Space Charge Behavior Research Articles

Space Charge and Current Behavior in Low-density Polyethylene Sheet under Mechanical Force

Space Charge and Current Behavior in Low-density Polyethylene Sheet under Mechanical Force

Space Charge Behavior in Ethylene Propylene Rubber Under a 50-Hz AC Electric Field

In this study, the space charge behavior in ethylene propylene rubber (EPR) under a 50-Hz AC field is studied based on the variation of space charge response, FTIR spectra and impurity contents inside EPR with thermal treatment. The space charge was detected with the pulsed electroacoustic (PEA) method and the phase shift of detecting pulse under 50 Hz AC field was realized according to the automatic equal phase shift (AEPS) principle. The results show three main types of impurities in untreated EPR. Three distinct regions of space charge distribution are seen in untreated EPR under a 50-Hz AC field. The mean phase density of space charge ρ θ is increased with the enhancement of the applied field from 15 to 61 kV/mm. The maximum electric field distortion rate of untreated EPR tends to rise first and then decrease, ranging from 6.65% to 31.87%. The impurity content and the ρ θ decreased significantly for EPR thermal treated at 150 °C for 18 h. Corresponding, the maximum electric field distortion rate is effectively reduced to 0.66%. Finally, the space charge source and migration process are speculated based on the diffusion trend of the space charge isodensity line and the migration direction of various charges under the electric field.

Effect of XLPE/EPDM Interface on Space Charge Behavior in Cable Accessory

Space charges accumulate easily at insulation interface of cable accessories, which cause different electric field distribution and aging characteristics compared to single-layered cable. To explore the space charge behavior in double-layered insulation of cable accessories, a one-dimensional (1D) axisymmetric model for bipolar charge transport is built in this work, in which the effects of temperature gradient and electric field gradient in the radial direction are considered. Then the influences of electric field/temperature gradient and interface position on transport behavior of space charges are investigated. The simulation results suggest that the charges accumulate easily at the interface, which severely reduce the electric field in the inner layer of insulation and further inhibit the injection and migration processes of charges. The charge density at the interface increases with temperature and voltage, which accelerates the decay of the charge injection, furthers the interface charges to reach the dynamic equilibrium state quickly. The charge density at the interface and the electric field distribution are closely related to the interface position, and the electric field is more uniform when XLPE occupies the 1/3 to 1/2 inner part of the whole insulation.

Space charge suppression of polyethylene induced by blending with ethylene-butyl acrylate copolymer

High-voltage direct current (HVDC) power cables have been paid more attention due to the big issue concerning the main insulation materials. The residues in the industrial production process will cause space charge generation and accumulation in the materials, which has become one bottleneck to limit the development of power cables up to higher voltage levels. In this paper, the LDPE matrix was modified by three types of ethylene-butyl acrylate (EBA) copolymers (1.0 wt%) with different polarities (BA content) via melt blending to optimize the space charge behavior of the LDPE insulation. The micromorphology and structure of the blends are examined by polarized light microscope and differential scanning calorimetry. The space charge distribution is tested by the pulsed electroacoustic method. The trap level is studied by the thermally stimulated current. The results show that EBA introduces more deep traps and decreases the shallow traps. The medium-polar EBA (16% BA content) can effectively suppress charge accumulation, and have the same suppressed effect on XLPE/EBA blends. This study will provide important insights into the design and development of advanced HVDC power cable applications.

Improved DC Conductivity and Space Charge Characteristics of XLPE for HVDC Cable Application: Effect of Voltage Stabilizers

Electric field distortion caused by the DC conductivity variation and space charge accumulation under temperature gradient in the cross-linked polyethylene (XLPE) insulation is the most serious problem in HVDC cable application. In this paper, three different voltage stabilizers, Bis(4-fluorophenyl)-methanon, Bis(4-hydroxyphenyl) ketone, and Bis[4-(dimethylamino)phenyl] diketone, denoted as A, B, and C, respectively, were mechanically dispersed into the XLPE material with an addition of 0.5 wt%, aiming at improving the DC conductivity characteristics and space charge behaviors of the XLPE insulation at different temperature. The DC conductivity under different temperature and the electric field was tested by a three-electrode system. Meanwhile, space charge properties at 30 °C and 90 °C were investigated using PEA method, and the trap level distribution was estimated by the surface potential decay (SPD) process. The experimental results indicate that all the three voltage stabilizers have improved the DC conductivity characteristics of the XLPE, especially the voltage stabilizers B and C. In addition, the XLPEs modified by the voltage-stabilizers B and C with higher deep trap level demonstrate an excellent ability on space charge suppression at different temperatures. It is concluded that owing to the excellent voltage stabilizing effect, the voltage stabilizers B and C show much potential in HVDC cable application.

Mechanism of bulk charging behavior of ethylene propylene rubber subjected to surface charge accumulation

To investigate the bulk charging behavior of ethylene propylene rubber subjected to surface charge accumulation, surface potential decay (SPD) measurement and electro-acoustic method were employed, after the sample was treated by a corona discharge for different time durations. The space charge behavior and trap distribution were obtained. The results show that the surface potential decay (SPD) property correlated with charging time and surface charge polarity. The decay rate of surface potential correlates with charging time, where SPD rate decreases with the increase of charging time. The accumulated surface charges can be injected into the bulk state, and the bulk charging property has a strong correlation with corona treatment time duration. The long-time corona treated sample has a lower initial SPD value. Surface trap distribution influences the accumulation and injection characteristics of the carriers. A plurality of charges will be injected into the material and then trapped as space charges in the interior material. As a result, the trapped space charge can affect surface potential significantly, especially for samples with longer corona treatment duration.

Polyethylene Nanocomposites for Power Cable Insulations.

This review represents a comprehensive study of nanocomposites for power cables insulations based on thermoplastic polymers such as polyethylene congeners like LDPE, HDPE and XLPE, which is complemented by original results. Particular focus lies on the structure-property relationships of nanocomposites and the materials’ design with the corresponding electrical properties. The critical factors, which contribute to the degradation or improvement of the electrical performance of such cable insulations, are discussed in detail; in particular, properties such as electrical conductivity, relative permittivity, dielectric losses, partial discharges, space charge, electrical and water tree resistance behavior and electric breakdown of such nanocomposites based on thermoplastic polymers are described and referred to the composites’ structures. This review is motivated by the fact that the development of polymer nanocomposites for power cables insulation is based on understanding more closely the aging mechanisms and the behavior of nanocomposites under operating stresses.

The role of point defects and defect gradients in flash sintering of perovskite oxides

The present study investigates the impact of point defects and their redistribution on the flash sintering process. Strontium titanate was chosen as a model system for the group of perovskite ceramics. The characteristics of flash sintering of strontium titanate were analyzed with different acceptor dopant concentrations. The onset of flash sintering was found to be dependent on the acceptor dopant concentration, as expected by the increasing conductivity. A gradient in the microstructure was found after flash sintering with larger grain sizes at the negative electrode. TEM-EDS measurements indicated Ti enrichment at the positive electrode for undoped strontium titanate and strong acceptor segregation for doped strontium titanate. In contrast, grain boundaries at the negative electrode were found to be stoichiometric for the undoped case and the acceptor segregation was less obvious for the doped case.Based on these results and the space charge behavior of strontium titanate, we infer that a gradient of the oxygen vacancy concentration is induced by the electric field during flash sintering: at the positive electrode the oxygen vacancy concentration is higher than at the negative electrode. For strontium titanate it is well known that a high oxygen vacancy concentration reduces the space charge and, hence, acceptor segregation, which agrees well with the experimental findings. Overall, the present study highlights the importance of point defect gradients and space charge for flash sintering of strontium titanate, which may be applicable for many other functional ceramics.

Failure of nano-modified oil impregnated paper under repeated impulse voltage: Effects of TiO2 nanoparticles on space charge characteristics

Since the first application of oil-paper composite insulation in power transformers 100 years ago, researchers are now trying to improve the performance of transformer oil by suspending nanoparticles in the dielectric. However, the performance of paper impregnated with nano-modified oil remains unclear. We previously found nano-oil impregnated paper had a good capability to sustain repeated impulses. The amount of space charges accumulated in the nano-oil impregnated paper is much smaller than that in fresh oil impregnated paper. We considered this behavior to be the main reason for the better electrical performance of the nano-oil-impregnated paper. In this work, based on those achieved results, we focus on the modification mechanism of nano-oil impregnated paper. We analyzed the effects of TiO2 nanoparticles on the relative permittivity and their relationship with the charge injection process. We also discuss the influence of TiO2 nanoparticles on charge trap states and the space charge behavior. The dielectric characteristics and trap energy of both nano-oil-impregnated paper and fresh-oil-impregnated paper are determined in this work.

Effect of graphene nanoplatelets on space charge and breakdown strength of PP/ULDPE blends for HVDC cable insulation

In this paper, polypropylene (PP) is blended with ultra low density polyethylene (ULDPE) content to 5, 15, 25 wt% by melt blending to modify its stiffness and brittleness. The mechanical tests show that both the flexibility and tensile strength of PP are enhanced by the addition of ULDPE with a content of 15 wt%, but the other contents show a weaken elongation at break and tensile strength. Graphene nanoplatelets of 0, 0.005, 0.01 and 0.05 wt% are introduced into the PP/ULDPE (15 wt%) blend to improve its dielectric properties. The DC conductivity, space charge behavior, distribution of trap levels, and breakdown strength of PP/ULDPE/graphene nanocomposites were investigated. The results show that PP/ULDPE/graphene nanocomposites with the nanofiller content of 0.01 wt%, have lower DC conductivity, reduced space charge, and higher breakdown strength. The improved dielectric properties are largely attributable to the deep traps in the interfacial zones between the graphene nanoplatelets and PP/ULDPE blends which restricts the carrier injection and transportation in polymer. It is shown that the PP/ULDPE/graphene nanocomposites with 15 wt% ULDPE and 0.01 wt% graphene nanoplatelets show good dielectric properties for application in HVDC cable insulation.

A trap regulated space charge suppression model for LDPE based nanocomposites by simulation and experiment

Space charge suppression is one of the key goals for the development of HVDC cable insulation. This study intends to reveal space charge suppression mechanism in LDPE based nanocomposites by numerical simulation and experiments in terms of the effect of trap characteristics. The simulation model takes into account bipolar charge injection, transport, charge trapping, recombination and extraction. Both the experiments and simulations clearly indicate that a dramatic increase in the deep trap density can result in remarkable bulk space charge suppression. The large number of deep traps introduced in the LDPE based nanocomposite by nanofillers should be responsible for its excellent space charge suppression effect. The simulation results indicate that the increase in the trap density shows much more remarkable space charge suppression performance as compared to the trap depth, while the increase in both the trap density and trap depth shows the most remarkable space charge suppression performance. A trap regulated space charge suppression model has been proposed indicating that the trap density and depth dominate space charge behavior via regulating the balance between electrode injection current and bulk conduction current due to its control on the rates of charge trapping and detrapping processes, control on the number and location of the trapped charges. The dramatic increase in the trap density and depth results in extremely low bulk conductivity and carrier mobility. A large number of the injected charges will be trapped immediately around electrode interface forming a thin trapped charge layer. The trapped charge layer will produce a strong opposite electric field and enhance the potential for charge injection, leading to a significant decrease in the injection current and finally a balance between the injection current and the bulk conduction current at a lower current level, resulting in remarkable bulk space charge suppression. In addition, high bulk conductivity due to lower trap density and lower trap depth can also contribute to space charge suppression due to the quick relaxation of the bulk space charges, though it will increase the leakage current.

Effect of Temperature on Space Charge Distribution of Oil–Paper Insulation under DC and Polarity Reversal Voltage

The electric field distortion caused by space charge is an important factor affecting the operation reliability of oil–paper insulation in a converter transformer. To study the accumulation and decay characteristics of the space charge within oil-impregnated pressboard under DC and polarity reversal voltage, and consider the possible operating conditions of the converter transformer, the space charge behavior of oil-impregnated pressboard was measured by the pulsed electro-acoustic (PEA) method in the temperature range from −20 °C to 60 °C. The effect of temperature on the accumulation and decay characteristics of space charge is also analyzed. The space charge accumulated within the pressboard at low temperature is mainly homocharge injected by the electrode, while heterocharge formed by ion dissociation counteracts some of the homocharge at high temperature. Thus, the space charge of pressboard first increases, then decreases, with an increase in temperature. However, slow decay of the space charge causes severe distortion of the electric field distribution in the pressboard during voltage polarity reversal.

Influence of nano layer structure of polyimide film on space charge behavior and trap levels

Measurements of space charge density and trap levels are informative ways to predict failure conditions of insulation materials. The charges are injected from electrodes and penetrate from surface towards deep inside of the insulation material. In order to resist the charge injection, trapping, migration and increase charge recombination, detrapping and extraction rate, layer (nanocomposite-PI-nanocomposite) sandwich structure samples are used. Two techniques Pulse Electro Acoustic (PEA) and Thermal Stimulated Current (TSC) are combined to characterize space charge behavior and trap levels. Lab-view based program is used to detect the charge density distribution and total charge decay, different peaks of current using TSC technique are used to get activation energy and trap level density. The effects of structural interface (single, double, triple layers) and types of nano (Al 2 O 3 , SiO 2 ) fillers are analyzed to influence charge accumulation polarity and trap levels. Trap levels are calculated using TSC data as well as total charge decay data. The trap values calculated from both data match each other. Al 2 O 3 -PI-Al 2 O 3 sandwich structure showed remarkable improvements to resist accumulation of space charge for longer time. Mostly charges are shallow traps with 1.126 eV trap level and recombination rate is higher. In double layer structure in which interface is made up of SiO 2 and Al 2 O 3 , deep trap level of 1.26 eV and higher trap density is observed. The charges decay is faster in three-layer structure than double and single layer structure. It is concluded that the interface formed with different type of nano and structure of insulation, both are an important parameter for manufacturing industry to suppress space charges accumulation and traps.

Modeling of charge transport in insulating dielectric for gas-insulated transmission line based on multiple trap levels

Space charge behavior in polymeric materials is of interest to the electrical engineering community and has significant effects on the behavior of the polymers that are used in gas-insulated transmission lines (GILs). For example, the space charge in certain insulating materials can cause severe distortion of the electric field that may even lead to breakdown of the materials. It is therefore important to understand the trapped charge distributions in these materials because they can be used as novel indicators for effective evaluation of the aging status and the electric withstand strength of these insulating materials. This paper addresses recent developments in the modeling of charge transport in polymers for use in GIL applications. A numerical model of bipolar charge transport in polymeric insulators is presented in this paper. This model was studied based on use of multiple trap levels and consideration of symmetrical parameters for the generation and transport of electrons and holes.

Effect of graphene oxide particles on space charge accumulation in LDPE/GO nanocomposites

This paper focuses on the space charge characteristics of low-density polyethylene (LDPE)/ graphene oxide (GO) nanocomposites, as well as the corresponding trap level distributions. The relative permittivity, DC conductivity, DC breakdown strength and space charge behaviors of nanocomposites with GO filler content of 0, 0.001, 0.005, 0.01, 0.05, and 0.1 wt% were studied. The trap level distributions were investigated using the isothermal discharge current (IDC) method. The experimental results show that the 0.01 wt% LDPE/GO nanocomposites have a significantly lower relative permittivity, a lower DC conductivity, a higher breakdown strength and a smaller amount of space charge accumulation than that of the neat LDPE. With increased GO content, the space charge accumulation of the nanocomposites shows an increasing trend. It is indicated that the certain content of GO nanoparticles could introduce large quantities of deep carrier traps, which suppress the transport of charge carriers and further restrain the space charge injection and accumulation. The experimental results reveal that the GO nanoparticles show great promise for use in HVDC cable applications, which could reduce particle agglomeration due to its insignificant addition.

Improvement in space charge behavior of cellulose paper by coating with AlOx nanostructure film

The space charge characteristics of insulation paper are crucial under direct current. Thus, improving the space charge behavior of insulation paper is essential. In this study, a nanostructure AlOx film was coated on the surface of insulation paper via RF magnetron sputtering. The space charge distribution was measured using a pulsed electroacoustic instrument. After coating with nanostructure AlOx film, the calculated characteristics showed that space charge accumulation in the samples decreased, and the trap density on the surface of the insulation paper also decreased. Results showed that using the nanostructure AlOx film to improve the space charge performance of insulation paper was feasible and effective.

Failure Characteristics and Mechanism of Nano-Modified Oil-Impregnated Paper Subjected to Repeated Impulse Voltage.

Nano-modification is a prospective method for improving the electrical properties of transformer oil. In most situations, transformer oil combined with cellulose paper is used to construct an insulation system for power equipment, such as power transformers. However, the influence of nanoparticles on the electrical performance of oil-impregnated paper is still unclear. Therefore, in this paper, we identify the failure characteristics of both fresh and nano-modified oil/paper. Specifically, the accumulative failure characteristics of nano-oil-impregnated paper (NOIP) are experimentally determined. The space charge distribution and trap characteristics of fresh paper and NOIP were measured, and the effect of nanoparticles on the space charge behavior are then analyzed. Finally, we measure the microstructure of fresh paper and NOIP subjected to repeated impulses. The test results indicate that nano-titanium oxide (TiO2) particles have a limited effect on the breakdown voltage of NOIP. However, the particles can dramatically improve the resistant ability of NOIP against repeated impulses. For the NOIP with a nano-concentration of 0.25 g/L, the improvement reaches 62.5% compared with fresh paper. Under repeated applications of impulse voltages, the space charge density of NOIP is much lower than that of fresh paper. The deep trap density of NOIP is much higher than that of fresh OIP, whereas shallow trap density is relatively lower. Micropores are generated in paper insulation subjected to repeated impulses. The amount of the generated micropores in NOIP is lower than that in fresh paper. Nano-TiO2 particles suppress the accumulation of space charge in the oil paper insulation, which weakens the electric field distortion in the dielectric. However, nanoparticles reduce the accumulative damage caused by repeated impulses. The above two points are considered the main reasons to improve the resistant ability against repeated impulses.

Bipolar Charge Transport Model Optimized to Simulate the Space Charge Build-Up Within the Bi-Dielectric System

In this paper, finite element method correlated with boundary conditions has been used to solve the Poisson’s equation. Splitting method, solving the continuity equation in two-time step, with and without term source associated with new Schottky injection application, however, Runge–Kutta method with a Gaussian filter calculated the distribution of all charges whether mobile or trapped accumulated on the interface. Moreover, these equations are solved in order to simulate the space charge behavior accumulated in the bi-dielectric system with different nature of electrodes. This model takes into account trapping, detrapping, recombination as well as the diffusion. Moreover, this model is optimized in order to fit experimental data by considering the surface charge at electrodes. Simulation results showed that the injection mechanism based on both the electric field and the properties of either metal-electrode or the dielectric. Furthermore, the polarity of the interfacial charge trapped on the dielectric interface depends on both the dielectrics permittivities and the nature of the electrodes. Thereby, surface charge at electrode improving the performance of this model to simulate the other charge accumulated into the bulk of the bi-dielectric system rather the interfacial charge trapped on the dielectric interface.

Surface Layer Fluorination-Modulated Space Charge Behaviors in HVDC Cable Accessory.

Space charges tend to accumulate on the surface and at the interface of ethylene–propylene–diene terpolymer (EPDM), serving as high voltage direct current (HVDC) cable accessory insulation, which likely induces electrical field distortion and dielectric breakdown. Direct fluorination is an effective method to modify the surface characteristics of the EPDM without altering the bulk properties too much. In this paper, the surface morphology, hydrophobic properties, relative permittivity, and DC conductivity of the EPDM before and after fluorination treatment were tested. Furthermore, the surface and interface charge behaviors in the HVDC cable accessory were investigated by the pulsed electroacoustic (PEA) method, and explained from the point of view of trap distribution. The results show that fluorination helps the EPDM polymer obtain lower surface energy and relative permittivity, which is beneficial to the interface match in composite insulation systems. The lowest degree of space charge accumulation occurs in EPDM with 30 min of fluorination. After analyzing the results of the 3D potentials and the density of states (DOS) behaviors in EPDM before and after fluorination, it can be found that fluorination treatment introduces shallower electron traps, and the special electrostatic potential after fluorination can significantly suppress the space charge accumulation at the interface in the HVDC cable accessory.

Crystalline Modification and Its Effects on Dielectric Breakdown Strength and Space Charge Behavior in Isotactic Polypropylene.

Adding nucleating agents (NAs) is one of the most efficient ways to obtain improved mechanical, optical, and thermal properties of isotactic polypropylene (iPP). While it is well appreciated that electrical property is critically affected by crystalline modification, the role between them remains unclear. Here, we address this issue by incorporating commercial α-NA and β-NA into iPP, both of which exhibit strong nucleation ability, e.g., reducing the size of crystalline agglomerates from 45.3 μm (Pure-iPP) to 2.5 μm (α-iPP) and 7.6 μm (β-iPP), respectively. Mechanical testing results show that while β-modification decreases the tensile strength a little, it does enhance the elongation at break (200%) and toughness (25.3% higher), relative to its unfilled counterparts. Moreover, a well-dispersed β-iPP system obtains a comprehensive improvement of electrical properties, including dielectric breakdown strength, space charge suppression, and internal field distortion under a high external field (−100 kV/mm) due to newly-generated deep charge trapping sites. This crystalline modification strategy is attractive for future development of many engineering insulating polymers.