Space Charge Breakdown Research Articles

Effect of Functional Grafting Modification on Dielectric Properties of Polypropylene Insulation—Part II: Space Charge and DC Breakdown

Effect of Functional Grafting Modification on Dielectric Properties of Polypropylene Insulation—Part II: Space Charge and DC Breakdown

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.

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%.

Theoretical Investigation of the Phenomenon of Space Charge Breakdown in Electromembrane Systems

At present, it is customary to consider the overlimit operating modes of electromembrane systems to be effective, and electroconvection as the main mechanism of overlimiting transfer. The breakdown of the space charge is a negative, "destructive" phenomenon, since after the breakdown the size and number of electroconvective vortices are significantly reduced, which leads to a decrease in mass transfer. Therefore, electromembrane desalination processes must be carried out before space charge breakdown occurs. Thus, the actual problem arises of determining at which potential jumps a breakdown of the space charge occurs at a given concentration of the solution. Electromembrane systems are used for desalination at electrolyte solution concentrations ranging from 1 to 100 mol/m3. In a theoretical study of increasing the efficiency of the desalination process, mathematical modeling is used in the form of a boundary value problem for the system of Nernst-Planck and Poisson (NPP) equations, which refers to "hard" problems that are difficult to solve numerically. This is caused by the appearance of a small parameter at the derivative in the Poisson equation in a dimensionless form, and, correspondingly, a boundary layer at ion-exchange membranes, where concentrations and other characteristics of the desalination process change exponentially. It is for this reason that the numerical study of the boundary value problem is currently obtained for initial concentrations of the order of 0.01 mol/m3. The paper proposes a new numerical-analytical method for solving boundary value problems for the system of Nernst-Planck and Poisson equations for real initial concentrations, using which the phenomenon of space charge breakdown (SCB) in the cross section of the desalination channel in potentiostatic and potentiodynamic modes is studied. The main regularities of the appearance and interaction of charge waves, up to their destruction (breakdown), are established. A simple formula is proposed for engineering calculations of the potential jump depending on the concentration of the solution, at which the breakdown of the space charge begins.

INFLUENCE OF SPACE CHARGE BREAKDOWN ON THE PERFORMANCE OF ELECTROMEMBRANE SYSTEMS

It is known that the existing methods of using electromembrane systems (EMS) do not allow the most efficient water treatment and water treatment at overlimiting current modes, which is why there is a need to determine the operating parameters of electrodialysis plants to prevent complex destructive effects, it is essential limiting mass transfer, and reducing the efficiency of EMS. In addition, in practice, a contradiction is found in practice - an increase in the potential jump in the existing EMS does not provide an intensification of the transfer of salt ions under overlimiting current modes. The article is devoted to the study of the influence of one of the destructive phenomena - the breakdown of the space charge on the performance of electromembrane systems in highly diluted salt solutions. The aim of this work is a theoretical assessment of the effect of space charge breakdown on the transport of salt ions in electromembrane systems. A mathematical model based on the Nernst-Planck and Poisson equations has been developed. The process of electrodialysis desalination of a NaCl solution located in a flow chamber between an anion exchange membrane (AEM) and a cation exchange membrane (CEM) is considered. The solution flow is directed from bottom to top. Numerical modeling of the effect of space charge breakdown on EMS performance in highly dilute salt solutions has been carried out. The new mathematical model and software have been developed for numerical simulation of the effect of space charge breakdown on EMS performance in highly dilute salt solutions. In this work, using a 2D mathematical model and the formula for the current-voltage characteristic of the desalination channel, the effect of space charge breakdown on the transfer of salt ions in electromembrane systems at various values of the potential jump was determined for the first time. The ranges of currents are determined, which correspond to a high and slight increase in the performance of electromembrane systems, as well as a range of currents in which there is no increase. The obtained results can be used to determine the optimal value of the potential jump according to the criterion of maximum productivity. The results obtained can be applied to the design of industrial EMC to intensify mass transfer and increase productivity.

Effects of Pre-Crosslinking on Space Charge and Breakdown Characteristics of XLPE Cable Insulation

This study focused on the effect of pre-crosslinking on the electrical properties of XLPE insulation for HVDC cables. The XLPE samples were pre-crosslinked at 140, 150 and 160 °C. The space charge and DC breakdown characteristics of the XLPE samples with and without pre-crosslinking were investigated. The results show that the pre-crosslinked XLPE samples have an obvious space charge injection and transportation, resulting in more severe electric field distortion. Under the electric field of 100 kV/mm, the electric field distortion rates of the XLPE samples pre-crosslinked at 150 and 160 °C reached 51% and 72% respectively, which were much higher than that of the XLPE sample without pre-crosslinking (14%.) Compared with the XLPE without pre-crosslinking, the breakdown strengths of XLPE pre-crosslinked at 140, 150 and 160 °C were reduced by 11%, 6% and 7% respectively, at the temperature of 50 °C. It is concluded that the pre-crosslinking leads to imperfect crystallization and micro-defects in the amorphous region of XLPE, thereby leading to more space charge accumulation and reduced DC breakdown. The pre-crosslinking of XLPE insulation therefore should be prevented during HVDC cable manufacturing.

Mathematical Modeling of the Phenomenon of Space-Charge Breakdown in the Galvanostatic Mode in the Section of the Electromembrane Desalination Channel.

One of the ways to increase the efficiency of the desalination process in membrane systems is to use intensive current modes. Recently, the phenomenon of space-charge breakdown was theoretically described for desalination under intensive current modes. The space-charge breakdown is a decrease in the magnitude and size of the extended space charge regions (SCRs) of opposite signs, formed at the cation- and anion-exchange membranes in the desalination channel, when they approach each other. Therefore, this phenomenon negatively affects the intensity of electroconvection and the efficiency of mass transfer in membrane systems. We report the results of the first theoretical analysis of the space-charge breakdown in the galvanostatic electric mode, which is generally used in the research and operation of membrane systems. For this purpose, a one-dimensional model of the ion transfer of the electrolyte solution in the section of the desalination channel at the direct current is developed. The regularities of changes in the extended SCRs in the galvanostatic mode are determined. A relation is obtained for the onset time of the space-charge breakdown, which makes it possible to determine the parameters of the effective operation of the membrane system.

Space-Charge breakdown phenomenon and spatio-temporal ion concentration and fluid flow patterns in overlimiting current electrodialysis

Overlimiting current regimes are of practical interest for electrodialysis with ion-exchange membranes (IEMs) operating at current densities significantly higher than the limiting current density. Such high ion currents across the membranes allows reducing the investments into expensive IEMs and stacks. Electroconvection (EC) is the major contributor to the overlimiting current and concepts to utilize EC are highly desired. Most of the known theoretical works describing EC in electrodialysis channels are performed when only a single cation exchange membrane is considered. The presence of a neighboring anion-exchange membrane has not been considered so far.We report the results of direct numerical simulations of ion and water transport involving EC in an electrodialysis channel formed by a cation- and an anion-exchange membrane. A 2D “basic” model involving the Nernst-Planck-Poisson-Navier-Stokes equations is introduced. Details of spatio-temporal changes in the distribution of concentrations and space charge patterns as well as the interactions of EC vortices are analyzed.The presence of the anion-exchange membrane in the transition to a close-to-chaotic regime at high potential difference is examined. For the first time, we discover the phenomenon of space-charge breakdown where the space charges of opposite sign formed at the two membranes are short-circuited throughout the desalination channel. The details of the mechanism of this phenomenon are analyzed using the 2D model and a simplified 1D model. Simulations show that the space-charge breakdown leads to a decrease in the size and number of EC vortices in the region of breakdown, which results in a reduction of the local current density. This phenomenon, apparently, determines the upper limit of the possible increase in the mass transfer rate by electroconvection.

Effect of Thermal Ageing on Physico-Chemical and Electrical Properties of EHVDC XLPE Cable Insulation

This paper presents the influence of thermal ageing on the physico-chemical and electrical properties of 500 kV extra high voltage direct current (EHVDC) XLPE cable insulation. EHVDC XLPE cable sections are placed in a thermal oven and thermally aged at 106 °C (melting peak temperature) for 250, 500 and 1000 h. After ageing, the inner, middle and outer layers of the cable insulation are characterized by Fourier transform infrared (FTIR) spectroscopy, oxidation induction time, X-ray diffraction (XRD), space charge, DC conductivity and DC breakdown measurements. The physicochemical analysis results show that the cross-linking by-products in the cable insulation are difficult to be completely removed during the ageing process, whereas the thermal ageing cannot completely consume the antioxidants in the materials. The space charge measurement results show that the hetero charges are accumulated inside the unaged middle layer samples. However, the space charge accumulation and electrical conductivity of the samples are significantly reduced after 1000 h ageing. Moreover, the DC breakdown strength of the inner and middle layer samples is continually increased with the increase of the ageing time. It is elucidated that the thermal ageing leads to the re-crystallization and re-degassing of samples, improving the space charge behavior and the charge carrier trapping phenomenon. It is proposed that the temperature of 106 °C could be used as a suitable degassing temperature for the EHVDC XLPE cable production.

Moisture Absorption Characteristics of Nanoparticle-Doped Silicone Rubber and Its Influence Mechanism on Electrical Properties.

To further explore the long-term stability of nano-dielectrics, experiments were carried out to investigate the moisture absorption characteristics and electrical properties of silicone rubber (SiR) doped with different inorganic nanoparticles. Thermogravimetric analysis (TGA) is utilized to research the moisture absorption characteristics including mass fraction and binding forms. The trap depth and electron orbitals are calculated by density functional theory to explain the influence mechanism of water molecules on SiR. It is found that the addition of nanoparticles will increase the moisture content of SiR. Additionally, the nano-TiO2-doped SiR absorbs more water and binds with water relatively more loosely than nano-Al2O3. There is a degradation of space charge inhibition capability and breakdown strength after moisture absorption, which might be explained by shallow traps brought by water molecules and the narrowed forbidden bandwidth of SiR.

Investigation of charge transport and breakdown properties in XLPE/GO nanocomposites part 2: Effect of polarity reversal

In this paper, the effect of polarity reversal on the space charge behaviors and breakdown properties of cross-linked polyethylene (XLPE)/graphene oxide (GO) nanocomposites are studied. Five kinds of XLPE based nanocomposites doped with different combinations of GO quantum wells (QWs) and the polycyclic aromatic compounds (PAC) were prepared by mechanical blending. The dynamic space charge distributions and breakdown strength of the XLPE samples under polarity reversal were measured. The experimental results indicate that the injection and accumulation of space charges in XLPE show polarity dependence under polarity reversal. The breakdown strength of the XLPE samples decreases with the increase of the pre-applied hetero-polarity electric field. Both the GO QWs and PAC can inhibit charge transport and improve the breakdown properties of XLPE, and the synergistic effect between the two dopants is observed. As a result, XLPE samples modified by the PAC functionalized GO QWs show a minimum space charge accumulation and the highest breakdown strength under polarity reversal. The quantum chemistry calculation method was employed to clarify the charge transport characteristics in XLPE dielectrics, and the relationship between space charge distributions and breakdown properties of the nanocomposites under polarity reversal was discussed.

Effects of radical scavenger on space charge accumulation of PP/ULDPE composites for HVDC cable insulation

In this work, the space charge behavior and breakdown strength of polypropylene (PP)/ultralow density polyethylene (ULDPE) blends and three different radical scavenger modified composites are performed at 30, 60 and 90 °C. The trap distribution in the radical scavenger and the composites are investigated based on quantum chemistry calculation and isothermal discharge current test. The results show that, 0.3 wt% addition of radical scavenger has little effect on the thermal and mechanical property of the composites. The radical scavenger can introduce traps with different energy levels, which has greatly inhibited the space charge accumulation aggravated by the increasing temperature in the PP/ULDPE (PU) blends. Although the overall breakdown strength of the samples decreases with the rise of temperature, the performance of radical scavenger modified composites show the best, which suggests a great correlation with the traps they introduce, the molecular structure and chemical reaction of the additives themselves. The PP/ULDPE blends with 0.3 wt% C944 exhibit great potential for HVDC cable insulation.

Temperature-dependent DC conductivity and space charge distribution of XLPE/GO nanocomposites for HVDC cable insulation

Nanocomposites have been proven to be an important dielectric material, which provides advanced dielectric properties of insulation to power equipment. For the use of polyethylene (PE)-based nanocomposites in HVDC cable insulation, their temperature dependence needs to be considered, because it could deeply affect the conductivity and space charge behavior. To investigate the temperature dependence of conductivity and space charge properties of the cross-linked polyethylene (XLPE)/graphene oxide (GO) nanocomposites, XLPE samples with GO mass fractions of 0, 0.001, 0.01 and 0.1 wt% were prepared. The conductivity, space charge behavior and breakdown strength of the nanocomposites were tested. Then, the surface potential decay (SPD) was measured to elucidate the mechanism of the GO nanoparticles in the XLPE matrix according to the trap theory. The obtained experimental results showed that the XLPE/GO nanocomposites with the mass fraction of 0.01 wt% exhibited a lower conductivity, lower space charge accumulation, and higher DC breakdown strength than those of the neat XLPE. This observation is related to the nanoparticle-polymer interaction regions and the deep traps introduced by the GO nanoparticles. In addition, the temperature dependence of the electrical properties of nanocomposites decreased when appropriate GO amounts were added. This result indicates that the XLPE/GO nanocomposites are more suitable for use in HVDC cable insulation.

Investigation of the Space Charge and DC Breakdown Behavior of XLPE/α-Al2O3 Nanocomposites.

This paper describes the effects of α-Al2O3 nanosheets on the direct current voltage breakdown strength and space charge accumulation in crosslinked polyethylene/α-Al2O3 nanocomposites. The α-Al2O3 nanosheets with a uniform size and high aspect ratio were synthesized, surface-modified, and characterized. The α-Al2O3 nanosheets were uniformly distributed into a crosslinked polyethylene matrix by mechanical blending and hot-press crosslinking. Direct current breakdown testing, electrical conductivity tests, and measurements of space charge indicated that the addition of α-Al2O3 nanosheets introduced a large number of deep traps, blocked the charge injection, and decreased the charge carrier mobility, thereby significantly reducing the conductivity (from 3.25 × 10−13 S/m to 1.04 × 10−13 S/m), improving the direct current breakdown strength (from 220 to 320 kV/mm) and suppressing the space charge accumulation in the crosslinked polyethylene matrix. Besides, the results of direct current breakdown testing and electrical conductivity tests also showed that the surface modification of α-Al2O3 nanosheets effectively improved the direct current breakdown strength and reduced the conductivity of crosslinked polyethylene/α-Al2O3 nanocomposites.

Mathematical modelling of space charge breakdown in membrane systems taking into account the non-catalytic dissociation/ recombination reaction of water molecules

In electromembrane systems, a theoretical study of salt ion transfer usually uses mathematical models of salt ion transfer in the depleted diffusion layer of ion-exchange membranes. In this paper, a new mathematical model of ion transport in the cross-section of the desalination channel formed by two ion-exchange membranes – anion-exchange (AEM) and cation-exchange (CEM), taking into account the non-catalytic dissociation/recombination reaction of water molecules. The model is a boundary value problem for a non-stationary system of Nernst-Planck and Poisson equations. A numerical analysis of the boundary value problem is performed and the main regularities of the 1:1 salt ion transfer process are established, in particular, the occurrence and development of space charge breakdown is shown. The interaction of the space charge and the noncatalytic dissociation/recombination reaction of water molecules are theoretically investigated.

A space charge dynamics based model for lifetime evaluation of insulating polymers

To explain the generally observed and applied electric field dependent lifetime of insulating polymers, a model based on space charge dynamics has been proposed. Electric field strength affects trapping and detrapping coefficients, then the electromechanical energy released by space charge dynamics is influenced, which is a direct factor of molecule scission and dielectric breakdown. Hence static and dynamic space charge breakdown occurs under low and high electric fields, respectively. The combined consideration of charge trapping and detrapping helps to extract some adjustable parameters and give a description of lifetime in a wide electric field range. The simulation results show that the lifetime-electric field curve is piecewise linear in double logarithmic coordinates, indicating that charge dynamics are responsible for electric field dependent lifetime. Charge trapping accounts for low field lifetime while charge detrapping determines high field lifetime. The model reveals that it is improper to predict lifetime at low electric fields from lifetime test at high electric fields immediately.

Temperature dependent electrical properties of thermoplastic polypropylene nanocomposites for HVDC cable insulation

Polypropylene has been regarded as a potential recyclable insulation material for HVDC cable, which is beneficial to improve the working temperature and transmission capacity of HVDC cables. However, the electrical properties of polymeric insulation materials exhibit apparent temperature dependent characteristics. Temperature influences the generation and migration of charge carriers, thereby affecting the electrical properties. This paper focuses on the electrical properties of polypropylene nanocomposites at different temperatures, including volume resistivity, space charge and DC breakdown strength. The temperature dependent electrical properties of polypropylene are regulated by adding MgO nanoparticles. Based on the characterization of microstructure and trap characteristics, the mechanism of regulating the charge carrier transport process and the electrical properties of the nanocomposites by introducing MgO nanoparticles is explained. The polypropylene nanocomposites show decreased electrical conduction, suppressed space charge accumulation and increased DC breakdown strength, which are desirable for recyclable HVDC cable insulation application. This work may pave a way for tailoring the high temperature electrical properties of polymeric insulation material.

Modification mechanism of low-density polyethylene insulation by hydrophilic and hydrophobic porous SiO2 nanoparticles

To study the modification effect and mechanism of porous nanoparticles on electrical insulation performance, SiO2/low-density polyethylene (LDPE) nanocomposites with different filler concentrations are prepared by filling two kinds of porous SiO2 nanoparticles. The microstructure and dispersion of SiO2 in LDPE are characterized by scanning electron microscope [ the effect of nanofilling on the crystallinity of LDPE is analyzed by Differential Scanning Calorimetry (DSC)]. The mechanism of insulation modification of nanocomposites is studied by space charge and direct current conductance analysis of charge transport and Weibull distribution of breakdown field strength. By filling nanoscale SiO2 particles, the crystallinity of the LDPE matrix can be changed effectively. It is indicated that the hydrophobic SiO2 nanofillers with higher dispersity in LDPE matrix are more effective than hydrophilic SiO2 nanofillers to improve the crystallinity of LDPE matrix. The LDPE crystallinity of hydrophobic 1 wt% SiO2/LDPE nanocomposite approaches the highest value through heterogeneous nucleation. Compared with pure polyethylene, the conductivity of nanoporous SiO2 filled polyethylene composites is reduced obviously and the lowest conductivity is obtained for the nanocomposites filled with hydrophobic SiO2 nanoparticles in 1 wt% filling rate. The space-charge distribution and electric breakdown field strength further demonstrate that the filling of 1 wt% hydrophobic SiO2 nanoparticles can efficiently inhibit carrier injection and increase the breakdown field strength. The heterogeneous nucleation of porous SiO2 nanofillers increases the crystallinity of LDPE, and thus decreases the carrier mobility and increases the probability of charge charge trapping. The dispersion of nanofillers in composite is improved significantly by the symmetrical group –CH3 on the surface of hydrophobic SiO2 nanoparticles, which effectively suppresses the space-charge accumulation and evidently increases the electric breakdown field strength. It is proved that the special nanoscale interface in nanodielectrics can substantially improve the dielectric properties of polyethylene.

Temperature dependent space charge and breakdown strength of PP/ULDPE/graphene nanocomposites for HVDC extruded cable insulation

The space charge behavior, conductivity and breakdown strength are critical issues in the design of insulation for high voltage direct current (HVDC) extruded cables, which is greatly affected by the operating temperature at different load conditions. The space charge, DC conductivity and breakdown characteristics of polypropylene (PP) based insulation at various temperatures are investigated in this paper. The space charge, DC conductivity and the breakdown strength of pure PP, PP/ultralow density polyethylene (ULDPE) blends and its graphene nanocomposites were examined at 30, 60 and 90 °C. The results show that, at elevated temperature, the homo-charge injection in PP from both electrodes is enhanced. Significant charge accumulates in PP/ULDPE blends in the vicinity of the anode at 90 °C. A small amount (0.01 wt%) of graphene nanoplatelets in PP/ULDPE blends inhibit the injection of homo-charge at elevated temperatures. No dramatic change in the breakdown strength with increasing temperature is observed. The DC conductivity increases with increasing temperature, and the conductivity significantly increases from 60 to 90 °C compared to from 30 to 60 °C. The PP/ULDPE blends with 0.01 wt% graphene nanoplatelets exhibit good properties for potential HVDC cable insulation.

Numerical analysis on space charge and AC-DC combined breakdown strength in polyethylene

In high voltage direct current (HVDC) power transmission systems, electrical equipment at the valve side usually withstands complex stresses like AC-DC combined voltages. Understanding the charge migration and accumulation characteristics under complex stresses and their influences on the breakdown strength are of great significance. This paper presents an in-depth simulation study on the AC-DC combined breakdown strength using a modified bipolar charge transport model, which takes the deterioration of materials into consideration. Firstly, the simulation results of charge profiles under AC voltages agree well with experimental results, which partly illustrates the appropriate selection of parameters. Then the AC breakdown strengths with different ramping rates and frequencies are studied. Charges accumulated relatively close to the surface of samples can cause severe distortion of the electric field after the change of voltage polarity. After that, the breakdown voltages with different ratios of the AC to DC component are calculated and analyzed, especially on how AC and DC component influence the charge migration and accumulation during the evolution of the aging process eventually leading to breakdown. The simulation results of AC-DC combined breakdown voltages show good agreement with previous experimental results.