M. Tariq Nazir

Performance of silicone rubber composites with SiO 2 micro/nano-filler under AC corona discharge

Outdoor insulators are often subject to corona discharges and the problem is becoming more prevalent with the increasing use of higher transmission voltage levels. For polymeric insulators, exposure to such discharges can alter the chemical structure of basic polymer and degrade surface properties. This paper investigates the effect of micro and/or nano fillers in silicone rubber composites in suppressing such damage. Four different types of samples are fabricated: pristine silicone rubber (PR), 30wt% micron-sized silica/silicone rubber (MC), 27.5wt% micron + 2.5wt% nano silica/silicone rubber (NMC), and 5wt% nano silica/silicone rubber (NC) composites. Samples are exposed to AC corona using a needle to ground-plane electrode setup. Experimental results are analyzed based on five different measurement methods: phase-resolved partial discharge (PD), hydrophobicity loss-recovery, Scanning Electron Microscopy (SEM), Surface roughness and Fourier Transform Infrared Spectroscopy (FTIR). Results indicate that NC shows a strong resistance to partial discharges and hydrophobicity loss. In the area below the needle tip, higher hydrophobicity loss and higher recovery are observed as compared to the vicinity region. Variations in surface roughness, appearance of crackles, voids, pits, surface splitting into blocky structures and damages to chemical structure of silicone rubber are appreciably retarded in NC as compared to PR, MC and NMC. Based on NMC results, it is found that addition of nano-sized silica can be an attractive approach to improve the corona resistance of micron-sized silica filled silicone rubber.

AC corona resistance performance of silicone rubber composites with micro/nano silica fillers

Silicone rubber (SR) insulating surfaces are often subjected to corona partial discharges (PD) during outdoor operation which can change the surface chemistry significantly and result in considerable surface deterioration, hydrophobicity loss, and enhancement of PD activity. In this work, the effect of micro and nano-fillers addition on surface degradation of SR under AC corona discharge is examined by analyzing the PD characteristics, hydrophobicity and surface morphology. Nano precipitated silica and ground silica with particle size of ~20 nm and ~5 μm are adopted and incorporated in RTV silicone rubber by two different mixing techniques. Mechanical stirring followed by ultrasonic water bath are applied to achieve uniform dispersion of fillers in RTV SR matrix. The four different types of the composite, i.e. pristine SR, 30-wt% micro-silica/SR, 27.5-wt% micro with 2.5-wt% nano-silica/SR and 5-wt% nanosilica/SR composites are tested under AC corona discharge. It is found that, at the end of ageing, the nano-silica composite showed the best suppression against PD activity and higher resistance against surface damage as compared to other test samples. It is also found that resistance to hydrophobicity loss is offered by composites during the initial 48 h of corona treatment but it is completely lost at the end of 96 h of ageing.

Effect of AC corona discharge on hydrophobic properties of silicone rubber nanocomposites

In wet conditions, corona discharges may occur on composite insulator surfaces of well-designed power transmission equipment. Corona exposure gradually reduces the surface hydrophobicity and degrades the polymer surface. Recently, much attention has been paid on improvement of corona surface resistance by dispersing nanofillers in the base polymer. This paper presents experimental investigations to understand how silica filler loading can affect hydrophobic properties of RTV silicone rubber nanocomposites under AC corona discharge. Nano precipitated SiO2 with size of ~20 nm was used as filler. Silicone rubber nanocomposites with different nano SiO2 loading by weight were fabricated using mechanical stirring and ultrasonic water bath methods. A point-to-plane electrode geometry housed in a glass chamber was used to produce corona discharge at standard atmospheric pressure. The nanocomposite samples were placed between the electrodes, in direct contact with the plane electrode (ground) but separate from the point electrode by a fixed 5 mm air gap. The changes in hydrophobicity of test samples as a function of corona exposure time were determined by measuring the water static contact angle with a goniometer. The surface free energies of nanocomposite samples were calculated by considering the static contact angles of water and diiodomethane in relation to corona exposure duration. The surface morphology of nanocomposite samples was also studied by means of Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM).

Comparative AC tracking and erosion resistance of micro-A1N and BN filled silicone rubber composites

Thermal degradation due to dry band arcing in the discharge region causes tracking and erosion failure of composite insulators. This work investigates resistance against tracking and erosion of silicone rubber filled with micron sized aluminum nitride (AlN) and boron nitride (BN) particles. Their performances are evaluated by comparing the average tracking length, eroded mass and thermal distribution across the tested specimens. Micron AlN (5–10 μm) and BN (5 μm) particles were acquired for fillers. For each test sample, the inclined plane test (IPT) according to IEC 60587 was carried out, lasted for 4 hours with initial applied voltage of 3 kV and then increased at a rate of 0.25 kV/h. The experimental findings indicate that remarkable resistance to tracking length and eroded mass is achieved with higher filler loadings of 20wt% and 30wt% of BN as compared to AlN. Thermal distribution results indicate that heat is mainly accumulated in the discharge region and higher BN loadings suppressed it more effectively than AlN counterparts. The maximum temperatures in the discharge region at 3.5 h of IPT are measured at 384°C, 440°C and 130°C for 0wt%, 30wt%-AlN and 30wt%-BN, respectively. It is concluded that BN provides better thermal conductivity than AlN which helps in dissipation and reduction of heat in the discharge region and improves the tracking and erosion resistance.

Erosion resistance of micro-AlN and nano-SiO 2 hybrid filled silicone rubber composites

Erosion of weathershed composite materials is one of the key failure modes in outdoor insulators and it can adversely impact on the reliability of power system networks. To enhance resistance against erosion and dry band arcing, inorganic particles are often added to base polymeric matrix. This paper investigates the erosion resistance of micron-AlN and hybrid micron-AlN + nano-silica filled silicone rubber composites. Micron aluminum nitride (AlN: 5–10 pm) and nano-silica (SiO2: 20 nm) particles were obtained for fabrication of composites. Five samples for each type were tested by following the inclined plane test (IPT) procedure as described in IEC 60587. The step wise tracking voltage (method 2) was adopted for the IPT with initial voltage of 3 kV, ramping rate of 250 V/h and duration of 4 h. Results show that hybrid composites exhibit substantially lower eroded mass than micron-AlN filled composites. Moreover, the average leakage current in hybrid composites is noticeably lower than 30%m-AlN. Hence, it is expected that this may help restrict dry band arcing and thermal induced erosion degradation. Morphological results show that a pit appeared in 30%m-AlN with large depth and extensive cracks inside. Only minor erosion depth is noticed in 28%m-AlN+2%n-silica with compact structure on the edges of eroded region. Better thermal stability and thermal conduction ability can be possible reasons for the better performance of hybrid filled composites in IPT.

Ultraviolet weathering resistance performance of micro/nano silica filled silicone rubber composites for outdoor insulation

The degradation of high voltage (HV) outdoor polymeric insulators due to ultraviolet (UV) weathering environment has long been remaining a research subject for the reliable power delivery. Such weathering conditions produce chemical changes on the surface of insulators and significantly impact on the hydrophobicity, roughness and surface structure. This paper describes the experimental findings on the accelerated UV weathering resistance of micro and/or nano silica filled silicone rubber (SR). Four different types of composite are synthesized: unfilled (U-SR), micro-silica filled (M-SR), nano-silica filled (N-SR) and micro/nano silica mixture (MN-SR). All the composites are exposed to the synergetic effect of UV, elevated temperature and AC electric stress in a chamber for 104 days. Experimental findings indicate that N-SR significantly resists the drop in hydrophobicity. Also, considerable gain in the surface roughness of composites is observed at the end of weathering relative to the virgin state. A sharp gain in the average and peak surface roughness of M-SR and MN-SR is measured, respectively. Significant silica particle exposure is noticed in M-SR and MN-SR but a very smooth surface of N-SR is captured in the SEM results. Few surface crackles are noticed on the surface of M-SR and N-SR. From the above results, it can be concluded that nano-sized silica particles in N-SR compose a better UV reflecting layer which makes it superior over the other composites.

AC corona resistance of micro-ATH/nano-Al 2 O 3 filled silicone rubber composites

Alumina tri-hydrate (ATH) filled silicone rubber composite insulators are widely used for outdoor insulation purposes. They are often affected by corona discharges. A high reduction in hydrophobicity, surface mangling and roughness variation are ramifications of corona discharges. In this paper, resistance against corona damage of pure, ATH and ATH+nano Al2O3 filled silicone rubber is compared through SEM, surface roughness and hydrophobicity recovery performance. Micron ATH (∼5 μm) and nano Al2O3 (∼40 nm) are selected for composite fabrications. The three different samples, i.e. pure silicone rubber (USiR), 40wt%-ATH/silicone rubber (MSiR) and 39wt%-ATH+1wt%-nano Al2O3 (MNSiR) are fabricated and tested under AC corona discharge for 168 h. A pin-plane electrode geometry enclosed in a glass chamber is employed for corona discharge exposure. Test results show that severe defects in the form of surface rupture appeared in USiR. Such defects are not found in MSiR and MNSiR but in the form of voids and craters in MSiR. A smooth surface with minor cracks and low peak roughness is observed in MNSiR. Comparatively, higher increment in average and peak roughness is seen in USiR. Furthermore, it is found that the pace of hydrophobicity recovery is marginally lower in MSiR and MNSiR. Besides, hydrophobicity recovery in the region below the electrode tip is substantially higher in USiR followed by MSiR and MNSiR.