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In this paper, the electrical tree growth characteristics of silicone rubber (SiR) are studied under impulse superimposed on DC voltage and a temperature gradient (TG) field. The temperature difference between high voltage (HV) and ground (GD) electrodes varies from -90 to 90 °C resulting in a TG field in the samples. The electrical tree morphology, length and accumulated damage are discussed. Experimental results show that with the increase of absolute value of TG, the electrical tree length and accumulated damage continue to increase, and the tree morphology turns from branch-like tree to bush-branch hybrid tree. The positive TG has a more obvious acceleration effect on the tree growth than does the negative TG. Under the TG, the temperature dependent carrier transport results in space charge accumulation and electric field distortion at low temperature side, which is the main reason for the different growth characteristics of electrical tree. Besides, the accumulated damage of DC superimposed with heteropolar impulse voltage is larger than that under DC superimposed with homopolar impulse voltage. It is concluded that the temperature gradient and impulse superimposed on DC voltage promote the electrical tree growth of HVDC cable accessories insulation. [ABSTRACT FROM AUTHOR]

In this paper, graphene/silicone rubber (SiR) nanocomposites with filler contents of 0.001–0.005 wt% are prepared to improve electrical tree inhibiting ability under different temperature gradients. The electrical treeing test is performed under three temperature gradients by controlling the temperature of high voltage and ground sides. Experimental results indicate that the SiR sample with 0.003 wt% graphene presents the best performance on electrical tree resistance. The tree length and accumulated damage are both reduced with the increase of graphene content from 0 to 0.003 wt%, but increased with the further increase of filler content from 0.003 to 0.005 wt%. The tree outline tends to be circular in neat SiR under negative temperature gradient, while it is triangular under positive temperature gradient. However, the effect of temperature gradient is weakened when graphene exists. Based on quantum dot models, the role of graphene quantum wells (QWs) in the regulation of charge transport and electrical tree resistance in SiR is revealed. This research provides an effective method to suppress electrical trees of SiR for cable accessories. [ABSTRACT FROM AUTHOR]

This paper focuses on the effects of hindered phenolic antioxidants AO300 and AO736 with similar molecular structure on the electrical properties of isotactic polypropylene (iPP). Experiments of conductivity, space charge distribution, and DC breakdown strength after the hetero-polarity DC prestress are performed. The experimental results show that these two antioxidants can increase the conductivity-temperature coefficient, reduce the accumulation of space charge and improve the DC breakdown strength after the hetero-polarity DC prestress of iPP. By calculating the molecular orbitals of these two antioxidants, it is found that the antioxidants introduce local states in the forbidden band of iPP matrix to capture high-energy charges. Besides, the bond dissociation energy (BDE) of the hydrogen-oxygen bond in AO736 is lower than that in AO300, representing a stronger ability of proton transfer, which scavenges the radicals, inhibits the growth of microvoids and prevents the process of breakdown. It is concluded that the improvement of space charge and breakdown properties is attributed to the coupling effects of deep trap sites and radicals scavenging introduced by antioxidant. This research shows that AO736 has potential applications in PP insulation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Field grading materials (FGMs) with nonlinear conductivity, which can form a uniform local field in high-voltage direct current (HVDC) cables, have received considerable attention. This paper focuses on the temperature-dependent charge transport property of an FGM based on high-temperature vulcanized (HTV) silicone rubber (SiR). A nonlinearity in the conductivity is achieved by mixing silicon carbide (SiC) particles with the SiR matrix. The conductivities of the samples are measured at temperatures of 30–90 °C. In addition, the surface potential decay is analyzed. The experimental results show that the nonlinear conductivity is suppressed at high temperatures. Increased temperature can cause a lattice vibration, which scatters carriers and reduces the conductivity. With an increase in the homopolar impulse voltage, all samples experiences a rise in the initial surface potential. The increase in SiC content and the increase in impulse voltage can both accelerate the decay process. The heteropolar impulse voltage not only intensifies the accumulation of surface charge but also slows down the decay process. Neutralization occurs immediately between heterocharges. The positive charges are captured by traps, and the attraction between heterocharges is thought to be the reason for the observed phenomena. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

An electrical field distorted by the complicated cable accessory structure and non-uniform temperature distribution is a significant threat to high voltage direct current (HVDC) cable. Thus, the field grading material (FGM) with nonlinear conductivity which can uniform local field receives attention. This paper focuses on the temperature-dependent nonlinear conductivity of the FGM based on high temperature vulcanized (HTV) silicone rubber (SiR). The nonlinearity was achieved by mixing silicon carbide (SiC) particles with the SiR matrix. The conductivities of the samples were measured at temperatures of 30–90 °C. In addition, the carrier mobility and trap distribution of different samples at different temperatures were obtained by analyzing the surface potential decay (SPD). The experiment results showed that the SiR/SiC composites had nonlinear conductivity when the SiC content exceeded 30 phr in the entire temperature interval. The conductivity increased, and the threshold field of conductivity decreased at high temperature. In the SiC mixed samples, a convergence trend appeared at high field at temperatures below 70 °C. The trap distribution moved to a higher energy level, and a higher shallow trap proportion appeared at high temperature. The mobility increased with the increase in temperature. This trend became more obvious at high temperature. In addition, compared with voltage, the temperature had a more remarkable effect on the carrier mobility. [ABSTRACT FROM AUTHOR]

This paper proposes a method to retrieve the land surface bidirectional reflectivity ρb in the mid-infrared (MIR) from MODIS channels 22 and 23. A split-window-like algorithm was developed to determine the MIR ground brightness temperature [image omitted] without the contribution of the solar direct beam from ground brightness temperatures measured at two adjacent MIR channels. Comparing the actual values of ρb with those estimated using our proposed method, the root mean square error (RMSE) was determined to be 0.0022 for the solar zenith angle (SZA) at nadir. Similar results were obtained for other SZAs, indicating that the proposed method can retrieve ρb accurately. A detailed sensitivity analysis found that the effects of instrumental noise, variations in water vapour content in the atmosphere, and reasonable variations in horizontal visibility on the retrieval of ρb were negligible and an error of 1 K on the ground brightness temperature caused by the inaccuracy of atmospheric corrections can lead to a maximum error of 0.034 on the retrieved ρb. We compared the use of radiosonde data and European Centre for Median-range Weather Forecast (ECMWF) atmospheric data to perform atmospheric corrections. The results show that atmospheric corrections can be performed with ECMWF data instead of radiosonde data without losing the accuracy of the retrieved ρb. MODIS land surface temperature/emissivity product MOD11B1 data were also used to validate [image omitted] . Comparison of [image omitted] estimated respectively with the proposed method and with MOD11B1 data showed that the RMSE is less than 1 K for cloud-free skies. [ABSTRACT FROM AUTHOR]

This paper presents the procedure and results of the radiance-based validation approach for the Moderate Resolution Imaging Spectroradiometer (MODIS) Land-Surface Temperature (LST) product. Surface emissivity spectra were retrieved by a sun-shadow method from surface-leaving radiance spectra measured with a thermal infrared (TIR) spectroradiometer in the 3.5-14 µm spectral region under sunshine and sun-shadow conditions. By using the measured surface emissivity spectrum and atmospheric profiles obtained by radiosonde balloons, and the LST values at validation sites in the V5 MODIS level-2 LST products, radiative transfer simulations were made with the MODTRAN4 code to calculate the top-of-atmosphere (TOA) radiance values in MODIS band 31 (L31). By adjusting the LST input values in the simulations to match the calculated L31 values to the MODIS measured radiance (MOD L31) values, MOD L31 inverted LSTs can be obtained. The MODIS LST product was validated by comparison to the values of the MOD L31 inverted LSTs. This approach compares well with the conventional temperature-based approach. The results of the radiance-based validation indicate that the accuracy of the MODIS LST product is better than 1 K in most cases, including lake, vegetation and soil sites in clear-sky conditions. The errors in the split-window retrieved LSTs may be larger in bare soil sites and highly heterogeneous sites due to large uncertainties in surface emissivities. The results of the radiance-based validation also reveal the weakness of the split-window algorithm used for the generation of the MODIS LST product in two situations: one in cases where LSTs are larger than the air temperature at the surface level (T s-air) by more than 16 K and the columnar water vapour (cwv) is larger than 1.5 cm, and another in cases under the influence of thin cirrus clouds or heavy aerosol loadings. These two situations were not considered in the development of the current MODIS LST algorithm. [ABSTRACT FROM AUTHOR]

Three definitions of emissivity for non-isothermal pixels are evaluated, and their differences are analysed. On the basis of the experimental data and numerical simulation, a generalized formula is established to scale for land surface emissivity, reflectance, and other variables between pixel scale and sub-pixel scale. Finally, it is pointed out that isothermal concept of land surface temperature and homogeneous concept of some other parameters in one spatial scale are relative according to humans' capability of measurement, and that the heterogeneity in a spatial scale is absolute in the natural world. [ABSTRACT FROM AUTHOR]

Directional thermal observations from the ATSR-2 satellite sensor were used to estimate separate vegetation and soil temperatures for a number of cloud free scenes covering south-east Spain over five days in 1999. Underlying methodology for this is a simplified radiative transfer scheme and a concurrent estimate of the fraction of ground covered with vegetation. The vegetation and soil temperatures were used together with near-surface relative humidity measurements to adjust the root zone soil moisture content and roughness length for heat in a newly developed multi-component land surface parameterization scheme, embedded in a regional weather forecast model. The ATSR surface temperature data have a strong influence on the modification of the thermal roughness length. The optimal roughness length gradually changes over the growing season, as can be expected from the dependence of thermal roughness on vegetation density. Application of the method to a grassland scene in The Netherlands resulted in a much smaller adjustment to the thermal roughness length. The distribution of the roughness over the Spanish test area appeared to be consistent in time, as correlation coefficients of roughness values between two subsequent acquisition dates were significantly positive. Small improvements in the calculated surface energy balance appear from independent near-surface air temperature observations in the Spanish area. The use of bi-angular thermal infrared observations seems useful to improve the description of aerodynamic roughness properties on regional scales. [ABSTRACT FROM AUTHOR]