Weigen Chen

Wavelet Networks in Power Transformers Diagnosis Using Dissolved Gas Analysis

Wavelet networks (WNs) are an efficient model of nonlinear signal processing developed in recent years. This paper presents a comparative study of WN efficiency for the detection of incipient faults of power transformers. After 700 groups of training and testing gases-in-oil samples are processed by fuzzy technology, we compare and analyze the network training process and simulation results of five WNs which include two types of WNs with two different activation functions and evolving WN. A lot of diagnostic examples show that the diagnostic accuracy and efficiency of the proposed five WN approaches prevail those of the conventional back-propagation neural-network method and are suitable for faults diagnosis of power transformers, especially with the evolving WN achieving superior performance.

Gas sensing properties and mechanism of nano-SnO 2 -based sensor for hydrogen and carbon monoxide

Nano-SnO2 powder was prepared by the hydrothermal method in this paper. X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the composition of the crystalline phase and themorphology of the prepared gas-sensitive materials, respectively. In particular, the study focused on the sensing behaviors of nano-SnO2-based sensor towards power transformer fault gases such as hydrogen and carbon monoxide. The optimum working temperature for hydrogen and carbon monoxide is about 400°C and 360°C, separately. Further investigations into the adsorption process of gas molecule on SnO2 (110) surface based on the first principles were conducted. The calculations indicated that 1σ orbits of H2 split into several new electronic peaks and 5s orbits of CO almost degenerated completely in the adsorption process, which promoted charge transfer between gas molecule and SnO2 (110) surface. It provides a qualitative explanation for the prepared nano-SnO2-based sensor exhibiting different gas sensing properties towards H2 and CO.

Hydrothermal Synthesis of Various Hierarchical ZnO Nanostructures and Their Methane Sensing Properties

Hierarchical flower-like ZnO nanorods, net-like ZnO nanofibers and ZnO nanobulks have been successfully synthesized via a surfactant assisted hydrothemal method. The synthesized products were characterized by X-ray powder diffraction and field emission scanning electron microscopy, respectively. A possible growth mechanism of the various hierarchical ZnO nanostructures is discussed in detail. Gas sensors based on the as-prepared ZnO nanostructures were fabricated by screen-printing on a flat ceramic substrate. Furthermore, their gas sensing characteristics towards methane were systematically investigated. Methane is an important characteristic hydrocarbon contaminant found dissolved in power transformer oil as a result of faults. We find that the hierarchical flower-like ZnO nanorods and net-like ZnO nanofibers samples show higher gas response and lower operating temperature with rapid response-recovery time compared to those of sensors based on ZnO nanobulks. These results present a feasible way of exploring high performance sensing materials for on-site detection of characteristic fault gases dissolved in transformer oil.

Pd-Doped SnO2-Based Sensor Detecting Characteristic Fault Hydrocarbon Gases in Transformer Oil

Methane (CH4), ethane (C2H6), ethylene (C2H4), and acetylene (C2C2) are important fault characteristic hydrocarbon gases dissolved in power transformer oil. Online monitoring these gaseous components and their generation rates can present the operational state of power transformer timely and effectively. Gas sensing technology is the most sticky and tricky point in online monitoring system. In this paper, pure and Pd-doped SnO2 nanoparticles were synthesized by hydrothermal method and characterized by X-ray powder diffraction, field-emission scanning electron microscopy, and energy dispersive X-ray spectroscopy, respectively. The gas sensors were fabricated by side-heated preparation, and their gas sensing properties against CH4, C2H6, C2H4, and C2H2 were measured. Pd doping increases the electric conductance of the prepared SnO2 sensors and improves their gas sensing performances to hydrocarbon gases. In addition based on the frontier molecular orbital theory, the highest occupied molecular orbital energy and the lowest unoccupied molecular orbital energy were calculated. Calculation results demonstrate that C2H4 has the highest occupied molecular orbital energy among CH4, C2H6, C2H4, and C2H2, which promotes charge transfer in gas sensing process, and SnO2 surfaces capture a relatively larger amount of electric charge from adsorbed C2H4.

Hydrothermal synthesis and hydrogen sensing properties of nanostructured SnO 2 with different morphologies

In this work, nanoscale SnO2 with various geometrical morphologies, including pine needle-like, sphere-like, sheet-like, grapelike nanostructures, was prepared via a facile hydrothermal process. Microstructures and morphologies of all the as-synthesized products were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Meanwhile, the specific surface areas of the as-prepared SnO2 nanostructures were determined by Brunauer-Emmett-Teller (BET) analysis. Gas sensorswere fabricated and their gas sensing properties towards hydrogenwere systematically investigated.The results indicate pine needle-like SnO2 structure exhibits exclusive better gas sensing performances to hydrogen than the other morphologies, which can be attributed to its novel shape with a large specific surface area. Such an unexpected morphology is a promising candidate for the use of SnO2 as a gas sensing material in future hydrogen sensor applications.

Analysis of furfural dissolved in transformer oil based on confocal laser Raman spectroscopy

Furfural (Furan-2-carbaldehyde) dissolved in transformer oil is regarded as an important mark of the thermal and mechanical degradation in oil-paper insulation. Confocal laser Raman spectroscopy (CLRS) is an effective means of detecting trace liquid and has been widely applied in many fields. Using CLRS to detect furfural concentration can overcome many disadvantages of traditional detection methods and accelerate testing. Thus, the application of CLRS in the detection of furfural concentration was investigated in this research. A structure model of a furfural molecule was constructed with Gaussian 09W software. Then, the Raman spectrum of this molecule was analyzed theoretically. Meanwhile, a set of liquid test platforms was set up based on basic CLRS principles. Subsequently, standard transformer oil samples were tested by utilizing extraction technology together with the CLRS liquid test platform. As a result, the peak at 1677 cm-1 was selected as the Raman characteristic peak. The detection limit reached 0.10 mg/L. In addition, a quantitative analysis model was constructed between the concentrations and the Raman characteristic peak areas by applying external standard method and least square method. Finally, six types of oil samples with different furfural concentrations were detected; the test results were compared with those obtained through high-performance liquid chromatography. Comparison results showed that CLRS with extraction can quantitatively and effectively detect the furfural concentration in transformer oil (with a maximum error less than 11.7%). Therefore, a new detection tool is presented for assessing the degradation of insulating papers in power transformers.

A New Broadband Microcurrent Transducer for Insulator Leakage Current Monitoring System

In this paper, a type of high bandwidth microcurrent transducer designed for online monitoring of an insulator is described. According to the equivalent circuit model, the effects of transducer winding number, stray capacitance, and loading resistance on passbandwidth and sensitivity are discussed. The academic lower limit of the transducer is nearly zero and the upper limit is infinity, which are proven by the analysis of the electronic circuit and the simulation by saber. The test of the current transducer shows that the transducer can be used in most high-voltage insulations systems with broad passbandwidth, high linearity, high sensitivity, and noncontact.

Heterodimer nanostructures induced energy focusing on metal film

As an interesting surface plasmon phenomenon discovered several years ago, electromagnetic field redistribution in nanoparticle dimer on film system provides a novel thought to enhance the light power on a plain film which could been widely used in surface enhanced Raman scattering (SERS), solar cells, photo-catalysis, etc. Homodimers on film are mainly investigated in past years, while the properties of heterodimers on film are still unclear. In this work, size difference induced electromagnetic field redistribution in Ag nanoparticle dimer on Au film system is investigated first. The results obtained from finite element method indicate that the smaller nanoparticle has much greater ability to focus light energy on Au film, which even reached more than 5 time compared to the larger one. Further researches indicate that this energy focusing ability has a strong relationship to the wavelength and diameter ration in dimer. Similar focusing phenomenon is found in the system of thick wire-smaller particle on film. Later, the SERS spectra collected in the small nanoparticle-large nanowire system provide an experimental evidence for this theoretic predication. Our results strengthen the understanding of surface plasmon on plane film and have potential application prospects in the surface plasmon related fields.

Facile hydrothermal synthesis and basic gas-sensing properties of two three-dimensional nanostructures of SnO 2

The hierarchical SnO2 sphere-like architecture, consisting of numerous thin nanosheets, was successfully synthesized via a facile hydrothermal method. The structures and morphologies of this hierarchical architecture were characterized in detail by means of powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and Brunauer-Emmett-Teller (BET). Further comparative experiments of gas-sensing performances of the as-prepared SnO2 were investigated towards ethanol. It shows this three-dimensional, sheet-spheres, SnO2 as a potential gas-sensing material for a broad range of future sensor applications, like sensitive response to other gases such as hydrogen, carbonic oxide, and methane.

Using a sensitive optical system to analyze gases dissolved in samples extracted from transformer oil

A sensitive optical system which can be used to measure accurately the concentrations of fault gases dissolved in samples extracted from transformer oil is described, and some test results are presented.