Z. L. Li

Surface charge accumulation and decay of direct-fluorinated RTV silicone rubber

Silicone rubber (SiR), as a basic insulating material, is widely used in insulators for HVDC transmission lines. In these lines, corona discharge even occurs on well-designed hardware and insulators, which can inject charge into the insulator surface. The existence of surface charge has a marked effect on insulation degradation and plays an important role during the development of the surface flashover. Direct-fluorination is a method to change the chemical component, which could induce the corresponding changes in electrical properties of the surface layer, thus influencing the charge injection. This paper tries to study the effect of fluorination time on surface charge accumulation and decay of room temperature vulcanized (RTV) SiR. The samples were fluorinated in a laboratory vessel at 298 K (25°C) using a F2/N2 mixture with 12% F2 by volume at 0.05 MPa (500 mbar) for 5, 10, 15, and 20 minutes. The sample surface chemical composition was analyzed by means of ATR-IR. The behavior of surface potential under dc voltage was recorded at room temperature with a relative humidity of ~40%. Obtained results show a dependence of the surface potential and the dissipation time upon the fluorination time, varying with the charging time. It is suggested that the direct fluorination can significantly suppress the accumulation and accelerate the decay of the surface charge.

Surface charge and dc flashover characteristics of direct-fluorinated SiR/SiO 2 nanocomposites

Silicone rubber (SiR), as a basic insulating material, is widely used in insulators for HVDC transmission lines. Incorporation of nanoparticles could not only improve the insulating properties, but also alter surface charge behavior as a consequence. In HVDC transmission lines, corona discharge could even occur on well-designed insulators, which can inject charge into the insulator surface and influence dc flashover voltage. Direct fluorination is a method to change the surface chemical component of polymers, which could induce the corresponding changes in electrical properties of the surface layer, thus influencing surface charge behavior and dc flashover voltage. This paper tries to study the effects of fluorination time and mass fraction of nanoparticles on surface charge behavior and dc flashover voltage of SiR/SiO2 nanocomposites. Samples were prepared by dispersing nano-SiO2 into SiR matrix with the fraction of 0, 1, 3 and 5 wt% respectively. Then the direct fluorination of the samples were conducted in a stainless reaction vessel at 25 °C using a F2/N2 mixture gas with 12% F2 by volume at 0.05 MPa for 5, 10, 15 and 20 minutes. After corona discharge tests performed at room temperature with a relative humidity of ~ 40%, the decay curves of the surface potential were measured using an electrostatic voltmeter, and flashover tests were carried out under dc stress, of which the polarity was identical with the corona discharge. Obtained results show that the dc flashover voltage is affected by the surface charge, which varies with the fluorination time and mass fraction of nanoparticles. It is indicated that the direct fluorination can markedly improve the decay rate of surface charge, and raise the dc flashover voltage of SiR/SiO2 nanocomposites.

Hydrophobicity, surface charge and DC flashover characteristics of direct-fluorinated RTV silicone rubber

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.

Field-dependent conductivity and space charge behavior of silicone rubber/SiC composites

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.

Tracking and erosion resistance of silicone rubber/BN nanocomposites under DC voltage

Silicon rubber is widely employed as insulating materials in transmission lines due to its excellent electrical and mechanical properties. The discharges that happen inevitably during the operation of the composite insulators release too much heat that can cause electrical erosion on the surface of silicon rubber (SiR) insulators. It is known that the aim of improving the resistance to tracking and erosion of silicon rubber can be achieved by increasing its thermal conduction ability. In this study, samples were prepared by dispersing nano-BN particles into silicone rubber at concentrations of 0%, 2.5%, 5% by weight. According to IEC 60587, Inclined plane (IP) tracking and erosion tests were conducted to compare the phenomena occurring during the tests. The experimental results suggest that with the increase in concentration of nano-BN fillers from 0 wt% to 5 wt%, the thermal properties are improved. In addition, the erosion depth and the weight loss show a decreasing trend which proves that the resistance of silicon rubber to tracking and erosion is improved.

Trap Modulated Charge Carrier Transport in Polyethylene/Graphene Nanocomposites

The role of trap characteristics in modulating charge transport properties is attracting much attentions in electrical and electronic engineering, which has an important effect on the electrical properties of dielectrics. This paper focuses on the electrical properties of Low-density Polyethylene (LDPE)/graphene nanocomposites (NCs), as well as the corresponding trap level characteristics. The dc conductivity, breakdown strength and space charge behaviors of NCs with the filler content of 0 wt%, 0.005 wt%, 0.01 wt%, 0.1 wt% and 0.5 wt% are studied, and their trap level distributions are characterized by isothermal discharge current (IDC) tests. The experimental results show that the 0.005 wt% LDPE/graphene NCs have a lower dc conductivity, a higher breakdown strength and a much smaller amount of space charge accumulation than the neat LDPE. It is indicated that the graphene addition with a filler content of 0.005 wt% introduces large quantities of deep carrier traps that reduce charge carrier mobility and result in the homocharge accumulation near the electrodes. The deep trap modulated charge carrier transport attributes to reduce the dc conductivity, suppress the injection of space charges into polymer bulks and enhance the breakdown strength, which is of great significance in improving electrical properties of polymer dielectrics.

Effects of direct fluorination on space charge accumulation in HTV silicone rubber

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.

Nonlinear conductivity and interface charge behaviors between LDPE and EPDM/SiC composite for HVDC cable accessory

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.

Effects of nonlinear conductivity on interface charge in HVDC cable accessories

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 kV/mm. 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. The suppression mechanism is attributed to the nonlinear conductivity. As a consequence, the approximate SiC doped EPDM can availably suppress the interface charge accumulation and offer a possible method for the improvement of cable accessory performance.

Surface charge behaviors of SiR/SiC composites with nonlinear conductivity

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 tried to modify the surface charge behaviors of HTV SiR composites by incorporating silicon carbide (SiC) particles with different filler size and crystal morphology The influences of SiC filler size and crystal morphology on the dc conductivity and surface charge behavior of HTV SiR composites were studied. The results indicated that, for the α-SiC particles filled composites, with the increase of the filler size from 0.45 μm to 5.0 μm, the fleld-dependent conductivity and surface potential decay (SPD) is enhanced, due to an increase of shallow trap density and a decrease of deep trap density. Compared with α-SiC particles, the β-SiC whiskers could provide a larger nonlinear conductivity, resulting in an acceleration of surface charge dissipation.