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1 Introduction
As a new type of temperature sensor, optical fiber temperature sensor has many advantages, such as high measurement accuracy, anti-electromagnetic interference, safety and explosion-proof, fire insulation and so on, and has been widely used in many special occasions. Therefore, the research and development of optical fiber temperature sensor has always been one of the hot and difficult points in the field of optical fiber sensors. Especially in the power system, power generation, transmission and distribution systems are accompanied by harsh magnetic fields and temperature environments (such as high radiation and high voltage). ), so the optical fiber sensor made of insulating material plays an important role in parameter measurement and monitoring of power system. Many research institutions are committed to developing practical optical power sensors, and some of them have been successfully tested. LUXTRON has successfully developed an optical fiber sensor for large-scale monitoring of the hot spot temperature of transformer windings. The measurement accuracy of this system is 2℃ in the range of -30 ~ 200℃. By using this sensor, the temperature field of transformer is detected and measured in the process of R&D, life estimation and dynamic load temperature management. In order to further improve the accuracy, this paper puts forward an accurate temperature sensing technology, and develops a multi-channel optical fiber temperature sensor to meet the needs of multi-point temperature monitoring of large transformers.
2 Basic principles
Fig. 1 is the schematic diagram of an optical fiber temperature sensor, which consists of a broad spectrum light source, a coupler, a polarized fiber with high extinction ratio, an UNCED, and a temperature and optical power meter sensor head. Among them, the transmission fiber is polarization-maintaining fiber, which is mainly used to connect bias devices and sensors. The sensing head is a short section of optical fiber, and the angle between the optical fiber transmission and the stress principal axis of the optical fiber sensing head is 45 (Figure 65, 438+0 enlarged part). At the other end of the first sensor, there is a total reflection plated dielectric film. In the process of optical transmission, the polarization mode coupling process is shown in Figure 2, where the Y axis represents the slow axis, the X axis represents the fast axis, and the arrow represents the polarization direction. The light polarized by a broad-spectrum light source through coupling and polarizer propagates along the optical fiber, and its cross section is 1 in Figure 2. The optical fiber transmission link is connected to the sensor head, and the stress axes of the two polarization-maintaining fibers form an angle of 45. The optical fiber sensing will first be carried out between the two polarization eigenmodes, and the cross section is shown as 2 in Figure 2. The reflected light returns along the same route and reaches this point again, and each polarization mode fiber will excite two other orthogonal polarization modes during transmission, and its cross section is shown in Figure 2-3. Four different polarization mode phases: YY'X, YY'Y, YX'X and YX'Y continue to transmit to the optical fiber. After reaching the polarizer, only the polarization mode YX'Y parallel to the polarizer can pass YY'Y and finally reach the power juice, as shown in Figures 2 and 4. Phase difference of
△β is the deviation constant of the transmission polarization axes of the two main optical fibers, L is the length of the first polarization-maintaining optical fiber sensor, and δ polarization only occurs in the first optical fiber sensor. Output signal-to-interference ratio
Where k is the light intensity factor reaching the detector and γ (δ) is the coherence function of the broad spectrum light source. Polarization maintaining fiber, such as pandas or tie type, is set in the coating structure to maintain polarization stress. The results show that the temperature of △ β -200 ~ +400℃ decreases with the increase of linear number, and the linear coefficient is about 10-3, which is negative, while the fiber length changes linearly with the temperature, and the temperature coefficient is about 10-6, which can be ignored. Therefore, δ can be written as
Where a0, a 1 are model coefficients, which can be obtained through calibration experiments; Ambient temperature sensor. From (3) we can see that the interference sensitive to the temperature of optical fiber can be identified by measuring the intensity of output light, and the measurement range and sensitivity can be adjusted by changing the length of optical fiber.
3 experimental research
Based on the structure shown in figure 1, the experimental system is set up. The light source is an SLD broadband light source with an average wavelength of 65438±0300nm and a spectral width of 30nm；. The transmission fiber is panda polarization maintaining fiber; The random access length of the sensor head is11.3 mm. The optical power meter interferes with the output power measurement, and the actual temperature is measured by a thermometer, and the measured data can be recorded by a computer in real time. At -45 ~ +65℃, the output power and temperature curves are shown in Figure 3.
The amplitude change of signal interference is caused by the temperature dependence of the coherence function γ (δ) on δ. For the sake of simple fitting process, assuming that γ (δ) changes linearly, according to Equation (3), there may be the following objective functions.
Taking the test data as the sample (4) as the target, the curve in Figure 3 is fitted, and the model coefficients are listed in Table 1. C 1 stands for γ (δ) and temperature; A 1 is the temperature coefficient of the sensor head; C2 and c3 are deviation values. As can be seen from Figure 3, the measured curve and the fitting curve coincide well near the peak value, and the error is caused by the γ (δ) linear model.
In order to verify the stability of the prototype sensor, the sensor head was placed in a constant 0℃ environment formed by mixing ice and water, and the output change was measured. As can be seen from Figure 3, the output power is about 324μW at 0℃, which is close to the maximum temperature sensitivity coefficient of 46.7 1μW/℃. Fig. 4 shows the curve of time and optical power output, the measuring time is about 30 min, and the sampling interval is 65438±0s. Statistics show that the standard deviation of optical power is 0.45μW, and the corresponding temperature change is 0.0096℃.
More than 4 channels temperature sensor
In large power transformers, it is necessary to monitor all possible hot spot temperatures, so a multi-channel temperature sensor system is necessary. In addition, because the optical fiber components in the transformer are all placed in hot oil, the sensor must meet the requirements of high-voltage insulation, resistance to high temperature above 250℃ and hot oil.
4. 1 sensor program
The 8-channel optical fiber temperature sensing system is shown in Figure 5. The light source is a high-power erbium-doped fiber light source with an output power of more than 65438±00mw, an average wavelength of 65438±0545nm and a spectral width of 365438±0nm. 1 × 9 single-mode fiber power splitter will be divided into 9 equal parts, and one channel will monitor the change of light source; The other eight are used as 8-channel temperature sensing light sources, and each channel adopts the structure shown in figure 1. The photodetector receives the first reflected light signal from the sensor 8, and the output of the detector 9 is digitized by the A/D converter. Firstly, the microprocessor calculates the temperature value according to the model, then sends it to the display module for display, and communicates with the computer in real time through RS232 serial port. The sensing part consists of a polarizer, a 2-meter-long polarization-maintaining fiber transmission and a sensing head, in which the extinction ratio of the polarizer is higher than 28 dB and the insertion loss is less than 0.8 dB. The sensing part and the light source/detection part 8 are connected with the core wire through 100 m single-mode optical cable to realize remote measurement.
4.2 sensor head production and packaging process
In application, the optical fiber and the sensing head are placed in the hot oil of the transformer. Therefore, the panda polarization-maintaining fiber is encapsulated in silica gel to ensure high temperature resistance above 250℃. The length of polarization-maintaining fiber of this lens is 1.9 mm, the loss is 1. 1 dB/km, and the diameter is 250μm m.. Fig. 6 is a schematic view of the sensor head and its package. Fig. 6 (A) shows 8 channels of sensing for each channel, which consists of FC connector, optical fiber polarizer, 2-meter-long panda polarization-maintaining optical fiber and 0.5-mm-long optical fiber sensing head. The capillary Y-shaped seal of Shi Ying optical fiber sensor head and the rear package are shown in Figure 6 (B). Fig. 6 (C) shows a bare optical fiber sensor head with a diameter of 125 micron and a reflective optical film deposited at the end. A capillary tube with an inner diameter of 365,438+09 μ m and a diameter of 436 μ m is used to protect the optical fiber and the sensor head, and its outer surface is coated with a PMMA film with a thickness of 20 μ m.. They ensure that the sensor head is in a harsh environment and has good electrical insulation and durability.
4.3 Linear sensor head model
In equation (4), five parameters need to be calculated, and the temperature calculation process is complicated. By choosing the length of the probe and the precise cutting process, the response of the sensor is close to linear. After linearization, fig. 7 (a) shows the response curve of the sensor head 8. Therefore, the temperature sensor can be obtained by polynomial fitting. Cubic polynomial fitting, fitting the objective function.
T = b0 + b 1x + b2x2 + b3x3 (5)
Where b0 ~ b3 are the coefficients that need model calibration. Fig. 7 (b) is the fitted typical temperature error curve, with the maximum deviation of 0.37℃ between 40℃ and 220℃, and fig. 7 (c) is the temperature value calculated according to the model of formula (5), which shows a good linear relationship.
4.4 Results of prototype calibration and test
In order to verify the measurement accuracy of the optical fiber temperature sensor prototype, a calibration test was carried out in China Great Wall Institute of Technology, and the unit carried out large-scale and high-precision temperature measurement in the national first-class temperature measurement unit. The measuring temperature range is from 0 ~ 200℃ to about 20℃, with a total of 1 1 measuring points. Typical test error is shown in Figure 8, and the measurement error of 8 sensors is within 0.5℃.
5 conclusion
In this paper, a practical optical fiber temperature sensor and related technologies are introduced in detail. A multi-channel temperature sensor which can be used to monitor the winding temperature of power transformer is developed. The small-sized optical fiber temperature sensor developed by special coating, packaging and manufacturing processes meets the requirements of anti-harsh environment and practical application.
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