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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]

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]

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]

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]

Interface charges are easy to accumulate between two different dielectrics with various characteristics, which may cause accelerated degradation of insulation systems. Ethylene-propylene-diene terpolymer (EPDM) is used mainly for HVDC cable joint, which is the most vulnerable part of the cable system because of the interface. Particles with nonlinear conductivity can be doped into the polymer matrix to modify the interface charge behaviors through altering the conductivity under combined stresses. In this paper, silicon carbide (SiC) particles were dispersed into EPDM with 0, 10, 30 and 50 wt% respectively. The space charge behaviors at the interface between LDPE and EPDM filled with SiC particles was measured under 15 and 30 kV/mm. Besides, dielectric constant, dc conduction and trap distribution were introduced to elaborate the suppression mechanism with SiC doping. The SEM results show that the particles are well distributed in the EPDM. The permittivity increases with the fillgrade and the dc conductivity shows an obvious nonlinear trend under various electrical fields. SiC doping can effectively suppress the interface charge accumulation under different stresses. The suppression mechanism is attributed to the nonlinear conductivity and more shallow traps of EPDM/SiC composite. As a consequence, the approximate SiC doped EPDM can availably suppress the interface charge accumulation and offers a possible method for the improvement of cable accessory performance. [ABSTRACT FROM PUBLISHER]

High temperature vulcanized (HTV) silicone rubber (SiR) is widely used as HVDC cable accessory insulation considering its outstanding electrical properties along with elasticity and thermal resistance. However surface charge caused by corona would accumulate on SiR surface and distort the local field which in consequence led to accessory premature discharge and insulation failure. This paper focused on the behavior of surface charge of SiR composites and managed to modify it by incorporating silicon carbide (SiC) particles into SiR matrix. The content of SiC varied from 0 to 100 % and its effects on the field-dependent conductivity and surface charge behaviors were discussed. The outcomes of experiments showed that the SiR/SiC composites performed a nonlinear conductivity as a function of electric field when the content of SiC exceeded 30 %. It was also noticed that this field-dependent conductivity became more obvious when the content of SiC increased. Meanwhile, the decay rate of surface charge was observed to rise with the increase of SiC content and voltage. It was confirmed that the SiR/SiC composites could accelerate the decay of surface charge and increase the dc conductivity under high voltage, resulting from the modification of the trap level distribution. [ABSTRACT FROM PUBLISHER]

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]