Shaohe Wang

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 migration in oil-impregnated film insulation under thermal ageing

Moisture produced in thermal degradation of insulating materials in transformer causes damage to the insulation system. To study the moisture diffusion and balance process in transformer insulation system and evaluate alternative insulation materials, three types of samples, including Kraft insulation paper, PC and PET film, were impregnated in oil and thermally aged under 90, 110 and 130°C respectively. By analysing the moisture variation in oil and solid insulation, it was found that the moisture diffusion, balance and hydrolytic degradation process of oil-paper insulation are different from those of other two types of samples. During the thermal ageing process, the moisture produced by polymer is much more than that by oil, and most of which is absorbed by sample surface. The ageing temperature has considerable influence on the moisture migration in polymers.

Effect of polarity reversal on space charge properties of CB/LDPE composite under DC field

Voltage polarity reversal is a special condition in HVDC transmission system. In this study, low density polyethylene (LDPE) and nano-CB/LDPE composites films were manufactured, and the space charge distribution was measured under DC voltage with and without the polarity reversal respectively by pulsed electro-acoustic (PEA) method to reveal the doping effect of nano-CB on the space charge distribution. According to the experimental results of LDPE, before the polarity reversal, homo-charges are injected and accumulate around the electrodes, and these space charges cannot dissipate completely after the voltage polarity being reversed which resulting in more serious electric field distortion. According to the test results of CB/LDPE after voltage polarity reversal, the density of shallow trapped charges decreases whereas the density of deep trapped charges increases, and the average charge density is reduced. The LDPE with 0.03wt% of CB nanoparticles shows sound performance on inhibiting the space charge accumulation after the voltage polarity reversal.

Direct current electrical characteristics of nano-CB/LDPE composite

Electric field distortion caused by the space charge accumulation in the insulation is considered to be one of the obstacles for the operation of high voltage DC cables. In this paper, nano-CB/LDPE composites were fabricated by using the melt blending method. Space charge characteristics and field distortions of nanocomposite films were measured under direct current (DC) field by pulsed electro-acoustic (PEA) method. Dielectric properties and DC breakdown strength were also investigated. Experimental results indicate that, by adding appropriate doping content of CB nanoparticles, the space charge injection threshold of the films was increased, which suppressed homo-polarity space charge injection and ameliorated the field distribution. Breakdown strength of nanocomposite decreased whereas the dielectric permittivity and dielectric loss tangent increased slightly. However, too much CB nanoparticles mixed in LDPE could weaken the suppression effect on charge injection and field distortion, and the LDPE with 0.03wt% of CB nano-particle showed a relative better ability of inhibiting field distortion.

Accelerated aging of fast thermal cycle effects on the behavior of space charge in polyimide

The spacecraft often interacts with plasma, high energy particles and magnetic fields on orbit. It causes the charge injected in the insulation materials and forms a high electric field. The discharge phenomenon will happen when the internal electric field reaches the discharge voltage threshold. The insulation material is a key component of electrical equipment on the spacecraft, its electrical properties affects the reliability and lifetime of the whole spacecraft. The insulation material on spacecraft withstands fast thermal cycle during operation on orbit, which causes internal thermal stress in it. The accumulation of internal thermal stress changes the material molecular structure and depolarization characteristics, thus affecting the behavior of the space charge inside the insulation material. In this paper, the temperature environment was achieved by contact heating plant and liquid nitrogen immersion to do accelerated aging test based on the temperature of operating spacecraft. By the mean of this method, 0, 50, 100, 150 and 200 times of the fast thermal cycle was carried out. The molecular structure characteristics, the thermally stimulated current characteristics and the space charge characteristics of the samples with different cycling times were studied through the methods including XPS, XRD, TSC and PEA. The results show that with the increase of fast thermal cycling times, the relative content of the C-N and C=O bonds increase and it also increases the shallow energy level charges. The charges are more easily to move inside the sample. Fast thermal cycle also increases the degree of crystallinity, thus the interface between crystalline region and amorphous region expands. It also increases the charge of deep energy level, which increased the amount of space charge injected in the sample.

Vapor-liquid equilibria of (CF 3 I + CO 2 ) at the temperature of 243.150K

It has become an imperative case on seeking the substitute of SF₆ for its severe greenhouse effect in recent years, not only for scientists but also the electricity sectors. CF₃I, as a substitute gas for SF₆ with lower Global Warming Potential (GWP), has been investigated. The use of pure CF₃I in gas insulated switchgear (GIS) and gas insulated metal enclosed transmission line (GIL) is difficult because CF₃I has a high dew temperature. We have therefore mixed CF₃I with CO₂. In this paper, an apparatus was used to get the vapor-liquid equilibrium data for (CF₃I + CO₂) system at 243.150 K. All of the experimental data were correlated with the Peng-Robinson (PR) EoS using the classical van der Waals (vdW) mixing rule (PR-vdW model). The results show the changing rule for the vapor pressure and its corresponding mole fraction of (CF₃I + CO₂) gas mixtures to suit the operation environment in GIS/GIL and supply an accurate basis for the application of (CF₃I + CO₂) gas mixtures in electric equipment.

Study on thermal aging oil-paper & polymer insulation based on acid value test

Oil-immersed transformer is the main equipment of power system operation, whose insulation aging research has been a topic that deserves attention. In this paper, based on the self-developed thermal aging oven, the pure oil sample and those of 3 kinds of different oil-immersed material (paper, Polycarbonate, Polyester) were aged under three different temperature (90°C, 110°C, 130°C) for accelerated aging process. By using Metrohm 877 type automatic titrator, acid value of samples with different aging time will be tested. Results suggest that under the same aging temperature, acid value of oil sample increases along with aging time. Accordingly, the higher the aging temperature is, the larger the acid value of samples under the same aging time will be. Moreover, the increase trend becomes steep gradually as aging temperature increases. By comparison among different materials, we found that in 90°C/110°C aging process, the increasing rate of acid value of oil-paper sample is the fastest, which is followed by the PET, PC and pure one. Specifically, under the aging condition of 130 °C, the increase rate of acid value of oil-PC sample is the fastest, and that of the pure one is still the slowest.

A study of waveform recovery in space charge test of PET by PEA method

In space charge test by using PEA method, acoustic dispersion and attenuation are unavoidable when sound travel in media. In order to reveal real information of space charge in the dielectric, original measured signal is need to be recovered. In this paper, acoustic propagation characteristics in PET was studied, and transfer function was established based on attenuation factors; by using deconvolution calculation, high order transfer function was exploited to restore space charge waveform; in consideration of that acoustic propagation and hardware performance are affected by temperature variation, PET samples were tested by PEA under different temperature, and a space charge waveform recovery method with temperature factor, which is suitable for PET material study is put forward.

Analysis on FRP rod of composite insulators by XPS

Composite insulators are widely used in transmission lines. Performance degradation of material is more and more urgent with operation time increases. In China Southern Power Grid, some composite insulators in-service suffer partial abnormal temperature rise which has been a serious potential hazard to operation of transmission lines. In this paper, a 500 kV AC composite insulator with partial abnormal temperature rise, was analyzed. By dissecting these insulators, FRP rod corrosion with varying degrees were found along the string. After that, chemical compositions on FRP rod corrosion surface were examined by XPS (X-Ray Photoelectron Spectroscopy). Test results showed that the basic elements, carbon, nitrogen and oxygen changes according to certain rules and the chain number of Si-O increases when corrosion degree aggravate. Analysis on change of chemical element reveals that the FRP rod corrosion of the composite insulators with partial abnormal temperature rise is mainly due to oxidation.