Isothermal Surface Potential Decay Method Research Articles

Effect of polarity‐reversal voltage on charge accumulation and carrier mobility in silicone rubber/silicon carbide composites

Polarity-reversal voltage in high-voltage direct current transmission system would cause serious electric field distortion, accelerating charge injection. The effect of polarity reversal on charge transport characteristics in silicone rubber (SiR)/silicon carbide (SiC) composites with nonlinear conductivity at different temperatures is studied. The charge transport characteristics of silicone rubber/silicon carbide composites under unipolarity voltage and polarity-reversal voltage at 30°C, 70°C and 90°C are obtained by the isothermal surface potential decay method, and the carrier mobility and trap characteristics of the composites are further calculated. Studies have shown that a large amount of hetero-charge remains in silicone rubber materials under polarity-reversal voltage, which in turn leads to intensive charge neutralisation and severe electric field distortion. The silicon carbide doped composites exhibit excellent performance in suppressing the accumulation of residual charges and weakening the charge neutralisation under polarity-reversal voltage. In addition, high temperature enhances the conductivity of silicone rubber/silicon carbide composites and reduces the accumulation of residual charges, which restrains the distortion of the local electric field under polarity-reversal voltage. This research shows that the application of non-linear conductivity material is an effective measure to solve the problem of cable insulation under polarity-reversal voltage.

The Evolution of Trapping Parameters on Three-Layer Oil-Paper of Partial Discharge Degradation for On-Board Traction Transformers

Trap plays an important role in the charge transportation of three-layer oil-paper and the investigation of trap parameters at different degradation degree is of great significance for further research of the partial discharge (PD) mechanism. This article considers the turn-to-turn structure of on-board traction transformer. A defect model of three-layer oil-paper insulation was designed for PD test. The PD degradation samples are selected from the inception of PD to the breakdown of PD to study the characteristics of surface potential decay on degradation samples by using isothermal surface potential decay method. Given that, the trap distribution at different degradation stages was calculated and its influence on PD characteristics was investigated. The result shows that AC fluctuation voltages caused by inter-harmonic has a great influence on PD distribution, surface charge distribution and trap distribution. With the increase of degradation, the decay rate of surface potential increases and the decay rate of AC fluctuation voltages is larger than that pure AC. And also, the number of shallow trap with AC fluctuation voltages is larger than that with pure AC. The rate of free charge increases with the number of shallow trap increasing. More free electrons involves in the process of PD, accelerating the aging rate of three-layer oil paper samples. Therefore, it can be concluded that surface charge and trap distribution are helpful to clarify PD mechanism.

Plasma fluorination of BaTiO3 for enhancement of interfacial adhesion and surface insulation of epoxy resin

Composite dielectrics with excellent electrical properties are integral to today’s energy industries and power infrastructure. Epoxy resin filled with inorganic nano-fillers has been extensively studied for its excellent performances. In this paper, barium titanate (BaTiO3) nanoparticles are selected as the fillers and fluorinated by plasma treatment. The morphology, element content and chemical bond of BaTiO3 and fluorinated BaTiO3 fillers are characterized by SEM, EDS, XPS and FTIR, respectively. The influence of filler concentration and fluorination on flashover voltage of epoxy resin composites was studied. We found that the fluorinated BTO could improve the compatibility of filler and matrix and surface insulation of epoxy resin composites. Finally, the relationship between filler concentration, dielectric constant, fluorination treatment and charge dissipation was analyzed, and the trap distribution on the surface of composites was calculated by isothermal surface potential decay method. Fluorinated BaTiO3 particles as fillers blended with epoxy resin have great engineering application potential.

The energy distribution of trapped charges on the surface of cross-linked polyethylene thermally aged at different temperatures

In this paper, the energy distribution of trapped charges on the surface of cross-linked polyethylene (XLPE) thermally aged at 100 °C and 160 °C were studied by the isothermal surface potential decay method. It was found that the surface traps of unaged XLPE were mainly deep ones of both electron-type and hole-type. In the process of thermal aging at 100 °C, the surface traps of XLPE were still mainly deep traps. As the aging time increased, the density of shallow traps increased, and the density of deep traps didn’t change much. In the process of thermal aging at 160 °C, the density of shallow traps of both electron-type and hole-type ones increased greatly, and the density of deep traps of both electron-type and hole-type ones decreased sharply. At the same time, the energy levels of hole-type traps decreased. Under the thermal stress, the increase in the density of shallow traps may be due to an increase in aging products such as carbonyl (C=O); the decrease in the density of deep traps should be due to a decrease in chemical defects in the crystalline regions because of the destruction of spherulites. Differences of the energy distribution of trapped charges on the surface of XLPE aged at two different temperatures may be caused by the differences in the way that crystal structures deteriorate.

Aging characteristics of polymeric materials by repeated surface flashovers in vacuum under microsecond pulse

Surface flashover characteristics of insulation materials in vacuum are key factors to determine the behavior of high voltage electrical equipment. Although the surface flashover in vacuum has been investigated for decades, the surface insulation properties of polymeric materials after repeated surface discharges have shown little attention by researchers. In order to investigate the aging characteristics of polymeric materials caused by repeated flashovers in vacuum, several typical materials were used to flashover by microsecond pulses. The flashover voltages were obtained for polytetrafluoroethylene, polymethylmethacrylate and polyamide for thousands of flashovers. The surface morphology of samples was observed after different numbers of flashovers. The trap parameter distribution was obtained by the isothermal surface potential decay method. To analyze the flashover voltage change with discharge times, the Weibull distribution was used to determine the conditioning and degradation effects in repeated discharge experiments. The results show that the morphology near the cathode triple junction and deep traps together affect the subsequent flashover voltage. The conditioning and degradation phenomena often coexist in the aging process of polymers and affect the surface insulation properties. This study could provide a reference for the type selection, design, and modification of insulation materials in vacuum.

Characteristics of Trap Level Distribution in Graphene Oxide/Low Density Polyethylene Nanocomposites Based on the Isothermal Surface Potential Decay Method

Characteristics of Trap Level Distribution in Graphene Oxide/Low Density Polyethylene Nanocomposites Based on the Isothermal Surface Potential Decay Method

Investigation of charge trapping and detrapping dynamics in LDPE, HDPE and XLPE

In this paper, charge trapping and detrapping dynamics of PE-based materials, i.e., LDPE, HDPE, XLPE were investigated by isothermal surface potential decay method (ISPD). The whole charge transport process including ions generation process in the air, charge trapping in the top surface, the surface potential establishing process, the charge migration process in the bulk was considered in these PE-based materials. The results showed that the trapping process was closely related to the capture cross section, the injected carrier concentration and the trap level. The established surface potential was mainly determined by the captured carriers in the top surface of the PE-based materials. In addition, it was found that the mobility of LDPE was much larger than that of XLPE and HDPE. And it is shown that deep traps may play a more important role in the charge migration process than shallow traps in these PE-based materials. It is indicated that the discrepancy of charge trapping and detrapping dynamics between these PE-based materials may be attributed to the different microscopic morphology.

Characterizing traps distribution in LDPE and HDPE through isothermal surface potential decay method

The energy distribution of trapped charges and traps in low density polyethylene (LDPE) and high density polyethylene (HDPE) were investigated based on isothermal surface potential decay (ISPD) method in this paper. Corona charging measurement was conducted by a non-contact setup, which was specially designed to obtain the decay characteristics of thin polymer films with varied charging polarity implying the dynamics of electrons or holes. Considering space charge profile of polymers under high dc electric field and detrapping mechanism after removing the field, a model based on ISPD was proposed to obtain the trap distribution of polymers. By assuming different charge spatial distribution features, two kinds of trap energy distributions of LDPE and HDPE were achieved respectively, which reveal similar distribution characteristics. It is shown that there are mainly deep traps in LDPE and HDPE both for electron-type and hole-type traps. The mean trap depth of hole-type traps is lower than that of electron-type ones in LDPE, whereas the situation in HDPE is reversed. Additionally, more trapped charges are accumulated in HDPE with deeper trap depth than that of LDPE. It is indicated that the obtained results may imply essentially nature of hole/electron traps, i.e., the electron-type traps show an inter-chain character whereas the character of hole-type traps is intra-chain. Furthermore, the energy distribution of traps is further related to the unique aggregation structure of LDPE and HDPE.

Characteristics of trapped charge distribution in LDPE and XLPE used in DC cables based on isothermal surface potential decay method

Characteristics of trapped charge distribution in LDPE and XLPE used in DC cables based on isothermal surface potential decay method