Ende Hu

Bio: Ende Hu is an academic researcher from Chongqing University. The author has contributed to research in topics: Electrical insulation paper & Space charge. The author has an hindex of 4, co-authored 6 publications receiving 28 citations.

Effects of antioxidants and acids on copper sulfide generation and migration induced by dibenzyl disulfide in oil‐immersed transformers

Abstract: This study aims to explore the effect of the antioxidant 2,6-di-tert-butyl-p-cresol (DBPC) and acids on the generation and migration of copper sulfide on insulating windings as a result of the reaction of dibenzyl disulfide (DBDS) and copper in oil-immersed transformers. The heating experiments were conducted by adding different amounts of DBPC and different kinds of acids to the naphthenic mineral oil. The result indicates that acids can accelerate the generation of copper sulfide and that DBPC cannot effectively consume DBDS to reduce the corrosion phenomenon, but rather accelerates the deposition of copper sulfide onto the insulating paper. In particular, when the DBPC concentration is approximately 0.9% (w/w) at 130 °C for ten days, the amount of copper sulfide deposited on the insulating paper reaches its maximum. Moreover, excessive use of the antioxidant DBPC accelerates the aging of insulating oil, on the contrary, which adversely affects the mineral oil. Although the addition of DBPC can improve the antioxygenic property of mineral oil, too much of it is inadvisable because the transformers that contain DBPC and DBDS may be at high risk and suffer from breakdown. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Space Charge Behavior in Paper Insulation Induced by Copper Sulfide in High-Voltage Direct Current Power Transformers

Abstract: The main insulation system in high-voltage direct current (HVDC) transformer consists of oil-paper insulation. The formation of space charge in insulation paper is crucial for the dielectric strength. Unfortunately, space charge behavior changes because of the corrosive sulfur substance in oil. This paper presents the space charge behavior in insulation paper induced by copper sulfide generated by corrosive sulfur in insulation oil. Thermal aging tests of paper-wrapped copper strip called the pigtail model were conducted at 130 °C in laboratory. Scanning electron microscopy (SEM) was used to observe the surface of copper and paper. Pulse electroacoustic (PEA) and thermally stimulated current (TSC) methods were used to obtain the space charge behavior in paper. Results showed that both maximum and total amount of space charge increased for the insulation paper contaminated by semi-conductor chemical substance copper sulfide. The space charge decay rate of contaminated paper was significantly enhanced after the polarization voltage was removed. The TSC results revealed that copper sulfide increased the trap density and lowered the shallow trap energy levels. These results contributed to charge transportation by de-trapping and trapping processes. This improved charge transportation could be the main reason for the decreased breakdown voltage of paper insulation material.

The Investigation on Thermal Aging Characteristics of Oil-Paper Insulation in Bushing

Abstract: Bushing is the key link to connect outer and inner insulating systems and also the essential electric accessory in electric power system, especially in the high voltage engineering (AC 1000kV, DC 800kV). This paper presented the experimental research of thermal aging characteristic of oil- paper insulation used in bushing. A thermally accelerated aging experiment at 90℃ was performed. The bushing models containing five layers of paper were sealed into the aging vessels and further aged for 250 days. Then several important parameters associated with the aging were observed and evaluated. The results showed that the degree of polymerization (DP) of papers gradually decreased. The DP values of outermost layer and middle layer fit well into the second-order kinematic model and first-order kinematic model, respectively. Less deterioration speed of the inter-layer paper than outer layer was confirmed by the variation of DP. Hydrolysis was considered as the main cause to this phenomenon. In addition, the logarithm of the furfural concentrations in insulation oil was found to have good linear relationship with DP of papers. Interestingly, when the aging time is about 250 days and DP is 419, the aging process reaches an inflection point at which the DP approaches the leveling off degree of polymerization (LODP) value. Both tanδ and acid number of oils increased, while surface and volume resistivity of papers decreased. The obtained results demonstrated that thermal aging and moisture absorbed in papers brought great influence to the degradation of insulating paper, leading to rapid decrease of DP and increase of the tanδ . Thus, the bushing should be avoided from damp and real-time monitoring to the variation of tanδ and DP values of paper is an effective way to evaluate the insulation status of bushing.

The impact of corrosive sulfur concentration in transformer oil on the electrical properties of insulated windings

Abstract: Dibenzyl disulfide (DBDS) is considered as one of the sulfur compounds that can cause copper corrosion in transformers. The deposition on insulated windings is found to be more likely formed with an increase in DBDS concentration which changes the electrical properties of insulated windings dramatically. In this paper, three transformer insulating oils were prepared by adding with different DBDS concentrations in laboratory condition, subsequently a paper-wrapped copper winding model was immersed in oil followed by an accelerated thermal aging at 130°C. Spectrum analysis was adapted to determine the elements content that were deposited on the surface of insulating paper. A high resistance meter (keithley 6517B) was used simultaneously to test volume/surface resistivity of paper-wrapped insulated windings which describes the frequency dependence of permittivity. The results showed that higher DBDS concentration in transformer insulating oil accelerated both the deposition on the surface of the insulating paper and on copper. Furthermore, the volume/surface resistivity decreased significantly. Our study examines the change of electrical properties of insulated winding. The results should be and beneficial to understand the sulfur corrosion mechanism in transformer oils.

Test device and test method for electric heat joint aging of transformer winding

Abstract: The invention discloses a test device and a test method for electric heat joint aging of a transformer winding The test device comprises a sealing tank, a support plate and a high voltage draining device, wherein the sealing tank is used for supplying an aging environment; the support plate is used for supporting a transformer winding model; the high voltage draining device is used for supplying a high voltage power supply and a high voltage electrode; a baffle is arranged in the sealing tank and divides the sealing tank into two independent sealing spaces, and the support plate and the high voltage draining device are arranged on a same side of the sealing tank According to the test device and the test method, electric heat joint aging and heat aging test of the transformer winding can be simultaneously performed in a same sealing environment, so that influence of single factor variable-electric field on oiled paper aging can be better analyzed

Causes of transformer failures and diagnostic methods – A review

Abstract: This paper presents a review on the sources of failures of transformer in the substation. Different investigations and test analyses have been conducted to identify the root causes of failure of the transformer in the power system, and to identify the preventive measures to avoid these breakdowns. The review work has been presented with the focus on bushing-failure, which has been identified as one of the major causes of transformer breakdown in different reported studies. In determining the lifespan of the insulation of the high voltage bushing, along with different factors, moisture effect has also been considered in presenting the degradation analysis. On-line measurement has been preferred over off-line measurements due to its benefits measuring the reliability and uninterrupted functioning of the future generation, transmission and distribution systems. Post fire-outbreak, external inspection of the damaged bushing along with analytical results, obtained with the aid of Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) analysis, have also been included in this study. In the analysis, electric field and equipotential lines with different oil levels have been simulated using COMSOL Multiphysics software.

Reviews on sulphur corrosion phenomenon of the oil–paper insulating system in mineral oil transformer

Abstract: The active sulphur compounds in transformer oil would react with copper wires and produce cuprous sulphides, which then attach themselves to the surface of copper wires and insulating paper, thus reducing the electrical performance of oil–paper insulation and causing insulation failure. This article aims to make a comprehensive summary on the field of sulphur corrosion of oil–paper insulation in the mineral oil transformer, including the corrosion mechanism of sulphide on copper sheets, the deterioration mechanism of sulphide on oil–paper, the factors influencing sulphur corrosion reaction, the method of characterising the degree of sulphur corrosion, the protection technology of sulphur corrosion and the field operation measures for sulphur corrosion. Moreover, four key problems that need to be further solved were sorted out: the synergistic deterioration mechanism of sulphide and its transformation mechanism, the micro-process and forming path of sulphur corrosion reaction, the characteristic parameters and quantitative characterisation methods of sulphur corrosion degree, and the effective protection technology of sulphide corrosion. To solve these problems, efforts will be made for deeper understanding of the sulphur corrosion phenomenon and its deterioration mechanism based on theoretical background, the assessment methods and protection technology, endeavouring to put forward solid theoretical basis and feasible technical means for preventing sulphur corrosion insulation failure.

Application of response surface methodology for optimizing the oxidative stability of natural ester oil using mixed antioxidants

Abstract: Natural ester insulation (NEI) oils have been developed since the early 1990s due to the increasing environmental, health and safety concerns from the public. NEI oils are well-known for having high fire and flash points, great moisture tolerance, good biodegradability and non-toxicity. However, NEI oils also have several drawbacks compared to mineral insulation (MI) oils such as low oxidative stability, high viscosity, low pour point and low resistance towards lightning impulse. Previous researchers have discovered that the oxidative stability of NEI oils can be improved by the addition of mixed antioxidants. However, such studies are focused on only one criterion and they neglect the dielectric strength of the oils. On the other hand, the one-factor-at-a-time (OFAT) method is commonly used to determine the optimum concentration of mixed antioxidants and requires a large number of samples. In this study, response surface methodology (RSM) technique is used to determine the optimum concentration of mixed antioxidants which will enhance the oxidative stability of NEI oil. The dielectric strength of the NEI oil is evaluated based on the AC breakdown voltage (BdV). RSM is chosen in this study because it is capable of determining the optimum values of the parameters of interest, without the need for a large number of test runs which will incur high cost for experimentation. A regression model is developed in this study based on the AC breakdown voltage of the NEI oil, propyl gallate and citric acid antioxidant concentrations. The oxidative induction time (OIT) and partial discharge inception voltage (PDIV) are carried out to determine the effect of the oxidative stability and withstand voltage on the partial discharge, respectively. The results showed that there is enhancement in the oxidative stability and dielectric strength of NEI oil using the optimum concentrations of mixed antioxidants relative to the fresh NEI oil. It can be concluded that RSM is a feasible alternative to determine the optimum concentration of antioxidants for NEI oils, which in turn, helps improve the oxidative stability and dielectric strength of these oils.

A modified X-model of the oil-impregnated bushing including non-uniform thermal aging of cellulose insulation

Abstract: The frequency domain spectroscopy (FDS) technique is widely used for the aging condition assessment of cellulose insulation on the bushing core. Generally, these assessment activities could be completed by the analysis of the FDS data between the laboratory sample and field bushing with the help of the traditional X-model. However, the aging condition of cellulose insulation on the bushing core is non-uniform due to the existence of temperature gradient. In this case, the traditional X-model is considered inappropriate since it ignored the effect of non-uniform thermal aging. In view of this issue, a modified X-model is reported in this work. Relying on the theoretical analysis of the modified X-model, both the mathematical expression and the equivalent circuit is derived naturally. The reliability of the modified X-model is confirmed by the experimental researches. In addition, the effect of aluminum foil in the modified X-model on FDS data was also studied. Findings reveal that the addition of aluminum foil would cause a slight deviation of the FDS curves. Therefore, the modified X-model might utilize to study the information of the non-uniform thermal aging. In that respect, the contribution of this work is in the exploration of the modified X-model as a potential tool for connecting FDS data corresponding to the lab condition and field condition including non-uniform thermal aging of cellulose insulation on the bushing core.

Inhibition method for the degradation of oil–paper insulation and corrosive sulphur in a transformer using adsorption treatment

Abstract: This study investigated the inhibitory effects of adsorption treatment on the degradation of oil–paper insulation and corrosive sulphur in oil. A thermal ageing experiment at 130°C was conducted using five different adsorbents to adsorb some impurities in oil on the 15th day of a 30 day ageing test, the related characteristics of oil–paper insulation before and after the adsorption treatment were measured and analysed. Meanwhile, a thermal ageing experiment at 150°C was also conducted before using five different adsorbents to adsorb dibenzyl disulphide (DBDS) in oil, energy dispersive X-ray was used to evaluate the degree of the corrosion of windings. The result indicates that adsorption treatment is an effective method to inhibit the degradation of oil–paper insulation. A molecular sieve, silica gel and activated alumina can effectively improve the performance of oil–paper insulation. By contrast, the antioxidant (2, 6-di-tert-butyl-p-cresol) can be also adsorbed by adsorbents, which accelerate the ageing degradation of the insulating oil. On the other hand, the adsorbents can adsorb DBDS in oil, but cannot remove all DBDS. The effect achieved from silica gel is especially significant, which not only effectively improves the performance of oil–paper insulation but also adsorbs DBDS in oil.