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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 work, nano-titanium oxide (TiO2) is encapsulated into boron nitride nanosheets (BNNS) by the calcination process to fabricate core-shell structured TiO2 @BNNS nanofillers. The effects of TiO2 @BNNS fillers on the DC conductivity, space charge and breakdown strength of polypropylene (PP) composites are investigated. Experimental results show that PP/TiO2 @BNNS composites have a lower temperature-dependent conductivity with an activation energy of 0.64 eV, which is much lower than that of PP, 0.95 eV. The DC breakdown strength of PP/TiO 2 @BNNS samples is 22.2% higher than that of PP at 90 °C. The average space charge density and electric field distortion rate of PP/TiO 2 @BNNS are reduced by 6.6 C/m3 and 23.3%, respectively, compared with PP, under a polarization electric field of 40 kV/mm at 90 °C. It is proven that TiO2 @BNNS nanofillers form numbers of electrical potential wells for charge carriers, thereby, inhibiting the hot electron transport and their damage to polymer molecules. This work provides an effective method to improve the dielectric performance of PP cable insulation. [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]

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]

Nonlinear conductivity composites can be applied to modify the local electric field distribution and accelerate the surface and space charge dissipation in HVDC cables. This paper focuses on the effect of the nonlinear conductivity on the charge transportation under impulse superimposed on DC voltage. The surface-charge-accumulation property of silicone rubber (SiR)/SiC composites under impulse superimposed on DC voltage is studied. The surface potential decay (SPD) is measured after the sample is stressed by impulse superimposed on DC voltage. The effect of nonlinear conductivity on carrier mobility is analyzed. The results show that under DC voltage, the decay rate increases for the rise in the initial surface potential (ISP). Higher surface potential caused by higher impulse voltage leads to a larger nonlinear conductivity, which immediately dissipates the surface charge. The increase in DC voltage is shown to be more effective in accumulating surface charge than impulse. The vertical movement to the ground and the horizontal movement of surface charges are two factors affecting the ISP and SPD, which are both affected by the nonlinear conductivity of SiR/SiC composites. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The addition of silicon carbide (SiC) micro powder into silicone rubber (SiR) matrix has been found to introduce a marked nonlinear conductivity into the resulting composites, without a degradation of the insulating properties under low electric field. Owing to these properties, the SiR-based nonlinear conductive composites have a potential use as the field grading material (FGM) layer in high voltage direct current (HVDC) cable accessories, which is employed to uniform electric field and suppress charge accumulation at the interface between crosslinked polyethylene (XLPE) and accessory insulation. In this paper, the space charge transportation and dissipation behaviors of SiC/SiR composites were investigated by pulsed electro-acoustic (PEA) test, in view of the effects of SiC filler size and crystal morphology. The results indicated that with the enhancement of nonlinear conductivity, the charge transportation and dissipation processes were accelerated. Compared with ?-SiC particles, the ?-SiC whiskers could provide a larger nonlinear conductivity. Meanwhile, the trap level distributions of SiC/SiR composites showed that the ?-SiC whiskers introduced a shallower trap level at 0.81 eV than the ?-SiC particles (0.85 eV). In addition, for the ?-SiC particles filled composites, with the increase of the filler size from 0.45 ?m to 5.0 ?m, the shallow trap density increased, and the deep trap density decreased, resulting in an enhancement of space charge dissipation. [ABSTRACT FROM AUTHOR]

High temperature vulcanized (HTV) silicone rubber (SiR), due to its excellent electrical insulation, elasticity and temperature resistance, is used in cable accessories for HVDC cables. The space charge, generated under dc stress in SiR, would distort the local electric field, further influencing the characteristics of partial discharge and accelerating the insulation aging. This paper tried to modify the space charge behaviors of HTV SiR composites by incorporating silicon carbide (SiC) particles into the polymer matrix with the filler content of 10, 30, 50, 100 wt%. The effects of the SiC particle content on the field-dependent conductivity and the space charge behaviors of HTV SiR were studied. The conductivities under different electric fields were measured by a three-electrode system at room temperature and the space charge behaviors were observed by the pulsed electro-acoustic (PEA) method. The results indicated that the conductivity of SiR/SiC composites was a nonlinear function of electric field when the content exceeds 30 wt%. With the increase of filler content, the threshold field for nonlinear conductivity decreased and the nonlinear coefficient increased. Meanwhile, the amount of space charges was dramatically reduced. It is suggested that the SiR/SiC composites with nonlinear conductivity can improve the dc conductivity under high electric field, thus raising the carrier mobility and suppressing the accumulation of space charge. [ABSTRACT FROM PUBLISHER]

HTV silicone rubber (SiR), due to its better electrical insulation, elasticity, temperature resistance, is used in cable accessories for HVDC cables. The space charge, formed under dc stress in SiR, would distort the local electric field, and furtherly influence the characteristics of partial discharge and accelerate the insulation aging. Direct fluorination, as a solution to change the chemical component, could alter the electrical properties of the polymer surface layer. This paper focuses on the effect of the fluorination on the space charge accumulation in HTV SiR. In this investigation, the samples were fluorinated at 25 ° using a F2/N2 mixture gas with 20 % F2 by volume at 0.05 MPa for 10, 20, 30 and 40 minutes. The space charge behavior of the fluorinated samples was observed by PEA method and the trap level distribution was obtained by modified isothermal discharge current (MIDC) test. The results indicated that the fluorination for 20 minutes effectively suppressed the space charge injection in SiR. However, excessive fluorination (over 30 minutes) brought in abundant deep traps, and led to significant amount of space charge accumulation in the bulk of polymer. As a consequence, the appropriate fluorination has significant effect on the space charge accumulation in HTV SiR and has a potential application for inner insulation in premolded accessories of HVDC cable. [ABSTRACT FROM AUTHOR]