Hong-tu Song

Highly Sensitive Dissolved Hydrogen Sensor Based on Side-Polished Fiber Bragg Grating

To eliminate the time-consuming oil-gas separation process in online dissolved gas detection of power transformer, fiber Bragg grating (FBG)-based hydrogen sensor is proposed to be installed into the power transformer oil directly. In the sensor design, the FBG cladding is side-polished with a residual thickness of 20 μm and sputtered with palladium/silver. Since side-polished cladding is very sensitive to curvature strain induced by palladium in the presence of dissolved hydrogen, the proposed sensor is much more sensitive than the untreated FBG hydrogen sensor. Measurements prove that the sensitivity is as high as 0.477 (pm/(μL/L)), about 11.4 times higher than the conventional FBG hydrogen sensor. Furthermore, repeatability and short-term stability have been investigated. The performances satisfy the actual need of monitoring dissolved hydrogen concentration in power transformer oil.

Pd/Ag coated fiber Bragg grating sensor for hydrogen monitoring in power transformers

Compared with conventional DGA (dissolved gas analysis) method for on-line monitoring of power transformers, FBG (fiber Bragg grating) hydrogen sensor represents marked advantages over immunity to electromagnetic field, time-saving, and convenience to defect location. Thus, a novel FBG hydrogen sensor based on Pd/Ag (Palladium/Silver) along with polyimide composite film to measure dissolved hydrogen concentration in large power transformers is proposed in this article. With the help of Pd/Ag composite coating, the enhanced performance on mechanical strength and sensitivity is demonstrated, moreover, the response time and sensitivity influenced by oil temperature are solved by correction lines. Sensitivity measurement and temperature calibration of the specific hydrogen sensor have been done respectively in the lab. And experiment results show a high sensitivity of 0.055 pm/(μl/l) with instant response time about 0.4 h under the typical operating temperature of power transformers, which proves a potential utilization inside power transformers to monitor the health status by detecting the dissolved hydrogen concentration.

Research on High Sensitive D-Shaped FBG Hydrogen Sensors in Power Transformer Oil

Dissolved hydrogen is a symbol gas decomposed by power transformer oil for electrical faults such as overheat or partial discharges. A novel D-shaped fiber Bragg grating (D-FBG) sensor is herein proposed and was fabricated with magnetron sputtering to measure the dissolved hydrogen concentration in power transformer oil in this paper. Different from the RI (refractive index)-based effect, D-FBG in this case is sensitive to curvature caused by stress from sensing coating, leading to Bragg wavelength shifts accordingly. The relationship between the D-FBG wavelength shift and dissolved hydrogen concentration in oil was measured experimentally in the laboratory. The detected sensitivity could be as high as 1.96 μL/L at every 1-pm wavelength shift. The results proved that a simple, polished FBG-based hydrogen sensor provides a linear measuring characteristic in the range of low hydrogen concentrations in transformer oil. Moreover, the stable hydrogen sensing performance was investigated by X-ray diffraction analysis.

Tracing Acetylene Dissolved in Transformer Oil by Tunable Diode Laser Absorption Spectrum

Dissolved gas analysis (DGA) is widely used in monitoring and diagnosing of power transformer, since the insulation material in the power transformer decomposes gases under abnormal operation condition. Among the gases, acetylene, as a symbol of low energy spark discharge and high energy electrical faults (arc discharge) of power transformer, is an important monitoring parameter. The current gas detection method used by the online DGA equipment suffers from problems such as cross sensitivity, electromagnetic compatibility and reliability. In this paper, an optical gas detection system based on TDLAS technology is proposed to detect acetylene dissolved in transformer oil. We selected a 1530.370 nm laser in the near infrared wavelength range to correspond to the absorption peak of acetylene, while using the wavelength modulation strategy and Herriott cell to improve the detection precision. Results show that the limit of detection reaches 0.49 ppm. The detection system responds quickly to changes of gas concentration and is easily to maintenance while has no electromagnetic interference, cross-sensitivity, or carrier gas. In addition, a complete detection process of the system takes only 8 minutes, implying a practical prospect of online monitoring technology.

Tracing methane dissolved in transformer oil by tunable diode laser absorption spectrum

Dissolved gas analysis (DGA)in power transformers is related to the degradation of insulation materials and dissolved methane (CH 4 )is a symbol gas of low energy electrical or thermal faults in power transformer oil. Conventional online DGA equipment suffers from problems such as cross sensitivity, electromagnetic compatibility and reliability. Based on tunable diode laser absorption spectroscopy (TDLAS) technique, a novel optical method is proposed to detect dissolved methane in transformer oil. Oil samples with fault gases are prepared and dissolved methane detection has been carried out in the laboratory. 1653.72 nm is selected as the central wavelength of tunable diode laser, moreover, a multipass gas cell and wavelength modulation strategy is utilized to trace methane dissolved in oil. Results show that the resolution of detection reaches 0.28 µL/L, and accuracy is less than 2 µL/L at low concentration. In addition, the detection cycle is so time-saving, less than 5 minutes. The proposed measurement method shows tremendous advantages, such as immediate response to concentration changes, no cross interference, no carrier gases, which is proved to be a promising maintain-free technique instead of conventional DGA equipment.

Note: Dissolved hydrogen detection in power transformer oil based on chemically etched fiber Bragg grating

A fiber Bragg grating (FBG) sensor based on chemically etched cladding to detect dissolved hydrogen is proposed and studied in this paper. Low hydrogen concentration tests have been carried out in mixed gases and transformer oil to investigate the repeatability and sensitivity. Moreover, to estimate the influence of etched cladding thickness, a physical model of FBG-based hydrogen sensor is analyzed. Experimental results prove that thin cladding chemically etched by HF acid solution improves the response to hydrogen detection in oil effectively. At last, the sensitivity of FBG sensor chemically etched 16 μm could be as high as 0.060 pm/(μl/l), increased by more than 30% in comparison to un-etched FBG.

Dissolved hydrogen detection in power transformer based on etched fiber Bragg grating

Due to aging and degrading of insulation oil in power transformers, dissolved hydrogen, as a typical fault gas, would produce accompanied by discharges or overheating. Palladium (Pd) film deposited on the surface of chemically etched fiber Bragg grating (FBG) as sensing element by magnetron sputtering process is proposed in this paper. Volume expands when Pd film with 560nm thickness absorbs hydrogen molecules and wavelength shift caused by the strain could be measured. In this principle, hydrogen of low concentration can be obtained through wavelength shift of FBG. Different aspects have been taken into consideration including membrane thickness, polyimide coating and cladding diameter to obtain satisfactory sensitivity. Experimental results in the lab showed that this developed hydrogen could detect 30 μL/L at 1 pm approximately, which proved to be a prospective sensor as a on-line method to be utilized in power transformers.

Highly sensitive detection of methane based on tunable diode laser absorption spectrum

Traditional methane detection techniques in power transformer oil have the problems of cross sensitivity and insecurity factor. Based on Beer-Lambert spectral absorption law, low concentration methane detection with tunable diode laser absorption spectrum is proposed in this paper for the advantages of high sensitivity and resolution. Firstly, the central wavelength of 1653.72 nm in near infrared spectrum is selected. Then long optical length gas cell by means of refraction and reflection more than 20 times is built in the laboratory. At last, at different concentrations ranging from 50~1000 μL/L, the harmonic signal waveforms are obtained. Experimental results show that the lower detection limit of this developed methane system can detect 1.28 μL/L approximately and the deviation is less than 0.02 V, which proved to be a prospective sensing technique to be utilized in power transformers.

Dissolved hydrogen measurement with palladium-silver FBG sensor in transformer oil

A novel sensor used in detection of hydrogen concentration dissolved in transformer oil was proposed based on fiber Bragg grating (FBG). Palladium was introduced as hydrogen sensitive material and silver was alloyed to improve permeability in hydrogen sensitive layer. Meanwhile membrane of titanium was fabricated for enhancing the adhesion between fiber and hydrogen sensitive layer. The thin film on FBG was coated with magnetron sputtering. A novel sensor that was wavelength-sensitive to hydrogen dissolved in oil was fabricated and the experimental platform was set up in this paper. The sensitivity of the sensor was tested in transformer oil at 80 °C which was typically high operating temperature or local overheating temperature of power transformers. The experimental result demonstrated that the sensor performed well at low hydrogen concentration of 323 µL/L and the sensitivity of sensor was as high as 24.4 µL/L at every 1pm wavelength shift.

Dissolved acetylene detection in power transformer oil based on near-infrared absorption spectrum method

Acetylene is seen as a symbol gas to distinguish arcing/sparking occurrence and overheat inside the power transformer. However, conventional detectors suffer from gas consumption, unrepeatable measurement and regular calibration. Based on Lambert Beers law, this paper put forward near-infrared absorption spectrum method to detect acetylene. Experimental platform is set up in the laboratory, mainly including a tunable laser light source, two adjusted optical fiber collimators, a acetylene gas cell and a built-in InGaAs photodetector (PD). The strongest acetylene gas absorption wavelength of 1530.374 nm is determined. Results have demonstrated that it is possible to get access to dissolved acetylene by measurement of characteristic spectrum in power transformer oil with obvious advantages of time-saving and without consuming the to-be-measured gas.