Search

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]

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]

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]