Hydrophobicity improvement of contaminated high temperature vulcanization (HTV) silicone rubber is of great significance to avoid risk of pollution flashover during wetting events. In the present investigation, low-temperature plasma jet produced by dielectric barrier discharge (DBD) under atmospheric pressure was used to quickly increase the hydrophobicity of contaminated HTV surface. Effects of the processing time, non-soluble deposit density (NSDD) and transfer time on the hydrophobicity improvement were studied and the long term stability of hydrophobicity was investigated. Experimental results proved that: hydrophobicity of contaminated HTV silicone rubber was significantly improved after a quick exposure to low-temperature plasma jet; and with the increase of exposure time, hydrophobicity transfer increased and a larger contact angle was obtained for the same transfer time. For the low discharge power plasma treatment, NSDD had a negative effect on hydrophobicity improvement and with the increase of the NSDD, increment of the contact angle decreased. In addition, hydrophobicity improvement had long term stability. Control experiment proved that interaction of plasma jet with silicone rubber played an important role in accelerating the transfer of low molecular weight (LMW) silane chains from the silicone rubber to the pollution layer, which finally resulted in the hydrophobicity improvement.

Hydrophobicity improvement of contaminated HTV silicone rubber by atmospheric plasma jet treatment

Silicone rubber (SiR) insulators are employed in high-speed railway as roof insulators, which have to continuously face the challenge of strong wind. Corona discharge can degrade the SiR surface and lead to hydrophobicity loss. In order to research the influence of wind on corona behavior of SiR, high-temperature vulcanized silicone rubber (HTV-SiR) specimens were exposed to corona environment under different wind conditions. The variations in corona discharge intensity, discharge current and hydrophobicity changes were measured and analyzed. The recurrent plot (RP) method was introduced to extract features from discharge current. In addition, the effects of wind on hydrophobicity recovery rate were also investigated. The experimental results reveal that the hydrophobicity loss during corona can be inhibited or accelerated depending on the wind direction, and the wind with higher speed has greater influence on the corona performance. Moreover, it is found that the wind can blow away the low molecular weight (LMW) and slow down the hydrophobicity recovery, while the wind pressure can accelerate the hydrophobicity recovery.

Effects of wind condition on hydrophobicity behavior of silicone rubber in corona discharge environment

Abstract Ethylene-propylene-diene-monomer (EPDM) is one of the most frequently used outdoor insulating materials. Like other polymeric materials, EPDM and its composites also degrade in outdoor applications. For evaluation of aging of insulating materials, accelerated multistress aging/weathering is an efficient method. In the current paper, comparative multistress aging performance investigation of neat EPDM (NE), EPDM with 20% microsilica (microcomposite, EMC), EPDM with 5% nanosilica (nanocomposite, ENC) and EPDM 20% microsilica 5% nanosilica (hybrid composite, EHC) is presented. Materials prepared according to ASTM D3182-07 were subjected to uniform ultraviolet (UV) radiation, heat, humidity, salt fog and acid rain at 2.5 kV voltage for 5000 h in a specially fabricated weathering chamber. For timely analysis of the aged materials, Fourier transform infrared (FTIR) spectroscopy, Swedish Transmission Research Institute (STRI) hydrophobicity classification and scanning electron microscopy (SEM) were used along with the critical visual inspection. Increasing discoloration was found in all materials which was proportional to% wt of the filler in composites. Improved hydrophobicity and better surface smoothness was recorded in composites as compared to NE, which was higher in the case of the microcomposite and hybrid composite. FTIR results showed least reduction in hydrocarbon bonds and lowest variation in chalking index in the case of the microcomposite.

Accelerated aging investigation of high voltage EPDM/silica composite insulators

The surface flashover is affected by many factors, such as surface roughness, nanofillers and chemical functional groups. In this research study, polyamide mesh was utilised as a scaffold to systematically study the synergistic effect of micro structure/nanofiller/superhydrophobicity on the surface flashover. Based on the dissolution and resolidification method, nanofillers were partially embedded into the mesh fabrics. Here, Al2O3 nanoparticles were used as a typical non-conductive nanofiller, and carbon nanotubes were used as a typical conductive nanofiller. Moreover, the micro structure was determined by altering the mesh size, and chemical fluorination was utilised to change the surface chemical group. It was found that the polyamide mesh with 300# demonstrated best anti-flashover properties. Chemical fluorination could effectively improve the flashover voltage. By further adjusting the ratio of Al2O3 to carbon nanotubes, the flashover voltage can be increased by up to 30%. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1049/hve2.12229

The synergistic effect of micro structure/nanofiller/superhydrophobicity on the surface flashover

Due to hydrophobic properties of polymer insulator, formation of discrete water droplets and the induced surface discharges on the insulator are typical phenomena during the insulator operation. To investigate the performance of polymer insulator in presence of discrete water droplets, maximum value, cumulative charge and energy eigenvector of discharge current were investigated. Effects of droplet numbers, droplet location, volume and contamination on surface discharges were analyzed. Obtained results reveal that with the development of surface discharges from initiation of corona discharge to arcing discharge and surface flashover, the dominate components in discharge current are the low frequency (0-78 Hz) components. For the condition of both single droplet and double droplets, flashover voltage and maximum value, cumulative charge and energy eigenvector of discharge current show the increasing tendency with increasing the droplet volume from 10 to 30 μL, but show the decreasing tendency under the condition of increasing the droplet volume from 30 to 40 μL and increasing the droplet contamination from 1.0 to 3.0 mS/cm. However, for triple droplets, the flashover voltage and investigated characteristics of discharge current show the decreasing tendency under the condition of increasing the droplet volume from 10 to 40 μL and increasing the droplet contamination from 1.0 to 3.0 mS/cm. Meanwhile, based on the comparison of droplet effects, it can be found that both droplet numbers and droplet location have significant influence on surface discharges.

Energy eigenvector analysis of surface discharges for evaluating the performance of polymer insulator in presence of water droplets

In HVDC transmission lines, corona discharge could occur even on well-designed hardware and insulators, which can significantly damage the polymeric insulators. Under dc voltage, compared with ac voltage, the electrostatic force due to the constant electrostatic field makes more ions bump against the insulator surface. Due to these influence, the decrease even the loss of hydrophobicity will take place, which may cause flashover to affect operating reliability of power system. Much attention has been paid on the accurate evaluation for the insulator hydrophobicity. In this paper, the acoustic characteristics of surface discharge, based on the static droplet test (SDT) and the dynamic droplet test (DDT), were investigated to evaluate the hydrophobic properties of silicone rubber (SiR) insulator at different ageing degrees. The specimens were prepared by dc corona degradation. The acoustic signal of surface discharge was captured through applying using an acoustic sensor above the specimen surface. In order to quantify the acoustic signals, statistic methods were employed to establish the relationship between the pattern characteristic of signal waveforms and the hydrophobicity condition. The relationship among the flashover voltage, the signal power, the pulse number, and the corona degradation time was obtained via the static droplet test. For the dynamic droplet test, it is found that the pulse number, the voltage range, the maximum slope and the signal energy present an obvious tendency with the hydrophobicity deterioration or loss. Weibull distribution parameter (β) of the data distribution reflects the difference of specimen surface hydrophobicity. Therefore, the obtained results indicate that the acoustic characteristic of discharge is sensitive to the hydrophobic properties in the laboratory tests, which can be applied as a non-contact method for hydrophobicity evaluation of SiR insulator under dc voltage in the future.

Y. P. Li

Papers

Hydrophobicity evaluation of silicone rubber insulator using DC discharge induced acoustic wave