Xidong Liang

Effective length of counterpoise wire under lightning current

In a high soil resistivity area, counterpoise wires are applied to decrease the grounding resistance of tower grounding devices. If the conductor of counterpoise wire is very long, although the power frequency grounding resistance of the tower grounding device is decreased, the lightning protection performance of the transmission line is still not good. The influences of the length of grounding electrodes on the lightning transient characteristic were analyzed. The dynamic and nonlinear effect of soil ionization around the grounding electrode was considered in the analysis model of transient characteristics for the grounding electrodes under lightning impulse. The counterpoise wire has an effective length when lightning passes through it. When the length of a grounding electrode exceeds the effective length, the grounding conductor will not be utilized effectively. The simulating experiments were performed to analyze influences of the length of the counterpoise wire on the impulse characteristics. The formulae to calculate the impulse effective lengths of counterpoise wires were proposed. The model proposed in the paper has been validated by comparing the numerical results with experimental tests.

Electric field analysis of water drop corona

Water drops on surface of polymer insulation materials may cause corona because they can enhance the electric field nearby. Water drop corona plays an important negative role in the long-term performance of composite insulators. In this paper, based on a model with two parallel electrodes water drop corona is studied. The relationship between initial corona voltage and different sets of water drops is obtained from experiments. Then several factors, which may affect the electric field analysis of water drop corona, are studied by numerical calculation on computer. Calculation is mainly on a 2-D model. However, a 3-D model is also used to study the effect of water drops distributed in space.

Investigation on permeation properties of liquids into HTV silicone rubber materials

In this paper, the permeation properties of three types of liquids into HTV silicone rubber with different Alumina Tri-hydrate (ATH) contents had been investigated by weight gain experiments. The influence of differing exposure conditions on the diffusion into silicone rubber, in particular the effect of solution type, solution concentration, and test temperature were explored. Experimental results indicated that the liquids permeation into silicone rubber obeyed anomalous diffusion ways instead of the Fick diffusion model. Moreover, higher temperature would accelerate the permeation process, and silicone rubber with higher ATH content absorbed more liquids than that with lower ATH content. Furthermore, the material properties of silicone rubber before and after liquid permeation were examined using Fourier infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM), respectively. The permeation mechanisms and process were discussed in depth by combining the weight gain experiment results and the material properties analyses.

Application of non-thermal plasmas on toxic removal of dioxin-contained fly ash

Abstract In this paper, non-thermal nanosecond plasma has been applied to detoxicate dioxin-contained fly ash. The experimental results show that a positive pulse discharge provides a higher destruction effect on the compounds contained than does a negative one. Products after discharge are discussed and analyzed in detail. It was found that different isomer compounds show different toxic removal effects, and the higher the toxicity of the compounds is, the higher is the destruction efficiency. The destruction efficiency ranges from 80% to 20%. For dioxin congeners, the efficiency from high to low can be arranged as an order of T4CDDs, P5CDDs, H6CDDs, H7CDDs and O8CDD and for the furan ones as T4CDFs, P5CDFs, H6CDFs, H7CDFs and O8CDF. Among all of the isomers contained in the fly ash, the 2, 3, 7, 8-T4CDD compound which has the highest toxicity shows the highest destruction efficiency up to 81%. It was also found that ordinarily, with the increase of the concentration of the toxic compounds, the destruction efficiency increases. Based on the content analysis of the ambient air after discharge in an airproof container, the destruction effect on dioxin compounds in the fly ash has been further confirmed. From these results, we are looking forward to finding out methods of higher toxic removal, and the application of non-thermal plasma technology on detoxification of dioxin-contained flue gas or fly ash before they are released into the atmosphere or are buried into the ground is highly expected.

Annealing effect on DC conduction in polyethylene films

Abstract High-density polyethylene (HDPE) specimens with different crystallinity have been prepared by varying annealing conditions. Based on the conventional thermally activated electronic injection process, the dependence of DC conduction on the crystallinity of HDPE has been studied. We calculated that with the increase of the crystallinity from 53% to 68%, values of the metal/dielectric contact potential for samples of PE decrease from 2.75 to 2.24 eV . We presumed that the morphological microstructure of the transcrystals in the electrode–polymer interface is responsible for the difference between the potential energy of an electron at the Fermi level in the electrode and in the conductive state in the polymer bulk.

Failure analysis of decay-like fracture of composite insulator

In recent years, a new type of composite insulator mechanical failure appeared in high voltage transmission lines, which is a type of severe accident for power system, but only few studies on it have been reported all over the world. In this paper, this type of failure was named Decay-like Fracture by summarizing and analyzing the general features, and the decay-like fracture of composite insulator was treated and studied as a new type of composite insulator mechanical failure. By analyzing some field failed composite insulators in China, main features of decay-like fracture were then summarized. The fracture spot of decay-like fractured insulator became crisp and looked like decayed wood. The mechanisms of the normal fracture and brittle fracture of composite insulator cannot explain decay-like fracture, and boron-free Fiber Reinforced Plastic (FRP) core which has been widely used to prevent brittle fracture could not prevent decay-like fracture. Decay-like fracture is not a pure mechanical failure. The discharging and surface micro current on the core surface would finally lead to mechanical fracture of composite insulator. But the process and mechanism of decay-like fracture still need further researches.

Influence of temperature on the hydrophobicity of silicone rubber surfaces [outdoor insulator applications]

The hydrophobicity status of silicone rubber (SIR) is affected by various factors. In order to study the influence of temperature on the hydrophobicity of silicone rubber, three kinds of SIR samples, which are clean surface, contaminated, and corona treated ones, were used. The static contact angle of a water droplet on the surface of the SIR was measured and was used in the evaluation of hydrophobicity. Based on the hydrophobicity, hydrophobicity transfer property and hydrophobicity recovery property, the influence of temperature on the hydrophobicity of silicone rubber was discussed. The experimental results showed that the hydrophobicity of the SIR material decreases with a decrease in temperature, with the transfer and recovery rate of hydrophobicity also decreasing, simultaneously. This paper considered that the change of surface free energy of SIR with temperature is a main reason for the presence of temperature-dependent hydrophobicity and the different hydrophobicity transfer properties are inherently related with the transfer speed of low molecule weight (LMW) components at different temperatures.

Preliminary Recommendations on the Suitable Shed Profile for HVDC Station Insulators with Silicone Rubber Housing

The selection of suitable shed profiles for station insulators with silicone rubber housings is an important subject for the successful operation of the HVDC and UHVDC systems. This subject has been discussed on a Seminar with a group of invited experts on external insulation. The participants recognized the lack of research results in this area. However, considering the urgent engineering needs for the coming UHVDC projects, after a review of the operational experiences of the existing 500-600 kV HVDC stations, especially the stations in China, the participants concluded the Seminar with preliminary recommendations on the decisive parameters of suitable shed profiles and a test method for shed profile selection. These recommendations are considered to be applicable to vertically installed HVDC station insulators with silicone rubber housing and with relative large diameters. At the same time, the participants encourage and invite utilities, manufacturers, and research institutes to be active on this subject and to provide more reliable test results.

Annealing effect on the morphology of polyethylene materials and the tree initiation voltage

This paper presents the effects of different annealing conditions on the tree initiation of polyethylene (PE) films. It is well known that electrical trees are greatly influenced by the morphological structure in semi-crystalline polymers. Samples of PE films, including low-density PE, linear low-density PE and high-density PE have been measured. It was found that after re-crystallizing the crystallinity and the thickness of lamella increase, while the size change of the spherulites is invisible. It was also found that the tree initiation voltage increases after re-crystallizing. Finally we found that the increase of tree initiation voltage can be inherently related with the increase of the thickness of lamellae.

Study of Cracked Unidirectional Glass Fiber-reinforced Composites by Digital Speckle Correlation Method

Deformation and fracture behavior of glass fiber-reinforced composites with single-edge crack under tensile loading is studied by means of the traditional stress measurement method combined with digital speckle correlation method. After capturing the deformation image of the crack tip in different loading processes, the horizontal and vertical displacement fields are obtained. At the same time, using a three-element strain gauge, the strain responses near the crack tip during the whole damage and fracture process of the tensile specimen are recorded. The width of the local deformation region at the crack tip of the glass fiber-reinforced composite materials is determined. Then the analyses of deformation and fracture in a unidirectional fiberreinforced composite are performed. These results play an important role in understanding the deformation and fracture mechanism of the unidirectional glass fiber-reinforced composite materials.