Aging Of Silicone Rubber Research Articles

Study on the aging status of insulators based on hyperspectral imaging technology.

The acidic environment is one of the main factors leading to the aging of silicone rubber (SiR) insulators. Aging can reduce the surface hydrophobicity and pollution flashover resistance of insulators, threatening the safe and stable operation of the power grid. Therefore, evaluating the aging state of insulators is essential to prevent flashover accidents on the transmission line. This paper is based on an optical hyperspectral imaging (HSI) technology for pixel-level assessment of insulator aging status. Firstly, the SiR samples were artificially aged in three typical acidic solutions with different concentrations of HNO3, H2SO4, and HCl, and six aging grades of SiR samples were prepared. The HSI of SiR at each aging grade was extracted using a hyperspectral imager. To reduce the calculation complexity and eliminate the interference of useless information in the band, this paper proposes a joint random forest- principal component analysis (RF-PCA) dimensionality reduction method to reduce the original 256-dimensional hyperspectral data to 7 dimensions. Finally, to capture local features in hyperspectral images more effectively and retain the most significant information of the spectral lines, a convolutional neural network (CNN) was used to build a classification model for pixel-level assessment of the SiR's aging state of and visual prediction of insulators' defects. The research method in this paper provides an important guarantee for the timely detection of safety hazards in the power grid.

Characteristics of Aged Silicone Rubber Insulators Used in Outdoor for 20 Years

To collect aged silicone rubber (SiR) insulators with different numbers of years in use, which is for developing a degradation diagnostic system of SiR, is difficult. Therefore, it is desired to make degraded SiR surface which has similar characteristics to the degraded surface in outdoor. To realize the fabrication, we characterized the surfaces of aged SiR specimens cut out from a 22 kV line post insulator used in the coastal area for 20 years. The results showed that to introduce fine particles into the surface is necessary for making degraded surface having the minute surface roughness. Additionally, it was found that to activate the dispersion of LMW with foreign matters is also necessary. © 2024 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

Condition assessment criteria evaluation of asymmetric aged and fully aged silicone rubber insulators based on leakage current harmonics

AbstractThe leakage current characteristic monitoring of transmission line insulators is considered a worthy technique for insulator state prediction. In this paper, the harmonic analysis of leakage current signals has been performed to analyze the composite insulator condition. Experimental studies of asymmetrically aged, fully aged, and virgin insulators have been conducted. The effects of different aging types, pollution, and humidity on leakage current components, such as harmonics and the phase differences with the applied voltage, have been investigated. The analysis of the results shows that the effect of asymmetric aging on leakage current components is non‐linear, and this non‐linearity increases with the degree of pollution. Also, the type of insulator aging can be determined by the ratio of the third‐ and fifth‐order leakage current harmonics. For instance, the classification of insulators into virgin, asymmetrically aged, and fully aged can be determined by examining the corresponding value ranges of 0–0.5, 0.5–0.9, and > 0.9, respectively. Similarly, the phase difference values of < 15% and those > 15% indicate the clean and polluted operating conditions individually.

Application of Infrared Spectroscopy in Research on Aging of Silicone Rubber in Harsh Environment.

Polymer insulators using silicone rubber materials as sheds and sheaths are widely used in power systems to replace traditional porcelain and glass insulators which are heavy, inconvenient to install, and prone to pollution flashover. However, in recent years, polymer insulators that have been operating in harsh outdoor environments for many years have experienced different degrees of aging. The aging degree and aging products of silicone rubber are the focus of research. Fourier transform infrared spectroscopy (FTIR) is a technical method to analyze the internal molecular bonds and functional groups of materials, and it is often used to study the aging degree and aging products of silicone rubber. In this paper, the aging characteristics of silicone rubber samples in a high altitude area, salt fog environment, and acid environment were studied by FTIR. The results showed that the silicone rubber in a harsh environment, such as strong radiation, salt fog, and acid fog was degraded to some extent, and its main chain was cut off, the degree of polymerization was reduced, and the content of hydrophobic functional groups was reduced. Infrared spectroscopy can be used to analyze the aging phenomenon of polymers.

Dynamic mechanical characteristics of aged silicone rubber blend

The effect of aging on the mechanical properties of silicone rubber (SR) was investigated by means of ultrasonic, dynamic mechanical analysis, and FTIR techniques. Both longitudinal and shear (Ultrasonic wave velocities) were measured at room temperature and at frequencies of 2 MHz. Density, molar volume, ultrasonic wave velocities, tensile strength, mechanical properties, and FT–IR showed the improvement of the silicone rubber network with aging time from 0 to 70 days, while loosening of the network structure was observed at 14 days and 50 days aging. These behaviours were explained in terms of the change in cross-link density and average stretching force constant of bonds with aging. Thermogravimetric analysis and differential scanning calorimetric techniques showed quite low thermal stability and temperature performance for aged SR at 14 and 50 days than virgin SR which was confirmed by the cracks and voids appeared under scanning electron microscope.

Mechanical properties of thermo-oxidative aged silicone rubber thermally stabilized by titanium oxide based fillers

In this work, the mechanical properties of thermo-oxidative aged and thermally stabilized high consistency silicone rubber were investigated. The thermal oxidative stabilization is achieved by titanium oxide based fillers that were incorporated at different concentrations into the silicone compounds. Besides a pure titanium oxide filler, AEROXIDE® TiO2 P25, also an iron oxide doped heat stabilizer, AEROXIDE® TiO2 PF 2, was studied. The addition of the AEROXIDE® TiO2 fillers significantly improves the thermal stability of the compounds and maintains the mechanical properties on a good level after thermal aging. The thermal stabilization depends on the type and concentration of the AEROXIDE® TiO2 fillers. Due to iron oxide doping, the efficiency of AEROXIDE® TiO2 PF 2 is higher in comparison to AEROXIDE® TiO2 P25. However, a too high concentration, especially of AEROXIDE® TiO2 PF 2, induces a catalytic effect and favors the thermal degradation of the silicone. Therefore, the optimum concentration depends on the behavior of the heat stabilizer, its catalytic effect, and the thermal-oxidative conditions applied to the silicone. This study has revealed that titanium oxide based fillers are efficient heat stabilizers to preserve the mechanical properties of silicone rubber on a good level during applications at elevated temperatures.

A novel aging characterization method for silicone rubber based on terahertz absorption spectroscopy

To address the challenge of the lack of aging characterization methods for outdoor silicone rubber-based insulation materials in power engineering applications, this paper proposes a novel high precision non-destructive characterization method for aging of organic materials based on the vibration characteristics of silicone rubber materials in the terahertz frequency band. In order to analyze and verify the method proposed in this paper, the sources of vibration peaks in the THz band absorption spectrum of aged silicone rubber are discussed by accelerating test, microscopic characterization, and molecular simulation. Finally, the characteristic peak frequency and quantitative calculation method of aged silicone rubber are obtained. Compared to other aging assays, this method overcomes the limitations of the existing THz assay which requires testing in a nitrogen atmosphere, and has the advantage of not damaging the sample.

Coupling effects of gamma irradiation and moisture on the properties of SiO2 and TiO2 filled silicone rubber

This work mainly includes the effects of gamma irradiation and absorbed moisture on the properties of silicone rubber. The coupling effects have been analyzed by characterization of tensile, creep, and differential scanning calorimetry (DSC). With the same absorbed dose, the elongation at break, creep elongation, melting temperature, melting enthalpy and crosslinking density of the dewatered silicone rubber after irradiation show more obvious change than the pristine samples (the content of absorbed moisture is 0.3%), which means that the absorbed moisture is an important factor in the coupling aging of silicone rubber. The moisture is likely to reduce the chain mobility and restrain crosslinking reactions even with quite low absorbed moisture content.

Non-destructive aging evaluation of 500-kV field-serviced silicone rubber composite insulators with photothermal radiometry

In this article, a non-destructive photothermal radiometry (PTR) method is proposed to quantitatively evaluate the aging degrees of field-servicing silicone rubber composite insulators via a thermal diffusivity ratio and degradation layer thickness. A sensitivity analysis is performed to investigate the sensitivity of the PTR amplitude and phase to the thermo-physical, optical, and structural parameters of the aged silicone rubber composite insulators consisting of a degradation layer and a substrate layer. It is found that the PTR amplitude and phase are highly sensitive to the thermal diffusivity, effective infrared absorption coefficient, and thickness of the degradation layer as well as the thermal diffusivity of the substrate layer but are much less sensitive to optical absorption coefficients of the degradation and substrate layers and the effective infrared absorption coefficient of the substrate layer. Thus, parameters with high sensitivity coefficients are set as free parameters and determined by multi-parameter fitting the frequency dependences of the PTR amplitude and phase measured directly from the surfaces of the field-serviced silicone rubber composite insulators to a two-layer PTR theoretical model, while in the multi-parameter fitting, the parameters with low sensitivity coefficients are set as “known” parameters whose values are either measured separately or taken from literature. By doing so, the thermal diffusivity ratio and degradation layer thickness are accurately determined in a non-destructive way. Experimental results demonstrate that the thermal diffusivity ratio and degradation layer thickness determined by the proposed non-destructive PTR method are in good agreement with that determined by a conventional destructive PTR method, thus, proving the feasibility of the non-destructive PTR method for quantitative evaluation of aging degrees of the silicone rubber composite insulators being serviced in the high-voltage power transmission systems.

Research on Aging Dielectric Properties and Physical Testing of Electric Vehicle Insulating Silicone Rubber under First Principles

Aiming at the aging problem of insulating silicone rubber for new energy vehicles, this paper studies the equations of the aging of silicone rubber and its dielectric properties based on first-principles calculations. First, through the analysis of the microscopic mechanism of silicone rubber aging, the equations of the aging of silicone rubber and the rupture of the molecular main chain is obtained. Then the equations of silicone rubber molecular length and dielectric constant are solved by density functional theory. Ultimate, a molecular chain scission model of silicone rubber was built, and the micro-dielectric relationship was extended to the macroscopic level. The equation of the norm molecular length of silicone rubber and the dielectric constant is obtained. It provides an important theoretical basis for the aging detection of insulating silicone rubber for new energy vehicles based on electromagnetic waves.

Hydrophobic, dielectric and water immersion performance of 9000 h multi-stresses aged silicone rubber composites for high voltage outdoor insulation

Room temperature vulcanized silicone rubber (RTV-SiR) materials are broadly employed for high voltage outdoor insulation applications. Like other polymeric materials, such insulators degrade due to the multiple stresses caused by the environment. Therefore, for the investigation of multiple stresses presented in the environment such as UV radiations, salt fog, acid rain, heat, humidity, etc., a neat SiR, two aluminum trihydrate (ATH) filled composites of SiR (SMC1 and SMC2), one silica filled composite SiR (SMC3), and a co-filled composite (SMNC) having ATH and silica as fillers are fabricated and tested for 9000 h. After aging, the sample SMC3 and neat SiR show better hydrophobic behavior than the other samples. All the samples offer better leakage current suppression throughout aging. Dielectric breakdown strength (DBS) of SMC2 and SMC1 is measured better than their counterparts. Volume and surface resistivities of SMC1 remain the highest followed by SMC2 and SMC3. Scanning electron microscopy (SEM) results suggest that the surface of SMC2 retains excellent surface condition followed by neat SiR. SMC1, SMC2, and SMNC absorb higher water relatively after 24 h of immersion test whilst SMC1 and SMC3 exhibit a similar tendency after 2-h boiling test.

Electrical insulation properties of silicone rubber under accelerated corona and thermal aging

AbstractA contactless method based on energy shift of high‐energy cut‐off of the x‐ray bremsstrahlung, the so‐called Duane Hunt Limit and a conventional low voltage electrical technique (three‐probes technique) is applied on thermal and corona aged silicone rubber (SiR) to measure, respectively, the surface potential,Vs, and the surface resistivity,ρs. The effect of aging on these quantities, representing the dielectric properties, is studied. The results are highly reproducible and highlight a good correlation betweenVsandρs. It was observed that thermal aging combined with electrical aging deteriorates more the electrical properties of the polymer than thermal aging alone. Explanations for electrical characteristics (Vs,ρs) change with aging are supported by attenuated total reflection Fourier transform infrared spectroscopy spectra analysis and a chemical mechanism of aging in three steps (i.e., oxidation‐polycondensation, degradation, and thermal cracking). The surface degradation of the polymer is revealed by images of surface morphology obtained by using scanning electron microscopy (SEM). Roughness is greater for combined thermal and corona aging mode compared to thermal aging alone. In addition, the surface degradation of SiR polymer is confirmed by the loss of its hydrophobicity.

Six-year Trial of an HV Overhead Line Wide-bodied Composite Insulator

This paper addresses the long-term aging of silicone rubber shedded insulators mounted horizontally, and specifically relates the experience from a highly monitored six-year trial of two composite cross-arms designed for 400 kV. Weather conditions and leakage currents were continuously monitored in this moist and temperate area. In addition, the growth of algae on the surface over time is reported. As expected for the coastal location, strong seasonal variation was seen in leakage currents and this is associated with wind direction and resulting weather variations. Leakage currents remained low but increased linearly with time. Despite apparent large areas of algae growth on upward-facing surfaces, and some associated loss of hydrophobicity on some surfaces, the leakage currents on the insulators continue to be satisfactorily low. Cleaning one of the insulators after five-and-a-half years temporarily returned the current to its original value. In the short-term, leakage currents were heavily dependent on relative humidity, and peak currents are shown to be strongly dependent on the length of preceding periods without precipitation. It is suggested that measurement of these variables may be used to evaluate surface condition and be an effective asset management tool.

Photothermal Radiometry Depth-Profiling of Aged Silicone Rubber Composite Insulators

Thermal and structural properties of aged silicone rubber composite insulators widely used in high-voltage transmission power systems were characterized by photothermal radiometry (PTR) to evaluate the degree of aging. A two-layer PTR theoretical model was developed to extract the thermal diffusivity and the aged layer thickness from the PTR frequency-scan data. Results show that aging generated an aged layer with a reduced thermal diffusivity at the surface of the composite insulator. Further investigation into the depth dependence of the thermal diffusivity of aged composite insulators found a monotonically increasing thermal diffusivity along the depth direction, approximating to a hyperbolic tangential profile, which indicates a spatially continuous variation from surface to bulk during the aging process. The PTR depth-profiling of thermal and structural properties provides a deeper understanding of the aging process in composite insulators.

Effect of Aging under AC Superimposed Harmonic Voltage on Trap and Carrier Transport Properties of Silicone Rubber

The rapid growth of power grid capacity and the widespread use of a large number of power electronics and non-linear loads have led to harmonics in the power system. Harmonics in the power system will cause safety hazards to the normal operation of power equipment, exacerbate the aging of insulation materials, and reducing the overall operation reliability of the system. In the present work, we used power frequency ac voltage superimposed harmonics to carry out ageing experiments on power cable terminals. Then, we tested the infrared spectra, dielectric spectra, electrical conductivity, and surface potential decay characteristics of silicone rubber insulation materials on the cable terminals aged for different times. The experimental results show that the dielectric constant and dielectric loss of silicone rubber gradually increase with the aging time. In particular, the dielectric loss of silicone rubber changed greatly at low frequencies. The effect of dc conductance of aged silicone rubber on dielectric loss is significantly enhanced at low frequencies, which causes the dielectric loss to increase as the frequency decreases following an inverse power law. The surface potential decay rates of silicone rubber insulation after positive and negative corona charging accelerate with increasing the aging time, which is consistent with the experimental results of electrical conductivity. By analyzing the distribution characteristics of electron and hole traps in silicone rubbers, it is found that the trap energy levels of electron and hole traps become shallower as the operating time increases. The calculation of the carrier hopping conduction model shows that the shallow trap formed with increasing the aging time will lead to increases in both carrier mobility and conductivity. When the conductivity rises to a certain value, the silicone rubber will lose its insulation performance, resulting in insulation failure.

Charge transport dynamics and the effects on electrical tree degradation under DC voltages in thermally aged silicone rubber

Electrical tree is one of the major issues for insulation failure in high voltage power cables. In this paper, we investigate electrical tree degradation under DC voltages in thermally aged silicone rubber (SIR). Charge transport dynamics are revealed by means of space charge measurement and simulation to give a better insight into the electrical tree initiation and evolution. The results show that a thermal oxygen reaction enhances the SIR cross-linked networking and thus gives rise to a higher tree initiation voltage in the early stage of aging. Subsequently intensified thermal degradation and cracking reaction accelerate SIR cross-linked network breakdown and generate micro-cracks. The increased deep traps and significantly decreased charge carrier mobility after thermal aging result in greater homo charge accumulating near the needle tip. The mechanism of DC electrical tree initiation in SIR is clarified based on the trap theory. It is argued that more trapped charges in the low-density region near the tip in the late thermally aged stage would decrease the tree initiation voltage, while the resultant homo charges would in turn reduce the electric field around the tip. Therefore, after thermal aging, the reduction of tree initiation voltage under DC voltage is less than that under AC voltage.

Reliability assessment of aged SiR insulators under humidity and pollution conditions

In this paper, the effect of different pollution and humidity variations on the reliability of aged Silicone Rubber (SiR) insulators are investigated. Firstly, four different types of 20 kV SiR insulators have been selected to evaluate the effect of different insulator profiles on Flashover Voltage (FOV). Then artificially accelerated aging tests with 1000 h duration have been conducted for the aging of insulators. Numerous FOV tests have been carried out on the aged SiR insulators under different humidity and pollutions. Next, the risk of insulators utilization under different ambient conditions have been calculated by using the experimental test's data. Finally, the reliability of insulators under different conditions have been calculated and a new mathematical model for the reliability in different humidity has been proposed. The effect of SiR insulator profiles on FOV has been studied and the results have been shown that lower FOV is related to insulators with thicker profiles as well.

Influence of Salt Fog Test on Silicone Rubber Insulating Material Under AC and DC Voltages

The present work investigates the performance of electrically stressed silicone rubber insulating material in a cyclic salt fog test. Corona Inception Voltage due to water droplet initiated discharges changes after salt fog ageing. The Ultra High Frequency signals radiated during corona discharge activity under AC voltages have a frequency span of 0.8–1.5 GHz, whereas under DC voltages span lies in the range of 0.5–1.5 GHz. Contact angle reduces with the increase in number of salt fog ageing cycles. Surface potential accumulation studies indicate that salt fog aged silicone rubber shows characteristic variation in accumulated potential magnitude and the decay characteristics. Further tdv/dt of surface potential decay characteristics enables to understand the charge penetration depth. Weight loss studies and surface roughness analysis during salt fog test are used to quantify the amount of damage.

In‐situ and quantitative analysis of aged silicone rubber materials with laser‐induced breakdown spectroscopy

Due to their weak weather ability, the ageing process of organic silicone rubber composites remarkably influences the service performance of composite insulators and bushings, especially in high-voltage transmission lines. In this study, laser-induced breakdown spectroscopy (LIBS) was used for analysing ageing composite insulator sheds for potential on-site application. A spectrum was recorded after each laser pulse and the element composition of ablated materials with different depth was observed. A calibration curve that revealed linear relationship between C, Si, Al spectrum line intensity and corresponding element concentration was established with the result of X-ray photoelectron spectroscopy test. Regularly line intensities of C, Si and Al changed dramatically from the surface to the bulk layer. Given the number of laser pulses with varying element concentrations, as well as the linear relationship described above, the depth of ageing layer could be calculated immediately. Also, the result was verified by secondary ion mass spectrometry and scanning electron microscope. LIBS method can be a promising and efficient technique to assess the ageing state of composite materials on site.

Aging behaviors of silicone rubber composite materials under outdoor environment

We investigated the aging effect on the chemical structure of silicone rubber composite materials under outdoor environment. The variations of low molecular weight siloxanes in silicone rubber were probed by gas chromatography-mass spectrometry during the degradation process. The experimental results indicate that a series of cyclic siloxanes exist in both the virgin and aged silicone rubber samples, while the additional low molecular weight siloxanes (hexamethyl cyclotrisiloxane) only appear in the aged samples. Meanwhile, the total amounts of low molecular weight siloxanes in the aged samples are much less than those in the virgin ones. The loss of low molecular weight siloxanes is induced by the chain scission and depolymerization.