Xuetong Zhao

Effect of nano-Al 2 O 3 on the thermal aging physicochemical properties of insulating paper

Nano-Al₂O₃ is added into the insulating paper in preparation. The composite paper and ordinary paper samples are thermally aged in mineral insulating oil at the temperature of 130 °C for 31 days. The variation of degree of polymerization (DP) and tensile strength with aging time are obtained in this work. The furfural content and dissolved CO and CO₂ in oil are analyzed. The results show that, comparing with ordinary paper, the DP and tensile strength of composite paper decrease more slowly during the aging process. The furfural content and dissolved CO and CO₂ in oil which impregnate composite paper are less. It can be thought that the nano-Al₂O₃ can inhibit the thermal aging of insulating paper. Additionally, it is considered that the hydroxy on the surface of nano-Al₂O₃ can effectively adsorb H₂O and neutralize low molecular weight acid in the thermal aging process, which restrains the catalysis of H⁺ in thermal aging reaction and reduces the thermal aging of insulating paper.

Effect of temperature on 2-furfural partitioning in the oil-paper system of power transformers

Furfural content in oil is a valuable indicator of cellulose paper degradation and is widely utilized in aging assessment for power transformers. However, with the same aging degree of insulation paper, the actual 2-furfural content in oil is influenced by temperature because 2-furfural partitioning behavior in the oil-paper insulation system of power transformers changes with temperature. Thus, correction of the temperature effect on 2-furfural partitioning is crucial for accurate assessment of the aging of power transformers. In this research, oil-paper samples with different 2-furfural contents were prepared through an accelerated thermal aging test. A 2-furfural equilibrium experiment was conducted at different temperatures to construct a 2-furfural partitioning diagram. Results showed that 2-furfural partitioning behavior is influenced by temperature. At high temperatures, 2-furfural tends to migrate from paper to oil. To correct the temperature effect on 2-furfural partitioning, a 2-furfural correction factor was developed to normalize 2-furfural contents in oil at different temperatures to a reference temperature. And the 2-furfural correction factor was verified by experimental data.

Effects of temperature and aging on furfural partitioning in the oil-paper system of power transformers

Furfural content in oil is a valuable indicator of cellulose paper degradation and is widely utilized in aging assessment for power transformers. However, with the same aging degree of insulation paper, the actual furfural content in oil is influenced by temperature because furfural partitioning behavior in the oil-paper insulation system of power transformers changes with temperature. Thus, correction of the temperature effect on furfural partitioning is crucial for accurate assessment of the aging of power transformers. In this research, oil-paper samples with different furfural contents were prepared through an accelerated thermal aging test. A furfural equilibrium experiment was conducted at different temperatures to construct a furfural partitioning diagram. The mass fraction of furfural in oil and paper was calculated. Results showed that the mass fraction of furfural in oil increases with temperature rising and aging deepening, which means that furfural tends to migrate from paper to oil at high temperatures and deep aging conditions. The change in furfural partitioning between oil and paper at different temperatures causes a deviation in paper aging assessment. To correct the temperature effect on furfural partitioning, a furfural correction factor was developed to normalize furfural contents in oil at different temperatures to a reference temperature. The furfural correction factor was verified by experimental data and then introduced into the relation between degree of polymerization of paper and furfural content of oil. Verification results showed that the corrected equation can enhance the accuracy of paper aging estimation at different temperatures. The findings of this work indicated temperature effect on furfural partitioning should be considered when determining the furfural thresholds and evaluating the aging condition of power transformers.

Improving the anti-icing/frosting property of a nanostructured superhydrophobic surface by the optimum selection of a surface modifier

To understand the effect of chemical composition on the anti-icing properties of a nanostructured superhydrophobic surface (SHP), four SHP surfaces were prepared on glass, which was initially roughed by a radio frequency (RF) magnetron sputtering method and then modified with HDTMS (a siloxane coupling agent), G502 (a partially fluorinated siloxane coupling agent), FAS-17 (a fully fluorinated siloxane coupling agent) and PDMS (a kind of polysilicone widely used in power transmission lines). Results show that the anti-icing properties of these four SHP surfaces in glaze ice varied wildly and the as-prepared SHP surface which was modified with FAS-17 (SHP-FAS) demonstrated a superior anti-icing/frosting performance. Approximately 56% of the entire SHP-FAS remained free of ice after spraying it for 60 min with glaze ice, and the average delay-frosting time (the time taken for the whole surface to become covered with frost) was more than 320 min at −5 °C. Equivalent model analysis indicates that ΔG, defined as the difference in free energy of the Cassie–Baxter and Wenzel states, of the SHP-FAS is much lower than the other three SHP surfaces, giving priority to Cassie state condensation and the self-transfer phenomenon helping to effectively inhibit the frosting process by delaying the ice-bridging process, which is beneficial for improving the anti-frosting property. This work sheds light on and improves understanding of the relationship between anti-icing and anti-frosting properties and is helpful in making the optimum selection of a surface modifier for improving the anti-frosting/icing performances of a SHP surface.

Influence of nano-Al 2 O 3 on electrical properties of insulation paper under thermal aging

Composite papers (hand sheets) with 2wt% nanoAl<inf>2</inf>O<inf>3</inf> doped and blank papers were prepared and the accelerated thermal aging test under 130^oC with oil-blank paper and oil-composed paper were performed. The main electrical parameters of paper were tested. According to corresponding data analysis, it can be concluded that the addition of nano-Al<inf>2</inf>O<inf>3</inf> can improve the electrical properties and anti-aging ability of composite paper. The “electric double layer” structure of nanoAl<inf>2</inf>O<inf>3</inf> can weaken the interfacial polarization and dipole polarization in composite paper, thus reducing the permittivity and dielectric loss of composite paper.

A new accelerated thermal aging test for over-loading condition transformer

Transformer life span is severely affected by hot spot insulation failure. In operational transformers, heat mainly generated by windings and iron cores. Temperature of winding hot spot under over-load condition can reach a dangerous level. Due to oil circulation and heat convection, there is temperature gap between oil and windings. In addition, temperature gradients exist among windings and oil at axis direction, respectively. However, according to current method, both windings and oil are put into heating cabinet together to conduct accelerated thermal aging and no temperature gradients exist. For a better and more accurate oil-paper insulation aging model, one new method has been developed and initial testing has been made to conduct thermal aging test. In this procedure, winding sample is made into a `U type three-layer structure. The winding is fixed at the copper electrodes and then is immersed with oil into a sealed tank. Current produced by large current generator and flows through windings to heat up windings. On account of heat convection and oil circulation, there are temperature gradients at axis direction for both windings and insulating oil, respectively. Top layer windings temperature is precisely controlled within the preset temperature by adjusting current to simulate hot spot temperature status. Temperature distribution of oil and windings at axis direction in tank are recorded, respectively. Then 21-days thermal aging testing is conducted under 148°C. Degree of polymerization (DP) values of winding paper of each layer at different height during whole aging period are obtained.

Fabrication and anti-icing property of superhydrophobic coatings on insulator

Surface icing of transmission lines may inflict serious consequences such as tower collapse and power failure. Superhydrophobic (SHP) surface is believed to be potential candidate to realize anti-icing. Here we reported a method to fabricate SHP surface on insulator to enhance its anti-icing performance. The as-prepared SHP surface exhibits a contact angle (CA) of 161° and sliding angle (SA) of 1°. The anti-icing behavior of the as-prepared SHP surface was investigated by an artificial climate chamber. Results showed the as-prepared SHP surface can effectively mitigate the freezing process and reduce ice accumulation. Only isolated ice points formed on the SHP surface. Due to the micro/nano binary structures of the SHP surface, a large amount of air was trapped in the textures, which contributed to the superhydrophobicity. This method is easy to handle and doesnt need any special facility which may have potential future applications in transmission lines against ice accumulation.

PVDF energy-harvesting devices: Film preparation, electric poling, energy-harvesting efficiency

Poly(vinylidene fluoride) (PVDF) has been recently recognized as an attractive candidate for energy-conversion applications that are fabricated on its piezo- and pyroelectric activities. In this work, Commercial poly(vinylidene fluoride) (PVDF) films are uniaxially stretched with varying rates at 110 °C in order to endow PVDF piezo- and pyroelectric mainly via crystalline-phase transition from nonpolar α to polar β that are identified by infrared (IR) spectroscopy and X-ray diffraction (XRD). The fast stretching promotes the α-to-β phase transition and the quite high β-phase fraction in films resultantly. Additionally, fast stretching benefits to producing a slightly increase of γ phase. Sufficient high electric field applied during poling results in α-to-β phase transition according to the XRD observations. The quantified evaluation on XRD data illustrates that the poling accounts for size-increase and better chain-packing of the β-phase crystallites. The energy-conversion performance of the stretched films that were impacted by water drops as well its relation to the piezoelectric-voltage activity was evaluated by measuring the peak-voltage outputs.