Space Charge Injection Research Articles

Influence of maleic anhydride grafting on the positive temperature coefficient effect of semi‐conductive composites and space charge injection to XLPE insulation

AbstractTo suppress the resistivity positive temperature coefficient (PTC) effect of ethylene‐butyl acrylate copolymer (EBA)‐based semi‐conductive shielding layer and the injection of charge carriers to insulation layer, the polar molecule maleic anhydride (MAH) is grafted onto EBA macromolecules by melt blending and thermal grafting. The resistivity temperature stability of the grafted semi‐conductive composites, as well as the space charge distribution and direct current (DC) breakdown characteristics of cross‐linked polyethylene (XLPE) insulation using the composites as the electrode is investigated. The results show that MAH grafting can significantly reduce the volume resistivity of semi‐conductive composites, especially at a higher temperature, to suppress the PTC effect. And, the grafted semi‐conductive composites can prevent the injection of charge carriers to XLPE insulation from the semi‐conductive electrode to improve the space charge distribution and DC breakdown strength of XLPE insulation. The polar anhydride groups in the grafted MAH can enhance the interaction between EBA macromolecular chains and between EBA macromolecular chains and carbon black (CB) to improve the dispersion of CB in EBA matrix and the stability of the internal conductive network at the high temperature, improving the properties of EBA‐based semi‐conductive shielding layer and DC electrical properties of XLPE insulation layer.

Impact of plasma‐treated nano‐alumina on the thermal and electrical properties of epoxy resin at elevated temperatures

AbstractIn this study, to ramp up the thermal stability and insulation characteristics of epoxy resin (EP), the plasma‐treated EP/alumina composites with a mass fraction of 1 wt% were fabricated. The variations of functional groups in the EP composites with/without plasma treatment were characterized by Fourier transform infrared spectroscopy (FTIR), and the glass transition temperature of the specimens was analyzed by differential scanning calorimetry (DSC) analysis. The space charge distributions, the electrical conductivity and DC breakdown strength of the specimens were measured at 90°C. Moreover, the results were further analyzed in depth. The results indicated that the nanocomposites prepared with plasma‐treated nano‐alumina exhibited a higher concentration of OH and CO groups. At 90°C, the plasma‐treated alumina nanoparticles significantly increased the charge injection threshold of EP from about 26 nC (untreated) to 19 nC (treated); reduced the internal electric field distortion, decreased the charge accumulation and dissipation, and increased the breakdown strength from about 104 kV/mm (untreated) to 115 kV/mm (treated), and the conductivity from about 5.1 × 10−14 S/m (untreated) to 3.1 × 10−14 S/m (treated). In summary, it was elucidated that plasma‐treated nano‐alumina particles could effectively reduce deep trap density, trapped charges, charge injection and transportation and reinforce composites' insulation performance at high temperatures.Highlights The DBD plasma treatment is environmentally friendly with high efficiency. The testing adopted advanced characterization and analysis techniques. Plasma treatment of alumina nanoparticles enhanced the thermal stability. The treated nano‐alumina reinforced insulation performance of EP nanocomposites. The plasma treatment raised the space charge injection threshold and barrier.

Improved body and interface properties of EPDM insulation for HVDC cable accessory by grafted voltage stabilizer

As a key component in HVDC cable accessory, EPDM insulation is prone to breakdown both in body and along interface. To improve the comprehensive electrical strength of EPDM, voltage stabilizer 2-propenyl-4,6-dibenzoylresorcinol (ADR) was simultaneously grafted onto macromolecules through free radical reaction on its vinyl during crosslinking of EPDM. After modification, the mechanical strength and thermal stability of EPDM are improved. In terms of body properties, the injection and rapid transfer of space charge under normal electric field are weakened, the volume conductivity is reduced, the breakdown strength is improved and the growth of electrical tree is suppressed. In terms of interface properties, the normal electric field at the EPDM/XLPE interface is weakened, hence reduced potential risk of deterioration initiated from interface. Moreover, the surface conductivity of EPDM is reduced, the charge accumulation and migration along the EPDM/XLPE interface are suppressed, and the interface breakdown strengths under different interface pressures are significantly increased under tangential electric field. Molecular simulations and UV absorption characteristics indicate that the grafted ADR molecules not only introduce charge traps into EPDM, but also absorb the high energy from trapped electrons and consume it through multiple excitations, thus preventing the electron avalanche and subsequent degradation of EPDM.

Electrical properties enhancement of dually grafting modification for polypropylene cable insulation

AbstractPolypropylene (PP) is believed to be a rather promising cable insulating material for high‐capacity electric power system with low carbon emission due to its decent thermo‐resistance and recyclable nature. In this paper, a new dually chemical grafting modification strategy by methyl acrylate (MA) and acrylic acid (AA) is put forward to tailor the charge transportation behavior in PP, thus further enhancing the electrical properties. Experimental results indicate that the dually grafted PP with 2.3 weight percent (wt%) MA and 1.9 wt% AA shows enhanced volume resistivity and electrical breakdown strength than pure PP, and the space charge injection is significantly suppressed. This work further adopts thermally stimulated depolarization current (TSDC) test and computational analysis based on density functional theory (DFT) to reveal the mechanism of enhancement. The analysis shows that grafted chemical groups can introduce quantities of deep traps and electrostatic potential wells which are strongly correlated with the carbonyl group and would hinder the charge transportation thus improving the electrical insulating performances of PP. This work would provide a new route of PP‐based dually grafting modification for the development of high‐voltage cable insulation.

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.

Enhanced electrical insulation properties of 2-isopropenyl-2-oxazoline grafted polypropylene for high voltage direct current power cables

Polymers have been extensively used for high voltage direct current transmission systems as insulation power cables. Space charge accumulation consistently remains the primary factor contributing to the degradation of the electrical insulation properties of polymers. Grafting modification is a valid approach for suppressing the injection of space charges and free electrons by inducing deep traps. In this paper, the modified polypropylene grafted the group with low ionization energy is reported. The deep traps are revealed by the thermally stimulated depolarization current experiment and the band structure as well as the electrostatic potential by density functional theory simulation. The method with the innovative hybrid functional and basis set is adopted to establish the average local ionization distribution of the grafting group, thus confirming its strong electrophilicity. Furthermore, the modified polypropylene has been proved to obtain higher breakdown strength and operational stability by experiments. This work can provide insights for the design and selection of power cables with excellent electrical insulation properties.

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.

Electrical properties of MAH-g-PP modified PP/SEBS matrix semi-conductive composite materials

Polypropylene (PP) becomes a superior candidate material for new environmental-friendly cable insulation layer with its excellent heat resistance, electrical properties and degradability. At present, most of studies focus on the insulation layer of PP cable. However, there are few studies on semi-conductive shielding layer of PP cable which match its insulation layer. In this paper, polypropylene (PP)/styrene ethylene butylene styrene (SEBS) is adopted as matrix of PP cable semi-conductive layer to matching its insulation layer. Meanwhile, Maleic anhydride grafted polypropylene (MAH-g-PP) is used to cooperatively improve the compatibility between PP and SEBS. The physicochemical properties, space charge injection properties and compatibility of semi-conductive composites with different CB and MAH-g-PP contents is studied by combining experiment and simulation methods. The experimental results show that the surface roughness of the semi-conductive layer and the space charge inside the insulation layer decrease first and then increase with the increase of CB addition. Among them, the two parameters of 25 phr CB semi-conductive layer is the lowest, respectively 13.98 nm and 2.25 × 10−8C. These two parameters are significantly reduced after the addition of compatibilizer MAH-g-PP, which decreased by 29.47 % and 62.22 %, respectively. The simulation results show that the Flory-Huggins interaction parameters (χ) of MAH-g-PP with PP and SEBS are −73.3 and −45.2, respectively, which are 236.24 % and 107.34 % lower than those of PP/SEBS. The χ value of MAH-g-PP/CB is 8.20, which is 33.27 % and 9.59 % lower than CB/PP and CB/SEBS, respectively. It shows that MAH-g-PP can effectively improve the compatibility of matrix and filler. This work has important guiding significance for the research of semi-conductive layer formulation of PP cable.

Charge injection characteristics and charge migration mechanism of magnetic particle-modified EBA-based semi-conductive shielding layers

The space charge accumulation problem within insulation layer of high voltage direct current (HVDC) cable in engineering applications has attracted much attention. The inner semi-conductive shielding layer is located between the conductor and the insulation layer, which is the only way for charge injection into the insulation layer. The performance of the inner semi-conductive shielding layer has an important influence on the charge accumulation of the insulation layer. In this paper, the matrix resin EBA of high voltage grade DC cable shielding material is used as the research object. The magnetic particle SrFe12O19 is used to modify the EBA-based shielding material. The semi-conductive composites of CB/SrFe12O19/EBA with different SrFe12O19 concentrations (with different Lorentz Forces) are prepared. The mechanism of the influence of Lorentz Forces on the space charge injection characteristics of insulation layers and the mechanism of the charge migration are investigated. The trajectories of space charges in shielding layers are simulated using the finite element method. The experimental results show that SrFe12O19 can significantly reduce the space charge accumulation of the insulation layer. When SrFe12O19 is 5%, the shielding layer has the most obvious inhibition effect on the charge injection of the insulation layer. The simulation results show that the critical value of magnetic induction intensity is B′= 0.6 T when space charge moves in the shielding layer. This work provides a new way to solve the problem of space charge accumulation in HVDC cables, and has important reference significance for the development of HVDC cables.

The effect of Li(Ni0.9Co0.05Mn0.05)O2 on improving electrical properties of semi-conductive LDPE/EVA/CB composites

The conductive mechanism of the semiconductor shielding layer between the conductive core and the insulation layer of high-voltage direct current (HVDC) cables includes tunneling and over-diffusion theories; however, the ethylene vinyl acetate copolymer (EVA) matrix in the semi-conductive composite material expands when heated, leading to a break in the carbon black (CB) conductive networks, which results in an increase in resistivity and the formation of the positive temperature coefficient (PTC) effect. The addition of materials with negative temperature coefficient (NTC) properties can weaken the PTC effect while absorbing some free electrons to enhance the suppression of space charge injection into the insulating layer. Li(Ni0.9Co0.05Mn0.05)O2 (NCM) was prepared by sol–gel method, which was melted and mixed with low-density polyethylene (LDPE), EVA, and CB, and pressed and molded to obtain modified semi-conductive composites. The semiconducting composites were measured by resistivity, thermal stimulation depolarizing current method (TSDC), and pulsed electroacoustic method (PEA). The results showed that the composites had the best performance when the NCM doping was 2 wt%.

Investigation on AC and DC Breakdown Mechanism of Surface-Ozone-Treated LDPE Films under Varied Thicknesses.

Electrical breakdown is an important physical phenomenon in power equipment and electronic devices. Recently, the mechanism of AC and DC breakdown has been preliminarily revealed as electrode-dielectric interface breakdown and bulk breakdown, respectively, based on space charge dynamics through numerical calculations. However, the AC breakdown mechanism still lacks enough direct experimental support, which restricts further understanding and the design and development of electrical structures. Here, in this study, LDPE films with various thicknesses ranging from 33 μm to 230 μm were surface modified with ozone for different durations to experimentally investigate DC and AC breakdown mechanism. The results indicate that carbonyl groups (C=O) were introduced onto the film surface, forming shallow surface traps and leading to a decreased average trap depth and an increased trap density. Such a surface oxidation modulated trap distribution led to enhanced space charge injection and bulk electrical field distortion, which decreased DC breakdown strength as the oxidation duration went longer, in all film thicknesses. However, such decreases in breakdown strength occurred only in films below 55 μm under AC stresses, as the enhanced electrical field distortion at the electrode-dielectric interface was more obvious and dominating in thin films. These experimental results further confirm the proposed electrode-dielectric interface breakdown of dielectric films and provide new understandings of space charge modulated electrical breakdown, which fulfills dielectric breakdown theory and benefits the miniaturization of power equipment and electronic devices.

Effect of Li1.3Al0.3Ti1.7(PO4)3 on semiconductive composites: Positive temperature coefficient (PTC) performance and inhibition of space charge injection

The semiconductive layer located between the core and insulation layer of a high-voltage DC (HVDC) cable can inhibit the accumulation of space charge in the insulation layer. This helps increasing the electric field inside the cable, thus improving the stress distribution of the electric field on the inner and outer surfaces of the insulation layer and the electrical strength of the cable. An inevitable positive temperature coefficient effect (PTC) in the semiconductor layer leads to aging failure of the insulation layer. To improve this phenomenon, the semiconductive shielding layer was modified for suppressing its PTC effect and reducing the space charge injection into the insulation layer. Li1.3Al0.3Ti1.7(PO4)3(LATP) was prepared via a solid-phase reaction and added to a composite consisting of carbon black (CB), ethylene vinyl acetate copolymer (EVA), and low-density polyethylene (LDPE). Resistivity, thermally stimulated depolarization current (TSDC), and pulsed electroacoustic (PEA) measurements were performed on the modified sample. The results showed that when the LATP content was 4 wt%, the modification effect of the semiconductive shielding layer was most effective, the PTC effect was reduced by 11%, and the space charge injection into the insulating layer was improved significantly.

The impact of device length on the electron’s effective mobility

Within the framework of an electron transport regime classification scheme, we aim to explore the boundaries that occur between the ballistic, collision-dominated, space-charge injection, and non-space-charge injection electron transport regimes that are experienced by an electron within a semiconducting device, mapping out where these different electron transport regimes are. We do this by determining the electron’s mean free path and the relevant screening length. In order to make this analysis concrete, we perform this analysis for four representative semiconductor material systems, including silicon, gallium arsenide, the 4H-phase of silicon carbide, and the wurtzite phase of gallium nitride. The entire analysis is performed using a two-dimensional approach, this being representative of the electron transport that is experienced by an electron in the vicinity of a two-dimensional electron gas. Finally, following an evaluation of the dependence of the ballistic mobility on the device length scale for all four materials, an evaluation of the effective mobility as a function of the channel-length scale is pursued, a Matthiessen-rule based approach being employed for the purposes of this analysis.

Effect of Space Charge on Stationary and Transient Electric Field Distribution of HVDC Cable Joint

The conversion of AC XLPE cable lines into direct-current (DC) operation has been proven to be an effective means to improve the transmission capacity of the system. Prefabricated joints are the weakest points of a cable line because of the complex structures, and composite interfaces contained. Therefore, it is of great theoretical and engineering significance to study the field strength distribution of XLPE AC cable prefabricated joints under DC voltage, effectively evaluate their DC tolerance characteristics, and provide a basis for the construction and transformation of XLPE cable DC distribution network system. This paper takes the silicone rubber (SIR), and ethylene-propylene-diene monomer (EPDM) prefabricated joints of 10kV XLPE cables as an example, the cable joint temperature field, and electric field coupling simulation model is established by the finite element analysis software COMSOL Multiphysics. The steady and transient electric field simulation of the joints under different DC voltages and load conditions are carried out considering the influence of space charge. The results show that with the increase of applied voltages and load level, the space charge injection increases, and the maximum steady state field strength in the joint is concentrated at the root of the stress cone, showing an increasing trend. Compared with the SIR joint, the field strength in the EPDM joint is higher. The capacitive component of the field strength in the joint is proportional to the peak value of the impulse voltage under the DC superimposed impulse voltage. In other words, the transient electric field equation meets the superposition principle applied.

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.

Effect of the cross-linking agent on the cross-linking degree and electrical properties of cross-linked polyethylene

To study the effect of a cross-linking agent on the cross-linking degree and electrical properties of cross-linked polyethylene, different mass fractions of cross-linked polyethylene and their electrical properties were investigated. The cross-linking degree of cross-linked polyethylene with different mass fractions was tested by the extraction method. The breakdown field strength was tested by DC voltage breakdown tester, and the space charge injection of two kinds of cross-linked polyethylene was tested by Pulsed Electro-Acoustic. In addition, the resistivity was measured with conductivity test. The obtained results show that the crossing degree and breakdown field are the highest when the mass fraction of DCP is 2.5 wt% and BIPB is 1.5 wt%, which is 89.39% and approximately 310 kv mm−1, respectively, while the cross-linking degree and breakdown field are 88.28% and approximately 340 kv mm−1, respectively. At the same time, when DCP is used as the cross-linking agent, the amount of the same cross-linking agent is greater, which leads to more impurities contained in the material and increased space charge accumulation in the material; thus, the breakdown strength is lower than that of BIPB, and the conductivity is higher with the cross linking agent of DCP.

Evaluation of space charge distribution under a divergent electric field by combining experiment and simulation

Under divergent electric fields, space charge is locally accumulated and controls the initiation and evolution of insulation degradation phenomena, such as electrical trees and partial discharge. The space charge profiles provide crucial information for evaluating the insulation performance. However, the existing space charge measurement methods are not directly applicable under divergent electric fields. In this paper, a method combining experiment and simulation is proposed to evaluate the space charge distribution around a needle electrode. An experimental system is established, and the acoustic waves are measured with a hydrophone placed on the symmetric axis. In order to obtain the space charge distribution, the space charge injection process is simulated using the bipolar charge transport model to reduce the freedom of the distribution. Then, calculations are made for the acoustic waves produced by the simulated space charge distributions. Using the signals without the contribution of internal charge, a correspondence between the experiment and simulation is established. For each experimental signal, the matched calculated signal is obtained using the zero-crossing time as a reference. A perfect match between them is presented after the amplitude compensation, which indicates that the space charge distribution in the experiment is identical to that in the simulation model.

Higher speed, wider angle linear electro-optic deflection via domain engineered KTN crystals

Pulsed-biased higher speed (> 10 MHz @ 30 V, 100 ns pulse width) and DC-biased wider angle (105 mrads @ 410 V/mm) linear electro-optic (EO) deflection is reported in a thermally-controlled domain engineered (DE) ferroelectric (FE) potassium tantalate niobate [KTa1−xNb x O3, KTN] crystal. DE-FE KTN crystals can not only provide a higher transmittance and larger linear EO coefficient, but also enable higher speed (10X) and wider angle (2X) deflection than that of its paraelectric equivalent. The physical mechanism behind the optimization of injected space charge on high deflection angles at high speeds is also addressed. This significantly improves its use in megahertz EO applications.

Electrical treeing failure in silicone gel insulation for encapsulation under high frequency bipolar square-wave voltage

Silicone gel insulation has been widely used in power electronic devices, serving as the encapsulation material. Its dielectric strength under high-frequency voltage determines the reliable operation of the devices. Electrical tree is a typical failure of solid insulating materials, and the electrical tree propagation in silicone gel under bipolar square-wave voltage are investigated in this paper. The results show that electrical trees are pearl-line like, bush-like and bubble-like under varied voltage conditions. After the rapid growth period, the electrical trees stagnate under frequency lower than 20 kHz while develop steadily when the voltage frequency is higher than 25 kHz. The number of tree branch, fractal dimension and accumulated damage increase with the voltage frequency. In comparison, the electrical trees propagate slowly under sinusoidal-wave voltage. The partial discharge and photo-degradation corresponding to the fluorescence are found in main tree channel, which are related to voltage waveform and lead to different electrical tree behavior. The injection and transportation of space charge accelerates the electrical tree propagation, especially at the rising/falling edge of bipolar square-wave field due to polarity reversal. Besides, the amorphous carbon deposited near the needle tip and the bubbles in tree channels filled with hydrogen and carbon monoxide are all related with the electrical tree propagation.

Improved Wide-Temperature-Range Insulation Properties of Block Polypropylene by UV-Irradiated Cografting of Maleic Anhydride and 4-tert-Butylstyrene

Polypropylene (PP), as a recyclable thermoplastic polymer, is a promising insulating material for application as an environmentally friendly insulation layer in high-voltage-direct-current (HVDC) cables. However, the lack of toughness and the poor wide-temperature-range electrical properties under a DC electric field seriously restrict its development and application. Herein, a block polypropylene with a low elastic modulus is selected and grafted with organic molecules of maleic anhydride (MAH) and 4-tert-butylstyrene (St*) as a masterbatch (PP-g-(St*-co-MAH)) by an ultraviolet-irradiated radical reaction. The modified b-PP with different grafting contents was prepared by diluting the masterbatch through melt blending, and their DC electrical properties at a wide temperature range of 40–100 °C were tested. The results indicate that the introduction of a comonomer St* significantly improves the grafting rate of MAH and suppresses the degradation of PP chains during the grafting reaction; thus, the PP-g-(St*-co-MAH) possesses improved thermogravimetric performance and maintains the original mechanical properties and workability. The grafted functional groups introduce deep charge traps into the matrix and thus effectively suppress the space charge injection and accumulation, significantly reduce the conductivity, and enhance the breakdown strength of PP at wide temperature ranges. Among the tested materials, the PP grafted with 0.57 wt % St*-co-MAH exhibits the best electrical properties, while inadequate or excessive grafting content of St*-co-MAH relatively weakens the positive effects of grafting modification. The mechanism is analyzed based on calculation of the trap level and the three-dimensional (3D) electric potential.