Shaohua Li

Measurement of dynamic potential distribution during the propagation of a local arc along a polluted surface

A new method for measuring the dynamic potential distribution during the propagation of a local arc along a polluted surface is described. In order to study the role of electric stress during the pollution flashover processes, a flat polluted glass plate model was designed. The measuring electrodes made with a strip of aluminum foil were stuck on the bottom of the plate, opposite to the side on which the local arc propagated. The capacitance between the polluted surface and the measuring probe, which is in series with an additional capacitor, was used to construct a potential divider. By means of the dividers and a magnetic tape data recorder, the dynamic potential distribution, the position of local arc, and its propagation velocity were measured. The results show that the velocity of arc propagation along a polluted surface is 0.2 to 1 m/s and that the maximum average potential gradient in the pollution layer is approximately=0.5 kV/cm. >

Effect of alumina tri-hydrate surface modification on liquid permeation and electrical performance of silicone rubber

This paper investigates the liquid permeation performance of silicone rubber containing alumina tri-hydrate (ATH) that has been modified with N-[3-(trimethoxysilyl) propyl] aniline (Y-9669). From the change in weight during immersion in water, saline solution or nitric acid, and during subsequent drying in air as well as a second immersion test, it is revealed that the majority of the absorbed liquid exists in filler and interface for silicone rubber with original ATH. Surface modification of ATH filler can protect silicone rubber from liquid permeation. It is easier for liquid to permeate into silicone rubber than to escape from it due to the hydrophobicity of the silicone rubber matrix. A cycle of immersion and drying is found to have little impact on subsequent immersion in the case of water and saline solutions; however, damage inflicted on the matrix and filler by nitric acid means that the rate of permeation increases during subsequent immersion. The volume resistivity and surface resistivity of samples with unmodified ATH also decrease more appreciably than those with modified ATH after a cycle of immersion and air drying due to the amount of the residual absorbed liquid.

A modified 5000 h test procedure for silicone rubber insulator based on contamination and hydrophobicity change simulation

A large number of silicone rubber (SR) insulators have been used in inland areas. Long-term performance evaluation, pre-ranking and selection of these insulators are highly significant. IEC and other institutes have proposed several 5000 h tests with multiple stresses to evaluate long-term performances of composite insulators. However, these procedures have their own characteristics and limitations. None of the existing tests could provide a satisfactory simulation of the condition with severe contamination and high non-soluble deposit density (NSDD) in inland areas as well as the hydrophobicity change process of SR insulators. Based on modifications of the IEC test procedure, a novel 5000 h test procedure is proposed. There are two main modifications. The first one aims to replace the salt fog with a mixed contamination fog including both soluble and non-soluble pollutants. The second one aims to provide sufficient time for SR to recover and to transfer its hydrophobicity. A set of test apparatus based on this novel 5000h test procedure was designed and constructed. Two tests were conducted on 16 SR insulators with different material compositions and shed designs, including an AC and a negative DC test. Contamination accumulation on the surfaces of the specimens during both tests were measured. The NSDD and equivalent salt deposit density (ESDD) were close to those measured in the field in contaminated inland areas. Hydrophobicity reduction and recovery were well reproduced. Discharge activities were observed and leakage current was measured, which were consistent with the hydrophobicity statuses of the specimen. Tracking and erosion were found for some specimens, and the damage pattern was the same with that found in the field. The poor material properties and inappropriate shed designs could be identified successfully through the test. It was concluded that the modified 5000 h procedure could provide a satisfactory simulation of the desired contamination condition and hydrophobicity change process, and the insulators with different design defects are successfully screened out.

Investigations on the basic electrical properties of Polyurethane foam material

The Polyurethane (PU) foam materials have increasingly been used as a new kind of solid insulation material, which are always inserted into post hollow composite insulators as their inner insulation section. PU foam with PU as a base material which is homogenously filled with pores, PU foam material is a possible substitute material of the air, SF6 and N2. In order to achieve a better electrical performance, the electrical properties of PU foam should firstly been studied and understood in depth. In this paper, the electrical strength, volume resistivity, permittivity and partial discharge property of PU foam have been investigated. It is the porous structure characteristic with low density that distinguishes itself from other dielectric materials. Because of the pores distributed in the PU material, the partial discharge (PD) characteristics have been put more emphasis. Four kinds of PU foam materials with different pore size and pore geometry and one kind of PU elastomer have been used in our research. The micro-pore structure of PU foam was characterized by scanning electron microscopy (SEM) method. For PD property, we mainly focus on the PD inception voltage, PD extinction voltage, discharge magnitude with applied voltage as well as its relationship with physical structure of pores. Experimental results indicated that the electrical properties of PU materials have a closely relationship with pore structure, especially for the electrical strength and PD characteristics. The electrical properties with stress on the PD behavior of PU foam were discussed in depth by combing the electrical test result and the physical property of PU foam.

Aging and recovery of superhydrophobic silicone rubber under electrical and non-electrical stresses

Silicone rubber is widely used in the outdoor insulation for its hydrophobicity, at the same time, superhydrophobic surface with self-clean property is more ideal for outdoor insulating material. This paper reports an efficient way to fabricate the superhydrophobic silicone rubber whose contact angle reaches 151° and sliding angle reaches 5°. The long term stability of polymeric superhydrophobic insulating surfaces is examined under electrical and non-electrical stresses. The result shows that in the water aging process, the static contact angle of both SIR and superhydrophobic SIR can recover, but the dynamic contact angle shows a little recovery. In the corona aging process, both the static and dynamic contact angle of superhydrophobic SIR can almost restore to the original level, but the dynamic contact angle of normal SIR increases after restoration. The finding shows that micro-nano surface structure and the stability of the surface chemical composition are important for the anti-aging performance of the superhydrophobic SIR.