Understanding the influence of water droplet initiated discharges on damage caused to corona-aged silicone rubber

Understanding the performance of gamma-ray-irradiated epoxy nanocomposites

Abstract: Epoxy nanocomposites being used in the high-energy radiation zones as an insulant may undergo changes in their dielectric properties during service. In the present study, the performance of base epoxy resin (S1) is compared with epoxy resin with ion trapping particle (Sample S2) and epoxy resin with nanotitania (Sample S3) particle. The influence of gamma irradiation on nanocomposites was analysed. Corona inception voltage due to water droplet initiated discharge and contact angle reduces post-gamma-ray irradiation. Surface potential decay time constant reduced drastically for gamma-ray-irradiated specimens. Trap distribution characterisation indicated that charge mobility increases after irradiation. The surface roughness of the sample increases with the irradiation dosage. Dielectric relaxation spectroscopy shows that permittivity reduces and loss tangent increases with the gamma-irradiated specimens. Water diffusion rate increases for the gamma-ray-irradiated specimen. No change in elemental composition, measured using laser-induced breakdown spectroscopy, of test specimens was observed. The hardness of the material and plasma temperature formed during laser shine decreases with gamma-ray irradiation intensity for Sample S1, whereas samples S2 and S3 showed only marginal variation. The performance of Sample S2 is found to be better than Samples S1 and S3.

Dynamic formation mechanism of water droplet and induced surface discharges on silicone rubber composites

Abstract: Wettability has become one of the most important factors influencing power equipment insulation system in coastal power grid, which can seriously threaten the safety of power grid. This paper aimed at numerical simulation and experimental investigation on dynamic formation mechanism of water droplet and induced surface discharges on silicone rubber composites under different AC voltages, considering the factors of surface tension and droplet volume. Obtained results reveal that the spreading coefficient increases with increasing the electric field intensity from 1 to 5 kV/cm and with increasing the droplet volume from 10 to 50 µL. The spreading coefficient shows a decreasing tendency when the surface tension increases from 29.3 to 61.0 mN/m. Moreover, the maximum of surface electric field shows an increasing tendency under the condition of increasing both the droplet volume and the surface tension. On the basis of the physical model of droplet deformation, the electric field force and the surface tension play a significant role in the process of droplet deformation. Meanwhile, the experimental results are consistent with the simulation analysis, reflecting that the droplet deformation and the induced surface discharges mainly depend on two parameters: the droplet volume and the magnitude of applied voltage.

Assessment of Surface Degradation of Silicone Rubber Caused by Partial Discharge

Abstract: This paper reports experimental and analytical results of partial discharge degradation of silicone rubber sheets in accordance with proposed procedures. Considering the actual usage condition of silicone rubber as an insulating material of polymer insulators, an experimental procedure is established to evaluate long-term surface erosion caused only by partial discharge. Silicone rubber is subjected to partial discharge for 8 h using an electrode system with air gap. Voltage application is stopped for subsequent 16 h for recovery of hydrophobicity. The 24 h cycle is repeated 50 or 100 times. Deterioration of sample surface is evaluated in terms of contact angle and surface roughness. It is confirmed the proposed experimental procedure has advantage of no arc discharge occurrence, good repeatability of results, and possible acceleration of erosion. Surface erosion of silicone rubber progresses gradually and finally breakdown of silicone rubber occurs. Alumina trihydrate (ATH), an additive to avoid tracking and erosion by discharge, is not necessarily effective to prevent breakdown caused by partial discharge when localized electric field in air is enhanced by adding ATH. In such a situation, lower permittivity and higher resistance of silicone rubber seem dominant factors to prevent partial discharge breakdown and a careful insulation design should be required.

Understanding the dielectric and mechanical properties of self-passivated Al–epoxy nanocomposites

Abstract: Epoxy nano passivated aluminium composites with optimised size and filler contents were fabricated. Variation in contact angle and surface roughness is insignificant with increasing filler into nanocomposites but it showed a drastic reduction on corona ageing. Water droplet initiated corona inception voltage (CIV) is high under the negative DC voltage followed with positive DC and AC voltages. The bandwidth of ultra-high frequency signal generated due to water droplet initiated corona discharge lies in the range of 0.5–1.2 GHz. Surface potential measurements have shown that the decay in the potential was fast initially, and it became slower and sluggish subsequently. The trap energy density versus trap depth plot exhibits shallow traps and deep traps at around 0.8 and 0.87 eV, respectively. Permittivity, conductivity and loss factor have increased with an increase in the filler content in nanocomposites. The bulk resistance and capacitance of samples were determined for obtaining the equivalent parallel RC circuit model. Incorporation of nanofillers increases the glass transition temperature and reduces the tan δ with increasing frequencies as evident from dynamic mechanical analysis studies. A direct correlation is observed between the plasma temperature measured through laser-induced breakdown spectroscopy spectra and hardness of the material.

Understanding Hydrophobicity Recovery of Silicone Rubber Material Post Corona Ageing

Abstract: Surface hydrophobicity recovery of silicone rubber insulating material post corona ageing is reported in the present work. Recovery rate of hydrophobicity of uncleaned (S1) and cleaned sample (S2) after the corona ageing are compared. Results indicate that surface potential and its decay time constant reduces drastically after the corona ageing, as compared to the virgin specimen. However, the test specimens regain their hydrophobicity, though at a different rate, with time. Trap distribution characteristic shows that the characteristic time and charge trap depth reduces with the corona aged specimens and regain with rest period. A similar trend is observed with contact angle and corona inception voltage (CIV) of the test specimens. Pattern classification of Ultra High Frequency (UHF) discharge signal is done using modified Chaotic Analysis (CA).

Polydimethylsiloxane thermal degradation Part 1. Kinetic aspects

Abstract: A combination of traditional kinetic formal treatments and computer simulation has been made to analyze polydimethylsiloxane (PDMS) thermal degradation. It was shown that PDMS thermally decomposes to cyclic oligomers through Si – O bond scission in a chain-folded cyclic conformation energetically favored by overlapping of empty silicon d-orbitals with orbitals of oxygen and carbon atoms. Kinetic treatment shows that PDMS thermal volatilization, as rate of heating increases, becomes dominated by rate of diffusion and evaporation of oligomers produced on its decomposition. At high heating rate (e.g. 100°C min −1 .) thermal decomposition in nitrogen and in air tend to overlap because the rate of reaction between the material and oxygen is strongly reduced by low-oxygen solubility and high-thermal degradation rate. In nitrogen a small black residue is formed (silicon oxycarbide) which is produced by an alternative decomposition path leading to cyclic oligomers, made possible at high heating rate.

Hydrophobicity Recovery of Polydimethylsiloxane after Exposure to Corona Discharges

Abstract: A high-temperature-vulcanized polydimethylsiloxane (PDMS) elastomer has been subjected to corona discharges for different periods of time in dry air. The loss and recovery of hydrophobicity of the surface have been characterized by contact angle measurements. Immediately after exposure to corona discharges, samples showed a low surface hydrophobicity and, on storage in dry air, a continuous increase in hydrophobicity finally approaching the hydrophobicity of the unexposed material. The activation energy of the hydrophobicity recovery was two to four times greater than the activation energy of the diffusivity of low molar mass PDMS in PDMS elastomers, indicating that the diffusivity properties of the oxidized surface layer were different from that of the bulk. PDMS elastomers quenched in liquid nitrogen or subjected to small mechanical deformation (

Thermal polydimethylsiloxane degradation. Part 2. The degradation mechanisms

Abstract: The products of the thermal degradation of polydimethylsiloxane (PDMS) are determined by the heating conditions, since two competing mechanisms are involved. Cyclic oligomers are formed in the low degradation temperature range and during slow heating in programmed degradation. This involves molecular splitting of oligomers from loop conformations of the PDMS chain favoured by its flexibility, and assistance on the part of empty silicon d-orbitals. Methane and oligomers are formed in the high temperature range and during fast heating. This shows that homolytic scission of Si–CH3 also takes place and is followed by hydrogen abstraction.

The thermal degradation of polysiloxanes—I. Poly(dimethylsiloxane)

Abstract: Evolution of volatile materials from pure poly(dimethylsiloxane) heated under temperature programmed conditions (10° min−1 under vacuum) is detectable at 343° and reaches a maximum at 443°. The volatile products comprise a continuous mixture of cyclic oligomers from trimer upwards. Replacement of the terminal hydroxyl by trimethyl silyl structures (end-blocking) results in a considerable increase in stability but no change in the distribution of products. The reaction is strongly accelerated by KOH and methane then appears as a significant product. Thermal gravimetry and molecular weight measurements confirm that the hydroxyl terminated polymer degrades in a stepwise fashion from chain ends. These observations are discussed from a mechanistic point of view. The reaction is accelerated by oxygen and cross-linking occurs.

Hydrophobicity changes in silicone rubbers

Abstract: Water repellency, high surface resistivity, vandalism resistance, low density and good processability have made silicone rubbers based on polydimethylsiloxane (PDMS) very attractive materials in housings for outdoor insulation. Their ability to recover hydrophobicity after oxida. tion or contamination is of paramount importance and this is the topic of this review. A critical evaluation of the chemical and physical mechanisms responsible for hydrophobicity loss and recovery is presented.