrier on space charge inhibition. The interfacial barrier could improve the charge carrier dissipation and mitigating the space charge accumulation as confirmed by hopping conduction analysis. Moreover, DC breakdown strength of TPCE can also be enhanced by 12.3% and 6.1% with 1phr microcapsules at 30 °C and 70 °C compared to that of neat epoxy resin owing to the suppressed space charge. The results provide a potential approach of designing high-performance thermochromic phase change composites for smart electrical and electronic devices.

Space Charge Characteristics and Breakdown Properties of Nanostructured SiO2/PP Composites

Polypropylene (PP) has gained attention in the industry as an environmentally friendly material. However, its electrical properties are compromised due to space charge accumulation during operation, limiting its application in high-voltage DC cable insulation. This study investigates the effect and mechanism of SiO2 with a DDS surface hydrophobic treatment on space charge suppression and the electrical properties of PP composites. The PP matrix was doped with SiO2 nanostructures, both with a DDS surface hydrophobic treatment and untreated as a control group. The functional group structure and dispersion of nanostructured SiO2 in the matrix were characterized. The findings reveal that the incorporation of SiO2 nanostructures effectively mitigates charge accumulation in PP composites. However, a high concentration of unsurfaced nanostructures tends to agglomerate, resulting in inadequate space charge suppression and a diminished DC breakdown field strength. Nonetheless, surface treatment improves the dispersion of SiO2 within the matrix. Notably, the composite containing 1.0 wt% of surface hydrophobic SiO2 exhibits the least space charge accumulation. Compared to the base material PP, the average charge density is reduced by 83.9% after the 1800 s short-circuit discharges. Moreover, its DC breakdown field strength reaches 3.45 × 108 V/m, surpassing pure PP by 19.4% and untreated SiO2/PP composites of the same proportion by 24.0%.

Effect of magnetic particle size in semi-conductive layer on charge accumulation within insulation layer of HVDC cable

The key problem limiting the development of high-voltage direct current (HVDC) cable is the space charge accumulation within the insulation layer. A method is proposed to reduce the accumulation of space charge of cable insulation by a semi-conductive layer modified by magnetic particles. The physical essence is to change the charge migration path by the action of the Lorentz force in the semi-conductive layer. SrFe12O19 particles with different particle sizes from 80 nm to 1000 nm are prepared by the sol-gel self-combustion method. The magnetic properties and resistivity of the semi-conductive layer composites with various particle sizes are investigated. In addition, the effect of the semi-conductive layer electrode for charge accumulation in the insulation layer is studied. Finally, the relationship among SrFe12O19 particle size, magnetization intensity of the semi-conductive layer, and charge inhibition effect of the insulation layer is analyzed. The results of the experiments demonstrate that residual magnetization of magnetic particles has a significant impact on the space charge suppression. When SrFe12O19 particle size is 200 nm, the charge inhibition effect is the best. This work can provide a new view for the charge suppression of insulation layer.

Comprehensive Properties of Grafted Polypropylene Insulation Materials for AC/DC Distribution Power Cables

Polypropylene (PP) exhibits excellent insulation properties, high thermo-stability, and recyclable nature, thus expected to be the next-generation insulation material for HV cable application. Chemical grafting modification is an effective technology to improve the electrical properties of polypropylene. In this paper, we develop and report a new-type grafted PP insulation material by water-phase grafting technology. The comprehensive properties including electrical, thermal, and mechanical of it are tested and compared with traditional cable insulation material—crosslinked polyethylene (XLPE). The results show that the grafted PP holds superior thermal properties and enough mechanical flexibility. The electrical properties are of significant advantages, including resistivity enhanced by nearly two degrees of magnitudes, AC/DC breakdown strength raised by over 20%, and obviously suppressed space charge accumulation. These results indicate that grafted PP is very suitable for application in HV cable systems, either AC or DC. This research lays a foundation for the research and development of the next-generation recyclable polypropylene cables at higher voltage levels.

Effect of zinc oxide nanoparticle size on the dielectric properties of polypropylene‐based nanocomposites

AbstractTo develop new environmentally friendly and recyclable high‐voltage cable insulation materials, the effects of the particle size of zinc oxide (ZnO) on the mechanical and electrical properties of nanocomposites were investigated based on a feasibility study of polypropylene (PP) as a high‐performance cable insulation material. The small particle size of ZnO can synergistically toughen PP with a polyolefin elastomer (POE), but the large particle size of ZnO leads to lower elongation at break. Adding ZnO nanoparticles can improve the bulk resistivity of the composites and suppress space charge injection. 0.5ZnO200 has the highest direct current and alternating current breakdown field strengths, 36.6% and 14.7% higher than PP/POE, respectively. This improvement may be due to the ability of 200 nm ZnO to introduce more deep traps. However, as the nanoparticle content rises and the particle size decreases, the agglomeration becomes more frequent, leading to the overlap of inter‐nano interfaces and reducing the modification effect of the nanoparticles. This study on the effect law of nano‐ZnO with different particle sizes on the microstructure, mechanical properties, and electrical properties of PP can provide a reference for developing new recyclable high‐voltage insulated cable materials.

Remarkable modification of transferring characteristics for both positive and negative charges in XLPE-PS composite

Insulating materials with excellent performance have been one of the key factors that keep the safe operation of high-voltage direct-current (HVDC) cable systems, and investigating their charge trap distribution characteristics is of great significance to improve electrical properties, such as reducing conductivity, suppressing space charge accumulation, and elevating breakdown strength. In this paper, surface potential decay processes of low-density polyethylene (LDPE), low-density polyethylene/polystyrene (LDPE/PS), crosslinked polyethylene-polystyrene (XLPE-PS) and crosslinked polyethylene (XLPE) were investigated, and migration properties of positive charges and negative charges as well as related charge trap characteristics were analyzed. The results demonstrate that for positive charges and negative charges, the formation of featured structure in XLPE-PS composites reduces carrier mobility and deepens charge trap depth but does not change the width of charge trap energy distribution, which remains 0.27eV. Compared with XLPE, XLPE-PS shows a more unified trap distribution under positive and negative polarity, which may be attributed to the featured structure. The findings are helpful to clarify the relationship between multi-phase structure and electrical insulation performance of XLPE, and to provide a reference for the design of insulating materials for extruded HVDC cables.

Insight Into Space Charge Suppression by Interfacial Deep Traps in Polymer Nanocomposites

Polymer nanocomposites are attractive for HVDC insulation applications, especially for HVDC cables, due to their ability to suppress space charge accumulation through interfacial effects. However, direct evidence to support the existence of interfacial effects at the nanoscale is still lacking. Therefore, rational design and molecular engineering of the interfaces to improve the insulation properties of polymer nanocomposites remain unavailable. Here, we show that efficient space charge suppression can be achieved in polymer nanocomposites at temperatures up to 100 °C by introducing local deep traps through carefully designed nanoparticle/polymer interfaces. The local interfacial deep traps are directly detected at the nanoscale using intermodulation electrostatic force microscopy (ImEFM). This work provides a deep understanding of the interfacial effects in polymer nanocomposites and will enable the rational design of interfaces for high-performance insulation materials.

Space Charge Suppression Method of Insulation Layer of HVDC Cable Based on Semi‐Conductive Shielding Layer Modified by Nitrogen‐Doped Graphene

Space charge accumulation in the cable insulation will cause local electric field distortion, and accelerate the aging of the material. The inner semi‐conductive layer as an important structure of cable will affect the charge injection characteristics in the insulation layer. This work intended to explore the method of modifying the semi‐conductive layer with the Nitrogen‐doped graphene (NG) to suppress the charge accumulation utilizing the super‐smoothness of graphene and electron adsorption of N element. First, the NG is prepared using N‐modified graphene, and the semi‐conductive layer with different NG content is fabricated. Second, the physical and chemical properties of the semi‐conductive composite are carried out, then the charge accumulation characteristics in the insulation layer under the effect of the semi‐conductive layer are characterized. The results showed that an appropriate amount of NG doping can significantly inhibit the charge accumulation in the insulation layer. The surface roughness decreased by about 32.13% when the doped NG content is 0.5 phr. With the increase of NG content from 0 to 1 phr, the accumulation charges inside the insulation layer decreased by about 36.84%. This research provided a new approach for suppressing space charge accumulation in the insulation material of high voltage cable.

Space charge characteristics in epoxy/nano-MgO composites: Experiment and two-dimensional model simulation

Space charge accumulation in polymer dielectrics may lead to serious electric field distortion and even insulation failure during long-term operations of power equipment and electronic devices, especially under conditions of high temperature and direct current electric stress. The addition of nanoparticles into polymer matrices has been found effective in suppressing space charge accumulation and alleviating electric field distortion issues. Yet, the underlying mechanisms of nanoparticle doping remain a challenge to explore, especially from multi-dimensional composite insights. Here, a two-dimensional bipolar charge transport model with consideration of interface zones between organic/inorganic phases is proposed for the investigation into space charge behaviors of nanodielectrics. To validate the effectiveness and feasibility of the model, pulsed electroacoustic experiments are performed on epoxy/nano-MgO composites with different doping ratios of nanoparticles. Experimental observations match well with simulation anticipations, i.e., higher doping ratios of nanoparticles below the percolation threshold exhibit better capabilities to inhibit space charge accumulation. The deep traps (∼1.50 eV) generated in the interface zones are demonstrated to capture free charges, forming a reverse electric field in the region adjacent to electrodes and impeding the space charge migration toward the interior of the composite. This model is anticipated to provide theoretical insight for understanding space charge characteristics in polymer nanodielectrics and computing charge dynamics in extreme conditions where experiments are challenging to perform.

Improved DC Insulation Performance of XLPE With Graftable Voltage Stabilizer

Voltage stabilizers have the potential to improve the overall insulation properties of polymeric cables, but the poor compatibility with the polymer matrix limits their practical application. In this work, the reaction of 2,4-dihydroxybenzophenone (DHBP) with maleic anhydride (MAH) provides a novel and reactive aromatic ketone voltage stabilizer 4-(4-benzoyl-3-hydroxyphenoxy)-4-oxobut- 2-enoic acid denoted by MDVS, which can be grafted onto the backbone of polyethylene (PE) during the cross-linking process. The physicochemical and dc insulation properties of the cross-linked PE grafted with different addition of MDVS (XLPE-g-MDVS) are investigated, and the underlying mechanism is discussed. In comparison with the pristine XLPE, substantially suppressed space charge accumulation and decreased electrical conductivity are obtained in the XLPE-g-MDVS materials. As the content of MDVS is 1.2 wt%, XLPE-g-MDVS materials exhibit higher breakdown strength than that of the pristine XLPE. Furthermore, the graftable voltage stabilizer MDVS presents more efficient stabilizing effect than conventional voltage stabilizer DHBP before and after high-temperature degassing. Thermally stimulated current (TSC) tests and quantum chemical calculations are consistent with the experimental results of insulation properties, illustrating that capture and trap mechanisms cooperate in the improvement of XLPE dc insulation performance.

Improved Insulation Properties of Polypropylenes in HVDC Cables Using Aqueous Suspension Grafting.

Owing to its lack of crosslinking, polypropylene (PP) is considered an environmentally friendly alternative to crosslinked polyethylene as high-voltage direct current (HVDC) cable insulation. However, pure PP can accumulate space charges under a HVDC, and thus must be modified for use as an insulating material for HVDC cables. In this study, 4-methylstyrene is grafted onto PP using an aqueous suspension grafting method to improve its properties. The effects of the swelling time, reaction time, and 4-methylphenylene concentration on the reaction were investigated. The optimum process conditions were determined, including an optimum grafting ratio of 0.97%. The volume resistivity, ability to suppress space-charge accumulation, and DC breakdown strength of modified PP were also studied. Modified PP with a grafting ratio of 0.88% showed optimal space-charge suppression and the highest volume resistivity and breakdown strength. The work will facilitate the design and development of more efficient insulation materials for HVDC cables.

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.

Study of EPR-based nanodielectrics under operational conditions for DC cable insulation

A model DC material based on ethylene propylene rubber (EPR) including the pure EPR and the EPR-based nanodielectrics incorporated with two different nanoclays, Kaoline and Talc, under operational conditions was investigated. The operational conditions include a 20 kV/mm electric field at 25 °C, a 20 kV/mm electric field at 50 °C with a thermal gradient, and a 40 kV/mm electric field at 50 °C with a thermal gradient and polarity reversal. Space charge distribution, surface potential, and electrical conductivity were measured to characterize the model DC material and interpret the discrete charge dynamics in the bulk and at the interface of the three samples. The experimental results revealed that the electrical conductivity of Talc-filled nanodielectric has the least dependency on electric field and temperature, which reduces the conductivity gradient across the dielectric. Moreover, the successful suppression of space charge and the lower dielectric time constant in the Talc-filled nanodielectric result in a tuning electric field in the bulk not only under normal operating conditions but also more importantly under polarity reversal conditions. The maximum of absolute charge density decreases from 10.6 C/m3 for EPR to 2.9 C/m3 for the Talc-filled nanodielectric under 40 kV/mm with polarity reversal and at 50 °C with the thermal gradient. The maximum of local electric field enhancement for the mentioned condition reduces significantly from 97 kV/mm, 142% enhancement, for EPR to 45 kV/mm, only 12.5% enhancement, for the Talc-filled nanodielectric.

A review on polymeric insulation for high‐voltage application under various stress conditions

AbstractPolymeric material draws a great attraction in the development of insulation for High Votage Direct Current (HVDC) applications. Insulation material is the deciding key factor to increase the operating voltage and to transmit the bulk power with minimized power loss. Premature breakdown, high withstanding temperature, and impacts of space charge accumulation are all issues faced by the insulation material. Material qualities such as dielectric, electrical, structural, and mechanical properties are directly affected by the issues. Therefore, this paper provides an overview of the dielectric properties of polymeric materials and their composites under various stress conditions. The stress includes AC, DC, superimpose and combined (AC and DC) stress under which the dielectric constant, dielectric losses, electrical strength, and space charge distribution of polymeric insulating materials such as low‐density polyethylene, high‐density polyethylene, cross‐linked polyethylene, polypropylene, and random copolymer (RCP), as well as their composites, are explored. Polypropylene and RCP outperform all other polymers in terms of breakdown strength and space charge suppression under various stress conditions. RCP is a developing material that also has the potential to be an effective thermoplastic insulation material in the future.

Effect of Hydrophilic/Hydrophobic Nanostructured TiO2 on Space Charge and Breakdown Properties of Polypropylene.

Polypropylene (PP) has received more and more attention in the field of insulating materials as a recyclable thermoplastic. To further enhance the applicability of polypropylene in the field of insulation, it needs to be modified to improve its electrical properties. In this paper, the impact mechanism of AEROXIDE® TiO2 P 90 (P90) and AEROXIDE® TiO2 NKT 90 (NKT90) as nanosized hydrophilic and hydrophobic fumed titania from Evonik on the electrical properties of PP was studied mainly through the crystallization behavior and space charge distribution of PP nanocomposites. Two kinds of nanostructured TiO2 were melt-blended with PP according to four types of contents. The results of alternating current (AC)/direct current (DC) breakdown field strength of the two materials were explained by studying the microstructure and space charge characteristics of the nanocomposites. Among them, hydrophilic nanostructured TiO2 are agglomerated when the content is low. The spherulite size of the nanocomposite is large, the space charge suppression ability is poor, the charge is easy to penetrate into the pattern, and the AC/DC breakdown field strength is significantly reduced. However, hydrophobic nanostructured TiO2 has better dispersion in PP, smaller spherulites, more regular arrangement, and less space charge accumulation. The charge penetration occurs only when the nanostructured material content is 2 wt%, and the AC/DC breakdown strength increases by 20.8% at the highest when the nanostructured material content is 1 wt%. It provides the possibility to prepare recyclable high-performance DC PP composite insulating materials.