Guo-ming Ma

A Fiber Bragg Grating Tension and Tilt Sensor Applied to Icing Monitoring on Overhead Transmission Lines

A novel ice monitoring system for the overhead transmission lines based on fiber Bragg grating (FBG) sensing is proposed in this paper. Compared to the existing systems, this system has several unique advantages, such as unnecessary power supply onsite, excellent ability for avoiding electromagnetic interference, and long lifespan. First, two near-elliptical-shaped concavities with FBG in each side are designed on the column structure to improve accuracy in measuring eccentric load. Then, a high reliability and high resolution tilt sensing section is developed based on a beam of uniform strength where an FBG is fixed on. Finally, an unforced FBG is placed in the sensor to solve the cross-sensitivity of strain and temperature in FBG sensing. Tension and angle experiments are conducted in our laboratory to calibrate the sensor. The tension experiment results indicate that the sensor is sensitive to tension, and the sensitivity and resolution of the sensor are 0.0413 pm/N and 24.21 N. The results of the tilt angle experiment show that the sensitivity and resolution of the sensor is 16.17 pm/° and 0.0619°. The temperature effect on the tension and angle measurement, evaluated by putting the sensor in an oven, is less than ±0.3% and ±0.38% separately. A 250-h outdoor experiment was carried out in the testing field, and the results prove the sensor can work properly in harsh environments and no creep is observed during the experiment.

A Passive Optical Fiber Anemometer for Wind Speed Measurement on High-Voltage Overhead Transmission Lines

Measurement of wind speed is an essential part of overhead transmission line monitoring. To overcome the drawbacks of the existing anemometer, such as the requirement on field power, the sensitivity to electromagnetic interference, and the unavailability of real-time measurement, a novel anemometer is developed on the basis of the fiber Bragg grating (FBG) sensing technique. In the anemometer, a beam with uniform strength was used to measure the force generated by the wind; two naked FBGs were symmetrically glued on the upper and lower surfaces of the beam to solve the problem of cross-sensitivity between strain and temperature of FBG sensing. Furthermore, the temperature effect on the zero output was investigated by putting the FBG anemometer into a temperature-controlled cabinet, and the experimental results indicated that the temperature effect on the zero output of the wind speed was less than ±5 pm (-10° C to 50 °C). The results from the wind tunnel experiment indicated that the measurement error of the anemometer was less than ±0.5 m/s.

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.

Measurement of VFTO Based on the Transformer Bushing Sensor

Disconnector switching operations in large gas-insulated switchgear (GIS) systems may produce very fast transient overvoltages (VFTOs), causing high electric stress to other apparatuses in the substation, such as power transformers. This paper presents a VFTO measuring technique based on a transformer bushing sensor, which consists of the transformer bushing (C1) and a capacitor sensor (C2) installed at the test tap of transformer bushing. In order to solve the problem of the frequency limitation of the transformer bushing sensor for measuring VFTO, a convolution model on the square-wave response of the transformer bushing sensor is introduced to obtain the frequency response of the transformer bushing sensor, and a deconvolution model based on the incremental Wiener filter is applied to reconstruct the input transient voltage waveform at the tip of the bushing by using the output signals from the bushing sensor. A 110-kV transformer bushing sensor and a fast transient overvoltage source are built in our laboratory. The results from the laboratory indicate that a transient voltage can be measured correctly based on our technique with the 110-kV transformer bushing sensor. A 750-kV transformer bushing sensor is built in a 750-kV substation. The measuring results from the 750-kV substation by using our technique show that the peak value of the VFTO produced by the disconnector (AIS) switching off operations is about 775 kV, and the dominant oscillating frequencies of the VFTO are 3.5 MHz, 1.6 MHz, and 650 kHz.

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.

A Wireless and Passive Online Temperature Monitoring System for GIS Based on Surface-Acoustic-Wave Sensor

A novel temperature monitoring system for gas-insulated switchgears (GISs) based on surface-acoustic-wave (SAW) sensing is proposed in this paper. Compared with the existing systems, this system has several unique advantages, such as being wireless, passive, and at a proper cost with continuous monitoring. First, an accurate SAW temperature sensor is designed for the GIS conductor, where epoxy encapsulating and loop antennas are adopted in the SAW sensor to prohibit the potential partial discharge of the sensor itself. After that, a multifunctional reader antenna is developed, which can measure the conductor temperature and partial discharge by the time-division multiplexing technique. Experiments are carried out to examine the accuracy and response speed of the SAW sensors, and the results indicate that the uncertainty of the contact temperature measurement is smaller than 10 °C. Moreover, the insulation level and anti-interference ability are demonstrated based on experiments carried out on a 126-kV GIS. At last, temperature monitoring performance of the developed system is investigated by changing the current in the 126-kV GIS conductor. The experiment results prove that the developed system can meet the requirement of temperature monitoring in GIS.

A Reliable Wide-Bandwidth VFTO Sensor Based on Surface-Mounted Devices

A capacitor sensor with a high-voltage electrode inside the GIS tank has the best bandwidth among the sensors developed to measure the very fast transient overvoltage (VFTO). However, the low-voltage arm of the current measurement system is usually a handmade film capacitor which has disadvantages of unassigned capacitance value, temperature effect, and short lifespan. With a novel lower voltage arm based on surface-mounted devices, a wide-bandwidth, stable, and easy maintenance measuring system is proposed in this paper. It is found that the slight inherent inductance in the ground trace of the printed-circuit board can induce unexpected oscillations, which are demonstrated with theoretical analysis and experimental study. With the reforming of the printed-circuit board layout, the oscillation is reduced and 200-MHz high-frequency cutoff frequency is achieved. The environment experiment shows that the temperature effect on the developed sensor is much smaller than that on the conventional sensor. The VFTO generated by disconnector operation in 1100-kV GIS is measured by the developed system and a conventional broadband capacitor sensor. The contrastive results prove that the characteristic of the measuring system can meet the requirement of VFTO measurements. Thousands of measurements on ultra-high voltage GIS indicate that the developed measuring system is stable, and characteristics of the VFTO are gained.

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.