Hongwei Mei

Influence of sugar as a contaminant on outdoor insulation characteristics of insulators in a substation

This paper attempts to address the influence of sugar as a contaminant on outdoor insulation characteristics of insulators in a substation. Strong discharge activity was occurred under relative humidity less than 80% on insulators in a 500 kV substation at south China. A preliminary viewpoint that sugar as a contaminant leads to this strong discharge was drawn by investigating pollution sources around the substation and analyzing pollution layer conditions of discharged insulators. In order to further confirming this viewpoint and proposing the appropriate solutions for avoiding this discharge activity, the outward appearance, the surface conductivity, the discharge inception voltage and the flashover voltage of glass insulator and room temperature vulcanized (RTV) coated insulator under sugary and non-sugary contamination were investigated in detail in the lab. Furthermore, the hydrophobicity and hydrophobicity transference of RTV coated insulator were also investigated. The research results show that sugary contamination is very absorbent and viscous, outdoor insulation characteristics of insulators covered with sugary contamination are worse than that covered with non-sugary contamination, but outdoor insulation characteristics of RTV coated insulator covered with sugary contamination is still better than that of non-RTV coating insulator with non-sugary contamination. So it is still a specific solution to inhibit the discharge activity of insulators in the substation by spraying RTV coating. Of course, removing the sugar canes themselves from this area is the best solution.

Influence of environmental factors on hydrophobicity transfer property of silicone rubber material

Silicone rubber materials have been widely used as external insulation for power systems due to their good hydrophobicity and hydrophobicity transfer. However, pollution flashover accidents of composite insulators occurred from time to time in recent years. The influence of environmental condition on hydrophobicity and hydrophobicity transfer property of silicone rubber material needs further research. In this paper, incubator and saturated salt solution were used respectively to control ambient temperature and humidity precisely. Hydrophobicity measurement method were studied and regulated. Using incubator and special temperature control platform, hydrophobicity transfer property of silicone rubber material in different temperature was deeply studied, and the results were analyzed. The results showed that hydrophobicity transfer speed of silicone rubber slowed down with temperature decreased. Meanwhile, the results showed that ambient humidity had a great influence on hydrophobicity transfer property of silicone rubber. Flashover performance of silicone rubber samples with different hydrophobicity was analyzed. The results showed that the pollution flashover performance of composite insulators is closely related to the hydrophobicity state of the polluted surface.

Development of contamination flashover pre-warning system and analysis of operating experience

Pollution flashover has been a serious threat to the operation safety for the power system. Online monitoring is one most important way for the operation and maintenance of power system. The leakage current on insulator surface is a comprehensive reflection of the insulator surface contamination, environmental conditions and other affecting factors. The purpose of outdoor insulation state on-line monitoring could be achieved by measuring and monitoring the leakage current on insulator surface. This paper showed that close relationship existed between leakage current in both low humidity and saturated moisture conditions. A negative exponential relationship exists between pollution flashover voltage and a new parameter I* which could reflect the insulator shape and the maximum leakage current under saturated humidity. The flashover voltage under saturated humidity could be predicted through current obtained under low humidity conditions. A contamination pre-warning system was constructed in accordance using this mathematical model combined with some hardware system. Pilot systems were established in some typical polluted area in Jiangsu Province. The results showed that the system could achieve online monitoring purposes.

Prediction of flashover voltage based on leakage current under AC operating voltage

In this paper, a model to predict the AC flashover voltage of contaminated suspension insulators was presented. The prediction method is based on the maximum leakage current under AC operating voltage. Three kinds of widely used suspension insulators were tested in various contamination statuses such as pollution layers with different ESDD, different composition of the conductive components, different NSDD and different pollution distribution statuses to simulate the contamination statuses in nature. Effective ESDD was proposed and calculated. Influences of contamination statuses to maximum leakage current and flashover voltage were studied. Then, the relationships between flashover voltage and leakage current in these statuses are presented. Finally, considering the difference of insulator profiles, a new parameter is defined and a model to estimate the flashover voltage based on this parameter is developed. The model could be used to all kinds of suspension insulators in different contamination statuses and was validated by the test results.

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.

Electric field distribution characteristics of composite insulator with internal defect

In recent years, some events like insulator rupture failures were encountered in line operation. The inner defects, whose formation and development are heavily affected by electric field, would finally lead to the break of insulators. In order to study the electric field distortion of composite insulators with internal defects, the 3D electrical simulation models of 110kV AC composite insulator were built with the finite element software. According to the existence of some defects in statistics collected by the departments concerned, we have set several different microscopic internal defects of different lengths, different types (dry void, damp void or dielectric breakdown)and of different positions (high voltage side, low voltage side and middle part) between umbrella cover and rod. Besides, hole defects in umbrella cover were also set. Aimed at observing the characteristics of electric field distortion, the observation path was set through the defects in the model. After simulation, the simulation data such as electric filed distribution curves and electric field cloud charts were extracted and analyzed. The simulation results indicated that internal defects serve as the main reason of electric field distortion. Furthermore, the positions, lengths and dielectric constant of internal defects could play an important role on the extent of electric field distortion.

Non-destructive testing of internal defects in composite insulators using microwave technique

This paper introduces a kind of non-destructive detection method on detecting internal defects of composite insulator based on the microwave reflection method. The method emits the microwave to the sample, extracts the reflection wave with directional coupler, and detects its intensity. Flat plate shape samples made of silicone rubber and epoxy resin were used to study the stability the system, and the detection effect on defects of different sizes. On the other hand, the cylindrical shape samples were used to summarize the regularity of detection results on insulators without defects, and the detecting effect on the samples containing defects. Moreover, influence of surface contamination on this method was studied through experiments. The research results showed that the microwave detection method could detect the internal defect of composite insulators with a size of millimeter, and the surface contamination had a little influence on the detection.

Feasibility analysis of polymeric material insulator applied to phase to earth insulation of transmission line

Transmission lines exposed to the atmosphere in long terms may get easily affected by wind, icing and other factors which may lead to jumping and galloping, reduce the inter-phase clearance and cause flashover as a result. To avoid these, this paper applies polymeric material insulator to the line-to-earth, which means some spot through the span is linked to a ground anchor by the specialized polymeric material considering its better performance of anti-pollution flash-over, higher mechanical strength, lighter weight and less maintenance compared with traditional material. In order to investigate the effects of the line-to-earth applying the polymeric material insulator, the conductor-insulator-ground system has been simulated in this paper to calculate the sag, the inter-phase clearance and the dynamic amplitude. Different suspension points and material properties including diameter and mass have been analyzed and compared to provide proposal for line application. Furthermore, the simulation results have been compared with not only some other measures but also the results of the field test. The simulation results show that the dynamic amplitude gets smaller and the phase-to-phase clearance is enough when the polymeric material insulator is installed and the comparisons verify the accuracy of the simulation method, the feasibility and effectiveness of this measure. So it is a useful solution to reduce jumping and galloping by using specific polymeric material insulator.

A novel method of detecting contamination state on the hydrophobic surface

Partial surface conductivity is an ideal way for long-term contaminated degree monitoring of insulators. This method keeps the pollution layer from being destroyed in the measuring process. However, for hydrophobic surface, which is repelled from water, this method is hard to apply. In order to realize the measurement of partial surface conductivity on the hydrophobic surface, an improved measuring probe is proposed in this paper. The improved probe is composed of a handle and a cylindrical electrode. Based on the convenient turning over operation of this probe, the pollution layer can be wetted by dripping quantitative water into the probe and turning over the probe on the surface, which is more stable and reliable. The influence of water volume and measurement time of the new probe to the measuring results is elaborated in this paper. Test results demonstrate that partial surface conductivity increases as measurement time increases and finally becomes stable, and using too much or too little water both lead to the lessening of measuring results. It is noted that the suggested water volume is 0.06ml and the suggested measurement time is 2 minutes, in which case partial surface conductivity has good correlation with equivalent salt deposit density (ESDD) on artificial pollution samples. Contamination state can also be detected effectively on the hydrophobic surface. The method can be used for long-term monitoring and research studying of contaminated degree on the hydrophobic surface.