Ying-ting Luo

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.

Fiber bragg grating sensor for hydrogen detection in power transformers

A novel hydrogen sensor is designed on the basis of Fiber Bragg grating (FBG) sensing technique. The sensor can be arranged inside the transformer. It has several advantages, such as fast fault detection and location, immunity to electromagnetic interference, quasi-distribution measurement and real-time monitoring. The principle of the proposed FBG hydrogen sensor is based on changes of the physical properties of palladium films which absorb hydrogen. A thick palladium layer prepared by magnetron sputtering is used to achieve high sensitivity. Meanwhile, polyimide is added into the adhesive layer to improve the reliability of the sensor. Partial discharge experiments demonstrated that the wavelength shift of the FBG hydrogen sensor varies linearly with the concentration of hydrogen dissolved in the transformer oil. It is hardly disturbed by other factors. The hydrogen sensing experiment in oil at a temperature of 80°C revealed that the sensitivity of the sensor remains same as the temperature varies, ranging from room temperature to operating temperature of the power transformer. Thus, the proposed sensor can work properly under the operating temperature of power transformers.

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.

High Sensitive FBG Sensor for Equivalent Salt Deposit Density Measurement

Online monitoring of the contamination condition of power transmission line is important to prevent insulation from flashover. The conventional online monitoring methods are easily disturbed by the strong electromagnetic interference around the high voltage line. In this letter, Bragg wavelength of polyimide coated fiber Bragg grating (FBG) is found to be slightly restrained by the salt deposited on the polyimide surface. Based on the effect, a special designed FBG sensor is proposed for detecting equivalent salt deposit density (ESDD) on insulators of power transmission line. To improve the sensor sensitivity, multiple polyimide layers are coated on the naked FBG, and in the curing process a 0.15-N force is applied on the free end of FBG to form uniform layers. Experiment results show the restrain wavelength varies with the ESDD. The resolution can be improved with multiple polyimide layers.

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.