Sihang Gao

Bio: Sihang Gao is an academic researcher from Chongqing University of Posts and Telecommunications. The author has contributed to research in topics: Transformer oil & Corrosion. The author has an hindex of 7, co-authored 18 publications receiving 85 citations. Previous affiliations of Sihang Gao include Chongqing University.

Corrosion Mechanisms for Electrical Fields Leading to the Acceleration of Copper Sulfide Deposition on Insulation Windings

Abstract: Numerous failures of high-voltage transformers and reactors are caused by copper sulfide formation in oil-immersed insulations. This study explored the effect of electrical fields on copper sulfide formation. Accelerated aging experiments were conducted for mineral oil that contains dibenzyl disulfide, which was aged along with insulation windings under different conditions, including single thermal aging and electrical–thermal aging. The corrosive sulfur deposits were quantified using SEM/EDX and ICP-AES. The properties of the insulation oils were also measured and analyzed. Corrosion mechanisms for electrical fields leading to the acceleration of copper sulfide deposition on the oil-immersed insulation were proposed.

Thermal aging test for transformer oil–paper insulation under over-load condition temperature

Abstract: The accelerated thermal aging test is an effective method to simulate insulation material degradation in oil-immersed transformers. Traditionally, cellulose paper and mineral oil are heated up together at the same temperature when a thermal aging test is conducted. However, variation occurs between laboratory results and field conditions. Temperature gradients exist in real transformers, and each part of the winding is aged under various conditions. In order to rebuild temperature gradients and conduct a thermal aging test under this condition. First, 3D models with designed U-shape wingding which can guarantee different parts aged simultaneously under different temperatures are calculated for temperature field analysis. Then a thermal aging platform with a large current generator is proposed to construct temperature gradients in a thermal aging test. The large current generator helps to heat up the copper winding. Temperature sensors are installed in the aging tank to monitoring winding and oil temperature and control temperature. The aging temperature is selected according to the standard IEC 60076-7 for the over-load condition. The degree of polymerisation of the insulation paper is measured and properties of each layer of the insulation paper are analysed. Other aging products under both traditional test and method in this study are compared.

Effects of sulfur corrosion on the properties of oil–paper insulation induced by dibenzyl disulfide

Abstract: As a corrosive sulfur, dibenzyl disulfide (DBDS) is an efficient antioxidant in oil-immersed insulation. This study explored the reaction mechanism of corrosive sulfur with copper through heating experiments stipulated by IEC 62535. Thermal aging experiments were further conducted at 130 °C under nitrogen to investigate the comprehensive effect of the oxidation resistance and corrosivity of DBDS on the properties of oil-paper insulation. Experimental results showed that DBDS can decompose into corrosive benzyl mercaptan at a high temperature. Dibenzyl sulfide cannot corrode copper to form copper sulfide at 150 °C. Furthermore, the oxidation resistance and corrosivity of DBDS depend on the dynamic change in DBDS concentrations. This study suggests that 25 mg/kg should be used as the maximum additive content of DBDS in oil. With this additive content, the oxidation resistance of oil can be improved, and the corrosion damage of insulation winding can be controlled to a certain extent.

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.

Failure mechanism of transformer oil-immersed cellulosic insulation induced by sulfur corrosion

Abstract: The life of transformer oil-paper insulation is determined by the insulation property of transformer cellulosic insulation. Existing studies have shown that sulfur corrosion poses a serious threat to the properties of transformer oil-immersed cellulosic insulation due to the reaction of corrosive sulfur dissolved in oil with copper winding. This paper investigated the effect of sulfur corrosion on the characteristics of transformer cellulosic insulation, cellulosic insulation winding samples with different degrees of sulfur corrosion were obtained through an accelerated thermal aging experiment. Then, short-/long-term withstand voltage tests for the cellulosic insulation winding were conducted. Experimental results showed that the contamination of sulfur corrosion had a minimal effect on the power–frequency breakdown voltage of the cellulosic insulation winding but considerably decreased its long-term electrical life. Combined with the analysis of the electrical/physical and chemical parameters of oil-immersed cellulosic insulation, the failure mechanism of transformer oil-immersed cellulosic insulation induced by sulfur corrosion was synthetically discussed and analyzed.

Photoluminescence Spectroscopy Measurements for Effective Condition Assessment of Transformer Insulating Oil

Abstract: Condition assessment of insulating oil is crucial for the reliable long-term operation of power equipment, especially power transformers. Under thermal aging, critical degradation in oil properties, including chemical, physical, and dielectric properties, occurs due to the generation of aging byproducts. Ultraviolet-visible (UV-Vis) spectroscopy was recently proposed for the condition assessment of mineral oil. However, this absorption technique may involve all electronic states of the investigated material which typically yield a broad spectrum, and thus cannot precisely reflect the electronic structure of aged oil samples. It also cannot be implemented as an online sensor of oil degradation. In this paper, photoluminescence (PL) spectroscopy is introduced, for the first time, for effective condition assessment of insulating oil. The PL technique involves emission processes that only occur between a narrow band of electronic states that are occupied by thermalized electrons and consequently yields a spectrum that is much narrower than that of the absorption spectrum. Aged oil samples with different aging extents were prepared in the laboratory using accelerated aging tests at 120 °C, under which 1 day of laboratory aging is equivalent to approximately 1 year of aging in the field. These aged samples were then tested using PL spectroscopy with a wavelength ranging from 150 nm to 1500 nm. Two main parameters were evaluated for quantitative analysis of PL spectra: The full width at half-maximum and the enclosed area under the PL spectra. These parameters were correlated to the aging extent. In conjunction with PL spectroscopy, the aged oil samples were tested for the dielectric dissipation factor as an indication of the number of aging byproducts. Interestingly, we find a correlation between the PL spectra and the dielectric dissipation factor. The results of PL spectroscopy were compared to those of UV-Vis spectroscopy for the same samples and the parameters extracted from PL spectra were compared to the aging b-products extracted from UV-Vis spectra. Finally, the corresponding physical mechanisms were discussed considering the obtained results and the spectral shift for each spectrum. It was proved that PL spectroscopy is a promising technique for the condition assessment of insulating oil when compared to conventional transformer oil assessment measuring techniques and even to other optical absorption techniques.

Comparative Study on the Thermal-Aging Characteristics of Cellulose Insulation Polymer Immersed in New Three-Element Mixed Oil and Mineral Oil

Abstract: Cellulose paper, whose main component is cellulose polymer, has been widely used in oil-immersed power transformer that gradually deteriorates during transformer operation. Thermal aging is the main degradation form for cellulose paper immersed in insulation oil (oil–paper insulation) in a transformer. One of the most challenging issues in oil–paper insulation is inhibiting the aging of cellulose paper and extending its life. In this work, a comparative study was conducted on the thermal-aging characteristics of cellulose paper immersed in a novel three-element mixed insulation oil and mineral oil at 130 °C for 150 days. The key parameters of cellulose paper were analysed, including the degree of polymerization (DP), thermal-aging rate, surface colour, and AC breakdown voltage. The furfural content and acidity of the oil, as well as the AC breakdown voltage of the insulation oil were also analysed. The results show that the cellulose paper immersed in novel three-element mixed insulation oil had much higher DP values than that immersed in mineral oil after the same thermal-aging time. The mixed insulation oil could significantly inhibit the thermal aging of cellulose paper and prolong its life. The thermal-aging rate of the cellulose insulation polymer immersed in mixed insulation oil is significantly lower than that immersed in mineral oil, whether in the process of oil–paper insulation continuous aging or in the process of aging after oil replacement with unused insulation oil. The furfural generated by cellulose degradation in the novel three-element mixed insulation oil was also less than that in the mineral oil. The mixed insulation oil had a higher acidity value during the thermal-aging process, which was mainly due to the natural esters in the components of the mixed insulation oil. However, the AC breakdown voltage of the mixed insulation oil was always higher than that of the mineral oil. This study offers a new perspective in inhibiting the thermal aging of cellulose polymer in insulation oil.