In this paper, an advanced surface-grafting method is reported to improve the high-temperature performance of biaxially oriented polypropylene (BOPP) membranes. The leakage conductivity of the ...surface-grafted films decreases by 98% at 85 °C. The decline in the leakage loss contributes to the 99% charge-efficiency at 85 °C. The dielectric constant increases by 7%. The DC breakdown strength grows by 43% at 85 °C. The maximum energy storage density goes up from 1.45 to 2.77 J/cm 3 at 85 °C. The surface-grafted BOPP film exhibits outstanding energy density and charge-discharge efficiency characteristics. This research provides a theoretical reference for improving the performance of capacitor film based on surface modification.
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.
Space charge accumulation inside the cross-linked polyethylene (XLPE) insulation is one of the most serious problems in HVDC cable operation. In this paper, the voltage stabilizers that have ...excellent compatibility with XLPE are used to suppress the space charge in XLPE insulation. Three different voltage stabilizers, 4,4'-Difluorobenzophenone, 4,4'-Dihydroxybenzophenone, and 4,4'-Bis (dimethyl amino) benzyl, denoted as A, B, and C, respectively, were added into XLPE material with additive content of 0.5 wt%, and neat XLPE was used as a control. The differential scanning calorimeter (DSC) test, tensile test, DC conductivity and breakdown strength measurements were employed to characterize the thermal, mechanical and electrical properties of the samples respectively. Moreover, the space charge behaviors were investigated using the PEA method, and the trap-controlled carrier mobility was evaluated by the depolarization processes of the space charge. The mechanical and thermal analyses indicate that the presence of voltage stabilizers does not significantly change the microstructure of XLPE, as required for the preparation of HVDC cable insulation. Though the influence of voltage stabilizers on DC conductivity is not obvious, a higher breakdown strength was found in the voltage-stabilizer modified XLPE samples. In addition, XLPE modified by the voltage stabilizers B and C with higher trap-controlled carrier mobility show a good ability to suppress space charge in XLPE insulation, that is, less space charge accumulation and faster space charge dissipation than the neat XLPE. The experimental results reveal that the voltage stabilizers B and C show great promise for use in HVDC cable application.
In this paper, a novel modification method for biaxially oriented polypropylene (BOPP) membranes is proposed based on surface-grafting technology. The experimental results show that, compared with ...the pristine samples, the surface-grafted films show better breakdown strength under different voltage stress especially at higher temperatures (85 °C). The DC withstand strength of the surface-grafted PP membranes grows by 42.7% from 386.4 to 551.2 kV/mm. The breakdown strength climbs by 63.5% from 305.9 to 500.2 kV/mm under DC superimposed pulse voltage. Under DC superimposed harmonic stress, the breakdown strength is increased from 175.6 to 587.4 kV/mm by 234%. The influence of surface modification on charge transmission behavior is proposed based on the Density Functional Theory (DFT) simulation. This research provides a theoretical and methodological reference for performance optimization by surface modification for BOPP capacitors.
Space charges are likely to accumulate at the interface between insulation materials with different properties, which may lead to accelerated degradation of insulation systems. Cable accessories made ...of ethylene-propylene-diene terpolymer (EPDM) are considered to be the weakest part of HVDC cable system due to the existence of the interface between cable insulation and itself. Direct-fluorination can be applied to adjust the electrical properties of polymer insulation. Therefore, this paper intends to investigate the effects of direct-fluorination on the interface charge distribution between LDPE and EPDM composite insulation. The EPDM samples were fluorinated for 0, 15, 30 and 60 min, and LDPE for 0 and 30 min, respectively. The interface charge distribution were measured under 20 kV/mm. Obtained results show that the polarity of interface charge is the same with the voltage applied on EPDM sample, which can be verified by the Maxwell-Wagner-Sillars (MWS) theory. The fluorination for 30 min effectively suppresses the interface charge accumulation between LDPE and EPDM. However, excessive fluorination leads to significant accumulation of interface charge. Though fluorination of both dielectrics has the same effect with the single fluorinated EPDM on interface charge behaviors, more hetero-charges injection in the vicinity of cathode. The interface charge accumulation and dissipation process depends on not only the time constant but the surface state controlled by the fluorination time. As a consequence, the approximate direct fluorination can effectively suppress the interface charge accumulation of cable accessories and will make a potential application for the dc cable accessories.
Silicone rubber (SiR), as a basic insulating material, is widely used in insulators for HVDC transmission lines. The prominent characteristics of the SiR coating are the hydrophobicity and the ...hydrophobicity recovery, which can suppress the development of leakage currents and dry-band discharges, and prevent the flashover incidents. Fluorination is a method to change the chemical component of the surface layer of polymers, which could induce the corresponding change in hydrophobicity and some electrical properties, thus influencing the dc surface flashover characteristics. Room temperature vulcanized (RTV) SiR samples were fluorinated in a stainless reaction vessel at about 298K (25°C) using a F2/N2 mixture with 12% F2 by volume and 0.05 MPa (500 mbar) for respectively 5, 10, 15 and 20 minutes. The hydrophobicity of samples with different fluorination time was measured by the static contact angle tests after corona aging. The variation of hydrophobicity with the lapse of time was recorded. DC flashover tests in different conditions were carried out between two plane electrodes on top of the material sample. The flashover voltage was found to be associated with the surface hydrophobicity status of samples. Obtained results showed the effects of the fluorination on the surface charge, hydrophobicity performance and dc flashover voltage. It is suggested that the appropriate fluorination can improve surface hydrophobicity and suppress surface charge accumulation, thus raising the dc flashover voltage.
•It is to present comparative and sensitive analysis for PTCs with the MCRT method.•A detailed PTC optical model was developed based on a novel unified MCRT model.•Reference data determined by the ...divergence effect is useful to design a better PTC.•Different PTCs have different levels of sensitivity to different optical errors.•There are no contradictions between accuracy requirements of different parameters.
This paper presents the numerical results of the comparative and sensitive analysis for different parabolic trough solar collector (PTC) systems under different operating conditions, expecting to optimize the PTC system of better comprehensive characteristics and optical performance or to evaluate the accuracy required for future constructions. A more detailed optical model was developed from a previously proposed unified Monte Carlo ray-tracing (MCRT) model. Numerical results were compared with the reference data and good agreements were obtained, proving that the model and the numerical results are reliable. Then the comparative and sensitive analyses for different PTC systems or different geometric parameters under different possible operating conditions were carried out by this detailed optical model. From the numerical results it is revealed that the ideal comprehensive characteristics and optical performance of the PTC systems are very different from some critical points determined by the divergence phenomenon of the non-parallel solar beam, which can also be well explained by the theoretical analysis results. For different operating conditions, the PTC systems of different geometric parameters have different levels of sensitivity to different optical errors, but the optical accuracy requirements from different geometric parameters of the whole PTC system are always consistent.
The effect and mechanism of ultrasonic vibration on the microstructure and performance of laser cladding 316L coating has been studied, providing a theoretical basis and technical support for the ...application of the ultrasonic-assisted laser cladding process. The results show that ultrasonic vibrations with different amplitudes can effectively improve the macroscopic forming quality of the cladding layer, among which the ultrasonic vibration with 17.5 μm amplitude is the best. Ultrasonic vibration can also significantly improve the micro-forming quality of the cladding layer, such as homogenizing the structure, refining grains and reducing porosity. The wear mechanism of the cladding layer with an ultrasonic assistance of 17.5 μm is mainly abrasive wear, so the wear resistance has been significantly improved. The ultrasonic-assisted cladding layer exhibits significant passivation behavior in 3.5 wt% NaCl solution. It may be that the surface roughness reduction and grain refinement caused by ultrasonic assistance can improve the integrity and compactness of the cladding layer, so the dynamic equilibrium process of the dissolution and reformation of the passive film is changed, which can improve the corrosion resistance of the surface. There are some equiaxed grains in the columnar-grains area of the ultrasonic-assisted cladding layer, mainly because the inside of some primary columnar grains has gone through a process of lattice distortion → dislocation multiplication → substructure rotation → formation of small-angle grain boundary → misorientation accumulation → formation of new grains in the process of ultrasonic-assisted laser cladding of 316L stainless steel.
•Effect of ultrasonic vibration on 316 L cladding layer is described.•Action mechanism of ultrasonic vibration on 316 L cladding layer is explained.•Formation mechanism of equiaxed grains in the columnar-grains area is proposed.
In this paper, the surface nonlinear conductivity (SNC) spacer is proposed to regulate the electric field in a 500 kV DC-GIL, whose effects are verified by assigning a temperature gradient, a rough ...electrode, a metal particle and a polarity reversal voltage in the simulation model. The results show impressive behavior of the SNC spacer in suppressing surface charge and electric field distortion. To obtain a better performance, the regulation effect, the loss property and the transient response of the SNC spacer are explored by varying the ohmic surface conductivity and the nonlinear coefficient. The electric field utilization factor of the SNC spacer has a saturation region, but the surface loss increases with increases in the nonlinear parameters. Reducing the time constant of the SNC spacer restrains the electric field over-shoot under voltage polarity reversal. The parameter optimization of the SNC spacer is achieved by drawing a parameter map after fixing relevant indexes.
In order to improve the technology for predicting flashover risks of ice-covered insulators, this article delves into the nonlinear characteristics of leakage current (LC) and the phase changes of ...arcing discharges. The dynamic relationship between them is analyzed quantitatively. The dc ice flashover experiment is conducted in the CIGELE artificial climate room. The dynamic characteristics of arcs during the dc ice flashover process were studied. The chaos of the LC time series is examined by estimating the maximum Lyapunov exponent (<inline-formula> <tex-math notation="LaTeX">{L}_{\text {max}} </tex-math></inline-formula>) using embedded space theory and the phase space reconstruction method. An analysis of the dynamic behavior of LC is conducted using recurrent plot (RP) technology, leading to the development of an index to quantify the regularity of the LC. A quantitative analysis of the cooperative relationship between the LC and arc length, as extracted through image processing, is performed, confirming the regular patterns of LC variation. The findings indicate that variations in the LC and arc length are effective in distinguishing between the three stages of ice flashover. <inline-formula> <tex-math notation="LaTeX">{L}_{\text {max}} </tex-math></inline-formula> exceeds zero at all stages, indicating the chaotic nature of the LC, which is most pronounced during the early phase of the second stage. RP technology provides a clear, dynamic visualization of icing insulator discharge behavior through graphic texture transformation. The nonlinear indexes, RR and DET, of LC, demonstrate an increase alongside the development of arc discharge, indicating a growth of the regular and deterministic components of the discharge. Additionally, there is a synchronous increase in the LC and arc length, with a clear correlation exhibited during the four arc extension periods (T1-T4). As the flashover approaches, the degree of fitting between them increases significantly.
