Fuzeng Zhang

Effect of moisture on temperature rise of composite insulators operating in power system

The local heating phenomenon of composite insulator has drawn more and more attention gradually. From our analysis, the temperature rise of composite insulator is closely related to the moisture in atmospheric environment according to research results. Effect of moisture on the temperature rise of composite insulators is studied in this paper. The composite insulators, which have operated in power system for a long time and have been detected for local heating, and new insulator are taken as test samples. The temperature rise of operated composite insulators, which have been detected for local heating, increases with the rise of the relative humidity, while new insulator shows no temperature rise can be detected. The temperature rise of sample is not relevant to the core and the interface between core and housing material practically, but the high voltage end housing. The moisture content of the housing material is different at various relative humidity, and this leads to difference in dielectric loss, which is a significant source of local heating phenomenon. The difference of dielectric loss at different relative humidity leads to the temperature rise variation with the relative humidity.

Moisture induced local heating of overhead line composite insulators

There has been a growing concern over recent years regarding the phenomenon of local heating of composite insulators. In this paper, the influences of humidity, contamination and local electric field on the temperature and dielectric properties of field-aged insulators were studied experimentally. Aging of the silicone rubber (SIR) material on the weathershed surface, rather than resistive current flow from contamination, was found to be the major cause of local heating observed by thermography. The aged SIR material exhibited strong dielectric loss under high localized AC electric field and high relative humidity. A hypothesis was developed to mitigate the moisture induced local heating on composite insulators.

Investigation of the aging of line composite insulator sheds

The aging problem of material of composite insulators is one of key issues for power utilities. The most commonly used methods for aging evaluation are observation method, HC water spray classification method and static contact angle method and so on. However, there some subjective factor would be introduced by the operator of those methods. In order to investigate the aging state more objective, the thermally stimulated current (TSC) test has been used in our investigation. In this paper, the investigation on the aging of line composite insulator sheds has been carried out using the static contact angle method and TSC. This paper report the test results for the samples from the 110kV composite insulator sheds after 8 years of field operation. The test samples were prepared from the insulator sheds located in HV, LV and mid-section respectively, and the upside and downside of one shed has also been took care. According to the test, the result of hydrophobicity test is in accordance with the TSC test, and it was found that the aging degree of different sheds in the same line composite insulator is non-uniform. The HV end of the insulator generally has higher aging degree than that of LV end, and the mid-section of sheds aging is the lightest. This result is helpful for the comprehensive evaluation method of composite insulator sheds aging degree.

Evaluation of wetting condition and its effects on pollution flashover voltage of aerodynamic insulators

The flashover performance of insulators is significantly affected by the degrees and ways of wetting the pollution layer. However, there have only been a few reports on the wetting condition and its effects on the flashover characteristics of pollution on insulators to date. In this paper, a phase-angle difference-based method is proposed to test the wetting conditions of the pollution layer on insulators. A corresponding measuring device is also developed. The wetting process of the pollution layer on aerodynamic-type glass insulators is studied in an artificial climate chamber using the developed device. The related flashover performance is studied via the even-rising voltage method. Test results show that the phase-angle difference (PAD) curve accurately reflects the entire wetting process of the pollution layer from the initial stage, through the saturated stage to the oversaturated and washing away stages. Over the wetting process, the PAD curve initially drops sharply, and then remains stable for a few minutes before eventually rising slowly to the initial value. The PAD curve has its lowest value when the pollution layer is wetted to saturation, which occurs after approximately 5 min in our experiments. The flashover voltage characteristics of insulators are also significantly affected by the wetting conditions of the pollution layer. The flashover voltage in the saturated stage is shown to be lower than that in the other stages. Therefore, to improve the accuracy of the artificial pollution tests, quantitative methods (including PAD) should be used to obtain the exact time of wetting to saturation.

Research of nondestructive methods to test defects hidden within composite insulators based on THz time-domain spectroscopy technology

In this manuscript, we introduced a novel far-field nondestructive testing (NDT) for defects within composite insulators based on the terahertz time-domain spectroscopy (THz-TDS) technology. Impulse waves ranging from 0.02-2 THz were used to test onsite overheated 500 kV composite insulators and samples with artificial void defects. By analyzing the amplitude of time-domain waveform, as well as delay and Euclidean distance from the reference curve, defects as deep as 0.4 mm can be accurately positioned and the dimension of voids defects as small as 1.5 mm can be measured with the error less than 3%. Meanwhile, unlike the existing near-field nondestructive testing, the stand-off distance of system could be as far as 15 cm. It does not require much of the accuracy and the repeatability is amazingly high. Therefore it can carry out long distance detection towards composite insulators on the premise of enough power. In the end, this system was also utilized to measure the thickness of silicone rubber.

Research on aging evaluation and remaining lifespan prediction of composite insulators in high temperature and humidity regions

Composite insulators are the critical components of power transmission systems. 371 composite insulators with an operating duration between 3 and 22 years from the lines in high temperature and humidity regions were sampled. The material properties, mechanical properties, electrical properties and interface properties of these insulators were researched systematically. Moreover, 19 aging characteristic index were put forward to describe the aging conditions of insulators. We analyzed the correlation between the aging index and operating years, and corresponding aging mechanism was discussed in this paper. Meanwhile, according to the analysis results, we established lifespan prediction models based on physics and statistics respectively. The results showed: lifespan of the sampled insulators was determined by the thermal oxidation process of silicone rubber. Lifespan of the silicone rubber material was 14.6 years while the electrical lifespan of composite insulator was 18.9 years. With regard to the rest of the insulators in high-temperature and high-humidity regions, their remaining lifespan could be predicted in accordance with the mechanical load and Fourier translation infrared spectroscopy (FTIR) test results. In the end, we selected 20 insulators which had run for 14 years, and predicted their lifespan with the method mentioned in this article, the forecast error was 1 year.

Study on electrical properties and field solutions of water related heating of composite insulators on 500kV AC transmission lines

In 2011, a composite insulator in Shenzhen suffered fracture. Through infrared thermal test along the same and neighbor lines, 517 out of 1327 insulators presented with up to or over 0.5°C thermal increase. Early researches demonstrated that humidity was a factor to have effect on heating performance. In order to understand the potential mechanism of heating and provide precaution suggestions, we conducted FEM analysis on both thermal and electrical field; mechanical load test; electrical endurance test; mechanical, microscopic configurational as well as chemical analysis upon 9 problematic insulators and compared several ways to control end side field intensity. Defective interfaces were found. Polarization of insulation material after aging and water penetration was critical to heating. In order to reduce damage, decreasing the end side filed intensity was necessary to lessen heating loss as well as decelerate aging process.

Aging characteristic at different depths in a single composite insulators shed

Estimation of the degree of aging and the remaining lifetime of composite insulators is an important issue. In this paper, one shed of a 500 kV AC composite insulator that has operated for three years on a transmission line was studied. The insulator shed was cut longitudinally into four layers from the upper surface to the bottom surface and tested by the thermally stimulated current (TSC) method, and each layer was also tested by Fourier transform infrared (FTIR) spectroscopy. The results show that the first layer has fewer trap charges than the other layers in the TSC test, but it also has the highest trap level, and the trap depth of the first layer is deeper than that of the other layers. The trap levels and the quantities of shallow traps decrease successively from the second to the fourth layer. In the FTIR spectra, the absorption peaks of the first layer are lower than those of the other layers, and the other three layers have similar absorption peaks. The aging state of the first layer is more serious than that of the other layers. The study indicates that long-term contamination and ultraviolet (UV) radiation are the main reason for the aging differences among the different insulator shed layers.

Shed parameters optimization of composite post insulators for UHV DC flashover voltages at high altitudes

DC artificial pollution tests were carried out on 26 insulators with various shed parameters in high altitude areas to investigate the influence of shed parameters on the flashover performance of composite post insulators. The parameters tested included shed overhang, shed spacing and the core diameter for type A insulators with alternating single large and small sheds and type B insulators with alternating single large and dual small sheds. Ultraviolet photons emitted during partial discharge phenomena in the tests were characterized with an ultraviolet camera, and the arc development paths along the insulator surfaces were observed using a high-speed camera. The results show that the partial discharge phenomena become increasingly intense and arc bridging between the sheds becomes more serious with increasing shed overhang, but these phenomena become weaker with increasing shed spacing. The flashover voltage decreases with increasing core diameter, but increases initially and then decreases with increasing average shed overhang or large shed spacing. An optimal value for the ratio of large shed overhang to large shed spacing must therefore exist and is recommended based on the test results.

Study on the aging characteristics of insulators sheds

As composite insulators have been widely used in power system, the aging of composite insulator has become a threat to safe operation of transmission line. In order to investigate the influence of several aging factors on the sheds of high temperature vulcanized silicone rubber composite insulator, nine pieces of energized sheds on the high voltage end serviced on 500kV transmission lines have been selected as test samples. The salt deposit density (ESDD) and non-soluble deposit density (NSDD) measurements are adopted to measure the contamination accumulation characteristics of composite insulator sheds. In addition, thermally stimulated current (TSC) and hydrophobicity tests are also carried out on the shed samples. It indicates that TSC characteristics have the same tendency with the change of the contamination accumulation characteristics in both sides of one shed, but different from that of hydrophobicity. Combining with the contamination accumulation characteristics, hydrophobicity and TSC characteristics, a conclusion can be drawn that the contamination accumulation and ultraviolet radiation might be the main causes of aging, which are account for the difference of the aging characteristic between the two sides of one shed.